as the alkyl chain attached to OH group increases , electron releasing effect of C also increases . one C releases its electrons to second carbon ,then it releases them to next C and so on . the C attached to the OH releases its electrons to O atom thereby increasing electron density on O atom . As electrons were released by C to O ,there exists strong polarity between C and O , ultimately leading to decreased polarity between O-H. Due to the electron releasing effect, carbon develops high partial positive charge and so , even a weak nucleophile can easily attack on carbon ,forming it a carbocation and removing O-H group.HOPE THIS HELPS!!!