Question

Consider of following acids: 1\. \(\mathrm{HCN}\) 2\. \(\mathrm{HCOOH}\) 3\. \(\mathrm{CH}_{3} \mathrm{COOH}\) 4\. \(\mathrm{Cl}-\mathrm{CH}_{2} \mathrm{COOH}\) Correct order of acid strength is: (a) \(2>3>1>4\) (b) \(4>2>3>1\) (c) \(4>3>2>1\) (d) \(3>2>4>1\)

Short answer

(b) \(4>2>3>1\)

Step-by-step solution

Understand Acid Strength

The strength of an acid is largely determined by its ability to donate a proton (
H^+
) to a base. Stronger acids dissociate more completely in solution. Consider electronegativity and the inductive effect when comparing acids.

Evaluate
HCN

HCN
is a weak acid because the
CN^-
ion holds the hydrogen more tightly, making it less likely to donate protons compared to other acids listed. It has no strong electronegative groups pulling the hydrogen ion away.

Evaluate
HCOOH

HCOOH
, formic acid, is a stronger acid than
HCN
because the presence of the oxygen atoms increases its ability to release
H^+
ions due to resonance and the electron-withdrawing nature of oxygen.

Evaluate
CH_3COOH

CH_3COOH
or acetic acid is less acidic compared to
HCOOH
because the methyl group is electron-donating, which slightly reduces its ability to release protons.

Evaluate
Cl-CH_2COOH

Cl-CH_2COOH
or chloroacetic acid is the strongest among these acids. The chlorine atom is highly electronegative and provides a strong electron-withdrawing inductive effect, increasing the acid's ability to donate a proton.

Rank Acid Strength

Based on the evaluations:
Cl-CH_2COOH
>
HCOOH
>
CH_3COOH
>
HCN
. The inductive effect of chlorine in
Cl-CH_2COOH
makes it the strongest, followed by
HCOOH
and
CH_3COOH
, with
HCN
being the weakest.

Key concepts

Inductive Effect in Acids

The inductive effect plays a crucial role in determining the strength of an acid. This effect is essentially the influence of a substituent group on the electron cloud of the molecule, transmitted through a chain of atoms. When an electronegative atom or group, like chlorine, is attached to the chain, it pulls the electron density away. This is known as the electron-withdrawing inductive effect.
This effect has several impacts on acid strength:

• Increases the acidity: By pulling the electron density away from the acidic hydrogen, it makes the release of the proton ( H^+ ) easier and increases acid strength.
• Stabilizes the conjugate base: A more stable conjugate base is formed due to this effect, which in-turn promotes proton donation.

For example, in chloroacetic acid ( Cl-CH_2COOH ), the chlorine atom pulls electron density towards itself, enhancing the molecule's ability to release its proton more readily. This makes it a stronger acid than even acetic acid ( CH_3COOH ) or formic acid ( HCOOH ).

Proton Donation Ability

The ability of an acid to donate a proton is the cornerstone of its strength. An acid that can easily release its proton is considered strong because it fully or partially dissociates in water.
The key factors influencing proton donation ability include:

• Atomic Structure: How tightly the hydrogen is held by the rest of the molecule.
• Presence of Electron-Withdrawing Groups: Atoms like oxygen or chlorine that draw electrons away, making it easier for the hydrogen ion to dissociate.
• Substituent Effects: Groups that either stabilize or destabilize the release of H^+ .

Weak acids, like hydrogen cyanide ( HCN ), hold onto their protons because the CN^- ion does not strongly encourage release. In contrast, in chloroacetic acid ( Cl-CH_2COOH ), chlorine attracts electrons, facilitating proton release. This contrast shows why chloroacetic acid is stronger than hydrogen cyanide.

Electronegativity and Acid Strength

Electronegativity is a property of an atom that describes its ability to attract electrons. In the context of acids, it directly affects the acid's ability to donate protons.
Several points about electronegativity and acid strength include:

• The more electronegative a substituent, the stronger the acid: Highly electronegative atoms, like chlorine or oxygen, significantly enhance the acid strength by facilitating proton release.
• Stabilization of Conjugate Bases: An electronegative group can stabilize the negative charge on the conjugate base, strengthening the acid.

In acetic acid ( CH_3COOH ), the methyl group is less electronegative, making it a weaker acid. In contrast, chloroacetic acid ( Cl-CH_2COOH ) benefits from chlorine's high electronegativity. Its presence makes chloroacetic acid stronger by promoting electron withdrawal, thus reinforcing the acid's ability to dissociate and release protons.