Chloral hydrate is a gemdiol still it is highly stable.why?

Answer 1

The three electron withdrawing groups on the methyl group of chloral increase the polarized positive charge at the aldehyde carbon this favors an sp3 arrangement around that carbon CH(OH)2-CCl3 rather than the typical 0=CH-CCl3

Answer 2

The short answer is that the trichloro group on the adjacent carbon stabilizes the gem diol (or destabilizes the carbonyl that would result from elimination).

The long answer is:

This is a really interesting question. I'm going to assume that the geminal diol is thermodynamically more stable*, as opposed to kinetically more stable** because the back-and-forth reaction between carbonyl and gem diol should be reversible. Since it is a thermodynamic problem, you only need to consider the two end states (the gem diol and the aldehyde). The aldehyde right off the bat looks incredibly unstable - it's carbonyl carbon is extremely electrophilic due to that trichloro group (Cl is inductively electron withdrawing). Any molecule in aldehyde form would quickly be hydrolyzed by water.

* I'm not sure how advanced your orgo class is, but there is an important distinction between thermo stability and kinetic stability in chemistry. A substance that is more thermo stable than something else simply means that it has a lower delta G. Most of the time, thermo stability comes into play when an equilibrium can be established between the two species, and so you will always have more of the more thermo stable compound simply because it has a lower delta G (or more favorable Keq, etc.). However, it is important to remember that it is a DYNAMIC PROCESS - each molecule is constantly going back and forth between the two species, just think about a weak acid or base dissolved in water.

** A kinetically stable substance MAY OR MAY NOT be thermo stable. Kinetically stability simply means that the activation energy for the transition is so high, and the reaction goes so slow, that the substance is "effectively stable." So that means kinetic stability is STATIC (very few, if any molecules, are undergoing the reaction at any given time) Think about diamond vs graphite. Graphite is more thermo stable, so there is a favorable free energy transition for diamond to turn into graphite. However, the activation energy is so high (and the reaction so slow - a half life of like a billion years or something crazy), that the diamond turns out to be "stable" (kinetically. But it is technically NOT stable themodynamically).