The document deals with isomerism in organic chemistry. The various types of isomerism are examined such as structural isomerism, geometrical isomeris...
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TYPES OF ISOMERISM CHAIN ISOMERISM
STRUCTURAL ISOMERISM Same molecular formula but different structural formulae
POSITION ISOMERISM
FUNCTIONAL GROUP ISOMERISM
ISOMERISM GEOMETRICAL ISOMERISM Occurs due to the restricted rotation of C=C double bonds... two forms… E and Z (CIS and TRANS)) TRANS
STEREOISOMERISM Same molecular formula but atoms occupy different positions in space.
OPTICAL ISOMERISM Occurs when molecules have a chiral centre. Get two nonnonsuperimposable superimposable mirror images.
Note: structural isomerism also known as constitutional isomerism
STRUCTURAL ISOMERISM - INTRODUCTION
STRUCTURAL ISOMERISM - CHAIN
COMPOUNDS HAVE THE SAME MOLECULAR FORMULA BUT DIFFERENT STRUCTURAL FORMULA Chain
Positional
Two structural isomers of C 4H10.
different arrangements of the carbon skeleton similar chemical properties slightly different physical properties more branching = lower boiling point
BUTANE straight chain
2-METHYLPROPANE branched
same carbon skeleton same functional group functional group is in a different position similar chemical properties - slightly different physical
properties Functional Functio nal Group different functional group
different chemical properties different physical properties • Sometimes more than one type of isomerism occurs in the same molecule. • The more carbon atoms there are, the greater the number of possible isomers
STRUCTURAL ISOMERISM - CHAIN DIFFERENCES BETWEEN CHAIN ISOMERS Chemical
Isomers show similar chemical properties because the same functional group is present.
Physical
Properties such as density and boiling point show trends according to the of the degree of branching
Boilin Boi ling g Point Point
“straight “strai ght” ” chain chain isomers isomers have have higher higher value values s than bran branched ched ones ones the greater the degree of branching the lower the boiling point branching decreases the effectiveness of intermolecular forces less energy has to be put in to separate the molecules - 0.5°C 0.5°C straight chain
- 11.7°C 11.7°C branched
greater branching = lower boiling point
STRUCTURAL ISOMERISM - POSITIONAL • • • •
molecule has the same carbon skeleton molecule has the same same functional group... BUT the functional group is in a different position have similar chemical properties / different physical properties Example 1
POSITION OF A DOUBLE BOND IN ALKENES
1
2
PENT-1-ENE double bond between carbons 1 and 2
2
3
PENT-2-ENE double bond between carbons 2 and 3
There are no other isomers with five C’s in the longest chain but there are three other structural isomers with a chain of four carbons plus one in a branch.
1
STRUCTURAL ISOMERISM - POSITIONAL Example 2
STRUCTURAL ISOMERISM - POSITIONAL Example 3
POSITION OF A HALOGEN IN A HALOALKANE
1-CHLOROBUTANE halogen on carbon 1
2-CHLOROBUTANE halogen on carbon 2
1
RELATIVE POSITIONS ON A BENZENE RING
2
2
Moving the chlorine along the chain makes new isomers; the position is measured from the end nearest the functional group... the third example is 2- NOT 3-chlorobutane.
