Question

For the reaction sequence given below, the correct statement(s) is(are)

(In the options, X is any atom other than carbon and hydrogen, and it is different in P, Q and R)

• A. C-X bond length in P, Q and R follows the order Q > R > P.
• B. C-X bond enthalpy in P, Q and R follows the order R > P > Q.
• C. Relative reactivity toward SN2 reaction in P, Q and R follows the order P > R > Q.
• D. pKa value of the conjugate acids of the leaving groups in P, Q and R follows the order R > Q > P.

Answer 1

Answer: (B)

B

Answer 2

1. Determine the structures of P, Q, and R:

• Formation of P: Cyclohexene reacts with HBr via an addition reaction to form bromocyclohexane.
  P = Cyclohexyl bromide (C-Br bond)
  C1CCCCC1Br
• Formation of Q (Finkelstein reaction): Finkelstein reaction involves halogen exchange, replacing a bromine with an iodine using NaI in acetone.
  Q = Cyclohexyl iodide (C-I bond)
  C1CCCCC1I
• Formation of R (Swarts reaction): Swarts reaction involves halogen exchange, replacing a bromine with a fluorine using metallic fluorides like AgF.
  R = Cyclohexyl fluoride (C-F bond)
  C1CCCCC1F

Summary of C-X bonds in P, Q, R:

• P: C-Br
• Q: C-I
• R: C-F

2. Evaluate each statement:

A. C-X bond length in P, Q and R follows the order Q > R > P.
Bond length is primarily determined by the size of the halogen atom. As we go down the group, atomic size increases, leading to longer bond lengths.
Order of halogen atomic size: F < Br < I
Therefore, the C-X bond length order is: C-F < C-Br < C-I.
In terms of P, Q, R: R (C-F) < P (C-Br) < Q (C-I).
So, the correct order is Q > P > R.
The given statement Q > R > P is incorrect.

B. C-X bond enthalpy in P, Q and R follows the order R > P > Q.
Bond enthalpy (bond strength) is inversely related to bond length. Shorter bonds are generally stronger.
Order of C-X bond lengths: C-I > C-Br > C-F
Therefore, the C-X bond enthalpy order is: C-F > C-Br > C-I.
In terms of P, Q, R: R (C-F) > P (C-Br) > Q (C-I).
The given statement R > P > Q is correct.

C. Relative reactivity toward SN2 reaction in P, Q and R follows the order P > R > Q.
For SN2 reactions, the leaving group ability is crucial. A good leaving group is a weak base and can stabilize the negative charge. Leaving group ability generally increases with the size of the halogen atom.
Order of leaving group ability: I⁻ > Br⁻ > F⁻ (F⁻ is a poor leaving group due to high bond strength and high electronegativity).
Therefore, the reactivity toward SN2 reaction follows the order: Cyclohexyl iodide > Cyclohexyl bromide > Cyclohexyl fluoride.
In terms of P, Q, R: Q > P > R.
The given statement P > R > Q is incorrect.

D. pKa value of the conjugate acids of the leaving groups in P, Q and R follows the order R > Q > P.
The leaving groups are F⁻ (from R), Br⁻ (from P), and I⁻ (from Q). Their conjugate acids are HF, HBr, and HI, respectively.
The acidity order of hydrogen halides is: HI > HBr > HCl > HF.
pKa is the negative logarithm of the acid dissociation constant (Ka), so a stronger acid has a lower pKa.
Therefore, the pKa order is: HF > HBr > HI.
In terms of P, Q, R: pKa(HF) > pKa(HBr) > pKa(HI).
So, R > P > Q.
The given statement R > Q > P is incorrect.

The only correct statement is B.