Factors equilibrium position for carbonyls Watch

KaylaB
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So I'm currently doing a tutorial which asks to describe the trend in reactivity for the following carbonyls.

I am aware of the positive inductive effect of the CH3 groups on the carbonyl, and that for the carbonyl on the furthest right the CF3 and the phenyl group both have negative inductive effects meaning that the conjugate base would be the most stable.
It mentions that the position of the equilibrium is dependant on the reactivity of the carbonyl which is understandable, but uses the ratio of percentage carbonyl to hydrate in order to prove this. I can tell from what I've written down in my lecture notes that the greater the ratio of the hydrate the more reactive the carbonyl molecule is but I'm not too sure why this is.
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MexicanKeith
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(Original post by KaylaB)
So I'm currently doing a tutorial which asks to describe the trend in reactivity for the following carbonyls.

I am aware of the positive inductive effect of the CH3 groups on the carbonyl, and that for the carbonyl on the furthest right the CF3 and the phenyl group both have negative inductive effects meaning that the conjugate base would be the most stable.
It mentions that the position of the equilibrium is dependant on the reactivity of the carbonyl which is understandable, but uses the ratio of percentage carbonyl to hydrate in order to prove this. I can tell from what I've written down in my lecture notes that the greater the ratio of the hydrate the more reactive the carbonyl molecule is but I'm not too sure why this is.
Firstly, hydration is under thermodynamic control, that is, the species concentrations reflect their stabilities (as appose to kinetics where it would be determined by rates of reaction)

Three main factors to consider when explaining where the equilibrium lies:

  • as you identified, inductive effects, a withdrawing group makes the carbonyl carbon more electrophilic so the amount of hydrate is increased at equilibirum (because this means that the carbonyl is more unstable (reactive) relative to the hydrate). Opposite applies for inductive donation.
  • Mesomeric effects, If the carbonyl is stabilised by mesomeric effects then this stabilisation is lost on hydration and the proportion of hydrate will correspondingly decrease.
  • steric effect, large groups on the carbonyl get pushed together on hydration (bond angle decreases from ~120o to ~109o) and so the larger these groups, the less favourable the hydrate will be. This can be used to atleast partly (along with inductive effects) explain why ketones have less hydrate than aldehydes at equilibrium.

This is representative of the more general reactivity of carbonyl's because the more hydrate present at equilibrium, the more readily that carbonyl will react (eg with alcohols to for esters).

Hope that makes sense
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KaylaB
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(Original post by MexicanKeith)
Firstly, hydration is under thermodynamic control, that is, the species concentrations reflect their stabilities (as appose to kinetics where it would be determined by rates of reaction)

Three main factors to consider when explaining where the equilibrium lies:

  • as you identified, inductive effects, a withdrawing group makes the carbonyl carbon more electrophilic so the amount of hydrate is increased at equilibirum (because this means that the carbonyl is more unstable (reactive) relative to the hydrate). Opposite applies for inductive donation.
  • Mesomeric effects, If the carbonyl is stabilised by mesomeric effects then this stabilisation is lost on hydration and the proportion of hydrate will correspondingly decrease.
  • steric effect, large groups on the carbonyl get pushed together on hydration (bond angle decreases from ~120o to ~109o) and so the larger these groups, the less favourable the hydrate will be. This can be used to atleast partly (along with inductive effects) explain why ketones have less hydrate than aldehydes at equilibrium.


This is representative of the more general reactivity of carbonyl's because the more hydrate present at equilibrium, the more readily that carbonyl will react (eg with alcohols to for esters).

Hope that makes sense
Yeah that's ace, thank you so much
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