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Why does $\ce{C6H5COOH}$ in benzene form dimers? Also why only in benzene, why not in other solvents such as water?

In general, what are the properties of compounds which form dimers or polymers in benzene?

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  • $\begingroup$ Hint: somehow it didn't occur to you that your asking about hydrogen bonds. $\endgroup$
    – Mithoron
    Nov 1 '15 at 16:49
  • $\begingroup$ Yes, but why in benzene, and why not in water? $\endgroup$
    – Shodai
    Nov 1 '15 at 16:54
  • $\begingroup$ en.wikipedia.org/wiki/Carboxylic_acid $\endgroup$
    – Mithoron
    Nov 1 '15 at 17:02
  • $\begingroup$ I see. So they will form dimers in non-polar solvents. Can you explain this please? "whereas higher carboxylic acids are less soluble due to the increasing hydrophobic nature of the alkyl chain." Thanks! $\endgroup$
    – Shodai
    Nov 1 '15 at 17:05
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    $\begingroup$ That should be an entirely separate question. However, if you bear in mind that water is a pretty polar solvent, and alkyl groups are non-polar (hydrophobic basically means non-polar), I suppose you could deduce the answer. $\endgroup$
    – orthocresol
    Nov 1 '15 at 17:09
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In polar solvents such as water, the water molecules will solvate the carboxylic acid. Thus, hydrogen bonds will form between the 2 molecules and the carboxylic acid will dissolve.

The water is in large excess as it is the solvent so it will solvate most of the carboxylic acid molecules.

In a non-polar solvent such as benzene, no such hydrogen bonds can take place with the carboxylic acid and benzene molecules. As a result, the carboxylic acids simply interact with themselves and form dimers.

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