 Compounds with the same molecular formula but a different structural formula.

 In structural isomerism, the molecules differ in terms of the order in which the atoms are joined together. For example, there are two possible isomers with the molecular formula C 2 H 6 O.

 Clearly, the two substances belong to different homologous series and will have very different chemical and physical properties.

 In stereoisomerism, the molecules differ only by their orientation in space.  They have identical molecular formulae and the atoms are bonded together in the same order.  However, because the arrangements of the atoms in space is different, the molecules are non-superimposable.  This means that no matter how hard you try it is impossible to superimpose the image of one molecule on top of the other.  We will consider the following types of stereoisomers: geometric and optical.

 These generally arise due to lack of free rotation around a bond, especially a carbon to carbon double bond.  As was seen earlier, rotation around a double bond is restricted because it would involve breaking the pi bond.

 Both of the molecules on the previous slide could be called 1,2-dichloroethene which would usually suggest that they are identical molecules.  However, they are clearly non-superimposable and are, therefore, stereoisomers.  If the substituents are on opposite sides, the isomer is called the trans-isomer.  If the substituents are on the same side, the isomer is called the cis-isomer.

 Generally display differences in physical properties.

 The difference in the m.p. can be explained in terms of the differences in their shapes.  It appears likely that the molecules of the trans-isomers are able to pack more closely together in the solid state than the cis-isomers.  This close packing increases the London dispersion forces between the molecules and hence increase the m.p.  On the other hand, differences in b.p seem to be due to slight differences in polarity between cis and trans isomers.

 Fats and edible oils are naturally occurring esters on glycerol and long-chain carboxylic acids called fatty acids.  The fatty acids are almost exclusively straight-chain molecules with an even number of carbons, usually ranging from about 10 to 20.  The chains can be saturated, monounsaturated or polyunsaturated.

 Recent medical research has linked diet that is high in saturated fats with high levels of cholesterol in the blood.  This leads in a build up of fatty deposits in the arteries and an increase in the incidence of heart disease.  Unsaturated fats have not been similarly implicated, so health authorities have advised replacing saturated with unsaturated fats.  Most important fatty acids have a cis-arrangement around their C=C.

 These are not found naturally (exceptions meat and dairy).  Behave in a similar way to saturated fatty acids.  Most trans-fatty acids are “man made” and are used to either “harden” or to extend shelf-life.

 Optical isomers are possible when an asymmetric carbon is present somewhere in the molecule.  An asymmetric carbon is one with 4 different groups or atoms attached to it.  The molecule is described as being chiral.

 Your hands should be “identical” but they are mirror images of each other and it is impossible to superimpose on hand on top of the other.  Similarly a chiral molecule can have a mirror image version which is identical but not superimposable.  The two optical isomers are usually referred to as enantiomers.

 Amino acids are good examples of chiral molecules, with a hydrogen atom, a carboxyl group and an amino group all attached to the central atom. The fourth group (R) is what makes each amino acid unique.

 The importance of the spatial arrangement of these groups is demonstrated by the activity of receptor sites which can only interact with molecules in a specific orientation.

 During manufacture, a 50:50 mixture – referred to as a racemic mixture – of both enantiomers will often be produced.  Only one enantiomer will be the active ingredient in the drug whilst (ideally) the other enantiomer will be totally inactive.  Unfortunately, sometimes, the “inactive” enantiomer turns out to be able to interact with other receptors elsewhere in the body.

 Developed in the 1950’s with the main use of making users sleepy and relaxed.  It seemed to be the perfect tranquiliser.  The drug also reduced morning sickness, so became popular with pregnant women.  By the 1960’s, it was found to damage the development of unborn babies, especially if it was taken in the first 4 to 8 weeks of pregnancy.  The drug led to the arms and legs of the babies being very short or incompletely formed.

 More than 10,000 babies were affected around the world.  As a result of this disaster, thalidomide was banned.  Drug testing was also made more rigorous than before.  Thalidomide has been tested, but not with pregnant women.

 In recent years, Thalidomide has been relicensed and can be used in the treatment of leprosy, AIDS and certain cancers.

 The naming system for optical isomers relies on rules that rank the groups in order of priority.  With the lowest priority group relegated to the rear, it is determined whether the order of the other groups is “clockwise” or “anticlockwise” – the R or S configuration.

 Though very similar, there can be significant differences in properties between enantimors.

 Chiral molecules are also optically active.  One enantiomer will rotate plane polarised light by a certain number of degrees in a clockwise (+) direction, while the other will rotate by the same number of degrees but in an anticlockwise (-) direction.  A racemic mixture would be optically inactive.