Question

Substitution constant for meta benzoic acids is denoted by

• A. σ_m
• B. рKа_(H)
• C. ρ_m
• D. pHm

Answer 1

The Correct Option is A

The question asks about the substitution constant for meta benzoic acids, which plays a significant role in the study of reaction mechanisms, particularly in the field of pharmaceutical and organic chemistry.

In Hammett equation context, substitution constants are important for understanding the effects of substituents on the rates of reaction and equilibria. These are denoted by the symbol \(\sigma\). Specifically, for meta-substituents in benzoic acid derivatives, the substitution constant is \(\sigma_m\). 

Let’s analyze the given options:

1. \(\sigma_m\): This denotes the substitution constant for meta-substituents in the Hammett equation, which is correct for meta benzoic acids.
2. \(\text{pK}\alpha_{(H)}\): This symbol represents the negative logarithm of the acid dissociation constant for an acid, which generally denotes acidity but not substitution effects in meta benzoic acids.
3. \(\rho_m\): This represents the reaction constant, which is used in the Hammett equation to describe how the reaction rate or position of equilibrium is affected by substituents, rather than the specific substitution constant itself.
4. \(\text{pHm}\): This refers typically to a pH measurement and is unrelated to the substitution constant notation.

The correct answer is \(\sigma_m\) because it specifically denotes the substitution effect at the meta position in benzoic acid derivatives. The substitution constant is part of the Hammett equation, \((\log K = \rho \sigma)\), where \(\rho\) is the reaction constant and \(\sigma\) is the substituent constant that varies for ortho, meta, and para positions.

Therefore, the substitution constant for meta benzoic acids is correctly denoted by \(\sigma_m\).