Making nitriles from aldehydes and ketones

Making nitriles from aldehydes and ketones Aldehydes and ketones undergo an addition reaction with hydrogen cyanide. The hydrogen  cyanide adds across the carbon-oxygen double bond in the aldehyde or ketone to produce  a  hydroxynitrile. Hydroxynitriles used to be known as cyanohydrins.     For example, with ethanal (an aldehyde) you get 2-hydroxypropanenitrile: With propanone (a ketone) you get 2-hydroxy-2-methylpropanenitrile:     In every example of this kind, the -OH group will be on the number 2 carbon atom -  the one next to the -CN group. The reaction isn't normally done using hydrogen cyanide itself, because this is an extremely  poisonous gas. Instead, the aldehyde or ketone is mixed with a solution of sodium or   potassium cyanide in water to which a little sulphuric acid has been added. The pH of  the solution is adjusted to about 4 - 5, because this gives the fastest reaction. The  reaction happens at room temperature. The solution will contain hydrogen cyanide (from the reaction between the sodium or  potassium cyanide and the sulphuric acid), but still contains some free cyanide ions.   This is important for the mechanism.
These are useful reactions because they not only increase the number of carbon atoms in  a chain, but also introduce another reactive group as well as the -CN group. The -OH group  behaves just like the -OH group in any alcohol with a similar structure. For example, starting from a hydroxynitrile made from an aldehyde, you can quite easily  produce relatively complicated molecules like 2-amino acids - the amino acids which are  used to construct proteins.

 

reducing nitriles to primary amines

REDUCING NITRILES TO PRIMARY AMINES This page looks at the reduction of nitriles to primary amines using either lithium tetrahydridoaluminate(III) (lithium aluminium hydride) or hydrogen and a metal catalyst. The reduction of nitriles using LiAlH4 The reducing agent Despite its name, the structure of the reducing agent is very simple. There are four hydrogens ("tetrahydido") around the aluminium in a negative ion (shown by the "ate" ending). The "(III)" shows the oxidation state of the aluminium, and is often left out because aluminium only ever shows the +3 oxidation state in its compounds. To make the name shorter, that's what I shall do for the rest of this page.
The structure of LiAlH4 is: In the negative ion, one of the bonds is a co-ordinate covalent (dative covalent) bond using the lone pair on a hydride ion (H-) to form a bond with an empty orbital on the aluminium.
The overall reaction The nitrile reacts with the lithium tetrahydridoaluminate in solution in ethoxyethane (diethyl ether, or just "ether") followed by treatment of the product of that reaction with a dilute acid. Overall, the carbon-nitrogen triple bond is reduced to give a primary amine. Primary amines contain the -NH2 group. For example, with ethanenitrile you get ethylamine:     Notice that this is a simplified equation - perfectly acceptable to UK A level examiners. [H] means "hydrogen from a reducing agent".

The reduction of nitriles using hydrogen and a metal catalyst

The reduction of nitriles using hydrogen and a metal catalyst The carbon-nitrogen triple bond in a nitrile can also be reduced by reaction with hydrogen  gas in the presence of a variety of metal catalysts. Commonly quoted catalysts are palladium, platinum or nickel. The reaction will take place at a raised temperature and pressure. It is impossible to give exact details because it will vary from catalyst to catalyst. For example, ethanenitrile can be reduced to ethylamine by reaction with hydrogen in the  presence of a palladium catalyst.

The different kinds of amines

What are amines? The easiest way to think of amines is as near relatives of ammonia, NH3. In amines, the hydrogen atoms in the ammonia have been replaced one at a time by hydrocarbon groups. On this page, we are only looking at cases where the hydrocarbon groups are simple alkyl groups. The different kinds of amines Amines fall into different classes depending on how many of the hydrogen atoms are replaced. Primary amines In primary amines, only one of the hydrogen atoms in the ammonia molecule has been replaced. That means that the formula of the primary amine will be RNH2 where "R" is an alkyl group. Examples include:     Naming amines can be quite confusing because there are so many variations on the names. For example, the simplest amine, CH3NH2, can be called methylamine, methanamine or aminomethane. The commonest name at this level is methylamine and, similarly, the second compound drawn above is usually called ethylamine.     Where there might be confusion about where the -NH2 group is attached to a chain, the simplest way of naming the compound is to use the "amino" form. For example: Secondary amines In a secondary amine, two of the hydrogens in an ammonia molecule have been replaced by hydrocarbon groups. At this level, you are only likely to come across simple ones where both of the hydrocarbon groups are alkyl groups and both are the same. For example:   There are other variants on the names, but this is the commonest and simplest way of naming these small secondary amines. Tertiary amines In a tertiary amine, all of the hydrogens in an ammonia molecule have been replaced by hydrocarbon groups. Again, you are only likely to come across simple ones where all three of the hydrocarbon groups are alkyl groups and all three are the same. The naming is similar to secondary amines. For example: Physical properties of amines Boiling points The table shows the boiling points of some simple amines. type formula boiling point (°C) primary CH3NH2 -6.3 primary CH3CH2NH2 16.6 primary CH3CH2CH2NH2 48.6 secondary (CH3)2NH 7.4 tertiary (CH3)3N 3.5 We will need to look at this with some care to sort out the patterns and reasons. Concentrate first on the primary amines. Primary amines It is useful to compare the boiling point of methylamine, CH3NH2, with that of ethane, CH3CH3. Both molecules contain the same number of electrons and have, as near as makes no difference, the same shape. However, the boiling point of methylamine is -6.3°C, whereas ethane's boiling point is much lower at -88.6°C. The reason for the higher boiling points of the primary amines is that they can form hydrogen bonds with each other as well as van der Waals dispersion forces and dipole-dipole interactions.

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