3 февраля.Phenols

http://www.chemguide.co.uk/organicprops/phenol/acidity.html#top

THE ACIDITY OF PHENOL

This page explains why phenol is a weak acid and looks at its reactions (or in some cases, lack of reaction) with bases and with sodium metal.

Why is phenol acidic?
Compounds like alcohols and phenol which contain an -OH group attached to a hydrocarbon are very weak acids. Alcohols are so weakly acidic that, for normal lab purposes, their acidity can be virtually ignored.
However, phenol is sufficiently acidic for it to have recognisably acidic properties - even if it is still a very weak acid. A hydrogen ion can break away from the -OH group and transfer to a base.
For example, in solution in water:

Phenol is a very weak acid and the position of equilibrium lies well to the left.
Phenol can lose a hydrogen ion because the phenoxide ion formed is stabilised to some extent. The negative charge on the oxygen atom is delocalised around the ring. The more stable the ion is, the more likely it is to form.
One of the lone pairs on the oxygen atom overlaps with the delocalised electrons on the benzene ring.

This overlap leads to a delocalisation which extends from the ring out over the oxygen atom. As a result, the negative charge is no longer entirely localised on the oxygen, but is spread out around the whole ion.

Spreading the charge around makes the ion more stable than it would be if all the charge remained on the oxygen.
However . . . oxygen is the most electronegative element in the ion and the delocalised electrons will be drawn towards it. That means that there will still be a lot of charge around the oxygen which will tend to attract the hydrogen ion back again.
That's why phenol is only a very weak acid.

 

Activation of the ring

The -OH group attached to the benzene ring in phenol has the effect of making the ring much more reactive than it would otherwise be. For example, as you will find below, phenol will react with a solution of bromine in water (bromine water) in the cold and in the absence of any catalyst. It also reacts with dilute nitric acid, whereas benzene itself needs a nitrating mixture of concentrated nitric acid and concentrated sulfuric acid.

Figure: One of the lone pairs on the oxygen atom in the -OH group overlaps with the delocalised ring electron system giving a structure rather like the right.

The donation of the oxygen's lone pair into the ring system increases the electron density around the ring. A benzene ring undergoes substitution reactions in which the ring electrons are attacked by positive ions or the slightly positive parts of molecules. In other words, it undergoes electrophilic substitution. If you increase the electron density around the ring, it becomes even more attractive to incoming electrophiles. That's what happens in phenol.

Properties of phenol as an acid
With indicators
The pH of a typical dilute solution of phenol in water is likely to be around 5 - 6 (depending on its concentration). That means that a very dilute solution isn't really acidic enough to turn litmus paper fully red. Litmus paper is blue at pH 8 and red at pH 5. Anything in between is going to show as some shade of "neutral".

With sodium hydroxide solution
Phenol reacts with sodium hydroxide solution to give a colourless solution containing sodium phenoxide.

In this reaction, the hydrogen ion has been removed by the strongly basic hydroxide ion in the sodium hydroxide solution.

With sodium carbonate or sodium hydrogencarbonate
Phenol isn't acidic enough to react with either of these. Or, looked at another way, the carbonate and hydrogencarbonate ions aren't strong enough bases to take a hydrogen ion from the phenol.
Unlike the majority of acids, phenol doesn't give carbon dioxide when you mix it with one of these.
This lack of reaction is actually useful. You can recognise phenol because:

·         It is fairly insoluble in water.

·         It reacts with sodium hydroxide solution to give a colourless solution (and therefore must be acidic).

·         It doesn't react with sodium carbonate or hydrogencarbonate solutions (and so must be only very weakly acidic).

With metallic sodium
Acids react with the more reactive metals to give hydrogen gas. Phenol is no exception - the only difference is the slow reaction because phenol is such a weak acid.
Phenol is warmed in a dry tube until it is molten, and a small piece of sodium added. There is some fizzing as hydrogen gas is given off. The mixture left in the tube will contain sodium phenoxide.

The directing effect of the -OH group

The -OH group has more activating effect on some positions around the ring than others (for reasons which go beyond UK A level). That means that incoming groups will go into some positions much faster than they will into others.

The net effect of this is that the -OH group has a 2,4-directing effect. That means that incoming groups will tend to go into the 2- position (next door to the -OH group) or the 4- position (opposite the -OH group). You will get hardly any of the 3- isomer formed - it is produced too slowly.

Reaction with Bromine Water

If bromine water is added to a solution of phenol in water, the bromine water is decolorized and a white precipitate is formed which smells of antiseptic. The precipitate is 2,4,6-tribromophenol.

 

Notice the multiple substitution around the ring - into all the activated positions. (The 6- position is, of course, just the same as the 2- position. Both are next door to the -OH group.)

Reactions with Nitric Acid

The reactions with nitric acid are complicated because nitric acid is an oxidizing agent, and phenol is very easily oxidized to give complex tarry products. What follows misses all that complication out, and just concentrates on the ring substitution which happens as well.

With dilute nitric acid: Phenol reacts with dilute nitric acid at room temperature to give a mixture of 2-nitrophenol and 4-nitrophenol.

With concentrated nitric acid: With concentrated nitric acid, more nitro groups substitute around the ring to give 2,4,6-trinitrophenol (common name: picric acid).

Williamson ether synthesis

Ethers are produced from phenol by the Williamson method via an S _N mechanism.