Question

Which of the following is the stronger acid: \(\begin{array}{llll}\mathrm{CH}_{2} \mathrm{ClCOOH} & \text { or } \mathrm{CHCl}_{2} \mathrm{COOH} ? & \text { Explain your }\end{array}\) choice.

Short answer

The stronger acid is \(CHCl_{2}COOH\). This is because it has two chlorine atoms which exert a stronger inductive effect, leading to an easier removal of the hydrogen ion.

Step-by-step solution

Identify the compounds

The first acid, \(CH_{2}ClCOOH\), has one chlorine atom attached to the carbon while the second acid, \(CHCl_{2}COOH\), has two chlorine atoms.

Concept of electronegativity and inductive effect

Chlorine is an electronegative atom and bonds with carbon in both acids. Its electronegativity pulls electrons towards itself, thereby reducing the electron density around the carbon atom attached to the hydroxyl group (\(OH\)). This makes the hydrogen atom in the \(OH\) group acidic.

Compare the acidity

For \(CHCl_{2}COOH\), two chlorine atoms pull electrons towards itself more efficiently than one chlorine atom in \(CH_{2}ClCOOH\). Therefore, \(CHCl_{2}COOH\) is more acidic since the removal of a hydrogen ion (\(H^{+}\)) is easier due to the high effective negative charge on oxygen atom.

Key concepts

Electronegativity

Electronegativity is a term that refers to the tendency of an atom to attract a bonding pair of electrons. In the context of chlorinated carboxylic acids, like those in the given exercise, chlorine atoms exhibit high electronegativity. When attached to a carbon atom that is also bonded to a hydroxyl (OH) group, as in carboxylic acids, the chlorine atoms draw electrons towards themselves.

This electron-pulling effect is crucial for understanding why certain substances are more acidic than others. By attracting electrons, a chlorine atom influences the distribution of charge across the molecule, affecting how easily the molecule can donate a proton (H+ ion). The higher the electronegativity of the atoms attached to the acidic carbon, the more likely it is for the acid to release its proton, enhancing its acidity.

Inductive Effect

The inductive effect is a phenomenon in which the bond electrons are displaced along the carbon chain due to the electronegativity of a substituent. In chlorinated carboxylic acids, this effect plays a significant role in determining acid strength.

Chlorine's electronegativity induces a partial negative charge on itself and a corresponding partial positive charge on the adjacent carbons. This positive charge is relayed onto the carbonyl carbon, thus impacting the ease with which the hydroxyl hydrogen can be released as a proton (H+).

As such, the presence of multiple electronegative atoms like chlorine increases the inductive effect. The stronger the inductive effect, the greater the acid strength. Therefore, a carboxylic acid with more chlorine atoms, such as CHCl₂COOH, will usually be more acidic than one with fewer chlorine atoms, given that other structural factors remain constant.

Acid Strength Comparison

To compare the acid strength between CH₂ClCOOH and CHCl₂COOH, we examine the influence of chlorination on the carboxylic acid structure. The presence of chlorine, a highly electronegative atom, increases the overall acidity of the molecule due to the inductive effect.

With a greater number of chlorine atoms, as in CHCl₂COOH, the inductive effect is amplified. Consequently, this molecule is able to stabilize the negative charge that arises when the proton is lost more effectively than CH₂ClCOOH.

Thus, CHCl₂COOH is a stronger acid compared to CH₂ClCOOH. This enhanced acidity can be crucial in various chemical reactions where the strength of an acid is a limiting factor. Understanding this relationship between structure and acidity also helps in designing molecules with specific acid-base properties for industrial, pharmaceutical, or research applications.