(4) Addition of HBr to Alkenes in The Presence of Peroxides •
(Anti Markovnikov’s rule) When alkenes are treated with HBr in the presence of peroxides, ROOR (eg: H 2O2) the addition occurs in an anti-Markovnikov manner
•
The hydrogen atom of HBr attached to the doubly bonded carbon with fewer hydrogen atoms.
Example: H2C
CH CH3
+
ROOR HBr Br CH2CH2CH3
Complete the following reactions: (1) H3C
CH2C
+
CH2
HBr
H2 O2
CH3
(2) CH2
+
HBr
H2 O2
(5) Hydration of Alkene •
The acid-catalyzed addition of water to the double bond of an alkene (hydration of an alkene) is a method for the preparation of low molecular weight alcohols.
•
The acid most commonly used to catalyze the hydration of alkenes are dilute solution of sulphuric acid and phosphoric acid.
• The addition of water to water to the double bond follows Markovnikov’s rule. rule.
+
C C
+
H2O
H3O
C C H OH
Example: +
H3C
C
CH2
+
H3 O
H2 O
CH3 OH H3C
C CH3
CH3
Complete the following reactions: (1) CH2
+
H2 O
H3 O
+
(2) H2C
CH CH3
+
H2 O
H3 O
+
(6) Addition of Sulphuric Acid to Alkenes •
Alkenes dissolve in concentrated sulphuric acid to form alkyl hydrogen sulphates. sulphates .
•
Alkyl hydrogen sulfates can be easily hydrolyzed to alcohols by heating them with water.
•
The overall result of the addition of sulphuric acid to alkenes followed by hydrolysis is the Markovnikov addition of -H and -OH -OH..
H3C
CH3 C
H3C
C
CH3CH3 conc. H2 SO 4
H3 C
CH3CH3 heat
C
C H3
H OSO 3 H alkyl hydrogen sulphate
CH3
H2 O
C
H3C
C
C
H
OH
C H3
• Example:
HC
CH CH
2
OSO H
conc.H SO 2
3
4
HC
3
CH CH
3
2
H OH
H O 2
heat
H C
CH CH
3
2
H
6 ) Oxidation of Alkenes Alkenes
undergo a number of reactions in which the carbon-carbon double bond is oxidized
KMnO4
cold & dilute KMnO4
hot , OH- or H+
ozonolysis
i) With cold and dilute potassium permanganate, KMnO4 •
Potassium permanganate in base can be used to oxidize alkenes to 1,2-diols (glycols).
-
C
C
+
KMnO4 (purple)
C
C
+
OH OH
OH ,cold
MnO2 (brown precipitate)
•
This reaction is called Baeyer’s test. test.
•
It can serve as a test for the presence of carbon-carbon double bonds where the purple colour of the KMnO 4 decolourised, and brown precipitate of MnO 2 is formed.
Example: -
H2C
CH2
+ H
KMnO4 H
H
C
C
OH OH
OH ,H2 O cold
H
+
MnO2
ii) With hot potassium permanganate solutions to alkenes (oxidative cleavage of alkenes)
When oxidation of the alkene is carried out with KMnO4, in acidic rather than basic solution,, cleavage of the double bond occurs solution and carbonyl-containing products are obtained.
If the double bond is tetrasubstituted tetrasubstituted,, the two carbonyl-containing products are ketones ketones;;
If a hydrogen is present on the double bond,, one of the carbonyl-containing bond products is a carboxylic acid; acid;
If two If two hydrogens are present on one carbon, CO2 and H2O are formed.
Example: 1)
CH3 H3C
C
-
C
CH3
+
(i) OH ,heat
KMnO4
(ii) H3O
CH3 O H3C
C
+
O CH3
+
H3C
C
CH3
2) -
H2C
CH CH3
+
KMnO4
(i) OH ,heat (ii) H3O
+
O HO
C
CH3
+
CO2
+
H2 O
• The oxidative cleavage of alkenes can be used to establish the location of the double bond in an unknown alkene.
Example: • An unknown alkene with the formula C7H14 was found on oxidation with hot basic permanganate, to form a three-carbon carboxylic acid (propanoic acid) and fourcarbon carboxylic acid (butanoic acid). What is the structure of this alkene?
C7 H14
+
KMnO4
O H3C
C
CH2
(i) OH-,heat (ii) H3 O
+
O OH
+
CH2 C H3C CH2 OH
Answer: O H3C
C
CH2
O CH2 C H3C CH2 OH
OH
propanoic acid
H3C
butanoic acid
H
H
CH2C
C
CH2CH2CH3
3-heptene
Example • An unknown alkene undergoes oxidation by hot basic KMnO4, after acidification, gives the following product: O CH3CCH2CH2CH2CH2C
O OH
Deduce the structural formula for the unknown alkene.
iii) iii) Oz Ozon onol olys ysis is of of Alken Alkenes es (Oxidative Cleavage of Alkenes) •
A more widely used method for locating the double bond of an alkene alk ene involves the use of ozone (O 3).
•
Ozone reacts vigorously with alkenes to form unstable compounds called molozonides,, which rearrange molozonides spontaneously to form compounds as ozonides.
O
C
C
+
C
C
O3 O
O
ozonide
•
Ozonides: very unstable compounds •
•
can easily explode violently
•
they are not usually isolated but are reduced directly by treatment with water and in the presence of zinc z inc and acid (normally acetic acid) to give carbonyl compounds (either aldehydes or ketones).
O
H
C
C
+
Zn
H2 O,H
+
R
O
O
ozonide
H
C=O
+
O=C R
Example: CH3 H3C
C
(i)O3 CH CH3
(ii)Zn,H2 O/H
CH3 H3C
C
O
+
CH3
+
H
C
O
Example • Deduce the structural formula of an alkene that gives the following compound when it reacts with ozone in the presence of Zn / H+. O=CH-CH2-CH2-CH(CH3)CH=O
Exercise 1. Writ Write e the the stru struct ctur ure e of of alk alken ene e that that would produce the following products when treated with ozone followed by water, zinc and acid CH3COCH3 and CH3CH(CH3)CHO
2. Acid Acid--cata catallyzed yzed deh dehyd ydrratio ation n of neopentyl alcohol, (CH3)3CCH2OH, yields 2-methyl-2-butene as the major product. Outline a mechanism showing all steps in its formation.
Exercise • Compounds A, B, C and D are isomers with the molecular formula C4H8. A and B give a positive Baeyer test, test, while C and D do not. A exist as cis- and trans- isomers, isomers, while B does not have geometrical g eometrical isomers. C has only secondary hydrogen, hydrogen, while D has primary, secondary and tertiary hydrogen. hydrogen. Give the IUPAC names for A, B, C, and D.