Reaction Mechanism

REACTION MECHANISM

Contents Topic

Page No.

Theory

01 - 10

Exercise - 1

11 - 19

Exercise - 2

20 - 28

Exercise - 3

29 - 33

Exercise - 4

34 - 35

Answer Key

36 - 40

Syllabus Organic Reagents, Alkyl halides : Nucleophilic substitution reactions, elimination reactions, Alcohols : dehydration, reaction with sodium, phosphorus halides, ZnCl 2/concentrated HCl, Ethers : Preparation by Williamson’s Synthesis; Nucleophilic substitution reactions.

Name : ____

____ ______ ____ ____ ____ ____ ____ Contact ____ ____ ____ ____ No. __ _________________ __________________ _

REACTION MECHANISM (Alkyl halides, Alcohols & Ethers) KEY CONCEPTS Types of solvents

Polar pr protic so solvent

Polar ap aprotic so solvent

Capable of H-bonding

Incapable of H-bonding

• even thought polar protic solvents are not the best SN2 and E2, these reactions are often run in these solvents because because nucleophiles and bases bases are often easily prepared prepared in these solvents.

Physical & Inorganic By

NV Sir B.Tech. IIT Delhi

A-479 Indra vihar, kota Ph. - 9982433693 (NV Sir) 9462729791(VKP Sir)

Organic Chemistry By

VKP Sir M.Sc. IT-BHU

REACTION MECHANISM # 1

REACTION MECHANISM (Alkyl halides, Alcohols & Ethers) KEY CONCEPTS Types of solvents

Polar pr protic so solvent

Polar ap aprotic so solvent

Capable of H-bonding

Incapable of H-bonding

• even thought polar protic solvents are not the best SN2 and E2, these reactions are often run in these solvents because because nucleophiles and bases bases are often easily prepared prepared in these solvents.







Solvents

Non Pol ar

Polar 

Protic 

H2O CH3OH CH3CH2OH H–COOH CH3 –COOH NH3

7.

× × × × × × ×























×



8.

×



×

DMSO–Dimethyl sulphoxide

9.

×



×

DMF DMF – Dimethy Dimethyll formamide formamide

10.

×



×

DMA – Dimethyl Dimethyl acetamide acetamide

11.





×



12.





×



13.

C–C–C–C–C–C 



×



1. 2. 3. 4. 5. 6.




Aprotic

– – – – – –




Organic Reagents Organic reagents can be classified in two categories : (1) (1) Elect Electroph rophile ile : Electron pair acceptor are electrophile. (2) (2) Nucle Nucleoph ophile ile : Electron pair donor are nucelophile.

( 1)

Electrophi les It can be classified into two categories : (A) Charged electrophiles electrophi les (E ) (B) Neutral electrophi l es (E) (A) Charged el electrophiles : Positively charged species in which central atom has incomplete octet is charged electrophile H



, X  R ,

,

= O,

Note : Generally All cations are charged electrophiles except cations of  A, A,  A A group elements, elem ents,

Al+++ and

.

Neutral electrophi le les : It can be classified into three categories : ( B) Neutral covalent compound in which central atom has incomplete octet is neutral

(B)

electrophil e,

••

••

AlX3, C H2 , C X 2 BF3, BCl3, BBr 3, BI 3

( B) Neutral covalent compound in which central atom has complete or expended octet and

central atom has unfilled -d-shell -d-shell is neutral electrophile SnCl 4, SiCl 4, SbCl 5 Note :

(i) Cl2, Br 2 and I 2 also behave as neutral electrophiles. (ii) Electroph Electrophiles iles are are Lew Lewis is acids. acids.

( 2)

Nucleophil es Nucleophiles can be classified into three categories : (A) Charged nucleophil es es : Negatively charged species are charged nucleophiles. , O , R – (B)

, C

3,

,

H, R–

Neutral nu nucleophil es : It can be classified into two categories :

Neutral covalent compound, in which central atom has complete octet, has at least one lone pair of electrons el ectrons and and all atoms present present on central atom should not be electronegative, is neutral nucleophile. , R–

, R2

H– –H, R– –H, H– –H, H3, (C)

R– R–

R H2,

–H –H,

,

R3 ,

–NH2

R– –R R –

R2 H,

(Nitrogen nucleophile) (Ox ygen nucl eophi l es)

– R R3

(Sul phur nucl eophi l es) (Phosphorus nucleophiles)

Ambident nucl eo eophile : Species having two nucleophilic centres, one is neutral (complete

octet and has at least one lone pair of electrons) and other is charged (negative charge) behaves as ambident nucelophile.





–

  

Note :

(A) Organometallic compounds are nucleophiles. (B) Nucleophiles are Lewis bases. Physical & Inorganic By






Organic compounds which behave as electrophile as well as nucleophile :

Organic compound in which carbon is bonded with electronegative atom (O, N, S) by multiple bond/ bonds behaves as electrophile as well as nucleophile. O O || || R CH, R CR, O || R  C  OR ,

O || R  C  Cl

O || R  C  OH ,

O || R  C  NH2 ,

R–CN,

R–  Note : During the course of chemical reaction electrophile reacts with nucleophile.

NUCLEOPHILIC SUBSTITUTION REACTIONS (S N) AT SATURATED CARBON NUCLEOPHILIC SUBSTITUTION REACTIONS (SN) : Replacement (displacement) of an atom or group by an other atom or group in molecule is known as substitution reaction. If substitution reaction is brought about by a nucleophile then it is known as nucleophilic substitution reaction. Generally substitution takes place at sp 3 carbon. R–g +



R – Nu +

UNIMOLECULAR NUCLEOPHILIC SUBSTITUTION REACTIONS (SN1) : Nucleophilic substitution which involves two step process (a) First step : - Slow step involves ionisation to form carbocation

R–g  R+ + g – (b) Second step : - Fast attack of nucleophile on carbocation to result into product .

R+ + Nu –  R–Nu

SN1 Reaction of Alkyl halide Mechanism :

Ionisation of alkyl halide

       

Slow step (rds)

Characteristics of S N1 reactions : 1. It is unimolecular, two step process. 2. Carbocation intermediate is formed so rearrangement is possible in S N1 reaction. 3. It is first order reaction 4. Kinetics of the reaction

Rate  [Alkyl halide]

Rate of SN1 reaction is independent of concentration and reactivity of nucleophile.







5. Energetics of the S N1

Figure : Free energy diagram for the S N1 reaction.

SN1 Reaction of Alcohols (A)

Reaction with hydrogen halides

A common method is to treat the alcohol with a hydrohalic acid, usually HI or HBr. These acids are used to convert alcohols to the corresponding alkyl halides. (i) In acidic solution, an alcohol is in equilibrium with its protonated form. Protonation converts the hydroxy group from a poor leaving group to a good leaving group (H 2O). If the alcohol is protonated all the usual substitution and elim ination reactions are feasible, depending on the structure (1°, 2°, 3°) of the alcohol. (ii) Halides are anions of strong acids, so they are weak bases. Solutions of HBr and HI contain nucleophilic

and

ions.

