1.6 Isomerism

Question 1

Define structural isomerism

Answer 1

same molecular formula, different structural formula

Question 2

define chain isomerism

Answer 2

isomers have different arrangements of the carbon chain

Question 3

define positional isomerism

Answer 3

isomers have the same carbon skeleton but the functional group is in different positions in the molecule

Question 4

define functional group isomerism

Answer 4

same molecular formula,, different functional groups

Question 5

define stereoisomerism

Answer 5

same molecular formula, but atoms occupy different positions in space

Question 6

define geometrical isomerism

Answer 6

molecules with the same structural formula but different arrangement of atoms due to the presence of one or more C=C bonds, around which there is restricted rotation for the two different groups bonded to each carbon atom

Question 7

define optical isomerism

Answer 7

isomers exist as non-superimposable mirror images due to the presence of one or more asymmetric centres

Question 8

What are the two main types of isomerism?

Answer 8

structural isomerism

steroisomerism

Question 9

name types of structural isomerism

Answer 9

chain isomerism
positional isomerism
functional group isomerism

Question 10

name types of stereoisomerism

Answer 10

geometrical isomerism

optical isomerism

Question 11

Why is geometrical isomerism found in molecules with a C=C bond?

Answer 11

it is an energy barrier to free rotation

Question 12

what is required in geometrical isomerism

Answer 12

a C=C bond where each carbon in the bond is attached to two different groups

Question 13

How are priorities assigned in geometrical isomerism?

Answer 13

• On each carbon the groups are labelled 1 or 2 depending on priority - the atom with the highest atomic number bonded to the carbon is assigned 1.
• If highest priority groups are on the same side they are known as Z (zusammen)/cis, and if on opposite sides they are known as E (entgegen)/trans

Question 14

define chiral/assymetric centre

Answer 14

a carbon atom with four different atoms or groups bonded to it

Question 15

define optical isomers/enantiomers

Answer 15

molecules which exist as non-superimposable mirror images

Question 16

define optically active

Answer 16

a sample that rotates the plane of plane polarised light

Question 17

When drawing optical isomers, what is the chiral centre labelled with?

Answer 17

an asterisk

Question 18

how can optical isomers be distinguished?

Answer 18

The optical isomers of a compound will have the same physical properties and can only be distinguished by their ability to rotate the plane of plane polarised light

Question 19

what are the two types of optical isomer?

Answer 19

• D (dextrorotatory) - rotates plane of plane polarised light to the right
• L (laevorotatory) - rotates plane of plane polarised light to the left

Question 20

define plane polarised light

Answer 20

visible light, or other forms of electromagnetic radiation, whose vibrations occur in a single plane

Question 21

define racemic mixture/racemate

Answer 21

a 50:50 mixture of two optical isomers

Question 22

Describe the optical activity of a racemate

Answer 22

it will be optically inactive. This is due to the optical isomers in the mixture rotating the plane of plane-polarised light by equal amount in opposite directions.

Question 23

Name two examples of optical isomers in medicine

Answer 23

thalidomide

citalopram

Question 24

describe thalidomide

Answer 24

• Introduced in 1957 to treat morning sickness during pregnancy
• Withdrawn after it was found to cause severe birth defects
• Contains an asymmetric centre that gives rise to two optical isomers - R-thalidomide and S-thalidomide
• R-isomer - effective in treatment of morning sickness, plus a mild sedative
• S-isomer - none of the therapeutic effects of the R-isomer and was responsible for limb deformities and other severe birth defects

Question 25

Describe citalopram

Answer 25

• An effective treatment for depression - administered in the form of tablets containing a racemic picture of optical isomers.
• The S isomer has a shape that allows it to bond with receptor sites in the body, which then signals an increase in the production of serotonin.