Chapter 6: Amines (C1687952)

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1 Description

Amines ([mathjax]R-N-R'R''[/mathjax]) are derivatives of ammonia ([mathjax]NH_{3}[/mathjax]), with one or more alkyl or aryl (involving the aromatic ring) groups bonded to the nitrogen.

 

Formula

Image

Primary amine

[mathjax]\ce{NH2R}[/mathjax]

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Secondary amine

[mathjax]\ce{NHR2}[/mathjax]

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Tertiary amine

[mathjax]\ce{NR3}[/mathjax]

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Quaternary amine

[mathjax]\ce{NR4^+}[/mathjax]

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Only Ammonia ([mathjax]\ce{NH3}[/mathjax]), primary and secondary amines can form hydrogen bonds [as they have hydrogens]. The hydrogen bonds formed by amine is not as strong as those formed by water and alcohols, as the nitrogen is less electronegative than oxygen. The ability to hydrogen bond increases the boiling point, and the water solubility of amines. Amines can act as a Lewis base (electron pair donors, discussed ) by donating its lone electron pair, which by definition [as a Lewis base], make it a nucleophile (electron pair donating). Alkyl groups are electron donating (discussed ), and should hence strengthen the basicity of amines. However, tertiary amines experience steric hindrance, lowering basicity.

2 Important reactions

Amines like to act as nucleophiles [donating its electron pair, as just stated ]. In contrast, aldehydes and ketones like to act as electrophiles, undergoing nucleophilic addition (stated ). Occurring in 3 steps:

  • In the presence of acid, the amine acts as a nucleophile, attacking the carbonyl carbon [of the aldehyde or ketone]
  • The carbonyl oxygen is protonated by the acid, creating a hydroxyl group
  • A proton is then removed from the amine, and the hydroxyl group is protonated, creating a water-leaving group. With one less proton [and therefore related bond], the carbonyl carbon forms a double bond to the nitrogen. This therefore forms an imine, which is a compound with a carbon-nitrogen double bond

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    A hydrogen on the alpha carbon of the imine, can shift to the nitrogen, creating an enamine, in a tautomer reaction analogous to the keto-enol tautomerism

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The amine is such a good nucleophile [donating its electron pair, as just stated], that it can undergo nucleophilic substitution reaction with any carboxylic acid derivative (acyl halide, acid anhydride, ester, etc.) [which like others of the carbonyl group, are good electrophiles, stated ], to create an amide [noting substitution usually only occurs where there are good leaving groups]. Amides ([mathjax]R-CO-N-R'R''[/mathjax]) are an amine ([mathjax]-N-[/mathjax]) attached to a carbonyl ([mathjax]-CO-[/mathjax]).

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Since nucleophilic substitution occurs depending on whether there are good leaving groups, the best to worst leaving groups [of the carboxylic acid derivatives] are:

Carboxylic acid derivative

Leaving group

Reactivity

Acyl halide

Chloride ion leaving group, which is very stable, and therefore a good leaving group

Most reactive

Acid anhydride

Carboxylate ion leaving group, of, next most stability

 

Carboxylic acid

Hydroxide ion leaving group, which requires protonation to leave, and is therefore a poor leaving group

 

Ester

Alkoxide ion leaving group, which is an even stronger base than the hydroxide ion, making it the next least reactive

 

Amide

[mathjax]NH_{2}^-[/mathjax] ion leaving group

Least reactive

An alkyl halide will react with ammonia to form a primary amine, and an alkyl halide will react with a primary amine to form a secondary amine, and so forth.

Nitrile [mathjax]C\equiv N[/mathjax] is a compound with a carbon tripled bonded to a nitrogen. Like carboxylic acid, the carbon is partially positive.

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Nitriles can be hydrolyzed (add water) in the presence of acid, to form a carboxylic acid.

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Nitriles can be reduced to form amine.

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3 General principles

 




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Chapter 6: Amines - Organic chemistry - Pre-med science - MR. SHUM'S CLASSROOM