Greater the number of electron donating alkyl groups (+I effect), greater is the stability of carbocations. +I effect is in the order : +I effect is in the order :
CH3−
CH3
|
C
|
CH3
−>CH3−
CH3
|
CH
−>CH3−CH2−>CH3−
Hence, the order of stability of carbocations is 5<4<3<1<2