Although anti conformation is more stable than the gauch conformation but in some cases gauch or skew conformation is more stable than the anti confirmation why?

Gauch or skew conformation is more stable than the anti because of the intramolecular hydrogen bonding which is geometrically possible only in the gauch conformation.

For examples:

Ethane-1,2-diol, 2-Fluoroethan-1-ol, 2-chloroethan-1-ol and 2-Aminoethanol etc.
2-chloroethane-1-ol:

Ethane-1,2-diol:

Realated Questions:

How many cyclic isomers possible of (C6H12) Hexene?

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Conversion of phenolic esters of aliphatic and aromatic carboxylic acids into ortho- and para-acylphenols in the presence of FCR catalyst (AlCl3). Is know as Fries rearrangement.

Related questions:
What is difference between Houben–Hoesch acylation reaction and Gatter- mann–Koch formylation reaction? 

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Reactivity order of Pyrrole, Furan and Thiophene towards Electrophilic substitution :

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