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Chapter 4: Problem 38

The decreasing reactivity order of halides towards dehydrohalogenation is (a) \(\mathrm{RI}>\mathrm{RCl}>\mathrm{RBr}>\mathrm{RF}\) (b) \(\mathrm{RI}>\mathrm{RBr}>\mathrm{RCl}>\mathrm{RF}\) (c) \(\mathrm{RF}>\mathrm{RCl}>\mathrm{RBr}>\mathrm{RI}\) (d) \(\mathrm{RF}>\mathrm{RI}>\mathrm{RBr}>\mathrm{RCl}\)

Short Answer

Expert verified
Answer: The correct decreasing reactivity order of halides towards dehydrohalogenation is RI > RBr > RCl > RF.

Step by step solution

01

Reviewing Factors Influencing Dehydrohalogenation Reactivity

The reactivity of halides towards dehydrohalogenation depends on two primary factors: (1) The bond dissociation energy (BDE) of the C-X bond (X being the halide), and (2) The leaving group ability of the halide. The weaker the C-X bond, the greater its tendency to undergo dehydrohalogenation, as it requires less energy to break. Moreover, a better leaving group will promote dehydrohalogenation reaction as it is more likely to leave the molecule.
02

Comparing Bond Dissociation Energies

The bond strengths between carbon and the halides decrease in the following order: \(\mathrm{C-F}>\mathrm{C-Cl}>\mathrm{C-Br}>\mathrm{C-I}\). Therefore, the weaker the bond, the higher its reactivity towards dehydrohalogenation. In this case, a weaker bond means it takes less energy to break the C-X bond.
03

Comparing Leaving Group Abilities

Leaving group ability is directly related to how well the group can accommodate the negative charge once it leaves the molecule. In this case, the leaving group ability of halides increases as \(\mathrm{F^-}<\mathrm{Cl^-}<\mathrm{Br^-}<\mathrm{I^-}\). Therefore, iodide is the best leaving group and fluoride is the worst leaving group in this series.
04

Combining Factors and Determining the Order

Combining both factors of bond dissociation energy and leaving group ability, we can deduce the decreasing reactivity order of halides towards dehydrohalogenation. Reactivity will increase as C-X bond strength decreases and leaving group ability increases. Therefore, the correct order is \(\mathrm{RI}>\mathrm{RBr}>\mathrm{RCl}>\mathrm{RF}\).
05

Choosing the Correct Option

Based on the analysis above, the correct decreasing reactivity order of halides towards dehydrohalogenation is option (b) \(\mathrm{RI}>\mathrm{RBr}>\mathrm{RCl}>\mathrm{RF}\).

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Most popular questions from this chapter

Directions: Each question contains Statement- 1 and Statement-2 and has the following choices (a), (b), (c) and (d), out of which ONLY ONE is correct. (a) Statement- 1 is True, Statement- 2 is True; Statement- 2 is a correct explanation for Statement-1 (b) Statement- 1 is True, Statement- 2 is True; Statement- 2 is NOT a correct explanation for Statement-1 (c) Statement- 1 is True, Statement- 2 is False (d) Statement- 1 is False, Statement- 2 is True Statement 1 Nucleophilicity does not always match with basicity. and Statement 2 While nucleophilicity is more related to size, polarizability and steric factors, the basicity is the ability to abstract a proton from a substrate.

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Directions: Each question contains Statement- 1 and Statement-2 and has the following choices (a), (b), (c) and (d), out of which ONLY ONE is correct. (a) Statement- 1 is True, Statement- 2 is True; Statement- 2 is a correct explanation for Statement- 1 (b) Statement- 1 is True, Statement- 2 is True; Statement- 2 is NOT a correct explanation for Statement- 1 (c) Statement- 1 is True, Statement- 2 is False (d) Statement- 1 is False, Statement- 2 is True Statement 1 \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{O}-\mathrm{CH}_{2} \mathrm{Br}\) undergoes hydrolysis faster than \(\mathrm{CH}_{3}-\mathrm{O}-\mathrm{CH}_{2}-\mathrm{CH}_{2} \mathrm{Br}\) and Statement 2 The carbocation formed by ionization of \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{OCH}_{2} \mathrm{Br}\) is stabilized by resonance.
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