1,2-DICHLOROBENZENE ortho dichlorobenzene
1,3-DICHLOROBENZENE meta dichlorobenzene
1,4-DICHLOROBENZENE para dichlorobenzene
There are 2 more structural isomers of C 4H9Cl but they have a longest chain of 3
STRUCTURAL ISOMERISM – FUNCTIONAL GROUP • • • •
STRUCTURAL ISOMERISM – FUNCTIONAL GROUP
molecules have same molecular formula molecules have different functional groups molecules have different chemical properties molecules have different physical properties
ALCOHOLS and ETHERS
ALCOHOLS and ETHERS Name
ETHANOL
METHOXYMETHANE
Classification
ALCOHOL
ETHER
Functional Group
ALDEHYDES and KETONES
R-OH
Physical properties
p ol ar O- H b on d g iv es ri se to hydrogen bonding. g et h ig he r b oi li ng po in t and solubility in water
Chemical properties
Lewis base Wide range of reactions
ACIDS and ESTERS
R-O-R N o h yd ro ge n bo nd in g low boiling point in so lu bl e i n w at er Inert
MORE DETAILS FOLLOW
STRUCTURAL ISOMERISM – FUNCTIONAL GROUP
STRUCTURAL ISOMERISM – FUNCTIONAL GROUP
ALDEHYDES and KETONES
CARBOXYLIC ACIDS and ESTERS
Name
PROPANAL
PROPANONE
Classification
ALDEHYDE
KETONE
R-CHO
R-CO-R
Functional Group Physical properties
p ola r C =O b on d g iv es dipole-dipole interaction
p ola r C =O b on d g iv es dipole-dipole interaction
Chemical properties
easily oxidised to acids of same number of carbons
undergo oxidation under extreme conditions only
re duc ed to 1° a lcoh ols
re duc ed to 2° a lcoh ols
Name Classification Functional Group
PROPANOIC ACID CARBOXYLIC ACID R-COOH
METHYL ETHANOATE ESTER R-COOR
Physical properties
O-H bond gives rise to hyd ro gen bo nd in g. get higher boiling point and solubility in water
No hydrogen bonding inso lu ble in water
Chemical properties
acidic react with alcohols
fairly unreactive hydrolysed to acids
2
GEOMETRICAL ISOMERISM IN ALKENES
STEREOISOMERISM
Introduction Molecules have the SAMEMOLECULAR FORMULA but the atoms are joined to each other in a DIFFERENT SPACIAL ARRANGEMENT - they occupy a different position in 3-dimensional space. There are two types...
• an example of stereoisomerism • found in some, but not all, alkenes • occurs due to the RESTRICTED ROTATION OF C=C bonds • get two forms...
• GEOMETRICAL ISOMERISM • OPTICAL ISOMERISM
CIS (Z) Groups/atoms are on the SAME SIDE of the double bond
TRANS (E) Groups/atoms are on OPPOSITE SIDES across the double bond
GEOMETRICAL ISOMERISM
GEOMETRICAL ISOMERISM
RESTRICTED ROTATION OF C=C BONDS
RESTRICTED ROTATION OF C=C BONDS
Single covalent bonds can easily rotate. What appears to be a different structure is not. It looks like it but, due to the way structures are written out, they are the same.
C=C bonds have restricted rotation so the groups on either end of the bond are ‘frozen’ in one position; it isn’t easy to flip between the two. Animation doesn’t work in old versions of Powerpoint
ALL THESE STRUCTURES ARE THE SAME BECAUSE C-C BONDS HAVE ‘FREE’ ROTATION
This produces two possibilities. The two structures cannot interchange easily so the atoms in the two molecules occupy different positions in space.
Animation doesn’t work in old versions of Powerpoint
GEOMETRICAL ISOMERISM IN ALKENES CIS-TRANS
CIS / TRANS
Should only be used when there are two H’s and two nonhydrogen groups attached to each carbon. cis
non-hydrogen groups / atoms on the SAME side of C=C bond
trans
non-hydrogen groups / atoms on OPPOSITE sides of C=C bond
GEOMETRICAL ISOMERISM Isomerism in butene There are 3 structural isomers of C 4H8 that are alkenes*. Of these ONLY ONE exhibits geometrical isomerism.
* YOU CAN GET ALKANES WITH FORMULA C4H8 IF THE CARBON ATOMS ARE IN A RING
3
OPTICAL ISOMERISM
OPTICAL ISOMERISM
- is a form of stereoisomerism that occurs when compounds have non-superimposable mirror images
• Left and right hands are an example of non-superimposable mirror images.
CHIRAL CENTRES
a
a
C* b
d
C
d
c
c
b
• the two different forms are known as optical isomers or enantiomers • This usually happens when a molecule contains a C atom with four(4) different groups attached (chiral / asymmetric carbon [often shown by an asterisk (*)). • Such molecules are said to be chiral or optically active
Note: superposable and superimposable are identical in meaning
Chiral and Achiral
21
• For chiral molecules the mirror image is a different molecule (the mirror image is non-superimposable).
