Benzene shows electrophilic substitution reactions. Normally, anything that can generate a carbocation shows, for example, a Friedel-Crafts alkylation reaction. I wanted to know if this reaction is also possible: taking an aldehyde or a ketone with an acid, which will protonate the oxygen atom and generate a carbocation - followed by electrophilic attack on benzene. This reaction can even go further by protonation of the alcohol and forming another carbocation for a second substitution - as I've shown in the picture below. I couldn't find this anywhere, though it seems a simple way to make a benzyl alcohol - why isn't it more frequently carried out like other cases like Friedel-Crafts Alkylation, or Acylation, etc.? Something almost identical happens in synthesis of DDT using chloral and chlorobenzene.


2 Answers 2


I haven’t ever seen a reaction directly like that. To generate the alcohol you drew on your piece of paper, I would probably use phenyl lithium or phenyl magnesium bromide and add that to acetone; it is easier and faster. (Note that the reaction would stop at the tertiary alcohol and not react further under elimination of water.)

Note that the reaction to synthesise DDT starts from trichloroacetaldehyde. The three chlorine atoms draw electron density away from the carbonyl group, further strengthening the cation. I would suppose that this is not seen often because of the activation a simple aldehyde would need. Let’s face it: benzene is a very bad nucleophile.


I think that it is quite difficult to generate the carbocation in the second case as the carbon atom of benzene is $\mathrm{sp^2}$ hybridised which is more electronegative than an $\mathrm{sp^3}$ hybridised carbon atom. It makes the carbon atom that is attached to it more unstable if it carries a positive charge. So formation of a carbocation could not take place.

  • 6
    $\begingroup$ Actually formation of such carbocations occurs quite readily, they are resonance stabilized benzylic cations. $\endgroup$
    – ron
    Jul 6, 2015 at 14:13
  • $\begingroup$ The 7 member tropylium ion formed from the benzylic one mentioned above is still aromatic $\endgroup$
    – Beerhunter
    Jul 11, 2015 at 18:06

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