Stereoisomerism AH Chemistry Unit 3(c)

Occurs due to the restricted rotation of C=C double bonds... two forms… CIS and TRANS STRUCTURAL ISOMERISM STEREOISOMERISM GEOMETRICAL ISOMERISM OPTICAL ISOMERISM CHAIN ISOMERISM Same molecular formula but different structural formulae Occurs when molecules have a chiral centre. Get two non- superimposable mirror images. Same molecular formula but atoms occupy different positions in space. POSITION ISOMERISM FUNCTIONAL GROUP ISOMERISM

Geometric isomers

In alkenes CIS Groups/atoms are on the SAME SIDE of the double bond TRANS Groups/atoms are on OPPOSITE SIDES across the double bond

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. ALL THESE STRUCTURES ARE THE SAME BECAUSE C-C BONDS HAVE ‘FREE’ ROTATION

RESTRICTED ROTATION OF C=C BONDS 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. This produces two possibilities. The two structures cannot interchange easily so the atoms in the two molecules occupy different positions in space.

cis transcis trans

Physical properties Geometric isomers display differences in some physical properties e.g. melting point, boiling point Geometric isomerism also influences some chemical properties

Optical isomers

All molecules have a mirror image – but for many molecules it is the same molecule.

For some molecules the mirror image is a different molecule (the mirror image is non-superimposable).

Left and right hands are an example of non- superimposable mirror images.

This usually happens when a molecule contains a C atom with four different groups attached (chiral / asymmetric C). Such molecules are said to be chiral or optically active.

The optical isomers are called enantiomers. These are distinguished by +/-, D/L or more correctly R/S. A 50/50 mixture of the two enantiomers is called a racemic mixture and is optically inactive.

TASK Some of the following molecules are optically active. For each one, click its name below and decide whether it is optically active or not. Click again to see if you are correct. a) propan-2-ol b) 2-chlorobutane c) 1-chlorobutane d) 3-methylhexane e) butanone f) 2-methylbutanoic acid g) butan-2-ol h) 1-chloro-3-methylpentane

propan-2-ol NOT OPTICALLY ACTIVE Click here to go back to the optical isomerism task

2-chlorobutane OPTICALLY ACTIVE Click here to go back to the optical isomerism task

1-chlorobutane NOT OPTICALLY ACTIVE Click here to go back to the optical isomerism task

3-methylhexane OPTICALLY ACTIVE Click here to go back to the optical isomerism task

butanone NOT OPTICALLY ACTIVE Click here to go back to the optical isomerism task

propan-2-ol NOT OPTICALLY ACTIVE Click here to go back to the optical isomerism task

2-methylbutanoic acid OPTICALLY ACTIVE Click here to go back to the optical isomerism task

butan-2-ol OPTICALLY ACTIVE Click here to go back to the optical isomerism task

1-chloro-3-methylpentane OPTICALLY ACTIVE Click here to go back to the optical isomerism task

Molecules that are optical isomers are called enantiomers. Enantiomers have identical chemical and physical properties, except: their effect on plane polarised light their reaction with other chiral molecules

Light is a form of electromagnetic radiation.

The wave vibrations are perpendicular to the direction of travel of the wave.

Optical isomers rotate the plane of plane polarised light.

A Light source produces light vibrating in all directions B Polarising filter only allows through light vibrating in one direction C Plane polarised light passes through sample D If substance is optically active it rotates the plane polarised light E Analysing filter is turned so that light reaches a maximum F Direction of rotation is measured coming towards the observer AB C D E F POLARIMETERS can be used to analyse the effect optical isomers have on plane polarised light:

How optical isomers can be formed

Chiral molecules often react differently with other chiral 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. Many natural molecules are chiral and most natural reactions are affected by optical isomerism.

For example, most amino acids (and so proteins) are chiral, along with many other molecules. In nature, only one optical isomer occurs (e.g. all natural amino acids are rotate polarised light to the left).

Many drugs are optically active, with one enantiomer only having the beneficial effect. In the case of some drugs, the other enantiomer can even be harmful, e.g. thalidomide.

In the 1960’s thalidomide was given to pregnant women to reduce the effects of morning sickness. This led to many disabilities in babies and early deaths in many cases.

S thalidomide (effective drug) The body racemises each enantiomer, so even pure S is dangerous as it converts to R in the body. R thalidomide (dangerous drug)

S carvone (caraway seed)R carvone (spearmint) Caraway Seed has a warm, pungent, slightly bitter flavour with aniseed overtones.

S limonene (lemons) R limonene (oranges)