Question

1a. Arrange the compounds in order of decreasing PKa. Highest First.
a. CH3CH2OH
b. ClCH2CH2SH
c. CH3CH2SH

1b. Arrange the compounds in order of decreasing PKa. Highest First.
a. Cl2CHCH2OH
b. CH3CH2OH
c. ClCH2CH2OH

Answer 1

Concepts and reason

Strength of an acid is expressed in terms of ${{\rm{K}}_{\rm{a}}}$(acid dissociation constant) value or ${\rm{p}}{{\rm{K}}_{\rm{a}}}$(inverse logarithm value of${{\rm{K}}_{\rm{a}}}$) value.

The following factors determine the strength of an acidic compound.

1. The strength of an acid increases with increase in size of the atom to which donor hydrogen atom is attached in acidic compound.

2. Acidic strength increases with increase in electronegativity of the atom to which donor hydrogen atom attached.

3. Type of substituent and position of substituent from donor hydrogen atom will play role in determining acidic strength due to inductive effect.

Fundamentals

A proton (${{\rm{H}}^{\rm{ + }}}$) donor is an acid. A compound which can donate a proton or hydrogen atom very easily is strongly acidic compound.

The structure of an acidic compound will affect its ${\rm{p}}{{\rm{K}}_{\rm{a}}}$value.

Proton donating capacity of an acid is represented with ${\rm{p}}{{\rm{K}}_{\rm{a}}}$value. A strong acid will have a small ${\rm{p}}{{\rm{K}}_{\rm{a}}}$value and a weak acid will have a large ${\rm{p}}{{\rm{K}}_{\rm{a}}}$ value.

Part 1.a

Compare the acid strength of ${\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{OH}}$ and${\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{SH}}$. The compound ${\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{OH}}$is less acidic than${\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{SH}}$. The ${\rm{p}}{{\rm{K}}_{\rm{a}}}$ value of ${\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{OH}}$ is higher than the ${\rm{p}}{{\rm{K}}_{\rm{a}}}$ value of${\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{SH}}$, that is${\rm{p}}{{\rm{K}}_{\rm{a}}}\left( {{\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{OH}}} \right) > {\rm{p}}{{\rm{K}}_{\rm{a}}}\left( {{\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{SH}}} \right)$.

The compound ${\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{SH}}$is less acid than${\rm{ClC}}{{\rm{H}}_{\rm{2}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{SH}}$. The ${\rm{p}}{{\rm{K}}_{\rm{a}}}$value of ${\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{SH}}$is higher than the ${\rm{p}}{{\rm{K}}_{\rm{a}}}$ value of${\rm{ClC}}{{\rm{H}}_{\rm{2}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{SH}}$, that is ${\rm{p}}{{\rm{K}}_{\rm{a}}}\left( {{\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{SH}}} \right) > {\rm{p}}{{\rm{K}}_{\rm{a}}}\left( {{\rm{ClC}}{{\rm{H}}_{\rm{2}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{SH}}} \right)$.

Part 1.b

Compare the acid strength of ${\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{OH}}$ and${\rm{ClC}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{OH}}$. The compound ${\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{OH}}$ is less acidic than${\rm{ClC}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{OH}}$. The ${\rm{p}}{{\rm{K}}_{\rm{a}}}$ value of ${\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{OH}}$ is higher than the ${\rm{p}}{{\rm{K}}_{\rm{a}}}$ value of${\rm{ClC}}{{\rm{H}}_{\rm{2}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{OH}}$, that is ${\rm{p}}{{\rm{K}}_{\rm{a}}}\left( {{\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{OH}}} \right) > {\rm{p}}{{\rm{K}}_{\rm{a}}}\left( {{\rm{ClC}}{{\rm{H}}_{\rm{2}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{OH}}} \right)$ .

The compound${\rm{ClC}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{OH}}$is less acid than${\rm{C}}{{\rm{l}}_2}{\rm{CHC}}{{\rm{H}}_{\rm{2}}}{\rm{OH}}$. The ${\rm{p}}{{\rm{K}}_{\rm{a}}}$value of ${\rm{ClC}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{OH}}$is higher than the ${\rm{p}}{{\rm{K}}_{\rm{a}}}$ value of${\rm{C}}{{\rm{l}}_2}{\rm{CHC}}{{\rm{H}}_{\rm{2}}}{\rm{OH}}$, that is ${\rm{p}}{{\rm{K}}_{\rm{a}}}\left( {{\rm{ClC}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{OH}}} \right) > {\rm{p}}{{\rm{K}}_{\rm{a}}}\left( {{\rm{C}}{{\rm{l}}_2}{\rm{CHC}}{{\rm{H}}_{\rm{2}}}{\rm{OH}}} \right)$ .

[Part 1.b]

Ans: Part 1.a

Part 1.a

The decreasing order of ${\rm{p}}{{\rm{K}}_{\rm{a}}}$ values of the given compounds is${\rm{p}}{{\rm{K}}_{\rm{a}}}\left( {{\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{OH}}} \right) > {\rm{p}}{{\rm{K}}_{\rm{a}}}\left( {{\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{SH}}} \right) > {\rm{p}}{{\rm{K}}_{\rm{a}}}\left( {{\rm{ClC}}{{\rm{H}}_{\rm{2}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{SH}}} \right)$.