Eduqas Chemistry for A Level Year 2: Student Bk

115 OA1.1 Stereoisomerism The distinction between structural isomerism and stereoisomerism, and the two types of stereoisomerism Structural isomers are compounds that have the same molecular formula but differ in their arrangement of atoms. One of the simplest types is chain isomerism, where the chains are straight or branched. CH 3 CH 2 CH 2 CH 2 CH 3 CH 3 CH 3 pentane dimethylpropane H 3 C C CH 3 If the compounds contain a functional group then this may be at a different position on the chain. CH 3 CH 2 CH 2 OH CH 3 CH(OH)CH 3 propan-1-ol propan-2-ol The compounds may have the same molecular formula but a different functional group is present. For example, both cyclohexanol (a secondary alcohol) and hexanal (an aldehyde) have the molecular formula C 6 H 12 O. CH 3 CH 2 CH 2 CH 2 CH 2 C H OH O cyclohexanol hexanal One form of stereoisomerism is E-Z isomerism. Stereoisomerism occurs where the isomers have the same shortened formula but have a different arrangement of the atoms in space. E-Z isomerism is studied during the first year of this course. This type of isomerism is shown in alkenes. Dichloroethene can have both chlorine atoms bonded to the same carbon atom or each carbon atom can be bonded to one chlorine atom. H 2 C CCl 2 ClHC CHCl 1,1-dichloroethene 1,2-dichloroethene These two compounds are structural isomers. However, 1,2-dichloroethene can be bonded so that each chlorine is opposite each other or both atoms are on the same side of the double bond. C C C C Cl Cl H Cl H H Cl H ( E )-1,2-dichloroethene ( Z )-1,2-dichloroethene The π bond restricts rotation about the double bond and, as a result, 1,2-dichloroethene exists in two different forms, called E-Z isomers. The other type of stereoisomerism is called optical isomerism and occurs where the two isomers have different effects on plane polarised light. 1 Knowledge check The displayed formula of methylcyclobutane is shown below. H C C CH 3 H C C H H H H H Give the displayed formula of another compound that has the same molecular formula. 2 Knowledge check Citronellal is a lemon-scented oil that is used in perfumery. (CH 3 ) 2 C CHCH 2 CH 2 CHCH 2 C H O CH 3 State whether citronellal exists as E-Z isomers giving a reason for your answer. & S C Stretch & Challenge Give the displayed formula of two isomers whose molecular formula is C 4 H 8 O but which contain different functional groups. YOU SHOULD KNOW › › › ››› the difference between structural isomerism and stereoisomerism ››› that E-Z isomerism can occur with alkenes St u dy point For E-Z isomerism to occur the atoms/groups bonded to the carbon atoms at each end of the carbon to carbon double bond, need to be different. 1,1-Dichloroethane has two atoms the same bonded to each carbon atom of the double bond and so does not exist as E-Z isomers. The atoms/groups do not have to be the same at either side of the double bond. Both C C c a d b C C b a d c and have E-Z isomers.