Edexcel A Level Chemistry:复习笔记7.1.1 Chirality

When unpolarised light is passed through a polariser, the light becomes polarised as the waves will vibrate in one plane only

The major difference between the two enantiomers is:

One enantiomer rotates plane polarised light in a clockwise manner and the other in an anticlockwise fashion

A common way to differentiate the isomers is to use (+) and (-), but there are other systems using d and l, D and L, or R and S

The rotation of plane polarised light can be used to determine the identity of an optical isomer of a single substance

For example, pass plane polarised light through a sample containing one of the two optical isomers of a single substance

Depending on which isomer the sample contains, the plane of polarised light will be rotated either clockwise or anti-clockwise by a fixed number of degrees

Each enantiomer rotates the plane of polarised light in a different direction

A racemic mixture (or racemate) is a mixture containing equal amounts of each enantiomer

One enantiomer rotates light clockwise, the other rotates light anticlockwise

A racemic mixture is optically inactive as the enantiomers will cancel out each others effect

This means that the plane of polarised light will not change

Racemic mixtures are optically inactive

Racemic mixtures and drugs

In the pharmaceutical industry, it is much easier to produce synthetic drugs that are racemic mixtures than producing one enantiomer of the drug

Around 56% of all drugs in use are chiral and of those 88% are sold as racemic mixtures

Separating the enantiomers gives a compound that is described as enantiopure, it contains only one enantiomer

This separation process is very expensive and time consuming, so for many drugs it is not worthwhile, even though only half the of the drug is pharmacologically active

For example, the pain reliever ibuprofen is sold as a racemic mixture

The structure of ibuprofen showing the chiral carbon that is responsible for the racemic mixture produced in the synthesis of the drug

SN1 mechanism

The SN1 mechanism is a two-step reaction

In the first step, the C-X bond breaks heterolytically and the halogen leaves the halogenoalkane as an X- ion

This leaves a trigonal planar, tertiary carbocation

In the second step, the planar, tertiary carbocation is attacked by the nucleophile

The nucleophile is able to attack from either side of the planar carbocation, which results in the formation of a racemic mixture

Therefore, a reaction with an SN1 mechanism will produce a racemic mixture
SN1 Optical Isomers Mechanism

SN2 mechanism

The SN2 mechanism is a one-step reaction

The nucleophile donates a pair of electrons to the δ+ carbon atom of the halogenoalkane to form a new bond

At the same time, the C-X bond is breaking and the halogen (X) takes both electrons in the bond

The halogen leaves the halogenoalkane as an X- ion

For example, the nucleophilic substitution of bromoethane by hydroxide ions to form ethanol

The SN2 mechanism of bromoethane with hydroxide causing an inversion of configuration

The bromine atom of the bromoethane molecule causes steric hindrance

This means that the hydroxide ion nucleophile can only attack from the opposite side of the C-Br bond

Attack from the same side as the bromine atom is sometimes called frontal attack

While attack from the opposite side is sometimes called backside or rear-side attack

As the C-OH bond forms, the C-Br bond breaks causing the bromine atom to leave as a bromide ion

As a result of this, the molecule has undergone an inversion of configuration

The common comparison for this is an umbrella turning inside out in the wind

Inversion of configuration - umbrella analogy

Therefore, if a reaction with an SN2 mechanism starts with an enantiopure reactant then an enantiopure products will be formed