optical isomerism & aldehydes + ketones

Question 1

What are the 2 types of isomerism and what can they broken down into

Answer 1

Structural isomerism -> Chain isomerism, Positional isomerism, Functional group isomerism
Stereoisomerism - geometrical isomerism (E/Z) & optical isomerism

Question 2

Define structural isomerism

Answer 2

The same molecular formula
Different structural formula

Question 3

Define stereoisomers

Answer 3

The same structural formula
Different arrangement of atoms in space

Question 4

What are the common tests can be used to distinguish between aldehydes and ketones.

Give the name
Reagent
Conditions
Observation with aldehyde
Observation with ketone

Answer 4

Silver mirror test

Reagent - Tollen’s Reagent or Ammonical Silver nitrate
Conditions - warm gently
Observation with aldehyde - silver mirror forms
Observation with ketone - no visible change

Fehlings test

Reagent -Fehlings solution
Conditions - Warm gently
Observation with aldehyde - Blue solution to brick red precipitate
Observation with ketone - no visible change

Question 5

Why can nucleophilic addition to aldehydes and asymmetrical ketones result in a racemic mixture of optical isomers?

Answer 5

The bonding is about a __ __ c = o group in an aldehyde and ketone is planar.
• This means there is a 50:50 chance of the nucleophile attacking from one side of the molecule or the other.
• This results in equal proportions of each optical isomer forming, and so a racemic mixture is formed.

Question 6

What are some key points on optical isomerism

Answer 6

• Optical isomerism is a form of stereoisomerism.
• Stereoisomers have the same structural formula but a different arrangement of atoms in space.
• Optical isomers are non-superimposable mirror images.

Question 7

When does optical isomerism occur

Answer 7

when a carbon atom bonds to four different groups.

Question 8

What is optical isomerism

Answer 8

Molecules with chiral centres
That are non-superimposable mirror images
Rotate the plane of polarised light in opposite directions
One isomer will rotate it clockwise, the other isomer will rotate it anti-clockwise.

Question 9

What similar properties do optical isomers share

Answer 9

• Same melting point and boiling point.
• Same solubility in a range of solvents.
• Same Mr
• Same peaks in their NMR spectrum, IR spectrum and Mass spectrum.
  Many optical isomers will have the same chemical reactivity, however if they are used as a drug both isomers may not work in same way when inside cells.

Question 10

Define chiral

Answer 10

A molecule which can exist as a pair of optical isomers is said to be CHIRAL

Question 11

Define chiral centre

Answer 11

The carbon which is attached to four different groups is called the CHIRAL CENTRE

Question 12

Define stereogenic centre

Answer 12

Another name for a chiral centre is a STEREOGENIC CENTRE

Question 13

Define asymmetric carbon

Answer 13

Another name for a chiral centre is a ASYMMETRIC CARBON

Question 14

Define enantiomer

Answer 14

Each optical isomer can also be called an ENANTIOMER

Question 15

Define enantiomerically pure

Answer 15

If a sample contains only one optical isomer it is said to be ENANTIOMERICALLY PURE

Question 16

Define optically active

Answer 16

An enantiomerically pure compound is OPTICALLY ACTIVE as it will rotate the plane of polarised light

Question 17

Define race mix mixture

Answer 17

A mixture which contains quantities of enantiomers is called a racemix mixture