Isomers

What are Structural isomers:

same molecular formula different structures (or structural formulae)

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Structural isomerism can arise from

•Chain isomerism •Position isomerism •Functional group isomerism

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Functional group isomers

: Compounds with the same molecular formula but with atoms arranges to give different functional groups

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Aldehydes and ketones of the same chain length would be classed as

functional group isomers- e.g. Propanal and propanone (both C3H6O)

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Questions often involve functional group isomers of

carboxylic acids: including esters etc

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Stereoisomers have

the same structural formulae but have a different spatial arrangement of atoms

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There are two types of stereoisomerism:

geometrical (E- Z isomerism) and optical isomerism

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Optical isomerism occurs in

carbon compounds with 4 different groups of atoms attached to a carbon (called an asymmetric carbon).

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A carbon atom that has four different groups attached is called

a chiral (asymmetric) carbon atom

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These four groups are arranged

tetrahedrally around the carbon.

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This causes two different isomers that are

not superimposable to be formed. They are mirror images

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A mixture containing a 50/50 mixture of the two isomers (enantiomers) is described as being

a racemate or racemic mixture.

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Two compounds that are optical isomers of each other are called

enantiomers.

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Many naturally occurring molecules contain chiral C atoms, but are usually found in nature as a

pure enantiomer

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Optical isomers have similar , but they

physical and chemical properties, but they rotate plane polarised light in different directions.

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One enantiomer rotates it in§

one direction and the other enantiomer rotates it by the same amount in the opposite direction.

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One optical isomer will rotate light The other will rotate it

clockwise (+)(called dextrorotatory). anticlockwise(-)(called laevorotatory).

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-ve enantiomer

Anticlockwise rotation

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+ve enantiomer

clockwise rotation

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Racemate

no rotation

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A racemic mixture (a mixture of equal amounts of the two optical isomers) will

not rotate plane-polarised light.

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A racemate will be formed in a reaction mechanism when

a reactant or intermediate has a trigonal planar group in the molecule is approached from both sides by an attacking species

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There is an equal chance of

either enantiomer forming so a racemate forms. No optical activity is seen

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Nucleophilic addition of HCN to aldehydesandketones(unsymmetrical) when the trigonal planar carbonyl groupisapproachedfrombothsidesby the HCN attacking species: results in the formation of a

racemate

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Mechanism for the reaction

(drawn the same for both enantiomers)

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A racemate can also be formed in the AS reaction of the

electrophilic addition of HBr to an unsymmetrical alkene

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The bromide can attack this planar carbocation from

both sides leading to a racemate

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If the alkene is unsymmetrical,

addition of hydrogen bromide can lead to isomeric products.

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Drug action may be . and

determined by the stereochemistry of the molecule. Different optical isomers may have very different effects

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One enantiomer of thalidomide causes

birth defects in unborn children whilst the other had useful sedative problems. Unfortunately it was given in a racemic mixture when first used.

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Isomers

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