Question

The increasing order of stability of the following free radicals is

• A. $(C_6{H}_5{)}_2\dot{C}H<(C_6{H}_5{)}_3\dot{C}<(C{H}_3{)}_3\dot{C}<(C{H}_3{)}_2\dot{C}H$
  
• B. $(C{H}_3{)}_2\dot{C}H<(C{H}_3{)}_3\dot{C}<(C_6{H}_5{)}_2\dot{C}H<(C_6{H}_5{)}_3\dot{C}$
  
• C. $(C{H}_3{)}_3\dot{C}<(C{H}_3{)}_2\dot{C}H<(C_6{H}_5{)}_2\dot{C}H<(C_6{H}_5{)}_3\dot{C}$
  
• D. $(C_6{H}_5{)}_3\dot{C}<(C_6{H}_5{)}_2\dot{C}H<(C{H}_3{)}_3\dot{C}<(C{H}_3{)}_2\dot{C}H$
  

Answer 1

The correct option is B $(C{H}_3{)}_2\dot{C}H<(C{H}_3{)}_3\dot{C}<(C_6{H}_5{)}_2\dot{C}H<(C_6{H}_5{)}_3\dot{C}$


The order of stability of free radicals
$(C_6{H}_5{)}_3\dot{C}>(C_6{H}_5{)}_2\dot{C}H>(C{H}_3{)}_3\dot{C}>(C{H}_3{)}_2\dot{C}H$
The stabilisation of first two is due to resonance and last two is due to hyperconjugation.
For first two free radical: $3^o$ free radical will have more resonance due to more phenyl group than $2^o$ free radical which leads to more resonating structure thus more stability.
Similarly, for last two free radical: $3^o$ free radical will have more $\alpha -$H than $2^o$ free radical which leads to more hyperconjugative structure thus more stability. Hence correct option is (b).