Explain the order of reactivity of $1^o$,$2^o$,and $3^o$ halides towards the $S_N2$ reaction.

(N/A) The order of reactivity for the $S_N2$ reaction is: $CH_3X > 1^o$ halide $> 2^o$ halide $> 3^o$ halide.
In an $S_N2$ reaction,the nucleophile approaches the carbon atom attached to the leaving group. The bulky substituent groups present on this carbon atom create steric hindrance. As the size of these groups increases,the $S_N2$ reaction becomes slower.
Methyl halides react the fastest in $S_N2$ reactions because they only have three small hydrogen atoms attached to the carbon. Tertiary $(3^o)$ alkyl halides are the least reactive in $S_N2$ reactions because the bulky groups provide maximum steric hindrance to the incoming nucleophile.