Question

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 Fluorobenzene undergoes nucleophilic substitution much faster than chlorobenzene. and Statement 2 Eluorine being more electronegative causes more decrease in electron density at the carbon leading to a faster attack by the nucleophile.

Short answer

Short Answer: Both Statement 1 and Statement 2 are false. Fluorobenzene does not undergo nucleophilic substitution faster than chlorobenzene, and the higher electronegativity of fluorine does not lead to a faster nucleophilic substitution. The correct response is (d) Statement- 1 is False, Statement- 2 is False.

Step-by-step solution

Analyze Statement 1

Fluorobenzene and chlorobenzene both belong to the category of halobenzenes. Nucleophilic substitution in halobenzenes is dictated by the electron-withdrawing characteristics of the halogen atom. Due to the presence of a partial positive charge on the carbon atom, any nucleophile will be attracted to it. The reactivity of nucleophilic substitution will depend on the electron-withdrawing power of the halogen which is directly related to the electronegativity of the halogen. The general trend of electronegativity for halogens is F > Cl > Br > I. But contrary to this, the nucleophilic substitution reactivity trend of halobenzenes is F < Cl < Br < I. Therefore, statement 1 is false because fluorobenzene does not undergo nucleophilic substitution much faster than chlorobenzene.

Analyze Statement 2

Fluorine is indeed more electronegative than chlorine, but the statement indicates that this would lead to faster nucleophilic substitution, which is incorrect. The higher electronegativity of fluorine will cause a higher electron density decrease at the carbon atom, but this actually results in a slower nucleophilic substitution, as the C-F bond is also stronger. Thus, statement 2 is also false. With both statements being false, the correct answer is: (d) Statement- 1 is False, Statement- 2 is False

Key concepts

Nucleophilic Substitution Reactivity

Understanding the reactivity of nucleophilic substitution is crucial for grasping organic chemistry reactions, especially when it comes to halobenzenes like fluorobenzene and chlorobenzene. The reactivity in nucleophilic substitution is primarily influenced by how easily the halogen attached to the benzene ring can be replaced by the nucleophile, a species that donates an electron pair to a positive center.

In halobenzenes, the situation is a little counter-intuitive. Though the general assumption might be that a more electronegative halogen would enhance nucleophilic substitution due to a stronger partial positive charge beckoning nucleophiles, the opposite is true. Reactivity in nucleophilic substitution decreases with increasing electronegativity of the halogen. This is because the bond strength between the halogen and the carbon atom becomes stronger as well, making it more difficult for the nucleophile to displace the halogen.

The correct understanding lies in the balance of two opposing factors: halogen electronegativity, which increases electron-withdrawing effects, and bond strength, which makes the halogen harder to displace. This interplay dictates that fluorobenzene actually undergoes nucleophilic substitution slower than chlorobenzene, contradicting the common misconception.

Electron-Withdrawing Effects

The electron-withdrawing effect is a fundamental concept in organic chemistry that explains how substituents on a benzene ring affect its reactivity. Substituents that are capable of withdrawing electron density from the benzene ring make the ring less reactive to nucleophiles. This is because nucleophiles are electron-rich species, and they preferentially attack regions of positive electron density.

In the context of halobenzenes, the electron-withdrawing effect is moderated by the electronegativity of the attached halogen. Despite fluorine being the most electronegative element, its strong electron-withdrawing capacity doesn't necessarily translate to increased nucleophilic substitution reactivity. Instead, it stabilizes the negative charge on the halogen, diminishing the positive character of the adjacent carbon atom and makes the carbon-halogen bond stronger, which is less favorable for a nucleophile to attack.

Therefore, a stronger electron-withdrawing effect can ironically result in decreased reactivity towards nucleophilic substitution, illustrating why we observe a lower reaction rate for nucleophilic substitution in fluorobenzene compared to chlorobenzene.

Electronegativity of Halogens

Electronegativity is a measure of an atom's ability to attract and hold onto electrons within a chemical bond. Among the halogens, the trend in electronegativity is clear: fluorine is the most electronegative, followed by chlorine, bromine, and iodine. In theory, higher electronegativity should result in a greater tendency for an atom to pull electron density towards itself, creating more pronounced partial charges within molecules.

In halobenzenes, the highly electronegative fluorine prominent in fluorobenzene does pull electron density away from the benzene ring, but this heightened electronegativity also strengthens the bond between the fluorine and the carbon. Stronger bonds are more difficult to break, rendering fluorobenzene less reactive to nucleophilic substitution than one might initially expect. This demonstrates that while electronegativity is a critical factor in determining molecular behavior, its consequences on reactivity are not straightforward and must be considered in the broader context of bond strengths and overall molecular stability.