Advanced Higher ChemistryAdvanced Higher Chemistry

Unit 3Unit 3

Geometric isomersGeometric isomers

IsomerismIsomerism

Isomerism is an important feature of Isomerism is an important feature of Organic Chemistry.Organic Chemistry.

It occurs whenever there is more than one It occurs whenever there is more than one way to organise a given number of atoms way to organise a given number of atoms in a molecule.in a molecule.

Isomers are defined as compounds with Isomers are defined as compounds with the same molecular formula but different the same molecular formula but different structural formulae.structural formulae.

Structural IsomersStructural Isomers

Methoxymethane and ethanol are structural Methoxymethane and ethanol are structural isomers as they have the same molecular isomers as they have the same molecular formula (Cformula (C22HH66O) but different structures.O) but different structures.

In methoxymethane the oxygen atom lies In methoxymethane the oxygen atom lies between the two carbon atoms whereas in between the two carbon atoms whereas in ethanol the two carbon atoms are bonded ethanol the two carbon atoms are bonded together with the oxygen at the end.together with the oxygen at the end.The two substances belong to two The two substances belong to two different homologous series (ethers and different homologous series (ethers and alcohols respectively) and so will have alcohols respectively) and so will have very different chemical and physical very different chemical and physical properties.properties.

StereoisomerismStereoisomerism

In stereoisomerism, the molecules have the In stereoisomerism, the molecules have the same molecular formula and the atoms are same molecular formula and the atoms are bonded together in the same order. However, bonded together in the same order. However, the atoms have a different three dimensional the atoms have a different three dimensional arrangement in space.arrangement in space.They are non-superimposable, i.e. they cannot They are non-superimposable, i.e. they cannot fit exactly on top of each other.fit exactly on top of each other.Stereoisomers fit into two categories – Stereoisomers fit into two categories – geometric and optical.geometric and optical.

Geometric IsomerismGeometric Isomerism

In geometric isomerism, there is a lack of free In geometric isomerism, there is a lack of free rotation around a bond (e.g. around a C=C).rotation around a bond (e.g. around a C=C).

Rotation around a double bond is restricted Rotation around a double bond is restricted because it would involve breaking the pi bond.because it would involve breaking the pi bond.

1,2-dichloroethane has two geometric isomers.1,2-dichloroethane has two geometric isomers.

Geometric isomers can be distinguished in two ways:Geometric isomers can be distinguished in two ways:cis -cis - both groups are on the same side of the both groups are on the same side of the

double bond.double bond.trans -trans -the groups are on the opposite sides of the groups are on the opposite sides of

the double bond.the double bond.

Molecule (a) is the trans isomer Molecule (a) is the trans isomer

(trans-1,2-dichloroethene)(trans-1,2-dichloroethene)

Molecule (b) is the cis isomer.Molecule (b) is the cis isomer.

(cis-1,2-dichloroethene)(cis-1,2-dichloroethene)

Trans isomers tend to have higher melting Trans isomers tend to have higher melting points than cis isomers thus showing that points than cis isomers thus showing that they have stronger intermolecular forces.they have stronger intermolecular forces.The trans molecules are more closely The trans molecules are more closely packed and have stronger van der Waals packed and have stronger van der Waals forces.forces.Differences in boiling point can also be due Differences in boiling point can also be due to slight differences in polarity.to slight differences in polarity.

Melting points

In both geometric isomers of 1,2-In both geometric isomers of 1,2-dichloroethene the C-Cl bonds are polar.dichloroethene the C-Cl bonds are polar.

In the cis-isomer, both bonds lie on the In the cis-isomer, both bonds lie on the same side of the double bond and so the same side of the double bond and so the molecule itself will be polar.molecule itself will be polar.In the trans-isomer, the molecule is In the trans-isomer, the molecule is symmetrical and so the polarities cancel symmetrical and so the polarities cancel each other out. The molecule is non-polar.each other out. The molecule is non-polar.As a result of these extra polar-polar As a result of these extra polar-polar attractions the cis-isomer has a higher attractions the cis-isomer has a higher boiling point than the trans-isomer.boiling point than the trans-isomer.

Reactions of geometric isomersReactions of geometric isomers

In certain cases, geometric isomers can also In certain cases, geometric isomers can also display different chemical properties e.g. the display different chemical properties e.g. the dehydration of the two isomers of dehydration of the two isomers of but-2-enedicarboxylic acid.but-2-enedicarboxylic acid.When the cis-isomer, which has two carboxyl When the cis-isomer, which has two carboxyl groups on the same side of the double bond, is groups on the same side of the double bond, is heated an internal condensation reaction takes heated an internal condensation reaction takes place producing but-2-enedioic acid.place producing but-2-enedioic acid.This reaction is not possible for the trans isomer This reaction is not possible for the trans isomer since the carboxyl groups are on opposite sides since the carboxyl groups are on opposite sides and there is restricted rotation about the carbon and there is restricted rotation about the carbon to carbon double bond.to carbon double bond.

The condensation of two carboxyl groups, with The condensation of two carboxyl groups, with the elimination of water, produces an acid the elimination of water, produces an acid anhydride e.g. two molecules of ethanoic acid anhydride e.g. two molecules of ethanoic acid can condense to give ethanoic anhydride.can condense to give ethanoic anhydride.

Cis and trans fatty acidsCis and trans fatty acids

Fats and edible oils are naturally occurring Fats and edible oils are naturally occurring esters of glycerol (propane-1,2,3-triol) and long esters of glycerol (propane-1,2,3-triol) and long chain carboxylic acids called fatty acids.chain carboxylic acids called fatty acids.The fatty acids are almost always straight-chain The fatty acids are almost always straight-chain molecules, with an even number of carbon molecules, with an even number of carbon atoms usually ranging from 10 – 20.atoms usually ranging from 10 – 20.The chains may be saturated, monounsaturated The chains may be saturated, monounsaturated or polyunsaturated.or polyunsaturated.Animal fats tend to have a higher degree of Animal fats tend to have a higher degree of saturated fatty acids whereas vegetable oils saturated fatty acids whereas vegetable oils have a higher degree of unsaturated fatty acids.have a higher degree of unsaturated fatty acids.

Recent medical research has linked a diet Recent medical research has linked a diet high in saturated fats with high levels of high in saturated fats with high levels of cholesterol in the blood. This leads to a cholesterol in the blood. This leads to a build-up of fatty deposits in arteries and an build-up of fatty deposits in arteries and an increase in the risk of heart disease.increase in the risk of heart disease.Health authorities have been advising Health authorities have been advising people to cut down on the total amount of people to cut down on the total amount of fat in their diet and replace sources of fat in their diet and replace sources of saturated fats with unsaturated fats. saturated fats with unsaturated fats.

Most of the important unsaturated fatty acids Most of the important unsaturated fatty acids have a cis-arrangement around their carbon to have a cis-arrangement around their carbon to carbon double bonds.carbon double bonds.

A few trans-fatty acids are found naturally, A few trans-fatty acids are found naturally, mainly in meat products and dairy foods.mainly in meat products and dairy foods.

Current evidence suggests that they Current evidence suggests that they behave in a similar way to saturated fatty behave in a similar way to saturated fatty acids in raising blood cholesterol levels.acids in raising blood cholesterol levels.