why alkanamines are stronger bases than ammonia?
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Comparing the basicity of alkyl amines to ammonia. Because alkyl groups donate electrons to the more electronegative nitrogen. The inductive effect makes the electron density on the alkyl amine's nitrogen greater than the nitrogen of ammonium.
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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:
An ammonium ion is formed together with hydroxide ions. Because the ammonia is only a weak base, it doesn't hang on to the extra hydrogen ion very effectively and so the reaction is reversible. At any one time, about 99% of the ammonia is present as unreacted molecules. The position of equilibrium lies well to the left.
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.
Comparing the strengths of weak bases
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:
Remember - the smaller the number the stronger the base. Comparing the other two to ammonia, you will see that methylamine is a stronger base, whereas phenylamine is very much weaker.
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:
the ease with which the lone pair picks up a hydrogen ion,
the stability of the ions being formed.
Why are aliphatic primary amines stronger bases than ammonia?
Methylamine
Methylamine has the structure:
The only difference between this and ammonia is the presence of the CH3 group in the methylamine. But that's important! Alkyl groups have a tendency to "push" electrons away from themselves. That means that there will be a small amount of extra negative charge built up on the nitrogen atom. That extra negativity around the nitrogen makes the lone pair even more attractive towards hydrogen ions.
Making the nitrogen more negative helps the lone pair to pick up a hydrogen ion.
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:
In the methylammonium ion, the positive charge is spread around the ion by the "electron-pushing" effect of the methyl group. The more you can spread charge around, the more stable an ion becomes. In the ammonium ion there isn't any way of spreading the charge.
To summarise:
The nitrogen is more negative in methylamine than in ammonia, and so it picks up a hydrogen ion more readily.
The ion formed from methylamine is more stable than the one formed from ammonia, and so is less likely to shed the hydrogen ion again.
Taken together, these mean that methylamine is a stronger base than ammonia.
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:
pKb
CH3NH2 3.36
CH3CH2NH2 3.27
CH3CH2CH2NH2 3.16
CH3CH2CH2CH2NH2 3.39
Why are aromatic primary amines much weaker bases than ammonia?
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 lone pair on the nitrogen touches the delocalised ring electrons . . .
. . . and becomes delocalised with them:
That means that the lone pair is no longer fully available to combine with hydrogen ions. The nitrogen is still the most electronegative atom in the molecule, and so the delocalised electrons will be attracted towards it, but the intensity of charge around the nitrogen is nothing like what it is in, say, an ammonia molecule.
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:
An ammonium ion is formed together with hydroxide ions. Because the ammonia is only a weak base, it doesn't hang on to the extra hydrogen ion very effectively and so the reaction is reversible. At any one time, about 99% of the ammonia is present as unreacted molecules. The position of equilibrium lies well to the left.
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.
Comparing the strengths of weak bases
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:
Remember - the smaller the number the stronger the base. Comparing the other two to ammonia, you will see that methylamine is a stronger base, whereas phenylamine is very much weaker.
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:
the ease with which the lone pair picks up a hydrogen ion,
the stability of the ions being formed.
Why are aliphatic primary amines stronger bases than ammonia?
Methylamine
Methylamine has the structure:
The only difference between this and ammonia is the presence of the CH3 group in the methylamine. But that's important! Alkyl groups have a tendency to "push" electrons away from themselves. That means that there will be a small amount of extra negative charge built up on the nitrogen atom. That extra negativity around the nitrogen makes the lone pair even more attractive towards hydrogen ions.
Making the nitrogen more negative helps the lone pair to pick up a hydrogen ion.
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:
In the methylammonium ion, the positive charge is spread around the ion by the "electron-pushing" effect of the methyl group. The more you can spread charge around, the more stable an ion becomes. In the ammonium ion there isn't any way of spreading the charge.
To summarise:
The nitrogen is more negative in methylamine than in ammonia, and so it picks up a hydrogen ion more readily.
The ion formed from methylamine is more stable than the one formed from ammonia, and so is less likely to shed the hydrogen ion again.
Taken together, these mean that methylamine is a stronger base than ammonia.
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:
pKb
CH3NH2 3.36
CH3CH2NH2 3.27
CH3CH2CH2NH2 3.16
CH3CH2CH2CH2NH2 3.39
Why are aromatic primary amines much weaker bases than ammonia?
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 lone pair on the nitrogen touches the delocalised ring electrons . . .
. . . and becomes delocalised with them:
That means that the lone pair is no longer fully available to combine with hydrogen ions. The nitrogen is still the most electronegative atom in the molecule, and so the delocalised electrons will be attracted towards it, but the intensity of charge around the nitrogen is nothing like what it is in, say, an ammonia molecule.
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