(iii) Concentrated hydrobromic acid rapidly converts t-Butyl alcohol to t-Butyl bromide. The strong acid protonates the hydroxyl group, converting it to a good leaving group. The hindered tertiary carbon atom cannot undergo SN2 displacement, but it can ionise to a tertiary carbocation. Attack by bromide ion gives the alkyl bromide. The mechanism is similar to other S N1 mechanism. (iv) 1-Butanol reacts with sodium bromide in concentrated sulfuric acid to give 1-Bromobutane by an S N2 displacement. NaBr , H SO

2 4 CH3 (CH2 )2  CH2OH         1  bu tan ol

CH3 (CH2 )2  CH2Br

1  bromobutane (90%)

Protonation converts the hydroxy group to a good leaving group, but ionization to a primary carbocation is unfavourable. The protonated unbranched primary alcohol is well suited for the S N2 displacement. (v) Secondary alcohols also react with HBr to form alkyl bromides usually by the S N1 mechanism. e.g .

HBr    

(vi) HCl (Hydrochloric acid) reacts with alcohols in much the same way that as the hydrobromic acid. (vii) Chloride ion is a weaker nucleophlile than bromide ion because it is small er and less polarizable. Lewis acid, such as ZnCl 2, is sometimes necessary to promote the reaction of HCl with primary and secondary alcohols.







Mechanism :

R –OH

  RDS     R  O H2  R –H2O

Reactivity of

HX :

Reactivity of

ROH :

R– X

HI > HBr > HCl allyl

benzyl > 3° > 2° > 1°

SN1 REACTIONS OF ETHERS (A)

Reaction with HX Ethers are unreactive towards most bases, but they can react under acidic conditions. A protonated ether can undergo substitution or elimination with the expulsion of an alcohol. Ethers react with conc. HBr and HI because these reagents are sufficiently acidic to protonate the ether, while bromide iodide are good nucleophiles for the substitution. If R or R’ is 3º then mechanism will be S N1 otherwise SN2 . Mechanism : R – O – R`

   

– ROH

X     R` – X

SN2 Reaction of Alkyl halide :

 

Mechanism :

+

Characteristic of S N2 1. It is bimolecular, one step concerted process 2. It is second order reaction because in the rds both species are involved

rate  [alkyl halide] [nucleophile] rate = k[alkyl halide] [nucleophile] If the concentration of alkyl halide in the reaction mixture is doubled, the rate of the nucleophilic substitution reaction is double. If the concentration of nucleophile is doubled the rate of reaction is also double. If the concentration of both are doubled then the rate of the reaction quadriples.

3. Kinetics of the reaction

4. Energetics of the reaction :

Figure : A free energy diagrams for S N 2 reaction 5. No intermediates are formed in the SN2 reaction, the reaction proceeds through the formation of an

unstable arrangment of atoms or group called transition state. Physical & Inorganic By






As we seen earlier, in an SN2 mechanism the nucleophile attacks from the back side, that is from the side directly opposite to the leaving group. This mode of attack causes an inversion of configuration at the carbon atom that is the target of nucleophilic attack. This inversion is also known as Walden inversion .

6. The stereochemistry of SN2 reactions

Inversion      

SN2 Reaction of Alcohol (A)

Reaction with HX : The protonated  unbranched primary alcohol is well suited for the S N2 reaction. Mechanism :

X     R – X + H2O R  O H2 

R –OH

CH3 CH2 CH2CH2CH2Cl

e.g CH3 CH2 CH2CH2CH2 OH n  Pentyl alcohol

(B)

n  Pentyl chloride

Reaction with phosphorus trihalides Several phosphorus halides are useful for converting alcohols to alkyl halides. PBr 3, PCl3, & PCl5 work well and are commercially available. Phosphorus halides produce good yields of most primary and secondary alkyl halides, but none works well with tertiary alcohols. The two phosphorus halides used most often are PBr 3 and the P4/I2 combination.

3R – OH + PX3

Mechanism :

3R – X + H3PO3

Step : 1

Step : 2

RCH2X+ HOPX2

Remarks

The mechanism for the reaction involves attack of the alcohol group on the phosphorus atom, displacing a halide ion and forming a protonated alkyl dihalophosphite In second step a halide ion acts as nucleophile to displace HOPX 2, a good leaving group due to the electronegative atoms bonded to the phosphorus. (C)

Reaction with thionyl chloride in presence of pyridine Thionyl chloride (SOCl 2) is often the best reagent for converting an alcohol to an alkyl chloride. The by products (gaseous SO 2 and HCl) leave the reaction mixture and ensure that there can be no reverse reaction.

O || Heat   R – Cl + SO2 + HCl R – OH + Cl  S  Cl     Pyridine







Mechanism :



– H    

 

.. .. R–O – S ..

.. O:

+ HCl

Cl

Chlorosulphite ester

  R – Cl + SO2

In the first step, the nonbonding electrons of the hydroxy oxygen atom attack the electrophilic sulphur atom of thionyl chloride. A chloride ion is expelled a proton and gives test of chloro sulphite ester. Second step is an S N2 mechanism SOCl

2     

e.g.

Py

Reaction with thionyl chloride

ROH + SOCl2   RCl + SO2 + HCl In this mechanism an internal nucleophile attacks from the same side of leaving group , means retension of configuration . It is an S Ni mechanism , where i means internal Mechanism :



 

– H    

.. .. R–O – S ..

.. O:

+ HCl

Cl

Chlorosulphite ester

  R – Cl + SO2

e.g.

SN2 Reaction of Ether (A)

Reaction with HX

A protonated ether can undergo substitution reaction. Ether react with conc. HBr and HI because these reagents are sufficiently acidic to protonate the ether. If R or R’ is 3º then mechanism will be S N1 otherwise SN2.







Mechanism :

+

 

+

X  R

HX     X – R + X – R

alkyl halide

SN Reaction of Epoxide Epoxides are much more reactive than ether because of angle strain in three membered ring therefore epoxide readily undergo nucleophilic substitution reaction. In basic medium mechanism is S N2. Nucleophile atacks on less hindered carbon. Mechanism :

Nu Nu | |  H R — CH — CH2     R — CH — CH2 | | O OH

e.g.

