Amines

Question 1

How are amines named?

Answer 1

Amines are named by adding the suffix -amine to the name of the alkyl group

In a secondary or tertiary amine, where there is more than one alkyl group, the groups are named as N-substituted derivatives of the larger group.

Question 2

Describe the basicity of amines

Answer 2

Amines have an unbonded electron pair and three attached groups

The lone pair is responsible for much of amine’s reactivity

• The lone pair is stabilized by electron donating effects of alkyl groups
• Thus, the lone pair is more stable in tertiary amines than in secondary amines, which is more stable than primary amines

In aqueous solutions, amines are weak bases and can accept a proton.

Question 3

How can amines with carbocations be stable?

Answer 3

The lone pair on the nitrogen can delocalize the positive charge

Question 4

The relative basicity of amines is determined by what? (2)

Answer 4

• If the free amine is stabilized relative to the cation, the amine is less basic
• If the cation is stabilized relative to the free amine, the amine is more stable, the the stronger base

Question 5

How does the base strength of amines differ between different functional groups attached to the amine?

Answer 5

• Electron withdrawing groups (eg. halides) decreases availability of lone pair on nitrogen (decrease basicity)
• Electron releasing groups (eg. alkyl groups) increase availability of lone pair (increase basicity)

For example:

Strongest 
 - tertiary amine
 - secondary amine
 - Primary amine
 - NH3
 - phenol-CH2-NH2
 - phenol-NH2
 - NO2-phenol-NH2
Weakest

Question 6

Describe amine nitrogen bonding and what this means for amine boiling point.

Answer 6

• Nitrogen atom can hydrogen bond to other hydrogens using its lone pair OR with hydrogens bonded to it (to N, O, F or Cl)
• Primary and secondary amines can hydrogen bond with each other, but tertiary amines cannot. This leads to boiling points which are higher than to be expected for 1’ and 2’ amines
• Hydrogen bonding also renders low weight amines soluble in water and allows for dipole moments

Question 7

How does the solubility of quaternary amine salts differ with increasing molecular weight?

Answer 7

Decreases with increasing molecular weight. The quaternary structure has steric hindrance and the lone pair electrons on nitrogen are not available for hydrogen bonding, thus their solubility is much reduced compared to other amines or even alkyl ammonium salts.

Question 8

The ability of NH3 to form coordination compounds with transition metal ions can best be accounted for by the fact that NH3:

Answer 8

Acts as an electron pair donor.

In a coordination compound between ammonia (:NH3) and a transition metal, a covalent bond is formed between the nitrogen atom of ammonia and the metal ion.

The formation of such a bond is often called a Lewis acid-base reaction. The metal ion is deficient in electrons and ammonia has a pair of electrons available for bonding. Therefore, both electrons that comprise the new covalent bond come from ammonia (ammonia is an electron pair donor or Lewis base).

Question 9

The pKa value of one compound is 15.7 (eg. ammonia), the pKa of a the other is 1.7 (eg. methyl alcohol). What would you expect to happen when you bring these together?

Why must you be aware of pKa values with nucleophiles?

Answer 9

The 1.7 pKa compound will be deprotonated by the 15.7 compound.

You must be careful when this is a possibility, as deprotonation of nucleophiles destroys them (in terms of nucleophilicity)