Benzene does not react with bromine except with Lewis acid catalysis. Phenols react in a very different manner: no Lewis acid is needed, the reaction occurs very rapidly, and the product contains three atoms of bromine in specifi c positions. All that needs to be done is to add bromine dropwise to a solution of phenol in ethanol. Initially, the yellow colour of the bromine disappears but if, when the colour just remains, water is added, a white precipitate of 2,4,6-tribromophenol is formed.

The product shows that bromination has occurred at the para position and at both ortho positions. What a contrast to benzene! Phenol reacts three times, without catalysis, at room temperature. Benzene reacts once, and needs a Lewis acid to make the reaction go at all. The difference is, of course, the enol nature of phenol. The non-bonding lone pair of electrons at oxygen contribute to a much higher-energy HOMO than the low-energy bond ing electrons in a benzene ring. We should let our mechanism show this. Starting in the para position:

Notice that we start the chain of arrows with the lone pair electrons on the OH group and push them through the ring so that they emerge at the para position to attack the bromine molecule. The benzene ring is acting as a conductor, allowing electrons to flow from the OH group to the bromine molecule. Now the reaction is repeated, but this time at one of the two equivalent ortho positions:

Again, the lone pair electrons on the OH group are fed through the benzene ring to emerge at the ortho position. A third bromination in the remaining ortho position—you could draw the mechanisms for this as practice—gives the final product 2,4,6-tribromophenol.

If you want to put just one bromine atom into a phenol, you must work at low tempera ture (<5 ْC) and use just one equivalent of bromine. The best solvent is the rather danger ously inflammable carbon disulfide (CS₂), the sulfur analogue of CO₂. Under these conditions, para-bromophenol is formed in good yield as the main product (which is why we started the mechanism for bromination of phenol in the para position). The minor product is ortho-bromophenol.

The OH group is said to be ortho, para-directing towards electrophiles. No substitution occurs in either meta position. We can understand this by looking at the curly arrow mechanisms or by looking at the molecular orbitals. we looked at the π system of an enolate and saw how the electron density is located mainly on the end atoms (the oxy gen and the carbon). In phenol it is the ortho and para positions that are electron-rich (and, of course, the oxygen itself). We can show this using curly arrows.

مواضيع ذات صلة

Reaction conditions

Conjugated alkenes can be electrophilic

Alkenes conjugated with carbonyl groups

Two reasons to avoid a Friedel–Crafts alkylation

Two or more substituents may cooperate or compete

Alkyl and acyl substituents can be added to a benzene ring by the Friedel–Crafts reaction

Sulfonation of benzene

Nitration of benzene

Benzene and its reactions with electrophiles

enols and phenols

Reactions of silyl enol ethers with halogen and sulfur electrophiles

Reactions of enol ethers Hydrolysis of enol ethers