In acidic medium mechanism is S N1 type. Nucleophilic attacks on more substituted carbon. Mechanism :

H

   

Nu | R — CH — CH2 | OH

ELIMINATION REACTIONS : E1 Reaction : Proton and leaving group depart in two different step. (a) First step : - Slow step involves ionisation to form carbocation (b) Second step : Abstraction of proton Mechanism : Step 1 : Formation of the carbocation (RDS)







Step 2 : Base (

) abstracts a proton (fast) + B –H

E2 Reaction :

Dehydrohalogenation is the elimination of a hydrogen and a halogen from an alkyl halide to form an alkene. (i) Hot alcoholic solution of KOH, EtO¯/EtOH (ii) NaNH2 

(iii) t-BuO¯ K in t-BuOH This is one step reaction, hydrogen and leaving group depart simultaneously Mechanism :

 

 

+ BH

(iii)E1 cB Reaction (Unimolecular conjugate base reaction) In the E1 cB, H leaves first and then the X. This is a two step process, the intermediate is a carbanion. Mechanism : Step - 1: Consists of the removal of a proton,

, by a base generating a carbanion

H | | CC X | | Step - 2: Carbanion looses a leaving group to form alkene  

– C  C – | |

Condition : For the E1 cB, substrate must be containing acidic hydrogens and poor leaving groups.







PART - I : OBJECTIVE QUESTIONS * Marked Questions are having more than one correct option.

Section (A) Organic reagent A-1.

Which of the following molecules can behave both as a nucleophile and an electrophile ? (A) CH3NH2

A-2.

(B) CH3Cl

(C) CH3CN

(D) CH3OH

Which one of the following statements is not correct for electrophile : (A) Electron deficient species are electrophile (B) Electrophiles are Lewis acids (C) All + ive charged species are electrophile (D) AlCl 3 and SO 3 are electrophiles

A-3.

Which of the following is an electrophilic reagent ? (B) OH –

(A) H2O

A-4.

(C) NO2+

(D) none

Which of the following reaction intermediates are electrophilic in character ?

A-6.

(a) Carbocation

(b) Carbanion

(c) Free radicals

(d) Carbenes

(A) Only b

(B) a and c

(C) a and d

(D) a, b, c and d

Which one of the following has maximum nucleophilicity ? (A)

A-7.

(B)

(C)

(D)

Which one of the following has maximum nucleophilicity ? (A) CH3 S 

A-8.

(B)

(C) Et3N

(D)

Correct arrangement of the following nucleophiles in the order of their nucleophilic strength is (A) C6H5O – < CH3O – < CH3COO – < OH – (B) CH3COO – < C6H5O – < CH3O – < OH – (C) C6H5O – < CH3COO – < CH3O – < OH –

A-9.*

A-10.

The correct nucleophilicity order is/are : (A) (CH3)3 O  > CH3 

(B) CH3S > CH3SH

(C) CH3CH2CH2O > (CH3)3CO

(D) (CH3CH2)3N > (CH3CH2)3P

Which among the following species is an ambident nucleophile ? 

(A) CH 3 – C H2

A-11.

(D) CH3COO – < C6H5O – < OH – < CH3O –

(B) CH2 = CH2



(C) C N

(D)

••

NH3

Decreasing order of nucleophilicity of the following nucleophile is –

(1) CH3O -

(2) CN

(3)

(4) CH3 CO – 2

(A) 4 > 3 > 2 > 1

(B) 2 > 1 > 4 > 3

(C) 2 > 1 > 3 > 4

(D) 1 > 2 > 3 > 4







Section (B) SN1 Reactions B-1.

Following is the list of four halides. Select correct sequence of decreasing order of reactivity for SN 1 reaction using the answer codes given below :

(1) H5C6

CH

Br

(2) C6H5 –CH2 –Br

(3) H5C6

CH

CH3

I

(4) C6H5 –CH2 –Cl

CH3

Answers codes : (A) 3, 1, 2, 4

B-2.

(B) 1,3, 2, 4

(C) 2, 4, 3, 1

(D) 4, 2, 3, 1

Which one of the following compounds will be most reactiv e for SN1 reactions: I

(A)

Cl O

(B)

Cl

Br O

O

(C)

(D) O

B-3.

Arrange the following compounds in order of decreasing rate of hydrolysis for S N1 reaction: (I)

CH2 –Br

(II) H3C

H3C (III) CH3 –CH2

CH2 –Br

(A) II > III > IV > I

B-4.

(IV)

(B) IV > III > II > I

CH2 –Br

CH

CH2 –Br

H3C

(C) III > IV > II > I

(D) I > II > III > I

Among the bromides I–III given below, the order of reactivity in S N1 reaction is:

(I)

(II)

(III)

(A) III > I > II

(B) III > II > I

(C) II > III > I

(D) II > I > III

B-5.

Which of the f ollowing compounds is most rapidly hydrolysed by S N1 mechanism. (A) C6H5Cl (B) Cl–CH2 –CH = CH2 (C) (C6H5)3CCl (D) C6H5CH2Cl

B-6.

Which among the following will give S N1 reaction C6H5

CH3 (1) H3C

CH

Br

C6H5

(2) H3C

C

Br

(3) CH3 –CH2 –I



C

Cl

CH3

CH3

Select the correct answer from the codes given below : Codes : (A) 1, 2 and 3 (B) 1, 2 and 4 (C) only 3


(4) H5C6

(D) 2 and 4




B-7.

What will be the major product of the following reaction ? H CH3 | | CH3OH, 30C CH3 — C — C — CH — CH3        | | | H CH3 Br

B-8.

H CH3 C H3 | | | (A) CH3 — C — C — C — CH3 | | | OCH3 H H

H | (B) CH3 — C — | H

H | (C) CH3 — C — | H

H CH3 | | (D) CH3 — C — C — | | H CH3

CH3 CH3 | | C — C — CH3 | | OCH3 H

CH3 OCH3 | | C — C — CH3 | | CH3 H H | C — CH2 | | H OCH3

Which one of the following compounds will give (d) and ( ) form in S N1 reaction (as major product)

(A)

B-9.

(B)

(C)

(D)

(C)

(D)

Product,

Identify the major product :

(A)

(B)

Aq.

 (B) AgNO

B-10.

3

Which statement is correct about the above reaction ?

(A) Product (B) is

(B) Product (B) is

(I) by SN1 mechanism.

(II) by SN2 mechanism.

(C) Product (B) is (I) by SN 2 mechanism. (D) Product (B) is (II) by SN 1 mechanism.







Section (C) : SN2 Reactions C-1.

The given compound CH 3 –O–CH2 –Br gives which one of the following reactions: (A) Only SN1 (B) Only SN2 (C) SN1 as well as S N2 (D) E1

C-2.*

In the given pair in which pair the first compound is more reactive than second to S N2 reaction.

(A)

Cl

CH2Cl

Cl

(B)

Cl Cl

Cl (C)

C-3.

