Topic 17 - Organic 2

Question 1

What is a chiral carbon

Answer 1

A carbon attached to 4 different groups in a compound

Question 2

What is optical isomerism?

Answer 2

A form of isomerism where enantiomers rotate plane polarised light in opposite directions by the same amount

Question 3

What are enantiomers?

Answer 3

A pair of molecules containing one chiral centre which are non-superimposable mirror images of each other

Question 4

What name is given to an equal mixture of two enantiomers and what is the effect on plane polarised light?

Answer 4

A racemic mixture/ racemate
No overall effect on plane-polarised light

Question 5

Why can a racemic mixture be formed by an Sn1 reaction?

Answer 5

• The C+ intermediate has a trigonal planar shape around the C+
• The OH- nucleophile equally likely to attack from the left or the right
• creates two enantiomers in a 50/50 mixture

Question 6

Why can a racemic mixture be formed by a nucleophilic addition reaction?

Answer 6

• The aldehyde or ketone has a trigonal planar shape around the carbonyl carbon (C=O)
• The nucleophile equally likely to attack from the left or the right
• creates two enantiomers in a 50/50 mixture

Question 7

Why are short chain miscible with water but long chains are not?

Answer 7

Before mixing:
- water forms hydrogen bonds between molecules
- aldehydes form permanent dipole-dipole forces

When mixed:
- short chains can use the carbonyl to form H-bonds with water that are similar in strength the the H-bonds in water, so do mix
- long chains mainly form London forces with the long non-polar alkyl chain and water. Much weaker than the H-bonds between water molecules, so they don’t mix

Question 8

Reagents, conditions, products and mechanism:
Reduction of aldehydes and ketones

Answer 8

Reagent: LiAlH4
Conditions: dry ether. Follow with acid/water work up
Product: primary alcohol (aldehyde) or secondary alcohol (ketone)
Mechanism: nucleophilic addition

Question 9

Reagents, conditions, products and mechanism:
Cyanohydrin formation (aldehydes and ketones reacting with CN-)

Answer 9

Reagent: KCN (aq)
Conditions: H+ cat. Follow with acid/water work up
Product: hydroxynitrile (has an OH and CN group)
Mechanism: nucleophilic addition

Question 10

What is a Grignard reagent?
How do you make one?
Why is it useful?

Answer 10

R - Mg - X
(eg. CH3CH2 - Mg - Br)

React a halogenoalkane with Mg in dry ether and heat

Can be used to increase carbon chain length

Question 11

Reagents, conditions, products and mechanism:
Reaction of aldehydes and ketones with a Grignard reagent

Answer 11

Reagent: Grignard reagent (haloalkane + Mg)
Conditions: dry ether. Follow with acid/water work up.
Product: longer chain alcohol
Mechanism: nucleophilic addition

Question 12

Reagents, product and positive test:
Iodoform reaction

Answer 12

Tests for a methyl ketone (CH3-C=O)

Reagents: I2 + NaOH
Product: Iodoform
Positive test: Yellow ppt (iodoform), antiseptic smell

Question 13

What chemical tests differentiate between an aldehyde and a ketone?

Answer 13

Tollen’s
- ammoniacal silver nitrate
- aldehyde gives silver mirror (Ag+ is reduced to Ag(s))
- ketone gives no change

Fehling’s
- Fehling’s solution
- aldehyde makes blue solution turn into brick red ppt
- ketone gives no change

Question 14

What is a positive test with 2,4-DNP and what is it used for?

Answer 14

• used to test for aldehydes and ketones
• positive test: orange solution to orange ppt
• the melting point of the recrystallised 2,4-DNP derivative can be used to identify the specific aldehyde or ketone

Question 15

Give two ways to make a carboxylic acid

Answer 15

1) Oxidation of a primary alcohol or aldehyde
Reagents + conditions: K2Cr2O7, H2SO4 cat.
Positive test: Orange to green

2) Hydrolysis of a nitrile
Reagents + conditions: water, H+ cat. (HCl), heat

Question 16

Give 3 properties of carboxylic acids

Answer 16

1. Weak acids
2. High boiling points (can form two hydrogen bonds per molecule)
3. Miscible with water up to 4 carbons (butanoic acid)

Question 17

Explain the difference in boiling points of butane, propanal and propanoic acid

Answer 17

All have London forces
Propanal and propanoic acid can both form permanent dipole-dipole forces
Propanoic acid can form 2 H-bonds per molecule
Propanoic acid can also create a dimer which increases the size of London forces

Question 18

Reagents, conditions and product:
Reduction of a carboxylic acid

Answer 18

Reagents: LiAlH4
Conditions: dry ether, followed by acid work up
Product: primary alcohol

Question 19

Reagents, conditions and product:
Making an ester from carboxylic acids

Answer 19

Reagents: an alcohol, conc. H2SO4 cat.
Conditions: heat under reflux
Product: ester

Question 20

Give two ways to make an acyl chloride

Answer 20

1. PCl5 (must be dry conditions)
   Products: acyl chloride, POCl3 + HCl
2. SOCl2
   Products: acyl chloride, SO2 (g), HCl (g)
   This way is better as other products are gases so easier to separate

Question 21

How do you turn an acyl chloride back into a carboxylic acid?

Answer 21

Add H2O

Question 22

Reagents, conditions and product:
Making an ester from an acyl chloride

Answer 22

Reagents: alcohol
Conditions: room temp, pyridine (to neutralise HCl produced)
Product: ester