Topic 17 : Organic Chemistry II

Question 1

State what is meant by the term chiral

Answer 1

A chiral carbon has 4 different groups attached to it
Forms isomers which are non-superimposable

Question 2

Give the steps in drawing optical isomers

Answer 2

1. Identify the chiral centre
2. Draw one enantiomer in a tetrahedral shape
3. Draw its mirror image

Question 3

A substance shows optical activity if…

Answer 3

It rotates the plane of polarisation of plane polarised light

Question 4

What does a polarimeter measure ?

Answer 4

The amount of optical activity

Question 5

How does a polarimeter work

Answer 5

A monochromatic light source passes through a polarising filter which converts unpolarised light into plane-polarised light
The plane-polarised light then passes through a sample tube containing some of the substance in solution
If the substance is optically active, the plane of polarisation is rotated

Question 6

Describe the properties of enantiomers

Answer 6

Identical physical properties with 1 exception :
They both rotate plane-polarised light by equal angles but in opposite directions
Identical chemical properties with 1 exception:
The way in which they react with the enantiomers of other substances may be different for each enantiomer

Question 7

What is a racemic mixture

Answer 7

Equimolar mixture of 2 enantiomers that has no optical activity as rotations cancel each other out

Question 8

What are the features of an SN2 reaction mechanism?

Answer 8

• the groups around the central atom get inverted, therefore optical activity of product is different from that of reactant
• 1 step reaction
• we have a transition state
• nucleophile attacks from opposite side to the leaving group

Question 9

What are the features of an SN1 reaction mechanism

Answer 9

• 2 step reaction
• produces a carbocation with a planar shape so therefore there is equal chance of nucleophile attacking from either side in 2nd step of reaction
• produces 2 products, both of which are enantiomers in a racemic mixture
• optical activity is different from reactant

Question 10

What is the carbonyl group

Answer 10

A carbon atoms joined by a double bond to an oxygen atom (C=O)

Question 11

What are the only 2 types of compounds classified as carbonyls ?

Answer 11

Aldehydes and ketones

Question 12

Describe and explain the bonding in the carbonyl group (C=O)

Answer 12

Composed of a combination of the sigma and pi bond like a C=C bond
C=O bond, however, is polar due to the differing electronegativities of carbon & oxygen - causes a shift in electron density towards the oxygen

Question 13

Why are carbonyls prone to nucleophilic attack ?

Answer 13

Shift in electrondensity towards the oxygen in the C=O bond causes the carbon to be slightly electron deficient and therefore prone to nucelophilic attack

Question 14

Describe and explain the boiling temperatures of carbonyl compounds

Answer 14

Contain the polar C=O group so have permanent dipole-dipole attractions
No hydrogen bonding
Boiling temps increase with increasing molecular mass as the extent of London forces increases

Question 15

Describe and explain the solubility in water of carbonyl compounds

Answer 15

The smaller aldehydes and ketones are soluble in water as they can form hydrogen bonds with water molecules
The solubility decreases with increasing chain length as the hydrocarbon portion of the molecule becomes more significant

Question 16

How are aldehydes and ketones converted to alcohols ?

Answer 16

Reduction using the reagent lithium tetrahydridoaluminate (LiAlH4)
Reaction is carried out with both the carbonyl compound and the reagent dissolved in dry ether
The reducing agent can be represent by a [H] in equations

Question 17

What are the 4 different reagents that can be used to oxidise aldehydes to carboxylic acids

Answer 17

Acidified potassium dichromate (VI)
Fehling’s solution
Benedict’s solution
Tollens’ reagent

Question 18

What colour change can be seen for the oxidation of aldehydes with Fehling’s or Benedict’s solution

Answer 18

Deep blue -> red precipitate

Question 19

What colour change can be seen for the oxidation of aldehydes to carboxylic acids using Tollens’ reagent

Answer 19

Colourless solution —> silver mirror

Question 20

Describe the reaction of a carbonyl with iodine

Answer 20

Redox reaction
Carbonyl compound is added to an alkaline solution iodine
Mixture is warmed and then cooled
Pale yellow precipitate forms = positive result for ethanal or a ketone

Question 21

Describe the reaction of carbonyls with hydrogen cyanide

Answer 21

• carried out in an aqueous alkaline solution containing KCN
• reagent is HCl
• nucleophilic addition
• products are referred to as hydroxynitriles

Question 22

Describe and explain the nucleophilic addition mechanism of the reaction of carbonyls and HCl

Answer 22

Step 1 : involves the nucleophilic carrack by a :CN- on the carbon atom of the carbonyl compound
Step 2: involves the reaction bête en the intermediate and the H-CN molecule
Results in a product where the functional groups -OH and -CN are joined to a carbon atom forming hydroxynitriles

Question 23

Describe the reaction of Brady’s reagent (2,4 dinitrophenylhydrazine) with carbonyl compounds

Answer 23

Reacts with most carbonyl compounds to form a bright orange precipitate

Question 24

Describe and explain the boiling temperatures of carboxylic acids

Answer 24

Strong intermolecular forces (hydrogen bonding)
As carbon chain length increases so does boiling point die to increased London forces
Pure carboxylic acids can form hydrogen bonded dimers

Question 25

Describe and explain the solubility of carboxylic acids in water

Answer 25

Shorter chain carboxylic acids are soluble in water as they can form hydrogen bonds with water molecules
Solubility decreases with increasing chain length as the hydrocarbon portion of the molecule becomes larger