Cl

The decreasing order of rate of S N2 reaction is : CH3  C  CH2  Cl || O

CH3 –Cl (I)

(II)

(A) IV > III > II > I

C-4.

(D)

Cl

(B) II > III > I > IV

CH3 –CH2 –Cl (III)

(IV)

(C) II > I > IV > III

(D) none

In the following reaction the most probable product will be :

(A)

(B)

(C)

(D)

(C)

(D)

PBr 3

C-5.

     (X) , X is :

(A)



(B)





SOCl2      Product,

C-6.

Identify the product

(A)

C-7.

(B)

(C)

(D)

Consider the following reaction. SOCl 2      ether In the above reaction which phenomenon will take place :

(A) Inversion

(B) Retention

(C) Racemisation

(D) Isomerisation

C-8.*

Which of the foll owing conditions favour SN2 mechanism ? (A) Strong nucleophile (B) High conc. of nucleophile (C) 3º alkyl halide (D) Polar protic solvent

C-9.*

3OH Y  CH    

H O18

 2    X, Identify X & Y : H

H

(A) X =

(B) Y =

(C) X =

(D) Y =

vkSj Y gSa %

Section (D) : E1 Reaction D-1.

D-2.

Which one of the f ollowing compounds undergoes E1 reaction most readily ?

(A)

(B) CH3 – CH2 – CH2 – Br

(C) CH3 – CH2 – CH2 – I

(D)

Which of the following will be most reactive f or E1 reaction ?

(A)



(B)

(C)


(D)




D-3.

CH3OH       [X] 

In the given reaction,

[X] as the major product among the elimination products is : CH3 | C = CH2

(A)

(B)

(C)

(D)

H SO 4  2    

D-4.

In the above reaction the major product is shown, which is f ormed through the intermediate (carbocation) given below :

Which bond will migrate to form the above product ? (A) p (B) q (C) r

D-5.

(D) s

Major product is :

(A)

(B)

(C)

(D)

Section (E) : E2 Reaction E-1.

Which of the following cannot undergo E2 reaction ? (A)

E-2.

(B)

(C)

(D) none of these

Arrange the following in decreasing order of stability of their transition state during elimination by strong base

(A) II > I > III

(B) II > III > I

(C) I > III > II

(D) I > II > III

E-3.

2-Bromopentane is heated with potassium ethoxide in ethanol. The major product obtained is (A) 2-Ethoxypentane (B) pent-1-ene (C) cis-pent-2-ene (D) trans-pent-2-ene

E-4.

In which of the f ollowing reaction, regioselectivity can be observed.

CH3

CH3

alc. KOH /  (A) CH3–C–CH2–Cl      

alc. KOH /  (B) CH3–C–CH3      

H

Cl

CH3

CH3

alc. KOH /  (C) CH3–C–CH2–CH 3      

alc. KOH /  (D) CH3–C–CH2–Cl      

CH3

Cl Physical & Inorganic By






E-5.

The most probable product in the following reaction is :

Trans

(A)

E-6.

(B)

(C)

(D)

Which one of the following hexachlorocyclohexane is least reactive and which one is most reactive f or E2 reactions with a strong base for dehydrohalogenation.

(A) I least & II most

(B) II least & I most

(C) III least & I most

(D) III least & II most

(C)

(D) All of these

Section (F) : E1cB Reaction F-1.

E1 cB reaction is given by which of the f ollowing : (A) CF3 – CHCl2

(B)

PART - II : MISLLANEOUS QUESTIONS

Comprehensions Type Questions : Comprehension # 1 Read the following passage carefully and answer the questions. One of the most interesti ng and useful aspects of stereochemi stry is the study of what happens to optically acitive molecules when they react. The product isolated from the reaction of a chiral starting material can tell us a great deal about the reaction mechanism . W e observe SN2 --------- Inversion of configuration SN1 --------- Racemisation SNi --------- Retention of configuration

1.

In the giv en reaction, mention reaction mechanism respectively

()

()

 

 

 

()

 

(V)

(A) SN2 , SN1 , SN2 , SNi

(B) SN2 , SN1 , SNi , SN2

(C) SN1 , SN2 , SNi , SN1

(D) SN2 , SNi , SN1 , SN2



+





2.

The given reaction is an example of which type of Mechanism ? C3H7 Ether      D

Cl + SO2 + HCl C6H5

(A) SN2

3.

(B) SN1

(C) SNi

(D) None

In which of the f ollowing reaction retention of configuration is observed ?

H Nal / acetone      Br  

(A) CH3

 

(B)

D CH3

C2H5  Br + HOH 

(C) CH3

Na

   OTs  

(D) H

C2H5

C3H7

Comprehension # 2 Alcohols undergo acid catalysed elim ination reactions to produce alkenes. Because water is lost in the elimination , this reaction is called dehydration reaction. Secondary and tertiary alcohols always give E1 reaction in dehydration. Prim ary alcohols whose -carbon is branched also give E1 reaction. The reactiv ity of alcohol for elimination reaction is tertiary alcohol > Secondary alcohol > Primary alcohol.

4.

conc .H2SO 4        Alkenes

In the given reaction :



Total number of alkenes (Including stereo isomers) formed will be (A) Two (B) Three (C) Four

5.

(D) Five

Which of the foll owing dehydration product (major) is incorrect ? (A)

conc .H2SO 4        

conc .H SO

2 4       

(B)



conc .H SO

2  4 (C) CH3 CH2 CH2CH2OH      CH3 –CH=CH–CH3



(D)

6.

conc .H2SO 4        

Identify the product in the given reaction :

(A)



(B)

Product

(C)


(D)




Assertion / Reasoning Type Questions DIRECTIONS : Each question has 5 choices (A), (B), (C), (D) and (E) out of which ONLY ONE is correct. (A) Statement-1 is True, Statement-2 is True; Statement-2 is a correct explanation for Statement-1. (B) Statement-1 is True, Statement-2 is True; Statement-2 is NOT a correct explanation for Statement-1. (C) Statement-1 is True, Statement-2 is False. (D) Statement-1 is False, Statement-2 is True. (E) Statement-1 and Statement-2 both are False.

7.

Statement-1 : If the mixture of

and

reacts with the excess of NaSH in DMF,,

the molecuarity of the S N2 reaction will be two but not t hree. ..

Statement-2 : In the SN2 reaction two molecules e.g. R – X and N u take part in the formation of transition state.

8.

Statement-1 : Iodide (I –) is the strongest nucleophile (in protic solvent) among the halide anions Statement-2 : Iodide is the largest halide anion and is the most weakly solvated in a protic solvent

Match the column Type Questions 9.

Match the following :







PART - I : MIXED OBJECTIVE

Single Choice Questions 1.

Which one is the strongest nucleophilic site in the following species ?

(A) 1

2.

