Eduqas Chemistry for A Level Year 2: Student Bk

116 Eduqas A Level Chemistry Year 2: Component 2 Some terms used in optical isomerism Chiral centre A chiral centre is an atom that is bonded to four different groups or atoms. This is often a carbon atom and we call this a chiral carbon atom. In older books these chiral carbon atoms were called asymmetric carbon atoms. The four different atoms or groups bonded to this chiral centre can be bonded in two different ways, which are mirror images of each other. C c d b a C a d c b The dotted lines go backward into the page and the wedge shaped lines come out of the page towards you. A common example of a compound with a chiral centre is butan-2-ol, CH 3 CH 2 CH(OH)CH 3 . C CH 3 H H 3 CH 2 C OH CH H 3 C CH 2 CH 3 OH Enantiomer Enantiomers are stereoisomers that are non-superimposable mirror image forms of each other. These can also be called optical isomers. If a compound contains two or more chiral centres it is possible to have stereoisomers that are not mirror image forms of each other. These are called diastereoisomers, but are not within the scope of this specification. However, you should be able to identify chiral centres within this type of molecule. The diagram shows the formula of butane-1,2-diol with the chiral centres indicated by asterisks. H OH OH H HH H H H C C* C* C H Key Terms A chiral centre is an atom in a molecule that is bonded to four different atoms or groups. Enantiomers are non- superimposable mirror image forms of each other that rotate the plane of polarised light in opposite directions. Optical activity occurs in molecules that possess chiral centre(s). These molecules rotate the plane of polarised light. A racemic mixture is an equimolar mixture of both enantiomers that produces no overall rotation of plane polarised light. Extra Help Carbon atoms that have multiple bonds cannot act as chiral centres as they cannot be bonded to four other atoms or groups. As a result these compounds do not have a chiral centre. C O HO H C C F Cl F Cl H H H H H C C C C H & S C Stretch & Challenge The formula of 1,3-dichloropropan-2-ol is ClCH 2 CHCH 2 Cl OH Give the displayed formula of another isomer of formula C 3 H 6 Cl 2 O that has a chiral centre. YOU SHOULD KNOW › › › ››› that enantiomers affect the plane of polarised light ››› that a chiral centre occurs when a carbon atom has four different atoms or groups bonded to it