Why amines are not basic

Of course, the trimethylammonium ion is still less acidic than ammonia. The presence of three alkyl groups sharply diminishes the ability of the solvent to stabilize the corresponding ammonium ion, thus causing a reversal in the tendency of the alkyl groups to decrease acidity and increase basicity.

Amines are the most basic class of organic compounds. They are virtually the only organic compounds which are substantially basic in aqueous solution and which are completely protonated by dilute solutions of strong acids. Upon protonation, of course, the form salts of the alkyl ammonium ions, which are water soluble if the R groups are not too large. Consequently, amines can be separated from other classes of organic compounds like halides, ethers, alcohols, and ketone as well as alkanes, alkenes and alkynes, of course , by a simple extraction technique.

After drying and evaporation, the amine is obtained. Note that primary and secondary amines , like ammonia have protic hydrogens and therefore possess a degree of acidity unlike tertiary amines, which have no acidic hydrogen. We have previously seen that ammonia has a pK a value of about 38, and is a very weak acid.

Consequently, such amines are much more basic pK b about 4 than they are acidic pK a 38 , so that their aqueous solutions are rather strongly alkaline. We have seen that they are strong enough bases to be able to generate enolates of ketones quantitatively. It is interesting to note that, since the nitrogen atom of amines is tetrahedral, such a nitrogen can be a stereocenter if it has three different R groups attached. By definition, the fourth group is an electron pair, so that all four groups are different.

Certain amines, for which this inversion is especially difficult, can be prepared and are relatively stable as a single enantiomer. Consequently, they can be used effectively as nucleophiles in S N 2 reactions with alkyl halides. This also applies to ammonia , the inorganic parent of organic amines. This would produce a secondary amine, and then even further reaction with alkyl halide would give a tertiary amine. Thus, a mixture of primary, secondary, and tertiary amines would be generated unless ammonia is used in large excess.

This reacts readily with an alkyl halide to give an organic azide, which can be reduced with lithium aluminum hydride to the primary amine. We will not look into the specific mechanism of this latter reduction reaction.

Therefore, the organic azide, once formed, is unable to react with the alkyl halide. The result is that we do not have to use an excess of the nucleophile to get exclusively the primary amine. These nitriles can also be reduced with lithium aluminum hydride to the primary amine. In this case, the primary amine has one additional carbon atom than is contained in the alkyl halide. It can be synthesized as shown below. Replacing an alkyl group by a phenyl or other aryl group greatly diminishes the basicity of the amine function.

This means, of course, that the anilinium ion is a one-millionfold stronger acid than the methylaminium ion. Correspondingly, this means that aniline is a weaker base than methylamine, by a factor of a million!

The resonance structures for aniline are shown below, where it is shown that the ring becomes electron rich, with partial negative charge carbanion character at the ortho and para positions, while the nitrogen tends to become electron deficient partial positive charge. These electrons are then delocalized around the ring on to the positions indicated.

See the indicated overlap in the orbital picture shown below:. This makes aniline much more stable thermodynamically than methylamine or any alkylamine, and thus much less readily protonated weaker base. Note that the resonance structure on the right, below, is not a valid resonance structure. Pyridine is an aromatic amine, but in a very different sense from aniline.

Pyridine is essentially benzene with one of the CH groups of benzene replace by a N atom. Therefore, pyridine is less easily protonated than typical aliphatic amines such as piperidine. The pK a of pyridine is 5. Aniline, a typical arylamine, exhibits the resonance structures shown in Figure 1. As structures b through e in Figure show, delocalization of the unshared electron pair occurs throughout the ring, making these electrons less available for reaction.

As a result of this electron delocalization, the molecule becomes less basic. Previous Carboxylic Acids and Their Derivatives. Next Preparation of Amines. Removing book from your Reading List will also remove any bookmarked pages associated with this title.

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The easiest way of looking at the basic properties of amines is to think of an amine as a modified ammonia molecule. In an amine, one or more of the hydrogen atoms in ammonia has been replaced by a hydrocarbon group. Replacing the hydrogens still leaves the lone pair on the nitrogen unchanged - and it is the lone pair on the nitrogen that gives ammonia its basic properties.

Amines will therefore behave much the same as ammonia in all cases where the lone pair is involved. These are most easily considered using the Bronsted-Lowry theory of acids and bases - the base is a hydrogen ion acceptor.

We'll do a straight comparison between amines and the familiar ammonia reactions. Ammonia reacts with acids to produce ammonium ions.

The ammonia molecule picks up a hydrogen ion from the acid and attaches it to the lone pair on the nitrogen. If the reaction is in solution in water using a dilute acid , the ammonia takes a hydrogen ion a proton from a hydroxonium ion.

If the acid was hydrochloric acid, for example, you would end up with a solution containing ammonium chloride - the chloride ions, of course, coming from the hydrochloric acid.

Such things don't exist on their own in solution in water. If the reaction is happening in the gas state, the ammonia accepts a proton directly from the hydrogen chloride:. The nitrogen lone pair behaves exactly the same. The fact that one or more of the hydrogens in the ammonia has been replaced by a hydrocarbon group makes no difference. If the reaction is done in solution, the amine takes a hydrogen ion from a hydroxonium ion and forms an ethylammonium ion.

Alternatively, the amine will react with hydrogen chloride in the gas state to produce the same sort of white smoke as ammonia did - but this time of ethylammonium chloride. These examples have involved a primary amine. It would make no real difference if you used a secondary or tertiary one. The equations would just look more complicated.

The product ions from diethylamine and triethylamine would be diethylammonium ions and triethylammonium ions respectively. Again, it is easiest to use the Bronsted-Lowry theory and, again, it is useful to do a straight comparison with ammonia. Ammonia is a weak base and takes a hydrogen ion from a water molecule to produce ammonium ions and hydroxide ions.

However, the ammonia is only a weak base, and doesn't hang on to the hydrogen ion very successfully. The reaction is reversible, with the great majority of the ammonia at any one time present as free ammonia rather than ammonium ions. There is, however, a difference in the position of equilibrium. Amines are usually stronger bases than ammonia.

There are exceptions to this, though - particularly if the amine group is attached directly to a benzene ring.