Nitrogen Compounds

Question 1

Why are boiling points of amines higher than alkanes

Answer 1

• amines are polar compounds and can form intermolecular hydrogen bonds
• large amount of energy is required to overcome the stronger hydrogen bonds between amide molecules

Question 2

Basic properties of amines

Answer 2

• weak Lewis bases in which the lone pair of electrons on the N atom can form a co-ordinate bond with a proton from an acid
• basic strength of an amine depends on the availability of the lone pair of electrons on N atom to form a dative bond to H⁺

Question 3

Relative basicity of amines in gaseous medium and aqueous medium

Answer 3

• 3 degree > 2 degree > 1 degree > NH₃
• the more readily the lone pair of electrons on N atom accepts a proton —> larger the value of K_b—> stronger the base
• RNH₂ > NH₃ > C₆H₅NH₂

Question 4

Why is ethylamine a stronger base than ammonia

Answer 4

• ethyl group is electron-donating and increases the electron density around the N atom
• increases the availability of the lone pair of electrons on N atom to accept a proton
• stronger base than ammonia

Question 5

Why is phenylamine a weaker base than ammonia

Answer 5

• lone pair of electrons on N atom in phenylamine is delocalised into the pi electron cloud of the benzene ring
• decreases the availability of the lone pair of electrons on N atom to accept a proton
• weaker base than ammonia

Question 6

How does type of substitute to affect basicity

Answer 6

— electron-donating substituent
- increases the availability of the lone pair of electrons on N atom to accept a proton
- increase in basicity
— electron-withdrawing substituent
- reduces the availability of the lone pair of electrons on N atom to accept a proton
- decrease in basicity

Question 7

Factors affecting basicity

Answer 7

• type of substituent
• number of electron-donating/ withdrawing substituent
• proximity of electron-donating/ withdrawing substituent to NH₂ group

Question 8

Preparation of aliphatic amines
Preparation of aromatic amines

Answer 8

• reduction of nitriles (RCN)
• reduction of amides
• nucleophilic substitution of halogenoalkanes, RX
• reduction of nitrobenzene

Question 9

Reactions of amines

Answer 9

• salt formation (acid-base reaction)
• formation of amides with acyl chlorides
• electrophilic substitution in benzene ring (bromination)

Question 10

Properties of amides

Answer 10

• neutral solid
• lone pair of electron on nitrogen atom is delocalised into the pi bond of the adjacent C=O by resonance, hence not available to accept a proton
• high boiling point due to intermolecular hydrogen bonding

Question 11

Preparation of amides

Answer 11

• nucleophilic substitution of acyl chlorides
• acyl chlorides with ammonia to form 1 degree amide
• acyl chlorides with 1, 2 degree amines to from 2, 3 degree amides

Question 12

Reaction of amides

Answer 12

— hydrolysis
- acid hydrolysis
- base hydrolysis
— reduction of amide

Question 13

Properties of amino acids

Answer 13

• has 2 functional groups, a carboxylic acid group and a basic amino group
• naturally occurring amino acids are alpha-amino acids where the alpha-carbon is directly bonded to both -COOH and -NH₂ groups

Question 14

How to determine if amino acid is basic, acidic or neutral

Answer 14

• If R group contains one or more carboxyl group: acidic amino acid
• If R group contains one or more amino group: basic amino acid
• If R group contains neutral group: neutral amino acid

Question 15

Zwitterion formation in amino acids

Answer 15

• proton from -COOH can be donated to the -NH₂ group of the same molecule to give a zwitterion
• carries both a positive and negative charge

Question 16

Physical properties of amino acids

Answer 16

— crystalline solids with very high melting points
- due to presence of strong electrostatic forces of attraction between the oppositely charged ends of neighbouring zwitterion in a giant ionic lattice structure
— very soluble in water
- charged ends of zwitterion can from strong ion-dipole attractions with water molecules

Question 17

Acid-Base properties of amino acids
- in acidic solutions
- in alkaline solution

Answer 17

• amphoteric since they contain both acidic functional group and basic functional group
• acts as a base by accepting a proton from the acid to form a positive ion
• migrate to negative electrode(cathode) in a electric field
• acts as an acid by donating a proton to the base to form a negative ion
• migrates to the positive electrode in a electric field

Question 18

Properties of polypeptides and proteins

Answer 18

• two amino acids react together to form a peptide
• condensation reaction between -COOH group of one alpha-amino acid with -NH₂ group of the other alpha-amino acid to eliminate a small molecule of water
• amino acid that make up proteins is called side chains
• proteins have open ends structure unlike peptides

Question 19

Hydrolysis of polypeptides/proteins

Answer 19

• peptide bond in a protein can be cleaved to yield a mixture of amino acids
• acid hydrolysis
• base hydrolysis