*
chiral molecules (optically active)
OH C H
OH COOH CH3
(-) lactic acid in sour milk
TASK
HOOC H3C
C H
(+) lactic acid in muscles
Some of the following molecules are optically active. For each one, decide whether it is optically active or not.
achiral molecules (not optically active)
23
24
propan-2-ol
CH3 CH
CH3
OH
NOT OPTICALLY ACTIVE a) propan-2-ol
e) butanone
b) 2-chlorobutane
f) 2-methylbutanoic acid
c) 1-chlorobutane
g) butan-2-ol
d) 3-methylhexane
h) 1-chloro-3-methylpentane
4
2-chlorobutane
CH3 CH
CH2
CH3
25
26
1-chlorobutane
Cl
CH2CH3
CH2CH3
C
C
CH3 Cl
H
H3C Cl
CH2
CH2
CH2
CH3
Cl
NOT OPTICALLY ACTIVE
H
OPTICALLY ACTIVE
3-methylhexane
CH3 CH2 CH CH2 CH2 CH327 CH3
H
CH3
CH2CH2CH3
CH2CH2CH3
C
C
CH3 CH2CH3
CH3 CH3CH2
28
O
butanone
C
CH2
CH3
NOT OPTICALLY ACTIVE H
OPTICALLY ACTIVE
29
propan-2-ol
CH3 CH
2-methylbutanoic acid
CH3
CH3
CH2
CH2CH3
OH
C
NOT OPTICALLY ACTIVE CH3
H COOH
CH3
O
CH
C
30
OH
CH2CH3 H HOOC
C CH3
OPTICALLY ACTIVE
5
OH
butan-2-ol CH3
CH2
CH
CH2CH3 C CH3
H OH
H HO
CH3
CH3
CH2CH3
CH2CH3
C
C CH3
CH3
CH2
CH
Cl CH2
32
CH2
CH2CH3 C
H CH2ClCH2
H CH2CH2Cl
CH3
OPTICALLY ACTIVE
OPTICALLY ACTIVE
OPTICAL ISOMERS - DIFFERENCE
33
• Molecules that are optical isomers are called enantiomers.
• Enantiomers have identical chemical and physical properties, except:
• • • • •
isomers differ in their reaction to plane-polarised light plane polarised light vibrates in one direction only one isomer rotates light to the right, the other to the left rotation of light is measured using a polarimeter rotation is measured by observing the polarised light coming out towards the observer
• If the light appears to have
• Their effect on plane polarised light; • Their reaction with other chiral molecules
Examples Optical activity is common in biochemistry and pharmaceuticals • Most amino acids exhibit optical activity • many drugs must be made of one optical isomer to be effective - need smaller doses (safer and cost effective) - get reduced side effects - improved pharmacological activity
CH3
1-chloro-3-methylpentane
31
turned to the right DEXTROROTATORY d or + form
turned to the left LAEVOROTATORY l or - form
Racemate • a 50-50 mixture of the two enantiomers (dl) or (±) is a racemic mixture. The opposite optical effects of each isomer cancel each other out
OPTICAL ISOMERISM The polarimeter A
B C
D E F
A B C D E F
Light source produces light vibrating in all directions Polarising filter only allows through light vibrating in one direction Plane polarised light passes through sample If substance is optically active it rotates the plane polarised light Analysing filter is turned so that light reaches a maximum Direction of rotation is measured coming towards the observer
If the light appears to have
turned to the right DEXTROROTATORY(+)
turned to the left LAEVOROTATORY(-)
6
37
• Chiral molecules often react differently with other chiral molecules.
38
• For example, most amino acids (and some proteins) are chiral, along with many other molecules.
• This is like the idea that a right hand does not fit a left handed glove – the molecule must be the correct shape to fit the molecule it is reacting with.
• In nature, only one enantiomer usually occurs (e.g. all natural amino acids rotate polarised light to the left).
• Many natural molecules are chiral and most natural reactions are affected by optical isomerism.
39
• Many drugs are optically active, with one enantiomer only having the beneficial effect.
• In the 1960’s thalidomide was given to pregnant women to reduce the effects of morning sickness.
40
• This led to many deformaties in babies and early deaths in many cases.
• In the case of some drugs, the other enantiomer can even be harmful, e.g. thalidomide.
42
O
NH
O
O O
H2C
NH
O C
C N
CH2 H
O
N
• Thalidomide was banned worldwide when the effects were discovered.
H2C CH2
H
O
O
S thalidomide (effective drug)
R thalidomide (dangerous drug)
The body racemises each enantiomer, so even pure S is dangerous as it converts to R in the body.
