CIE A Level Chemistry复习笔记3.1.11 Stereoisomerism

Category: A-level课程, 教材笔记, 福利干货 Date: 2022年8月12日下午3:34

Stereoisomerism: Geometrical & Optical

Stereoisomers are compounds that have the same atoms connected to each other, however the atoms are differently arranged in space
There are two types of stereoisomerism:
- Geometrical (cis/trans) isomerism
- Optical isomerism

Geometrical (cis/trans) isomerism

Geometrical isomerism is seen in unsaturated (double bond containing) or ring compounds that have the same molecular formula and order of atoms (the atoms are connected similarly to each other) but different shapes
Cis/trans nomenclature is used to distinguish between the isomers
- Cis isomers have functional groups on the same side of the double bond/carbon ring
- Trans isomers have functional groups on opposite sides of the double bond/carbon ring

3.1-An-Introduction-to-AS-Level-Organic-Chemistry-Geometrical-Isomerism-Unsaturated-Compounds Geometrical isomerism in unsaturated compounds

3.1-An-Introduction-to-AS-Level-Organic-Chemistry-Geometrical-Isomerism-Cyclic-Compounds

Geometrical isomerism in cyclic compounds

This causes the compounds to have different chemical and physical properties
For example, they may have different reaction rates for the same reaction (chemical property) or different melting/boiling points (physical property)

Optical isomerism

Optical isomers arise when a carbon atom in a molecule is bonded to four different atoms or groups of atoms
The carbon atom is ‘asymmetric’ as there is no plane of symmetric in the molecule and is also called the chiral centre of the molecule
The two different optical isomers are also called enantiomers
- Just like the left hand cannot be superimposed on the right hand, enantiomers too are non-superimposable
- Enantiomers are mirror images of each other

3.1-An-Introduction-to-AS-Level-Organic-Chemistry-Optical-Isomerism

Both molecules are made up of the same atoms which are bonded to each other identically, however the chiral centre (carbon with four different groups) gives rise to optical isomerism

Optical isomers differ in their ability to rotate the plane of polarised light
- One enantiomer will rotate it clockwise and the other anticlockwise

A2 only

Normal light is unpolarised and consists of electric and magnetic fields that vibrate at right angles to each other in every possible direction
When the unpolarised light passes through a polariser, the light gets polarised causing it to vibrate in only one plane
A pair of optical isomers will rotate the plane of polarised light by equal amounts in opposite direction
When equal amounts of the enantiomers are present in solution, the plane of polarised light doesn’t change
- As the enantiomers cancel out each other’s effect
A solution with equal amounts of both enantiomers is also called a racemic mixture

3.1-An-Introduction-to-AS-Level-Organic-Chemistry-Unpolarised-Light Light consists of vibrations in all possible directions however when it passes through a polariser the light gets polarised and vibrates in only one plane: enantiomers cause the plane of polarised light to rotate clockwise or anticlockwise

Geometrical Isomerism in Alkenes

Unsaturated compounds

In unsaturated compounds, the groups attached to the C=C carbons remain fixed in their position
This is because free rotation of the bonds about the C=C bond is not possible due to the presence of a π bond

3.1-An-Introduction-to-AS-Level-Organic-Chemistry-Geometrical-Isomers-Unsaturated-Compounds

The presence of a π bond in unsaturated compounds restricts rotation about the C=C bond forcing the groups to remain fixed in their position and giving rise to the formation of geometrical isomers

Exam Tip

Geometrical isomerism is also possible in cyclic compounds because there is limited rotation about C-C single bonds that make up the rings.Therefore, the substitutions in cyclic compounds are fixed in their position (to stay either above or below the ring of carbon atoms).

转载自savemyexams