isomerism. isomers are compounds that have the same molecular formula but have different...

Isomerism

Isomerism• Isomers are compounds that have the same molecular

formula but have different arrangements of atoms in space.

• Isomers have different physical and chemical properties (depending on the type of isomerism).

• E.g. Butane and 2-methyl propane

Starter: make these molecules in the order given (5 minutes)

alkanes

alkenes

H CH3

| |H – C – C – H | | CH3 H

CH3 CH3

| |H – C – C – H | | H H

H CH3

| |H – C = C – H | | CH3 H

CH3 CH3

| |H – C = C – H | | H H

Do you notice whether it is easier to make 2 after 1 compared with 4 after 3? How is it different? Why is it different?

Challenge name these molecules

Same molecular formula but different arrangement of their atoms.

CHAIN ISOMERISM

POSITIONAL ISOMERISM

FUNCTIONAL GROUP

ISOMERISM

GEOMETRIC ISOMERISM

OPTICAL ISOMERISM

Atoms are bonded in a

different order.

Atoms are bonded in the same order but arranged differently

in space.

Rotation prevented by double bond.

Non- superimposable mirror images.

Stereoisomerism(Spatial isomerism)

SAME molecular formula, SAME functional groups, SAME bonds, but DIFFERENT arrangement in space.

(a)This often occurs with a double bond

Geometric Isomerism!(b) Or..(b) Or.. if a central atom is attached to 4 different groupsif a central atom is attached to 4 different groups

Optical Isomerism!

Geometric isomerism.o This type of isomerism results from restricted

rotation somewhere in the molecule (usually C=C).

o HOWEVER.......

Pi bonds are formed by a sideways overlap between p- orbitals, and if they are twisted then this

overlap is disrupted and the Pi bond is broken.

Therefore use this as a clue!

trans-1,2-dichloroethene

cis-1,2-dichloroethene

“on this side”

“across”

Cis and trans isomers may have different properties e.g. In the way they react with biological molecules.

Cis- MP -80ºC, BP 60ºCTrans – MP -50ºC, BP 48ºC

So what do we call this?

E-Z isomerism

If 2 groups with the highest priority are on the same side they are a Z isomer.

Z –Zusammen = together E –Entgegen = opposite

Rules

1. Separate the two sides of the double bond and treat them separately.

2. The atom with the highest atomic number is given the highest priority.

Examples

Comparing stereoisomerism and structural isomerism

Bob being a stereoisomer

Bob being a structuralisomer

Normal Bob

TASK:

Illustrate the structures of the three different isomers of C5H12.

The Same

or Not the SameSlides 2-17

• E.g.

Optical isomers• Specific to compounds with four

different groups on the same carbon atom

• called Chiral carbons

• Four groups allows two different arrangements in space

Non-superimposable mirror imagesor

Enantiomers

Optical isomers

• Chiral cmpds have the same properties as each other.

• Except for

• Optical activity

• Interaction with biological systems; i.e. enzyme action Bulb

Monochromatic light source

Polarising filter

Second ParallelPolarising filter

PlainSample insolution

detector

Plane Polarised Light

PlainSample insolution

Optical isomers

• Chiral cmpds have the same properties as each other.

• Except for

• Optical activity

• Interaction with biological systems; i.e. enzyme action Bulb

Monochromatic light source

Polarising filter

SecondNon-parallelPolarising filter

detector

Sample insolution

Optical isomers• The polarising filters are

perpendicular so no light should get through.

• However,

• With the optical isomer in place light is transmitted.

BulbMonochromatic light source

Polarising filter

SecondNon-parallelPolarising filter

detector

Optical isomers• One enantiomer or optical

isomer rotates the beam of plane polarised light clockwise.

• i.e. it is dextrorotatory

• e.g. d-glucose

• e.g. (+) glucose

• The second polariser must be rotated clockwise to stop light being transmitted again.

BulbMonochromatic light source

Polarising filter

SecondPolarising filter

Sample insolution

detector

Optical isomers• One enantiomer or optical

isomer rotates the beam of plane polarised light anticlockwise.

• i.e. it is laevorotatory

• e.g. l-glucose

• e.g. (-) glucose

• The second polariser must be rotated anticlockwise to stop light being transmitted again.

BulbMonochromatic light source

Polarising filter

SecondPolarising filter

Sample insolution

detector

• Optically active substances often occur as a 50/50 mixture of the two enantiomers.

• This is known as a racemic mixture It has no effect on plane polarised light.

C

H

CH3 C

H

OH OH

CO2H

CO2H

CO2H

OOHCH3 C

Cl

Cl

C

C2H5

CH3

Cl

C

H

CH3 C

F

OH OH

CH3

O

CH3

CO2H

OOH

Cl

C6H5 C

Cl

C6H5

C

C2H5

CH3

Cl

Optical isomers

• Implications for pharmaceutical compounds.

• Preparation

• Dosage

• Side effects

• e.g lactic acid

• e.g. thalidomide.

Some compounds exhibiting chirality have enantiomers with very different properties.

Eg. thalidomide

One enantiomer relieved morning sickness for mothers, the other hinder the development of babies and they were born with their limbs not formed properly.