(B) 2

(D) 4

Decreasing order of relativ e nucleophilicity of the following nucleophiles in protic solvent is ,

3.

(C) 3

,

,

(A)

>

> H2O >

(C)

>

>

, >

> H2O >

H2O (B)

>

>

(D)

>

>

>

> H2O

>

> H2O

Which of the following is not expected to be intermediate of the following reaction ?

H O

2    

(A)

4.

(B)

(C)

(D)

Which describes the best stereochemical aspects of the following reaction ? HBr

    Product

(A) Inversion of configuration occurs at the carbon undergoing substitution. (B) Retention of configuration occurs at the carbon undergoing substitution. (C) Racemization occurs at the carbon undergoing substitution. (D) The carbon undergoing substitution is not stereogenic.

5.

When the concentration of alkyl halide is tripled and the concentration of of SN2 reaction increases by : (A) 3 times (B) 2 times (C) 1.5 times




ion is reduced to half, the rate (D) 6 times




SOCl

6.

2      Product pyridine, 

Product of the above reaction is :

(A)

(B)

(C)

(D)

CH2OH

7.

Conc. H2SO 4         [X] as major product

In the given reaction :

[X] will be :

(A)

CH2

CH3

(B)

(C)

CH3

(D)

8.

The product X and Y are respectively :

(A)

&

(B)

&

(C)

&

(D)

9.

&

Which of the following will undergo fastest elimination reaction with alcoholic KOH.

(A)



(B)

(C)


(D)




10.

X and Y are respectively : (A)

and

(B)

(C)

and

(D)

and

and

More than one correct answer 11.

Which of these statements are correct about nucleophiles : (A) Nucleophiles have an unshared electron pair and can make use of this to react with an electron deficient species. (B) The nucleophilicity of an element (as electron donor) generally increases on going down a group in the periodic table. (C) A nucleophile is electron-deficient species (D) All good nucleophiles are good bases when we deal across the period.

12.

Which of the following is / are true for S N1 reactions ? (A) They occur through a single step concerted reaction. (B) They are favoured by polar solvents. (C) 3º alkyl halides generally react through this mechanism. (D) Concentration of nucleophile does not affect t he rate of such reactions.

13.

SN2 reaction will be negligible in Br

(A)

14.

15.

(B)

(C)

Rate of SN2 depends on (A) Conc of Nucleophile (C) Nature of leaving group

(D)

(B) Conc of substrate (D) Nature of solvent

Which of the following reaction is correct ? (A) 1, 4-Dichlorohexane + NaI

Acetone



1eq.

 CH3  CH2  CH  CH2  CH2  CH2 + NaCl | | I Cl

 (B) 1, 2-Dibromoethane + NaS – CH 2 – CH2 – SNa 

+ 2NaBr

Et O

2    C4H8ClONa  (C) 4-Chloro-1-butanol + NaH    

NH

CH I

3 3   C3H3Na      C4H6 (D) Propyne + NaNH 2   







PART - II : SUBJECTIVE QUESTIONS 1.

Arrange the given species in decreasing order of their nucleophilicity ? H2O

2.

Write nucleophilicity order in the f ollowing compound.

3.

Of the following pairs, which is the faster SN 1 reactions? (a) (c)

(a) (c)

4.

and

(b)

and

(d)

rFkk

and CH 3(CH2)5Cl and

(b)

rFkk

(d)

rFkk

CH3(CH2)5Cl

rFkk

Give the decreasing order of SN 1 reactions of the following compounds in H 2O.

(a) (i)

(ii)

(iii)

(b) (i)

(ii)

(iii)

(c) (i)

(ii)

(iii)

5.

(a)

(b)

6.

What eff ect do you expect due to following changes in S N1 reaction of (CH3)3CBr with CH3OH ? (a) The Concentration of (CH 3)3CBr is doubled and that of CH 3OH is halved. (b) The concentration of both (CH 3)3CBr and CH3OH are tripled.







7.

Which SN1 reaction of each pair would you expect to take place more rapidly ? Explain.  Me COH + HCl (a) (i) Me3CCl + H2O  3  Me COH + HBr (ii) Me3CBr + H2O  3  Me COH + HCl (b) (i) Me3CCl + H2O  3  Me COCH + HCl (ii) Me3CCl + CH3OH  3 3

8.

Explain how AgNO3 increases the rate of solvolysis in SN1 reactions of alkyl halide

9.

Complete the following reactions and predict the nature of solution after product formation.

H2O

(a)

10.

      P + Q Acetone 25C

(b)

R + S

Write the mechanism of the following reaction and mention the rate determining step.

(a)

(b)

HI

11.

  A + B, (1eq)

Identify A and B.

12.

Propose a mechanism for the following reaction

O (a)

O



 CH 3 O / CH 3 OH CH 3  CH  CH  CH 2     CH 3  CH  CH  CH 2OCH 3 + Cl | Cl



H (b) HO–CH2 –CH2 –CH2 –CH2 –OH   

13.

O

+ HOH

How many structural isomers of monobromo cyclohexene are possible. W hich will react most readily with NaCN?







14.

Which of the following reactions (a) and (b) would give a better yield of the desired ether as product.

 

(a)

 

(b)

15.

Explain why thionyl chloride is considered the best reagent for converting alcohols to alkyl chlorides.

16.

Predict the product of the f ollowing reactions using one equivalent of H I.

(a)

17.

(b)

(c)

Give the products of the foll owing reactions

(i)

18.

(d)

X

(ii)

Y

Few dialkyl ethers & cyclic ethers were allowed to react with excess of HBr with the following results. Identify the ether in each case.

HBr

   (P)  excess

+

CH3 – CH2 – CH2 – CH2 – Br

HBr

   (Q)  excess

HBr

   (R)  excess

HBr

   (S)  excess

19.

Observe the f ollowing reactions. O || CH3 – C – F O || CH3 – C – Br

H2O

    r 1

H O

 2  

O || CH3 – C – Cl

 2  

O || CH3 – C – I

 2  

r 3

H O r 2

H O r 4

Identify the order of the rate of reaction r 1, r 2, r 3 and r 4, give reason.







20.

Predict the products of the foll owing reactions

(i)

+

 

O || (ii) Ph  C  Cl

+

 

21.

Explain the order of the rate of esterification of the following acid with MeOH : MeCH2COOH > Me2CHCOOH > Me3CCOOH > Et3CCOOH >> (i-Pr) 2CHCOOH

22.

Write the correct reactivity order of nucleophilic substitution reaction for the following compounds.

23.

Write the correct reactiv ity order with NaOH of following compounds.

24.

When 1-Bromo-1-methylcyclohexane is heated in ethanol f or an extended period of ti me, three products result: one ether and two alkenes. Predict the products of this reacti on, and propose a mechanism for their formation. Also, mention the major elimination product.

