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Ammonia and Amines

Amines can be derivatives of ammonia where an hydrogen is replaced by a alkyl or aryl group. Like alcohols, they can be primary secondary or tertiary depending on how many carbons the nitrogen forms a bond with.


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Naming Amines

All primary amines have the general formula RNH2 where R can be an alkyl or aryl group.

Amines use the suffix -amine. e.g. methylamine (CH3NH2) and ethylamine (C2H5NH2)

Secondary amines have the general formula RR'NH for example    (CH3)2 NH is dimethylamine

Tertiary amines have the general formula RR'R''NH for example (CH3)3 NH is trimethylamine

Different substituents are written in alphabetical order:

CH3 (C3H7) NH is methylpropylamine

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Ammonia Angle

The angle of a perfect tetrahedron are 109.5 degrees. Ammonia's bond angles are 107 degrees because the lone pair of electrons on ammonia repels more than the bonding pairs of electrons in the N-H bond. Each lone pair of electrons reduces the bond angle by 2.5 degrees. All amines have the same shape of ammonia. (

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Boiling points of Amines

 ( Amines are polar so they hydrogen bond using their NH2 groups similar to alcohols with their OH groups. However nitrogen is not as electronegative as oxygen so these hydrogen bonds are not as strong as alcohols and water.

Thus the boiling points for amines are lower than the boiling points of alcohols.

Short chain amines are gases at room temp and long chain amines are volatile liquids. They smell fishy.

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Solubility of Amines

Primary amines with chain lengths up to C4 are very soluble in water and alcohols because they form hydrogen bonds with these solventds.

Most amines are also soluble in les polar solvents.

Phenylamine (C6H5NH2) however is not very soluble in water as the benzene ring cannot form hydrogen bonds.

(                                 (

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Reactivity of amines

Amines are proton acceptors and so are a Bronsted-Lowery base. Amines are also a lone pair donor and so are a Lewis base.

The lone pair on an amine can form a bond with:

1) A H+ ion so the amines is acting as a base.

2) An electron deficient carbon atom, so the amine can act as a nucleophile.

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Amines as bases

Phenylamine can react with hydrochloric acid (as it has dissociated to H+ and Cl-) to form pheynlammoniumchloride.

C6H5NH2 + H+  +  Cl-   --->    C6H5NH3         +    Cl-

                                             N has a positive charge

Amines react with acids to form salts.

Pheynlamine is an arylamine and is relitevly insoluble, but it dissolves in HCl because it forms a soluble salt (pheynlammoniumchloride). 

Then if a strong base is added, like NaOH, it removes the proton (H+) from the salt and regenerates the insoluble amine.

C6H5N+H3 +  Cl-   +  OH-   -->   C6H5NH2   + H2O  +  Cl-

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Comparing base strengths

The strength of a base depends on how readily it accepts a proton. Ammonia and amines all have lone pairs to attract a proton however there are different base strengths.

tertiary amine >  secondary amine > primary amine > ammonia > phenylamine

Alkyl groups release electrons away from the alkyl group and towards the nitrogen atom increasing its electron density (this is called the inductive effect) making it easier for the nitrogen atom to attract a proton. 

More alkyl groups attached to the nitrogen atom means more alkyl groups can release electrons increasing nitrogen's electron denisty further and thus its base strength.

Aryl groups however do the opposite as they attract electrons away from the nitrogen atom because the lone pair of electrons overlaps with the delocalised system on the benzene ring.

                                      amines with alkyl groups > ammonia > amines with aryl groups

tertiary amine >  secondary amine > primary amine > ammonia > phenylamine

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The inductive effect

In an alkylamine:

R --->---- NH2

In an arylamine:

C6H5 ---<--- NH2

---->---- shows the direction the electrons are moving.

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Amines as nucleophiles

The lone pair of electrons on an amine will attack positively charged carbon atoms so amines act as nucleophiles.

Ammonia reacts with a haloalkane to produce a primary aliphatic salt by nucleophilic substitution.

NH3   +   RX   -->   [RNH3]+X-

[RNH3]+X-    +   NH3   -->  RNH2   +   [NH4]+X-

However the primary amine produced is also a nucleophile and so will also react with the haloalkane to produce a secondary amine. The secondary amine will then react to form a tertiary amine. Finally the tertiary amine will react to form a quarternary ammonium salt.

R3N    +    RX   --->  [R4N]+X-

So a mixture of primary, secondary and tertiary amines and a quarternary ammonium salt are formed in this reaction which can be seperated by fractional distilation but this is not a very efficient method. A large excess of ammonia would give a better yield of a primary amine.

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Nucleophilic substitution of an amine

Same as any nucleophilic substitution mechanism.

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Preparation of primary amines

Primary aliphatic amines can be prepared from Haloalkanes in a two step process.

Step 1: Haloalkanes react with a cyanide ion in aqueous ethanol. The cyanide ion replaces the halide ion by nucleophilic substitution to form a nitrile.

RBr +  CN-  --->   RCN   +   Br-

 Step 2: Nitriles contain the functional group "C triple bond N" which can be reduced to primary amines by catalytic hydrogenation by Ni/H2

RCN     +  2H2   --->   RCH2NH2

This gives a purer product than RBr + NH3 as only the product is formed.

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Preparation of phenylamine

Phenylamine can be made from benzene.

Step 1: Benzen is reacted with a mixture of nitric and sulfic acid (both concentrated) which produces nitromenzene.

C6H6 + HNO3   --->  C6H5NO2   +   H2O

Step 2: Nitrobenzene is reduced to phenylamine using tin and HCl as the reducing agent.

Tin and HCl form hydrogen which reduces the nitrobenzene by removing oxygen atoms of the NO2 group and replacing then with hydrogen atoms.

C6H5NO2 + 6H  --> C6H5NH2  +   2H2O

Since the reaction is carried out in HCl the salt C6H5NH3+Cl- is formed and NaOH is added to liberate the free amine.

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