Question

What carbon radical is formed by homolysis of the$\mathbf{C}\mathbf{-}{\mathbf{H}}_{\mathbf{a}}$ bond in propylbenzene? Draw all reasonable resonance structures for this radical.

What carbon radical is formed by homolysis of therole="math" localid="1648540916945" $\mathbf{C}\mathbf{-}{\mathbf{H}}_{\mathbf{b}}$bond in propylbenzene? Draw all reasonable resonance structures for this radical.

The bond dissociation energy of one of the C-H bonds is considerably less than the bond dissociation energy of the other. Which C-H bond is weaker? Offer an explanation.

Short answer

Answer

a.

b.

c. The $\mathrm{C}-{\mathrm{H}}_{\mathrm{a}}$bond is weaker than the $\mathrm{C}-{\mathrm{H}}_{\mathrm{b}}$bond as the carbon radical formed by the $\mathrm{C}-{\mathrm{H}}_{\mathrm{a}}$bond is highly resonance stabilized.

Step-by-step solution

Step-by-Step SolutionStep 1: Homolysis

Homolysis, also termed homolytic fission, refers to the bond fission where each atom obtains an electron. Uncharged reactive intermediates are created due to homolytic fission.

Bond dissociation energy

The power required to fragment a covalent bond homolytically is termed bond- dissociation energy. The symbol that is utilized to denote the bond-dissociation energy is $\mathbf{∆}\mathbf{H}\mathbf{°}$.

Resonance structures and bond dissociation energy

a.The carbon radical formed by the homolysis of the$C-H_a$bond is given below

Carbon radical formed by the homolysis of the$\mathrm{C}-{\mathrm{H}}_{\mathrm{a}}$ bond

The resonance structure of the radicals formed by homolysis of the $\mathrm{C}-{\mathrm{H}}_{\mathrm{a}}$bond is given below:

Resonance structure of radical formed by homolysis of the bond

b.The carbon radical formed by the homolysis of the $\mathrm{C}-{\mathrm{H}}_{\mathrm{b}}$bond is given below:

Carbon radical formed by the homolysis of the$\mathrm{C}-{\mathrm{H}}_{\mathrm{b}}$bond

c. The carbon radical generated by the homolysis of $\mathrm{C}-{\mathrm{H}}_{\mathrm{a}}$bond is highly resonance stabilized. Hence, the $\mathrm{C}-{\mathrm{H}}_{\mathrm{a}}$bond is weaker than the $\mathrm{C}-{\mathrm{H}}_{\mathrm{b}}$bond, and the bond dissociation energy for $\mathrm{C}-{\mathrm{H}}_{\mathrm{a}}$is lower.