25.

Predict the major product of E1 elim ination of the following compounds.

(a)

(b)

(c)

(d)

26.

Why dehydration of alcohol takes place in acidic medium but not in basic medium.

27.

1º alcohols are poor starting material for synthesis of 1-Alkene. Explain

28.

What happens when ethanol is heated with conc. H 2SO4 at 453 K. Explain the mechanism of this reaction.







29.

Predict the major product of the sulphuric acid catalysed dehydration of the following alcohols : (a) (CH3)2C(OH)CH2CH3

(b) CH3CH2 CH2CH(OH)CH3

(c) (CH3)2C(OH)CH(CH3)2

(d) (CH3)3CCH2OH

H     Product, 

30.

Write the major product of the reaction

Conc. H3PO4         A (Major) + B (Minor),

31.



Write structures of A and B

32.

Among the various isomers of C4H9Br which would undergo elimination m ost readily and why ?

33.

A halide with formula C6H13I is found to give two isomeric alkenes 2-methyl-2-pentene and 4-methyl-2pentene on dehydrohalogenation with alcoholic KOH. Suggest its structure.

34.

Bromocyclodecane on heating with ethanolic KOH, produces two alkenes Write the two products also mention the major one.

35.

A compound (X) [mol formula (C7H13Br)] on dehydrohalogenation gives only v inylcyclopentane, Write the strucuture of X.

36.

Explain why more alkylated alkene is formed predominatly if base is is obtained majorly when

37.

base is used.

Give the product of elimination reaction with its stereochemistry

(a)

38.

, while less alkylated alkene

(b)

?

Which alkyl chloride would yield following pure alkene on reaction with alcoholic KOH ?







CH3 | (i) CH3  C  CH2

39.

(ii) CH3 – CH2 – CH2 – CH = CH2

(iii) CH3  CH2  C  CH2 | CH3

The sum of molecular weights of the major products P and Q form at t he followng reaction is

(i)

KOH /  + CH3I         P (organic product)

(ii)

KOH /  Q (organic product) + CH3CH2OH      

40.

What are the essential conditons for any reaction to show E1cB mechanism ?

41.

Write down the increasing order of reactivity of following compounds via E1cB mechanism in basic medium (a) C6H5 – CH2 – CH2 – Br

CF3 – CH2 – CF3

(b)

42.

NO2 – CH2 – CH2 – Br

If ethanol containing EtOD is used as solvent, then deuterium exchange take place in E1cB mechanism. Why?

43.

+

Write down the mechanism of the above reaction







PART - I : JEE

PROBLEMS (PREVIOUS YEARS)

* Marked Questions are having more than one correct option.

1.

[JEE-2001, 1/135]

An SN2 reaction at an asymmetric carbon of a compound always gives (A) an enantiomer of the substrate (C) a mixture of diastereomers

(B) a product with opposite optical rotation (D) a single stereoisomer

2.

Explain why 7-bromo-1, 3, 5-cycloheptatriene exist as an ion while 5-Bromo-1, 3-cyclopentadiene does not form any ion ev en in the presence of Ag + . Explain why ? [JEE 2004, 4/60]

3.

Compound (X)

is reacted with aqueous acetone it gives following

[JEE-(scr.)2005, 3/144]

products.

(A) K, L

4.

(B) K, M

(C) L only

(D) M only

[JEE(M)-05, 1/144]

Explain the following observations

Acidic solution

Neutral solution

5.

Which is the best reagent to convert cyclohexanol into cyclohexene. (A) conc. HCl (B) conc. HBr (C) conc. H3PO4

[JEE-2005, 3/144] (D) HCl + ZnCl 2

H     X, Identify X. 

6.




[JEE-2005, 2/144]




7.

Match the following (one term in column-I may m atch with more than one terms in column-II)

[JEE-2006, 6/184]

8.

[JEE-2008, 3/163]

The major product of the foll owing reaction is

(A)

9.

(B)

(C)

(D)

In the following carbocation; H/CH 3 that is most likely to migrate to the positiviely charged carbon is

[JEE-2009, 3/160]

(A) CH3 at C-4

10.

In the reaction (A) Br

(B) H at C-4

(C) CH3 at C-2

HBr OCH3     the products are :

OCH3 and H2 Br and CH3OH

(C)

(D) H at C-2

[JEE-2010, 3/163]

(B)

Br and CH3Br

(D)

OH and CH3Br

11.

The total number of alkenes possible by dehydrobromination of 3-bromo-3-cycl opentylhexane using alcoholic [JEE 2011 160/4] KOH is :

12.

The compound that undergoes decarboxylation most readily under mild condition is - [JEE-2012, 3/136]

COOH (A)



COOH CH2COOH (B)

COOH O

CH2 COOH COOH

(C)


O (D)




PART - II : 1.

AIEEE PROBLEMS (PREVIOUS YEARS)

Following reaction  (CH3)3COH + HBr (CH3)3CBr + H2O 

is an example of : (1) Elimination reaction (3) Nucleophilic substitution

2.

3.

[AIEEE-2002] (2) Free radical substitution (4) Electrophilic substitution

SN1 reaction is feasible in :

[AIEEE-2002]

(1)

 + KOH (aq.) 

(2)

(3)

 + KOH (aq.) 

(4)

 + KOH (aq.) 

 + KOH (aq.) 

[AIEEE-2002]

Maximum dehydration takes place that of :

(1)

(2)

(3)

(4)

4.

Tertiary alkyl halides are practically inert to substitution by S N2 mechanism because of (1) steric hindrance (2) inductive effect (3) instability (4) insolubility

[AIEEE-2005]

5.

The decreasing order of nucleophilicity among the nucleophiles

[AIEEE-2005]

6.

7.

(a)

(b)

(c)

(d)

(1) (c), (b), (a), (d)

(2) (b), (c), (a), (d)

(3) (d), (c), (b), (a)

(4) (a), (b), (c), (d)

[AIEEE-2005]

The decreasing order of nucleophilicity among the nucleophiles

(a)

(b)

(c)

(d)

(1) (c), (b), (a), (d)

(2) (b), (c), (a), (d)

(3) (d), (c), (b), (a)

(4) (a), (b), (c), (d)

Fluorobenzene (C 6H5F) can be synthesized in the laboratory (1) from aniline by diazotisation followed by heating the diazonium salt with HBF 4 (2) by direct fluorination of benzene with F 2 gas (3) by reacting bromobenzene with NaF solution (4) by heating phenol with HF and KF




[AIEEE-2006]




8.

–  CH – Nu + Br CH3Br + Nu –  3 The decreasing order of the rate of the above reaction with nucleophiles (Nu ¯ ) A to D is :

[Nu ¯ = (A)  PhO ¯ , (1) D > C > B > A

9.

