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The most stable of:

(a) $\ce{(CH3)2C(OH)2}$

(b) $\ce{CH3 C Cl (OH)2}$

(c) $\ce{CH3CN}$

is $\ce{CH3CN}$, because the other two are geminal diols. How can we tell that geminal diols are unstable?

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    $\begingroup$ See What makes C=O more stable that C(OH)₂ $\endgroup$ – ron Jan 1 '16 at 16:28
  • $\begingroup$ the question is on stability of different gem-diols not for comparison of stability of C=O and diols. $\endgroup$ – solanki... Jan 1 '16 at 16:42
  • $\begingroup$ @kiransolanki That's also discussed in the answer. $\endgroup$ – ron Jan 1 '16 at 17:13
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    $\begingroup$ I'm voting to close this question as off-topic because the answer is addressed in the link provided by Ron. $\endgroup$ – Todd Minehardt Jan 2 '16 at 15:26
  • $\begingroup$ also related chemistry.stackexchange.com/questions/42756/… $\endgroup$ – Mithoron Jan 2 '16 at 23:13
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Formation Of Gemdiol

Carbonyl compounds react with water to give gem diols. This reaction is catalysed by acid. The reaction is reversible reaction.

Gem diols are highly unstable compounds hence equilibrium favours the backward direction. The extent to which an aldehyde or ketone is hydrated depends on the stability of gem diol.

Stability of gem diols depend on the following factors:

(i) Steric hindrance by +I group around α-carbon decreases the stability of gem diols. +I group decreases stability of gem diol and hence decreases extent of hydration.

(ii) Stability of gem diols mainly depends on the presence of –I group on α-carbon. More is the –I power of the group more will be stability of gem diols.

(iii) Intramolecular hydrogen bonding increases stability of gem diols. –I groups present on carbon having gem diol group increases strength of hydrogen bond.

More is the strength of hydrogen bond more will be the stability of gem diol.

Overall stability : (CH3)2C(OH)2 < CH3CCl(OH)2 < CH3CN

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    $\begingroup$ Your stability order seems incorrect. I also suggest to add at least one (and better three or more) examples of stable gem-diols. $\endgroup$ – permeakra Jan 1 '16 at 16:24
  • $\begingroup$ explain ur saying. $\endgroup$ – solanki... Jan 1 '16 at 16:25
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    $\begingroup$ Ask me tomorrow again if you can't guess yourself. $\endgroup$ – permeakra Jan 1 '16 at 16:27
  • $\begingroup$ If you still didn't guess, elimination of HCl is much more preferable than of water. $\endgroup$ – Mithoron Jan 2 '16 at 22:02
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Geminal diols are unstable due to repulsion between the two oxygens present on the same carbon.If a compound is able to disperse the electron pair of oxygens by phenomenon like hydrogen bonding,resonance,-H effect(hyperconjugation) etc,their stability will increase.

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    $\begingroup$ Simply not true. $\endgroup$ – Mithoron Jan 1 '16 at 19:30
  • $\begingroup$ Well this is what i was taught in school, and it does help to solve questions based on the stability of diols.I must be wrong then,could anyone please tell me what's the correct concept? $\endgroup$ – Raksh23 Jan 1 '16 at 19:49
  • $\begingroup$ Earlier answer is better $\endgroup$ – Mithoron Jan 1 '16 at 21:41

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