Organic Chemistry - Isomerism Flashcards
What are isomers
isomers are compounds with the same molecular formula but a different arrangement of atoms in space.
functional group isomer
have different functional group
positional isomer
the functional groups are attached to main chain at different point
structural isomer
they have a different arrangement of carbon atoms in the skeleton of the molecule.
what are stereoisomers
two or more compounds have the same have the same structural formula. they differ in the arrangement of the bonds in space.
what are the two types of stereoisomerism?
E-Z isomerism and optical isomerism
what is optical isomerism?
optical isomerism is another type of stereoisomerism. stereoisomers have the same structural formula, but have their atoms arrange differently in space. optical isomers have a chiral carbon atom. a chiral(asymmetric) carbon atom is one that has four different groups attached to it.
what are enantiomers or optical isomers?
they are mirror images and no matter which way you turn them, they won’t be superimposed.
what happen when you pass plane-polarised light through an optically active mixture.
the molecules interact with the light and rotate the plane of the vibration of the light. the two enantiomers of an optically active molecule will rotate the plane-polarised light in opposite directions. one rotates it in a clockwise direction, and the other one rotates it in an anticlockwise direction.
what is a racemate?
it contains equal quantities of each enantiomer of an optically active compound.
do racemates show any optical activity?
the two enantiomers cancel each other’s light rotating effect.
the effects of different enantiomers to human
Drugs work by changing chemical reactions that are taking place in the body. most drugs do this by binding to an active site - usually on an enzyme or a specific receptor molecule. a drug must be exactly right shape to fit into the correct active site - only one enantiomer will do. the other one might fit into a enzyme, and could cause harmful side-effects or have no effect at all.
Isomers
Isomers are compounds with the same molecular formula but different structures.
Isomerism can be divided into two types
structural isomerism and stereoisomerism.
There are three types of structural isomerism
positional isomerism, chain isomerism and functional group isomerism.
There are two types of stereoisomerism
geometrical isomerism and optical isomerism
positional isomerism
In amides, acyl chlorides and carboxylate salts, the functional group is always at the end of the molecule and so these molecules cannot show positional isomerism. The other molecules, however, can show positional isomerism:
Now can you work out four isomers of C5H10O2
ethanoic anhydride
propyl ethanoate
ethyl ethanoate
methanoic propanoic anhydride
chain isomerism of C6H11O2
2-methylpropyl ethanoate
butyl ethanoate
functional group isomerism
Carboxylic acids and esters have the same general formula CnH2nO2.
ethyl ethanoate butanoic acid
E-Z isomerism
Geometrical isomerism was introduced at AS-level. It occurs in alkenes when both carbon atoms forming the double bond are attached to two different groups.
Z but-2-ene E but-2-ene
In a double bond, the second bond is a π-bond. This is caused by the side-on overlap of two p-orbitals:
The result is an overlap in two places. This means that the bond cannot be twisted and as a result there is restricted rotation about the π-bond. This is why the cis and trans isomers cannot be interconverted and are therefore different.
Distinguishing between the different enantiomers
Optical isomers show identical physical and chemical properties in most respects.
In fact there is only one physical method by which they can be distinguished.
Plane-polarised light is light which has been filtered into a two-dimensional plane:
If plane-polarised light is passed through a liquid containing a chiral molecule, the plane of the light will be rotated. This can be detected using a polarimeter.
Molecules which are not chiral will not rotate the plane of plane-polarised light.
Chiral molecules will rotate plane-polarised light. Two optical isomers will rotate plane polarised light equally, but in opposite directions.
It is this difference in physical properties which enables them to be distinguished.
It is not possible to predict the direction in which a particular optical isomer will rotate plane polarised light, but two optical isomers will always rotate plane polarised light in opposite directions.
A substance which can rotate plane polarised light is said to be optically active.
The isomer which rotates plane polarised light clockwise is given the prefix (+) or D-. The isomer which rotates plane polarised light anticlockwise is given the prefix (-) or L-.
Importance of optical isomers in biochemistry
Optical isomers show identical chemical properties in most reactions. However, certain biochemical processes require the molecule to have a specific orientation of groups. Many drugs and enzymes are chiral and so only one of the optical isomers will be able to interact effectively with the target molecule in the body. Different optical isomers may therefore have very different biochemical effects.
racemates
Optical isomers are often found together in a mixture in equal quantities. The opposite effect they have on the rotation of plane polarised light will thus result in no overall rotation. An equimolar mixture of two optical isomers will thus have no effect on plane polarised light and is thus not optically active.
Such mixtures are said to be racemic mixtures or racemates.
If the chiral substance is produced by an addition reaction, then the product will always be a racemate as the attacking nucleophile or electrophile can attack the planar molecule from above or below with equal probability:
Eg formation of 2-bromobutane from but-2-ene (electrophilic addition)
The bromide ion can attack the carbocation from above or below, producing an equimolar mixture of the two enantiomers – ie a racemate:
A racemic mixture is an equimolar mixture of two optical isomers. Racemic mixtures are not optically active.
Thus chiral molecules will only show optical activity if one isomer is present in greater quantities than the other.
It is possible to predict whether a single enantiomer or a racemate will be produced, provided that the mechanism for the reaction is known.
