Unit 4 Section 4 Reactions of Amines

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Formation of Amines from Haloalkanes - Part 1

Amines can be made by heating a haloalkane with excess ammonia.

You will get a mixture of primary, secondary and tertiary amines, and quarternary ammonium salts, as more than one hydrogen is likely to be substituted.

You can seperate the products using fractional distillation in order to make one particular amine.

This is a nucleophilic substitution reaction. The general mechanism is:

1. NH3 attacks the delta positive carbon on the haloalkane and donates its lone pair of electrons, forming a bond with the carbon.

2. As carbon can only have four bonds, the addition of ammonia causes the C-X bond to break, releasing a negatively charged halide ion and leaving an alkylammonium salt.

3. A second ammonia molecule then donates its lone pair of electrons to one of the hydrogens attached to the nitrogen.

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Formation of Amines from Haloalkanes - Part 2

4. This hydrogen breaks off from the alkyl ammonium salt and joins the ammonia to form an NH4+ ion.

5. This leaves the primary amine. The NH4+ ion forms an ionic bond with the X- ion to produce NH4+X-.

The amine product still has a lone pair of electrons on the nitrogen, so it is still a nucleophile.

This means that further substitution is possible.

They keep happening until you get a quarternary ammonium salt, which can't react any further as it doesn't have a lone pair of electrons.

The mechanism for these substitutions is the same - you just use an amine instead of ammonia.

The number of substitutions can be reduced by using excess ammonia. The more ammonia there is, the less likely it is that a haloalkane will meet and react with an amine rather than ammonia.

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Formation of Amines from Nitriles

You can reduce an amine to a nitrile using a strong reducing agent such as lithium aluminium hydride (LiAlH4) followed by some dilute acid.

Another way is to reflux the nitrile with sodium metal and ethanol.

LiAlH4 and sodium are too expensive for industrial use.

So industry uses a metal catalyst such as platinum or nickel with hydrogen at a high temperature and pressure - its called catalytic hydrogenation.

Forming amines by reducing nitriles is useful if you want a primary amine because there are no further substitutions.

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Formation of Aromatic Compounds

Aromatic amines are produced by reducing a nitro compound, such as nitrobenzene.

There are two steps to the method:

1. Heat a mixture of a nitro compound, tin metal and concentrated hydrochloric acid under reflux - this makes a salt.

2. To turn the salt into an aromatic amine, you need to add an alkali, such as sodium hydroxide solution.

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Nucleophilic substitution: A reaction where a nucelophile

Catalytic hydrogenation:

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