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# Which of the following is the correct order of decreasing $SN_2$ reactivity :

$(a)\;R_2CHX > R_3CX > RCH_2X \\ (b)\;RCHX > R_3CX > R_2CHX \\(c)\; RCH_2X > R_2CHX > R_3CX \\ (d)\;R_3CX > R_2CHX > RCH_2X$

$SN^2$ mechanism occurs as
In $SN^2$ reaction , in the transition state there will be five groups attached to the carbon atom at which reaction occurs.
Hence $SN^2$ reaction is favoured by small groups on the carbon atoms attached to halogens.
$RCH_2 X > R_2 CHX > R_3CX$