25.2 Optical Isomerism

Question 1

What are isomers

Answer 1

Compounds with the same molecular formula but different arrangement of atoms in space

Question 2

What is stereoisomerism

Two types

Answer 2

Where two or more compounds have the same structural formula but there is a different arrangement of atoms in space

Two types
Optical
E-Z isomerism

Question 3

When does optical isomerism occur

Answer 3

When there are four different substituents attached to one carbon atom

This results in two isomers that are non-superimposable mirror images of each other. But they are not identical

Eg bromochlorofluoromethane

Question 4

What does optical isomerism mean

Answer 4

Pairs of molecules that differ in the way they rotate the plane of polarisation of polarised light - either clockwise
(+ isomer) or anticlockwise (- isomer)

It only happens in 3D molecules

Question 5

What does chiral mean
What are enantiomers

What is the chiral centre

Answer 5

Optical isomers are chiral and the two isomers are called enantiomers

The carbon bonded to the four different groups is called the chiral centre or the asymmetric carbon atom.

It is often labelled as *

Question 6

Do amino acids have a chiral centre?

Is lactic acid? 2-hydroxypropanoic acid

Answer 6

All alpha amino acids except glycine

Yes lactic acid is because although the carbon is bonded to two carbon atoms, they are each part of different groups

Question 7

Light consists of vibrating magnetic and electric fields

What is a polaroid

Answer 7

Think of light as waves with vibrations coming out of it in all directions at right angles to the direction of motion

A polaroid is a special filter that light passes through
When light passes through it, all the vibrations are cut out except those in one plane

So it makes the light vertically polarised

Question 8

What do you use to measure optical rotation
Describe process

Answer 8

Polarimeter

. Polarised light is passed through two solutions of the same concentration, each containing a different optical isomer of the same substance

. One solution will rotate the plane of polarisation through a particular angle clockwise (this is the + isomer)

. The other solution will rotate the plane of polarisation by the same angle anticlockwise.
We call this the - isomer