Amines Questions Techniques

Strength of Amines as a base

• The strength depends on the availability of the lone pair on the Nitrogen

• Alkyl groups have a positive inductive affect, pushing electrons towards the nitrogen, making the lone pair more available

• When an Amine group is close to a benzene ring, the electrons delocalise into the ring, meaning that the lone pair on the Nitrogen is less available

• Order of strength: Ammonia<Primary<Secondary<Tertiary

• If an amine is attached to a phenyl group, the further away the nitrogen is from the benzene ring the stronger a base it is (regardless of other alkyl groups)

Why is the below order of compounds listed in order of pH incorrect, and what would be the correct order:

Ammonia > Phenylamine > Ethylamine

• Ethylamine > Phenylamine because there is no benzene ring present, no negative inductive effect. Therefore the lone pair is more available

• Ethylamine > Ammonia because there is an alkyl group attached, pushing electrons/Positive inductive effect, meaning that the lone pair is more available

• Ammonia > Phenylamine because there is no benzene ring present, no negative inductive effect present, lone pair is more available

Correct order is:

Ethylamine > Ammonia > Phenylamine

What is the order of base strength of these compounds?

Weakest to strongest.

• Compound E is the weakest base.

  • Attached to a benzene ring which has a negative inductive effect. Lone pair is less available as a result

• Compound G is the second weakest base.

  • It is a secondary amine. Alkyl group has a positive inductive effect, pushing electrons towards the nitrogen,

  • The Nitrogen’s proximity to the benzene ring decreases the compound’s strength as a base

  • the lone pair is delocalised into the ring

  • Benzene ring has a negative inductive effect, making the lone pair less available

• Compound F is the strongest base.

  • Far away from the benzene ring so that there is no interaction between the lone pair and the benzene ring.

  • No negative inductive effect

  • Alkyl group has a positive inductive effect

  • Lone pair on the nitrogen is more available

What is the mechanism that occurs between halogenoalkanes/haloalkanes and amines

Nucleophilic Substitution

Give an equation for the preparation of 1,6-diaminohexane by the reaction of 1,6-dibromohexane with an excess of ammonia

CHBr + 2NH →

Strength of Amines as bases order

Aromatic < NH₃ < 1^o < 2^o

Why are tertiary amines not the strongest bases

They are not soluble because they do not have a hydrogen available to form hydrogen bonds with water