(B)  AcO ¯ ,

(C)  HO ¯ , (D)  CH3O ¯ ]

(2) A > B > C > D

(3) B > D > C > A

[AIEEE-2006] (4) D > C > A > B

[AIEEE-2006]

The structure of the major product formed in the f ollowing reaction is NaCN DMF

    

CH2Cl

(1)

CH2Cl

(2)

CH2CN

(3)

CH2CN

(4)

CN 

10.

CN

CN



Reaction of trans 2-phenyl-1-bromocyclopentane on reaction with alcoholic KOH produces : [AIEEE-2006] (1) 2–phenylcyclopentene (2) 1–phenylcyclopentene (3) 3-phenylcyclopentene (4) 4-phenylcyclopentene

11.

[AIEEE-2006]

The alkene formed as a major product in the above elimination reaction is :

(1)

(2) CH2 = CH2

(3)

(4)

12.

Which of the following is the correct order of decreasing S N2 reactivity ? (1) RCH2X > R3CX > R2CHX (2) RCH2X > R2CHX > R3CX (3) R3CX > R2CHX > RCH2X (4) R2CHX > R3CX > RCH2X

[AIEEE-2007, 3/120]

13.

The organic chloro compound, which shows complete stereochemical i nversion during an S N2 reaction, is

[AIEEE-2008, 3/105]

(1) (CH3)3CCl

(2) (CH3)2CHCl

(3) CH3Cl

(4) (C2H5)2CHCl

14.

Which of the following on heating with aqueous KOH, produces acetaldehyde ? [AIEEE-2009, 4/144] (1) CH3CH2Cl (2) CH2ClCH2Cl (3) CH3CHCl2 (4) CH3COCl

15.

From amongst the following alcohols the one that would react fastest with conc. HC l and anhydrous ZnCl 2, is : [AIEEE-2010, 4/144] (1) 2-Butanol (2) 2-Methylpropan-2-ol (3) 2-Methylpropanol (4) 1-Butanol

16.

Consider the following bromides :

The correct, order of S N1 reactivity is (1) B > C > A (2) B > A > C



[AIEEE-2010, 4/144]

(3) C > B > A


(4) A > B > C




17.

[AIEEE-2010, 4/144]

In the chemical reactions : NH2 HBF4 NaNO 2   B       A    HCl, 278 K the compounds ‘A” and ‘B’ respectively are

18.

(1) nitrobenzene and fluorobenzene

(2) phenol and benzene

(3) benzene diazonium chloride and fluorobenzene

(4) nitrobenzene and chlorobenzene

[AIEEE-2010, 4/144]

The main product of the foll owing reaction is : conc. H2SO4 C6H5CH2CH(OH)CH(CH3)2       

19.

(1)

(2)

(3)

(4)

An unknown alcohol is treated with the “Lucas reagent” to determine whether the alcohol is primary, secondary or tertiary. Which alcohol reacts fastest and by what mechanism : [JEE Mains 2013] (1) secondary alcohol by S N1 (2) tertiary alcohol by S N1 (3) secondary alcohol by S N2 (4) tertiary alcohol by S N2







NCERT QUESTIONS 1.

Write the isomers of the compound having formula C 4H9Br.

2.

Write the equations for the preparation of 1-iodobutane from (i) 1-butanol (ii) 1-chlorobutane (iii) but-1-ene.

3.

What are ambident nucleophiles? Explain with an example.

4.

Which compound in each of the following pairs will react faster in S N2 reaction with –OH? (i) CH3Br or CH3I (ii) (CH3)3CCl or CH3Cl

5.

Predict all the alkenes that would be formed by dehydrohalogenation of the following halides with sodium ethoxide in ethanol and identify the m ajor alkene: (i) 1-Bromo-1-methylcyclohexane (ii) 2-Chloro-2-methylbutane (iii) 2,2,3-Trimethyl-3-bromopentane.

6.

Write the structure of the major organic product in each of the following reactions: acetone

(i) CH3CH2CH2Cl + NaI

 

(ii) (CH3)3CBr + KOH

 

heat

ethanol heat

(iii) CH3CH(Br)CH2CH3 + NaOH (iv) CH3CH2Br + KCN (v) C6H5ONa + C2H5Cl (vi) CH3CH2CH2OH + SOCl2 (vii) CH3CH2CH = CH2 + HBr (viii) CH3CH = C(CH3)2 + HBr

7.

water   aq.ethanol

   peroxide

  

Write the mechanism of the f ollowing reaction : 

E tOH H2 O nBuBr + KCN   nBuCN

8.

Arrange the compounds of each set in order of reactivity towards SN2 displacement : (i) 2-Bromo-2-methylbutane, 1-Bromopentane, 2-Bromopentane (ii) 1-Bromo-3-methylbutane, 2-Bromo-2-methylbutane, 2-Bromo-3-methylbutane (iii) 1-Bromobutane, 1-Bromo-2,2-dimethylpropane, 1-Bromo-2-methylbutane, 1-Bromo-3-methylbutane.

9.

Out of C6H5CH2Cl and C6H5CHClC6H5, which is more easily hydrolysed by aqueous KOH.

10.

p-Dichlorobenzene has higher m.p. and solubility than those of o- and m-isomers. Discuss.

11.

The treatment of alkyl chlorides with aqueous KOH leads to the formation of alcohols but in the presence of alcoholic KOH, alkenes are major products. Explain.

12.

Primary alkyl halide C 4H9Br (a) reacted with alcoholic KOH to give compound (b). Compound (b) is reacted with HBr to give (c) which is an isomer of (a). When (a) is reacted with sodium metal it gives compound (d), C8H18 which is different from the compound formed when n-butyl bromide is reacted with sodium. Give the structural formula of (a) and write the equations for all the reactions.







13.

What happens when (i) n-butyl chloride is t reated with alcoholic KOH, (ii) ethyl chloride is treated with aqueous KOH, (iii) methyl chloride is treated with KCN?

14.

Explain why propanol has higher boiling point than that of the hydrocarbon, butane?

15.

Alcohols are comparatively more soluble in water than hydrocarbons of comparable molecular masses. Explain this fact.

16.

Show how will you synthesise: (i) cyclohexylmethanol using an alkyl halide by an SN2 reaction. (ii) pentan-1-ol using a suitable alkyl halide?

17.

Give reason for the higher boiling point of ethanol in comparison to methoxymethane.

18.

Write the names of reagents and equations for the preparation of the following ethers by Williamson’s synthesis: (i) 1-Propoxypropane (ii) Ethoxybenzene (iii) 2-Methoxy-2-methylpropane (iv) 1-Methoxyethane

19.

Illustrate with examples the limitations of W illiamson synthesis for the preparation of certain types of ethers.

20.

