Question

All of the following compounds can react with acids. Without using a table of acidities, rank them in order of increasing acidity. Explain your ranking.

(a)

CH₃CH₂CH₂SO₃H

(b)

CH2CH₂CH₂OH

(c)

CH₃CH₂CH₂COOH

(d)

CH₃CH₂CHClCOOH

(e)

ClCH₂CH₂CH₂COOH

Short answer

CH2CH₂CH₂OH < CH₃CH₂CH₂COOH <ClCH₂CH₂CH₂COOH < CH₃CH₂CHClCOOH <CH₃CH₂CH₂SO₃H

Compound in order of increasing acidity

Step-by-step solution

Electronegativity

A negative charge is carried very easily by a more electronegative element which gives a more stable conjugate base and a stronger acid. Electronegativity value increases from left to right in a periodic table.

Size

The negative charge on an anion is considered to be more stable if it is spread over a wider region of space. As the size of the element increases, the acidity also increases down the column in a periodic table.

Inductive effect

The permanent displacement of the shared electron pairs in a carbon chain towards the group which is more electronegative is known as the inductive effect. The conjugate base can be stabilized by electron withdrawing atoms or groups through the sigma bond of the molecule. A stronger acid (lower value of pK_a ) results from the stabilization of the conjugate base. The magnitude of the inductive effect is directly proportional to the number of bonds between the electronegative element and the place of the negative charge.

Compound in order of increasing acidity

Carboxylic acid has higher acidity than alcohol because the negative charge on its conjugate base is stabilized by delocalization. There is no delocalization in the conjugate base of alcohol (alkoxide). The presence of the electronegative chlorine atom generally increases the acidity of the carboxylic acid group. Since the chlorine atom is more electronegative than hydrogen atom , therefore it is able to pull the electron density more towards itself. The chlorine substituent is taken as an electron withdrawing group. Inductive effect occurs via covalent bonds and decrease with distance. So, a chlorine atom three carbon away from a carboxylic acid has a decreased effect compared to a chlorine just one atom away from a carboxylic acid. Lastly, sulfonic acid has the highest acidity because the negative charge on it is delocalized in two sulfur-oxygen double bonds (S = O).

CH2CH₂CH₂OH < CH₃CH₂CH₂COOH <ClCH₂CH₂CH₂COOH < CH₃CH₂CHClCOOH <CH₃CH₂CH₂SO₃H

Compound in order of increasing acidity