isomerism · 2020. 10. 2. · 1. geometrical (cis-trans) isomerism • caution!: this compound does...

IsomerismWhat is an Isomer?

Isomerism

Isomerism

Structural Isomerism

Stereoisomerism

Types of Structural Isomerism

Structural Isomersim

ChainFunctional

GroupPositional

Structural Isomerism (follow along in pg 272 – 273 Textbook)

1. Chain Isomerism

Butane

Methylpropane /2-methylpropane

Structural Isomerism

2. Positional Isomerism

Butan-1-ol Butan-2-ol

Structural Isomerism

2. Positional Isomerism

Pent-1-ene Pent-2-ene

Structural Isomerism

2. Positional Isomerism

Isomerism and Bond Rotation

• These structures are NOT isomers of each other. Why?

Structural Isomerism

3. Functional Group Isomerism

Propanal(Aldehyde)

Propanone(Ketone)

Structural Isomerism

3. Functional Group Isomerism

Propanoic Acid(Carboxylic Acid)

Methyl Ethanoate(Ester)

You try:

• Draw and name the isomers for compounds with the molecular formula C5H12

• Draw and name the isomers for C5H10

• Draw and name the isomers for C6H14

Tips for Drawing Isomers

Start with the straight-chain isomer

Draw the isomers with 1 less carbon atom in the longest chain

Draw the isomers with 2 less carbon atoms in the longest chain

For longer-chain alkanes, continue in this way until you are sure that the isomer is not one that you have already drawn

Isomers of Aryl Compounds

We do not number the carbon atoms because only 1 alkyl group is attached to the benzene ring

Stereoisomerism

Stereoisomerism

Geometrical (Cis-Trans)

Optical

Stereoisomerism1. Geometrical (Cis-Trans) Isomerism

Organic compounds which exhibit geometrical isomerism must have:

1. A double bond (a C=C alkene linkage)

2. Two identical groups attached to adjacent C atoms

3. Two different groups attached to each C on the double bond

Stereoisomerism1. Geometrical (Cis-Trans) Isomerism

TRANS - the identical Cl atoms are on OPPOSITE sides of the C=C bond

CIS - the identical Cl atoms are on the SAME side of the C=C bond

Stereoisomerism

1. Geometrical (Cis-Trans) Isomerism

TRANS - the identical H atoms are on OPPOSITE sides of the C=C bond

CIS - the identical H atoms are on the SAME side of the C=C bond

Stereoisomerism1. Geometrical (Cis-Trans) Isomerism

• CAUTION!: This compound DOES NOT display geometrical isomerism because the two identical groups are attached to the SAME C atom.

• To exhibit geometrical isomerism, the identical groups must be attached to DIFFERENT carbon atoms, and each C atom must have 2 different groups attached to it.

Geometrical (Cis-Trans) Isomerism

• The two geometrical isomers have different physical properties and they may have chemical properties which are slightly different.

Isomers of C4H8

• Draw all the isomers of C4H8. Hint: Include any geometrical (cis-trans) isomers too

But-1-ene

But-2-ene

2-methylpropene

Cyclobutane

12

3

Recall these 4 structural isomers of C4H8

Isomers of C4H8

• But Note that there are 2 spatial arrangements of but-2-eneSo, in total, we have 5 isomers of C4H8 – not 6.

But-2-ene exists in either of these 2 different spatial arrangements (cis

or trans) with the SAME structural formula , so we don't add back

but-2-ene again

4 5

'Crab' notation

• Geometrical isomers should always be drawn using 'crab' notation, showing the C=C bond as a planar centre with the 4 groups as shown below.

'Crab' notation• Using 'crab' notation, it is easy to predict whether geometrical isomerism will

exist in molecules.

Compound Structure in Crab Notation

Whether or not Geometrical Isomerism Exists

1. Ethene No geometrical isomerism because all 4 groups around the double bond are the same. The Cs are not attached to 2 different groups

'Crab' notation• Using 'crab' notation, it is easy to predict whether geometrical isomerism will

exist in molecules.

Compound Structure in Crab Notation

Whether or not Geometrical Isomerism Exists

2. Propene No geometrical isomerism because it does not have 2 identical groups and 2 different groups

'Crab' notation• Using 'crab' notation, it is easy to predict whether geometrical isomerism will

exist in molecules.

Compound Structure in Crab Notation

Whether or not Geometrical Isomerism Exists

3. But-1-ene No geometrical isomerism because there are not 2 identical groups and 2 different groups

'Crab' notation• Using 'crab' notation, it is easy to predict whether geometrical isomerism will

exist in molecules.

Compound Structure in Crab Notation

Whether or not Geometrical Isomerism Exists

4. But-2-ene Yes - geometrical isomerism because there are 2 identical groups and 2 different groups. This can exist in cis-trans forms

Optical Isomerism

Optical Isomerism

Four different groups are attached to a central carbon atom

Optical Isomerism

• Isomers of bromochlorofluoromethane

Optical Isomerism – alaine (amino acid)