How is 1-propoxypropane synthesised from propan-1-ol? W rite mechanism of this reaction.

21.

Preparation of ethers by acid dehydration of secondary or tertiary alcohols is not a suitable method. Give reason.

22.

Write the equation of the reaction of hydrogen iodide with : (i) 1-propoxypropane (ii) methoxybenzene and (iii) benzyl ethyl ether.

23.

Explain the fact that in aryl alkyl ethers (i) the alkoxy group activates the benzene ring towards electrophilic substitution and (ii) it directs the incoming substituents to ortho and para positions in benzene ring.

24.

Write the mechanism of the reaction of HI with methoxym ethane.

25.

When 3-methylbutan-2-ol is treated with HBr, the foll owing reaction takes place:

Give a mechanism for this reaction. (Hint : The secondary carbocation formed in step II rearranges to a more stable tertiary carbocation by a hydride ion shift from 3rd carbon atom.







Exercise # 1

PART - I A-1.

(C)

A-2.

(C)

A-3.

(C)

A-4.

(C)

A-6.

(A)

A-7.

(A)

A-8.

(D)

A-9.* B-5. C-2.* C-9.* E-2.

(BC) (C) (BD) (AD) (A)

A-10. B-6. C-3. D-1. E-3.

(C) (B) (C) (D) (D)

A-11. B-7. C-4. D-2. E-4.

(B) (C) (B) (A) (C)

B-1. B-8. C-5. D-3. E-5.

(A) (B) (A) (C) (C)

B-2. B-9. C-6. D-4. E-6.

(A) (A) (A) (B) (B)

B-3. B-10. C-7. D-5. F-1.

(A) (D) (B) (C) (D)

B-4. C-1. C-8.* E-1.

(A) (C) (AB) (C)

(B)

6.

(C)

7.

(A)

(C)

6.

(A)

7.

(B)

PART - II 1. 8.

(B) (A)

2. 9.

(C) 3. (B) 4. (C) 5. (A) - p, r, s ; (B) - r, s ; (C) - r, s ; (D) - p, q

Exercise # 2

PART - I 1.

(D)

2.

(B)

3.

(A)

4.

(C)

5.

8.

(A)

9.

(A)

10.

(A)

11.

(ABD) 12.

14.

(ABCD) 15.

(BCD) 13.

(ABC)

(ABCD)

PART - II 1.

>

>

> H2O

2.

III > II > I > IV

(a) (i), (b) i, (C)i (d) i

4.

a. (iii) > (ii) > (i)

5.

(a)

6.

(a) Rate - doubled ;

7.

(a) ii, (b) i

8.

Ag+ ion has strong affinity for X – than a solvent molecule, the precipitation of AgX accelerates the dissociation of R–X bond.

9.

(a) P =

,

(b) R =

, S=



b. (iii) > (i) > (ii)

3.

c. (ii) > (i) > (iii) (allylic > 3° > 1°)

(b)

(b) Rate - tripled

Q=

pH is less than seven because acid HBr is formed.

pH is less than seven because acid HCl is formed





10.

CH3 | (a) CH3  CH  CH  CH3 | OH

Slow     

rds

3  Methyl  2  butanol

 2      HO

(b)

rds

+ CH3OH

11.



12.

(a)

CH O / CH OH

3 3     

S N 2

N 2 CH3  CH  CH  CH 2  OCH3 S    CH3–CH–CH–CH2–O–CH3    CH3–CH–CH–CH2–O–CH3 | |  O Cl OH O

H

 H

  (b) HO–(CH 2)3 –CH2OH     ( CH ) CH O   H – O 2 3 2 

H H

 H 2O

  





H O    H

O

13.

Three

will react most readily with NaCN.

14.

(a) Give better yield, because in the (b) nucleophile attacks on more hindered 2° carbon. So along with substitution, elimination also posible in the (b).

15.

 R–Cl + HCl  + SO2  ROH + SOCl2  gases Pure RCl is obtained, as the by products (HCl and SO 2) are gases and get evaporated from the reaction mixture.

16.

17.

(a) I–H2C – CH2 – CH2 – CH2 – CH2 –OH

(b)

(c)

(d)

(i) X = CH3CH(OH)CH2OH

(ii) Y = CH3CH(OH)CH2OMe




+ CH3I




18.

P=

Q=

R=

S=

CH2OCH2

19.

r 4 > r 3 > r 2 > r 1

rate of SN2 Th is inversly proportional to basicity of leav ing group.

20.

(i)

21.

As the size of the substituents on the  C increases, the tetrahedrally bonded intermediate becomes more crowded. The greater the crowding, the larger is H+ of the TS and the slower is the reaction.

22.

II > I > IV > III

23.

25.

(a)

(b)

26.

In basic condition very poor leaving group –OH will eliminate but in acidic m edium –OH will be converted  into – O H which is very good leaving group.

(ii)

II > III > I

24.

+

(c)

(d)

2



27.

1º carbocation R – CH2 would rearrange and 2-alkene would result. Ev en if 1-alkene is also formed but, it would tend to rearrange in acidic medium to 2-alkene.

28.

What happens when ethanol is heated with conc. H 2SO4 at 453 K. Explain the mechanism of this reaction.

Ans.

(a) CH3—CH2— O —H + H+ ..

..

Ethanol

CH3—CH2—

—H

Oxonium ion slow     CH2=CH2 + H2O

(b)

CH3

29.

(a)

CH3

C

CH3

(c)

CH3

CH3 C

CH3

(b) CH3 – CH2 – CH = CH – CH3

CHCH3

C

(d)

CH3

CH3

C

CHCH3

CH3 H

    H O

30.

31.

H3C

+

2

A :



– H     

B:





32.

Tert-butyl bromide would undergo elimination most readily as the alkene formed from it would be more stable.

33.

34.

+

E2    

35.

The product is formed only by E2 eli mination.

HBr

36.

t-

37.

(a)

38.

(i)

39.

196

is bulky base so Hoffmann product is formed as major product.

(b)

or

; (ii) CH3CH2CH2CH2CH2Cl ; (iii) CH3CH2CHCH2Cl | CH3

O

P=

CH3

[M.F. C7H14O

M.W. 84+14+16 = 114]

[M.F. C6H10

M. W. 72 + 10 = 82]

CH3

Q=

CH3

M.W. (P + Q) = 114 + 82 = 196 40.

Reactant should have acidic -hydrogen and poor leaving group.

41.

(a) C6H5 – CH2 – CH2 – Br < CF 3 – CH2 – CF3

42.

In E1cB m echanism carbanion is f ormed as intermediate and 1st step is rev ersible.

(b) NO2 – CH2 – CH2 – Br <

43.

2nd step

In E1cB 1st step is rev ersible step so deuterium exchange take place







Reaction Mechanism

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