How are phenols made?

Question

What is the equation and conditions for the synthesis of phenols? Would it work if we added $\ce{NaOH}$ to benzene and then refluxed?

Answer 1

That reaction will definitely not yield a phenol. Phenols are not usually prepared directly from benzene, but from benzene derivatives, such as toluene or chlorobenzene:

Oxidation of toluene, as developed by Dow Chemical:

$\ce{C6H5CH3 + 2 O2 → C6H5OH + CO2 + H2O}$

Hydrolysis of chlorobenzene, using base or steam (Raschig–Hooker process):

$\ce{C6H5Cl + H2O → C6H5OH + HCl}$

Actually, there are processes that allow us to obtain it directly from benzene, but these are rarely used:

Direct oxidation of benzene with nitrous oxide, a potentially "green" process:

$\ce{C6H6 + N2O → C6H5OH + N2}$

The most widely used process for industrial production (95%) involves the partial oxidation of cumene (isopropylbenzene) via the Hock rearrangement:

$\ce{C6H5CH(CH3)2 + O2 → C6H5OH + (CH3)2CO}$

• Source of the reactions: Wikipedia English page for phenol

Answer 2

There are many ways to synthesize phenols. The "best" way depends on what aromatic compound you have available as a starting material. This web page lists many different methods for preparing phenols; if you find a method that looks interesting, you can click on the example and obtain more detailed information.

A very nice, general method for the synthesis of substituted phenols is the Sandmeyer reaction. In this reaction you start with an aromatic amine. The amine is diazotized to produce a diazonium salt. When the salt is mildly heated it eliminates nitrogen producing a reactive phenyl cation. When the diazonium salt is decomposed in the presence of water, phenols are produced.

Answer 3

To add to the other answer: there is an other method to synthesize phenols from benzene, namely through the benzyne intermediate. First you can react benzene with $\ce{Br2}$, in the presence of a strong Lewis acid like $\ce{AlCl3}$, to form bromobenzene. $\ce{NaNH2}$, a really strong base ($\ce{pK_{a}}=38$; compared to $\ce{NaOH}$ with $\ce{pK_{a}}=13$), can be used to remove a proton ortho to the bromo-group, where the inductive effect is largest. Then bromide is eliminated, yielding benzyne. Benzyne is incredibly unstable (very deformed "$\pi$ bond") and is electrophilic enough for $\ce{OH-}$ to attack, giving the phenol after protonation by the formed $\ce{NH3}$.

Note that this only happens under the most vigorous conditions where $\ce{NaOH}$ and $\ce{NaNH2}$ are fused together.

Direct electrophilic addition of an alcohol group to benzene is not possible, because there is no reliable source of $\ce{OH+}$.

Answer 4

Phenol is C6H6O, essentially adding one oxygen atom to benzene. The cumene process allows the sharing of two oxygen atoms between benzene and propene and make phenol and acetone. Similarly the similar process by Exxon-Mobil allows the sharing between benzene and cyclohexane which is produced by hydrogenation of benzene.

Therefore, I think the readers can see the trend; if a new process can effectively allow the sharing of oxygen atoms with phenol, it can be good.

Another trend is looking at phenol as adding oxygen atom to the benzene ring. Here, there are journal articles covering electrosynthesis of adding oxygen atoms to benzene. Also, nitrous oxide was tried for adding the oxygen, and releasing the dinitrogen byproduct.

Another trend is looking at phenol as the combination of the phenyl and hydroxyl substituents. Here, substituted benzenes are converted into hydroxyl-substituted benzene. Chlorobenzene is one example.