Organic II- Topic 17

Question 1

What is a chiral carbon atom/chiral centre?

Answer 1

A carbon atom with 4 different groups attached to it.

Question 2

What are enantiomers?

Answer 2

Two different optical isomers of each other (mirror images) due to chiral compound

Question 3

What is true of enantiomers?

Answer 3

Non-superimposable

Question 4

What is optical activity?

Answer 4

The ability of a single optical isomer to rotate the plane of polarisation of plane-polarised monochromatic light in molecules containing a single chiral centre

Question 5

What is a racemic mixture?

Answer 5

A mixture with equal quantities of each enantiomer of a chiral compound

Question 6

What don’t racemic mixtures do?

Answer 6

Don’t rotate plane polarised light and the two enantiomers cancel each other’s light-rotating effect

Question 7

Difference between Sn1 and Sn2 mechanisms

Answer 7

Sn1= single enantiomer reactant produces a racemic mixture of 2 optical isomers of each other ( won't rotate plane-polarised light)
Sn2= single enantiomer reactant produces a single enantiomer product

Question 8

if butan2ol solution proceeds via SN1 mechanism, why doesn’t it rotate plane-polarised light?

Answer 8

SN1 mechanisms produce racemic mixtures. Therefore there will be equal amounts of both isomers. The enantiomers light-rotating effects will cancel

Question 9

How many steps are in an SN1 vs SN2 mechanism?

Answer 9

SN1 =  2 Step
SN2= 1 Step

Question 10

2,4 DNPH reacts with carbonyls and forms a ….. ….. ….

Answer 10

Bright orange precipitate.

Question 11

How can the results of 2,4 DNPH and a carbonyl forming the identity of carbonyl compound?

Answer 11

Purification of org ppt by recrystallisation. Melting point of crystals can be compared to melting points of all known possible derivatives/ known values on database

Question 12

How can you reduce carbonyls?

Answer 12

Using LiAlH4 in dry ether, to form:
aldehyde —-> primary alcohol
ketone—-> secondary alcohol

Question 13

How do carbonyls react with Iodine in the presence of alkali?

Answer 13

form a yellow ppt of CHI3 and an antiseptic smell.

Question 14

If something contains a methyl carbonyl group it must be either….

Answer 14

ethanal

or a ketone with at least 1 methyl group

Question 15

Explain why it is necessary to use HCN and KCN in nucleophilic addition? [1]

Answer 15

As HCN alone provides insufficient CN-

Question 16

Describe 3 tests that can be used to confirm the presence of an aldehyde and their positive results.

Answer 16

Warm with Tollens reagent, if carbonyl present then silver mirror will form.
Warm with Benedicts or Fehlings solution and if carbonyl present then blue solution forms red ppt
Warm with Acidified dichromate ions to form carbonxylic acid from the aldehyde (solution turns orange to green as the dichromate ions are reduced)

Question 17

A chiral molecule is….

Answer 17

non-superimposable on it’s mirror image