Question

which alkyl halide follow only SN2 hydrolysis mechanism

Answer 1

1° alkyl halides follow the SN2 mechanism.

Answer 2

Primary alkyl halide follows only ${ S }_{ N }^{ 2 }$ hydrolysis mechanism.

The halide ion leaves from the front side in the case of ${ S }_{ N }^{ 2 }$ reaction, while the nucleophiles attack from the back side.  

Because of this, ${ S }_{ N }^{ 2 }$ reactions are always accompanied by configuration reversal.  

Thus other enantiomers are formed by an optically active halide's ${ S }_{ N }^{ 2 }$ reaction. It retains optical activity but has an opposite configuration. Generally, primary alkali halides undergo ${ S }_{ N }^{ 2 }$ reaction mechanism.

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