This page explains why simple organic bases are basic and looks at
the factors which affect their relative strengths. For A'level
purposes, all the bases we are concerned with are primary amines -
compounds in which one of the hydrogens in an ammonia molecule, NH3, is replaced either by an alkyl group or a benzene ring. Why are primary amines basic? Ammonia as a weak base All of the compounds we are concerned with are derived from ammonia and so we'll start by looking at the reason for its basic properties. For the purposes of this topic, we are going to take the definition of a base as "a substance which combines with hydrogen ions (protons)". We are going to get a measure of this by looking at how easily the bases take hydrogen ions from water molecules when they are in solution in water. Ammonia in solution sets up this equilibrium: The ammonia reacts as a base because of the active lone pair on the nitrogen. Nitrogen is more electronegative than hydrogen and so attracts the bonding electrons in the ammonia molecule towards itself. That means that in addition to the lone pair, there is a build-up of negative charge around the nitrogen atom. That combination of extra negativity and active lone pair attracts the new hydrogen from the water. The strengths of weak bases are measured on the pKb scale. The smaller the number on this scale, the stronger the base is. Three of the compounds we shall be looking at, together with their pKb values are: Methylamine is typical of aliphatic primary amines - where the -NH2 group is attached to a carbon chain. All aliphatic primary amines are stronger bases than ammonia. Phenylamine is typical of aromatic primary amines - where the -NH2 group is attached directly to a benzene ring. These are very much weaker bases than ammonia. Explaining the differences in base strengths The factors to consider Two of the factors which influence the strength of a base are:
Methylamine Methylamine has the structure: What about the effect on the positive methylammonium ion formed? Is this more stable than a simple ammonium ion? Compare the methylammonium ion with an ammonium ion: To summarise:
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Note: This is a bit of a simplification for A' level purposes. As bases get more complex, another factor concerning the stability of the ions formed becomes important. That concerns the way they interact with water molecules in the solution. You don't need to worry about that at this level. | |||||||||||
The other aliphatic primary amines The other alkyl groups have "electron-pushing" effects very similar to the methyl group, and so the strengths of the other aliphatic primary amines are very similar to methylamine. | |||||||||||
Note: If you want more information about the inductive effect of alkyl groups, you could read about carbocations (carbonium ions) in the mechanism section of this site. | |||||||||||
For example:
An aromatic primary amine is one in which the -NH2 group is attached directly to a benzene ring. The only one you are likely to come across is phenylamine. Phenylamine has the structure: The other problem is that if the lone pair is used to join to a hydrogen ion, it is no longer available to contribute to the delocalisation. That means that the delocalisation would have to be disrupted if the phenylamine acts as a base. Delocalisation makes molecules more stable, and so disrupting the delocalisation costs energy and won't happen easily. Taken together - the lack of intense charge around the nitrogen, and the need to break some delocalisation - this means that phenylamine is a very weak base indeed. |
Friday, 5 April 2013
ORGANIC BASES
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