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