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JEE Advance - Chemistry (2021 - Paper 2 Online - No. 13)

The amount of energy required to break a bond is same as the amount of energy released when the same bond is formed. In gaseous state, the energy required for homolytic cleavage of a bond is called Bond Dissociation Energy (BDE) or Bond Strength. BDE is affected by s-character of the bond and the stability of the radicals formed. Shorter bonds are typically stronger bonds. BDEs for some bonds are given below :
The amount of energy required to break a bond is same as the amount of energy released when the same bond is formed. In gaseous state, the energy required for homolytic cleavage of a bond is called Bond Dissociation Energy (BDE) or Bond Strength. BDE is affected by s-character of the bond and the stability of the radicals formed. Shorter bonds are typically stronger bonds. BDEs for some bonds are given below :
The amount of energy required to break a bond is same as the amount of energy released when the same bond is formed. In gaseous state, the energy required for homolytic cleavage of a bond is called Bond Dissociation Energy (BDE) or Bond Strength. BDE is affected by s-character of the bond and the stability of the radicals formed. Shorter bonds are typically stronger bonds. BDEs for some bonds are given below :
Correct match of the C-H bonds (shown in bold) in Column J with their BDE in Column K is

Column J
Molecule		 Column K
BDE (kcal $$mo{l^{ - 1}}$$)
(P) H-CH($$C{H_3}$$)$$_2$$ (i) 132
(Q) H-CH$$_2$$Ph (ii) 110
(R) H-CH=CH$$_2$$ (iii) 95
(S) H-C $$ \equiv $$ CH (iv) 88
P - iii, Q - iv, R - ii, S - i
P - i, Q - ii, R - iii, S - iv
P - iii, Q - ii, R - i, S - iv
P - ii, Q - i, R - iv, S - ii

Explanation

A more stable radical means lesser the bond dissociation energy.

Radicals formed are

JEE Advanced 2021 Paper 2 Online Chemistry - Basics of Organic Chemistry Question 24 English Explanation

Radical formed from Q (PhCH2$$-$$H) is most stable due to resonance.

Stability of free radical decreases with increase in % s-character.

P radical $$\to$$ sp3 $$\to$$ % s-character 25%

R radical $$\to$$ sp2 $$\to$$ % s-character 33%

S radical $$\to$$ sp $$\to$$ % s-character 50%

Thus, order of stability of free radical is

Q > P > R > S

Order of bond dissociation energy is

S > R > P > Q

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