Functional Groups 5: Amines and Thiols

Question 1

Describe the stereochemistry of amines.

Answer 1

Amines are formally chiral
But they can’t be resolved since the lone pair inverts to the other side of the molecule

Question 2

Describe the pH of amines.

Answer 2

Basic
Basicity measured by the acidity of the ammonium ion

Question 3

Describe the effect of steric hindrance on amines.

Answer 3

Steric hindrance around H on N increases as you move down the table
Inductive electron donation of methyl groups leads to slightly higher basicity than ammonia

Question 4

List base strength of amines from ammonia to tertiary amines.

Answer 4

Secondary amines > Primary amines > Tertiary amines > Ammonia

Question 5

Why are secondary amines stronger bases than primary amines?

Answer 5

Alkyl groups on secondary amines increase inductive effects of electrons increasing the electron density on nitrogen so the lone pair is readily available
Primary amines have less inductive effects

Question 6

Why are tertiary amines weaker bases than secondary and primary amines?

Answer 6

Steric hindrance around the nitrogen reduces accessibility of the lone pair

Question 7

Why is ammonia a weak base?

Answer 7

Because it has no alkyl groups to cause inductive effects

Question 8

How can you deprotonate amines?

Answer 8

By using a sufficiently strong base
E.g. tert-butyl lithium

Question 9

How can you deprotonate ammonia?

Answer 9

Using sodium metal in liquid ammonia
Forms sodium amide, a useful strong base

Question 10

How can amines be synthesised by reduction?

Answer 10

1) Reduction of amides
2) Reduction of nitriles
3) Reductions of azides

Question 11

Describe how amines react with acid chlorides.

Answer 11

Primary or secondary amines can react with acid chlorides to form amides

Question 12

Describe how amines can react with aldehydes.

Answer 12

To form imines (Schiff bases)

Question 13

Why do primary, secondary, and tertiary amines have different physical properties?

Answer 13

Because they experience hydrogen bonds to differing extents
E.g. primary amines can form two hydrogen bonds and they have a relatively high boiling point
Secondary can form one, and tertiary amines cannot from any so have the lowest b.p

Question 14

What are amides?

Answer 14

CONH2

Question 15

What are the physical characteristics of amides?

Answer 15

Decreasing melting and boiling points as you move from primary amides to tertiary amides
However, tertiary amides can also act as hydrogen bond acceptors so all classes of amide are somewhat water-soluble

Question 16

What are thiols?

Answer 16

Compounds based on sulfur
Similar to the hydroxyl group but with a sulfur atom substituted for the oxygen

Question 17

Why does sulfur not induce additional polarity in thiols?

Answer 17

Because sulfur is similar in electronegativity to carbon

Question 18

How do neighbouring thiol groups join together?

Answer 18

By strong disulfide bridges
Vital role in stabilising the structure of proteins by firmly linking amino acids together to lock the protein in place

Question 19

Describe the biological importance of some thiols.

Answer 19

Hydrogen sulfide - gasotransmitter - signalling functions within the body
Nitric oxide - regulates brain activity as well as activity in lungs, liver, kidney, and stomach
Hydrogen sulfide - protects the nerves, retinas, and kidneys