Optical isomerism

Functional group definition:

Atom/group of atoms in an organic molecule responsible for the characteristic reactions of the molecule.

Homologous series definition:

Set of organic compounds with the same functional group. Compounds differ in length of hydrocarbon chains.

Draw a primary amine with 5 carbons:

What type of amine is dimethyl amine, and draw it:

Secondary amine

What type of amine is ethyldimethyl amine, and draw it:

Tertiary amine

What is the general structure of an acid anhydride?

Draw the general structure of an N-substituted amide:

General structure of an ester?

Hydroxynitrile general structure?

What are stereoisomers?

Molecules with the same structural formula but different 3D arrangement in space

When do optical isomers occur?

When there are four different substituents attached to one carbon atom, resulting in two isomers that are non-superimposable mirror images of each other, but non-identical.

Why are pairs of molecules that are non-superimposable called optical isomers?

They differ in the way that they rotate in the plane of polarisation of polarised light, either clockwise as a + isomer or anticlockwise as a negative isomer

What are enantiomers?

A pair of optical isomers

What is the central carbon bonded to the 4 groups known as?

Chiral centre or the asymmetric carbon atom, indicated by an asterix.

What happens to light waves if they pass through a polaroid?

The filter cuts out all vibrations except those in one place such as the vertical plane.

How do light waves vibrate relative to direction of motion of the light waves?

At a right angle

What can optical rotation be measured using?

A polarimeter

How does the polarimeter work to measure optical rotation?

1. Light, once vertically polarised will be affected by different optical isomers of the same substance

2. The light is passed through two solutions of the same concentration each containing a different optical isomer

3. One solution rotates the plane of polarisation through an angle clockwise, and the other solution rotates the plane of polarisation through an angle anticlockwise.

What is a racemic mixture?

A mixture containing a 50/50 mixture of the two enantiomers

Why are racemic mixtures optically inactive?

Because a racemic mixture has no effect on the plane of light as one enantiomer cancels out the other.

Why are racemic mixtures hard to separate?

Optical isomers have identical physical and chemical properties so are hard to separate.

What ratio of optical isomers is often produced in organic synthesis to produce optically active compounds?

50:50 mixture forming a racemic mixture.

Draw the enantiomers of lactic acid (2-hydroxypropanoic acid)

What is mechanism is the first stage of the synthesis of lactic acid?

Nucleophilic addition

What nucleophile does step 1 of lactic acid synthesis use?

Cyanide ion from hydrogen cyanide

Describe the first step of synthesis of lactic acid:

Hydrogen cyanide is added across the C=O bond of ethanal to form 2-hydroxypropanenitrile

Draw the mechanism for the reaction of the CN- ion with ethanal:

Why does a racemic mixture form when nucleophilic addition occurs with an aldehyde or asymmetrical ketone?

The cyanide ion can attack the planar C=O group equally from either above or below the plane, so equal numbers of the isomers are formed.

Describe the second stage of lactic acid formation?

2-hydroxypropanenitrile is reacted with water acidified with a solution of HCl under reflux (very dilute HCl). This is hydrolysis of a nitrile.

Why is the first stage of the formation of lactic acid important?

It increases the length of the carbon chain by one from 2 to 3.

What is the equation for stage 2 of the formation of lactic acid?

What is the equation for stage 1 of the formation of lactic acid?

Does lactic acid form optical isomers?

Yes, a racemic mixture of two optical isomers form, these are the same as the ones from stage 1 and are not affected by the hydrolysis reaction.

Why are naturally occurring molecules often only produced as one optical isomer?

Most of them are made using enzyme catalysts, which only produce one of the two optical isomers because they are chiral too

Why are drugs often sold as a racemic mixture?

• They are expensive and hard to separate

  • It is difficult to synthesise in a way that only makes the required isomer

Why is only one isomer often required when a drug is being synthesised?

A molecule of the active ingredient fits the cell receptor, and this has a 3D shape so only one of the pair of optical isomers will fit.

Why is sometimes necessary to separate the two optical isomers in drugs?

Sometimes one isomer is safe and effective while the other is toxic or with unacceptable side effects.