Amines

What is a primary amine?

• 1 carbon group attached to N and 2 hydrogens

• E.g R-NH₂

What is a secondary amine?

• 2 carbon groups attached to N and 1 Hydrogen

• E.g R-NH-R

What is a tertiary amine?

• 3 Carbon groups attached to N and no remaining H

How do you prepare aromatic amines?

• Reduction of nitro compounds

• Using Sn (catalyst) and HCL

• Room temp

Equation for the reduction of a nitro compound into an aromatic amine

• Nitro compound + 6 [H] → Amine + 2H₂O

• Reducing agent written as [H] in equation

Aromatic amines are used in the manufacture of..

• Dyes (specifically azo dyes)

Method 1 of preparing aliphatic amines

• 2 step synthesis with a halogenoalkane

• Final step is reduction of nitrile

Outline the 2 step synthesis of a halogenoalkane to make aliphatic amines

• Preparing the nitrile

-Via nucleophilic substitution

-KCN / aq +ethanolic

- e.g. CH₃CH₂Br + KCN → CH₃CH₂CN + KBr

• Reduction

-Add H₂ and nickel catalyst (or Pt/Pd)

-e.g. CH₃CH₂CN + 2H₂ → CH₃CH₂CH₂NH₂

Advantages and disadvantages of method 1 (2 step synthesis of halogenoalkane) to make aliphatic amines

• Advantages

• Higher % atom economy ( desired product is formed = primary amine)

• Disadvantages

• But reaction only produces primary amines and lengthens carbon chain

• 2 step reaction → lower % yield

( % yield tells you how much product has been obtained compared with the maximum possible amount and % atom economy tells you how many of the atoms in the reactants end up in the desired product )

What is method 2 for preparing aliphatic amines ?

• Reacting ammonia with halogenoalkanes

• Nucleophilic substitution

+When an excess of NH3 is used the major product is primary amine

What is the problem with method 2 (preparation of aliphatic amine)

• Amine produced can act as a nucleophile as it has a lone pair of electrons on N atom

• Even with an excess of NH3 further substitution can occur

• This reaction thus produces primary, secondary and tertiary amines and quaternary ammonium salts.

• A mixture of all these chemicals can be formed therefore product is impure

• Thus low % atom economy

In method 2 if excess halogenoalkane is used..

• The production of secondary and tertiary amines are favoured.

• But major product is quaternary ammonium salt

Why are amines bases and why weak bases

• Because N atom has a lone pair of electrons that can accept H+

• But lone pair is often less available to accept H+→ making it a weak base

What happens when you react amine with strong acid ?

• Reaction goes to completion

• E.g CH₃CH₂NH₂ + HCL → CH₃CH₂NH₃ ⁺Cl^-

What does base strength of amines depend on?

• Availability of the lone pair of electrons on the N atom

Why are aliphatic amines strong bases?

• Alkyl groups are electron releasing

• This is known as the positive inductive effect

• Lone pair is more available to accept H+ ( Electron density pushed towards N)

Why are aromatic amines the weakest base?

• In benzene there is a a delocalised pi electron system

• Lone pair of nitrogen is delocalised into the pi cloud of the benzene

• Lone pair is thus less available to accept H+

In aliphatic amines the more alkyl groups attached to the amine..

• The greater the positive inductive effect

• Thus tertiary amines are stronger bases

What are nucleophilic properties of amines

• Amines are nucleophiles

What are the 2 mechanisms for amine reactions

• Nucleophilic substitution

• Nucleophilic addition-elimination

How can quaternary ammonium salts form?

• In nucleophilic substitution reactions where there is a large excess of halogenoalkane

• E.g ( CH₃)₃ N + CH₃Cl → N (CH₃)₄Cl

What are quaternary ammonium salts used as?

• Cationic surfactants such as

→Fabric softener

→Hair conditioner

→Disinfectants

Why can Quaternary ammonium salts be used as cationic surfactants

• Because they contain a positively charged ammonium head and long hydrocarbon tails

• The charged ammonium head is attracted to H₂O

• While the non polar tail dissolves in grease