Factors Affecting Acid Strength | GOC Class 11

Factors Affecting Acid Strength

GOC | Principles of Acidity & Stability | Class 11

1. The Fundamental Principle

According to the Brønsted-Lowry concept, an acid ($HA$) donates a proton to form a Conjugate Base ($A^-$).

$$ HA \rightleftharpoons H^+ + A^- $$

                    Golden Rule: The strength of an acid is directly proportional to the Stability of its Conjugate Base ($A^-$). The more stable the anion $A^-$ is (by dispersing the negative charge), the stronger the acid.
                

2. Inductive Effect

The shifting of $\sigma$-electrons along a carbon chain due to electronegativity difference.

Electron Withdrawing Groups (-I Effect): Groups like $-NO_2, -F, -Cl, -CN$ pull electrons, dispersing the negative charge on $O^-$, thus Increasing Acidity.
Electron Donating Groups (+I Effect): Groups like $-CH_3, -C_2H_5$ push electrons, intensifying the negative charge on $O^-$, thus Decreasing Acidity.

$$ CCl_3COOH > CHCl_2COOH > CH_2ClCOOH > CH_3COOH $$

3. Resonance (Mesomeric) Effect

Delocalization of the negative charge through $\pi$-bonds greatly stabilizes the conjugate base.

Carboxylic Acids ($RCOOH$): The negative charge in $RCOO^-$ is delocalized over two electronegative Oxygen atoms (Equivalent Resonance). Very Stable.
Phenols ($PhOH$): The negative charge in Phenoxide ion ($PhO^-$) is delocalized over the benzene ring. Stable, but less than carboxylate.
Alcohols ($ROH$): No resonance. Alkoxide ion ($RO^-$) is destabilized by +I effect of R group. Least Stable.

Order: $RCOOH > PhOH > H_2O > ROH$

4. Hybridization (s-character)

Electronegativity increases with s-character ($sp > sp^2 > sp^3$). A more electronegative atom holds the negative charge more tightly.

Ethyne ($HC \equiv CH$): Carbon is $sp$ (50% s). Most acidic hydrocarbon.
Ethene ($H_2C = CH_2$): Carbon is $sp^2$ (33% s).
Ethane ($H_3C - CH_3$): Carbon is $sp^3$ (25% s). Least acidic.

5. Periodic Trends

A. Across a Period (Left to Right)

Factor: Electronegativity dominates.

As EN increases, the atom stabilizes negative charge better.

$$ CH_4 < NH_3 < H_2O < HF $$

B. Down a Group (Top to Bottom)

Factor: Bond Strength / Atomic Size dominates (NOT Electronegativity).

As size increases, H-A bond becomes longer and weaker, making it easier to break.

$$ HF < HCl < HBr < HI $$

Note: Although F is most electronegative, HI is the strongest acid because the H-I bond is the weakest.

Practice Quiz

Test your ability to compare Acid Strengths.

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