Answers

2015-01-09T13:22:56+05:30

Preparations of Amines

SN2 Reactions of amines with alkyl halides: This reaction is best used to prepare quaternary amines (ammonium salts) because an alkylamine is a better nucleophile than ammonia, and a dialkylamine is a better nucleophile than an alkylamine... The reaction cannot be cleanly stopped, so it is best used with an excess of alkyl halide to produce the quaternary amine. Reduction of azides (N3): Sodium azide is a good source of the nucleophilic azide ion. This can be used to substitute for alkyl halides (alkyl azides are no longer nucleophilic) and the product can be reduced (commonly with lithium aluminum hydride followed with water). Reduction of nitriles: Sodium cyanide can be used to substitute a nitrile for a leaving group. The product can then be reduced (commonly with lithium aluminum hydride followed with water). Note that since the nitrile group contains a carbon, this adds one carbon in between the alkyl group with the leaving group and the amine. Reduction of amides: Amides can be reduced (again with lithium aluminum hydride) to form amines. Gabriel synthesis: The Gabriel synthesis uses the conjugate base of phthalimide (shown below) as a nitrogen nucleophile to displace a leaving group. The conjugate base is much more nucleophilic than phthalimide itself so the reaction stops cleanly after the addition of one alkyl group. The phthalimide can then be hydrolyzed in aqueous base to free the amine.
Reductive amination: This process combines a reaction you've seen before - the nucleophilic attack of an amine at a carbonyl carbon - with reduction to remove the oxygen. An amine is mixed with a carbonyl compound in the presence of H2 and a metal catalyst. This results in the amine taking the place of the carbonyl group. This reaction can be used to prepare primary, secondary or tertiary amines. It is especially useful when different alkyl groups are needed on the nitrogen. (the reaction will be run several times, each time adding one of the desired alkyl groups) Arylamines are prepared by the reduction of nitrobenzenes. Several sets of reduction conditions are effective. (H2 with Pt, or Fe and HCl)

Reactions of Amines

Aliphatic amines are able to react:

1.     as bases

2.     as nucleophiles (mentioned above reaction with alkyl halides and discussed reacting with acid halides in chapter 21)

3.     as precursors to leaving groups: The Hofmann Elimination reaction
Note that neutral amines would have to become negative if they were to act as leaving groups. This is an even more unstable leaving group than hydroxide. The first step in turning an amine into a good leaving group is to let it react with an excess of methyl iodide to give the quaternary ammonium salt. This will then be neutral if it is pushed off as a leaving group. A base (Ag2O is a source of hydroxide ion and is traditionally used in this reaction) can then cause an elimination. The use of a large ammonium as the leaving group results in the LESS substituted alkene as the major product. This is unlike eliminations that use smaller leaving groups (such as a halide).

Arylamines are able to react:

1.     as substrates in electrophilic aromatic substitution. Two important considerations:

1.     The -NH2 group is highly activating and multiply-substituted products are common

2.     The -NH2 group forms a complex with some of the catalysts used (such as AlCl3 or FeCl3 and will prevent reactions using those catalysts

These problems can be avoided by converting the arylamine into an arylamide (using reactions described in chapter 21), doing the desired electrophilic aromatic substitution, and then converting the amide back to an amine (by hydrolysis as described in chapter 21).

2.     by conversion to diazonium salts (note that other amines can be converted to diazonium salts but the diazonium will usually decompose by loss of N2). The diazonium is the central structure in the following diagram.

 

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