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AS/A-Level Chemistry - Properties of amines

Properties of amines

· A-level Chemistry,amines,Organic Chemistry,functional group,chemical properties

Let's look into amines in A-Level Chemistry to see how are their properties!

 Amines are weak bases; the lone pair of electrons on the nitrogen atom can behave as a proton acceptor. The strength of amines varies, but all react with strong acids to make alkylammonium salts. 

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Amines have a lone pair of electrons which can form bonds with:

  1. An H

+ ion- hence act as Bronsted-Lowry base

  1. An electron- deficient C, hence acting as a nucleophile

 Reaction of amines with acids 

Amines react with acids in the same way as ammonia:  

NH3+ HCl ⇨ NH4Cl  

2NH3+ H2SO4 ⇨  (NH4)2SO4 

primary amines: 

R1-NH2 + HCl ⇨ R1-NH3Cl  

2R1-NH2+ H2SO4 ⇨ (R1-NH3)2SO4 

secondary amines: 

R1R2-NH + HCl ⇨ R1R2-NH2Cl  

2R1R2-NH + H2SO4 ⇨ (R1R2-NH2)2SO4 

tertiary amines: 

R1R2R3-N + HCl ⇨ R1R2R3-NHCl  

2 R1R2R3-N+ H2SO4 ⇨ (R1R2R3-NH)2SO4 

Eg: CH3NH2 + HCl ⇨ CH3CH2NH3+Cl- 

The charges on the ions are often shown, eg R1R2-NH2+Cl- 

 Comparison of base strength of different amines 

The basicity of ammonia and amines depends on the availability of the lone pair of electrons on the nitrogen atom. The more available the lone pair, the stronger the base.   

The availability of the lone pair in turn depends on how much electron density is on the nitrogen atom, and this depends on the adjacent atoms.   

Primary amines have one alkyl group attached to the N atom: 

The hydrogen atom is slightly more electropositive than the carbon atom. This means that the carbon atom has a slight surplus of electrons which the nitrogen atom can attract towards itself. This effect is known as a “positive inductive effect” and it means that the N atom has an excess of electron density: 

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 The greater the electron density on the N atom, the more likely the lone pair is to be released for bonding. Primary amines are thus stronger bases than ammonia, as the alkyl groups push electron density onto the N atom, making the lone pair more available for bonding.   

It follows that secondary amines, which have two electron-pushing alkyl groups, are stronger bases than primary amines and that tertiary amines are stronger bases than secondary amines. 

Reaction of alkylammonium salts with bases  

The alkylammonium salts are slightly acidic and can be converted back into amines on addition of alkalis:   

Primary ammonium salts: R1-NH3Cl + NaOH ⇨ R1-NH2 + NaCl + H2O 

Secondary ammonium salts: R1R2-NH2Cl +NaOH ⇨ R1R2-NH +NaCl + H2O 

Tertiary ammonium salts: R1R2R3-NHCl+ NaOH ⇨ R1R2R3-N+ NaCl + H2O   

Quaternary ammonium salts are not acidic as the N atom is not bonded to any hydrogen atoms. 

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Drafted by Eunice (Chemistry)

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