Table of Contents
Why are geminal diols unstable?
Gem diols are unstable because the OH(alcohol) groups are present on a single carbon. Hence because they are closer to each other lone pair- lone pair repulsion would be higher as their lone pairs would repel each other. Viccinal diols on the other hand have OH group at different carbon atoms.
Are Geminal diols stable?
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.
Why are acetals more stable than Hemiacetal?
Cyclic acetals are more stable than regular acetals because of the chelate effect, which derives from having both -OH groups of the acetal connected to each other in the diol. 5. Cyclic hemiacetals that form five- or six-membered rings are stable (as opposed to non-cyclic hemiacetals which are not stable species).
Why are acetals stable in basic conditions?
In the absence of acid, acetals are not susceptible to hydrolysis i.e. the reverse reaction. They are therefore quite stable to neutral and basic reaction conditions.
How are geminal diols formed?
It has been demonstrated that water, in the presence of an acid or a base, adds rapidly to the carbonyl function of aldehydes and ketones establishing a reversible equilibrium with a hydrate (geminal-diol or gem-diol).
Why are acetals unstable in acid?
Simple ethers have similar reactivity to acetals under basic/nucleophilic conditions. In acid, acetals are usually more reactive because they can form resonance stabilized oxocarbenium ions.
Why do acetals not form under basic conditions?
Protonation gives the hemiacetal. In order to proceed to the acetal, the alkoxide would have to displace the hydroxide in an SN2 reaction. Hydroxide is not a good enough leaving group for SN2 reaction, so it is impossible to form the acetal under basic/nucleophilic conditions.
Why are hemiacetals unstable?
You can see why hemiacetals are unstable: they are essentially tetrahedral intermediates contain- ing a leaving group and, just as acid or base catalyses the formation of hemiacetals, acid or base also catalyses their decomposition back to starting aldehyde or ketone and alcohol.