It has been found that a compopund containing two or more hydroxyl groups attached to the same carbon atom is unstable, and readily eliminates a molecule of water.

Any factor that  will shift the equilibrium to the right stabilises the hydrate. In case of chloral we find that the hydrate is stable due to strong -I effect of three Cl atoms. In other cases where +I effect is operating it will destabilize the hydrate leading to elimination of water molecule. Similarly steric effect also causes elimination of water as steric hinderance would destabilize the hydrate.