Question

Chlorobenzenc, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Cl},\) is a key intermediate in the manufacture of dyes and pesticides. It is made by the chlorination of benzene, catalyzed by \(\mathrm{FeCl}_{3}\), in this series of steps: (1) \(\mathrm{Cl}_{2}+\mathrm{FeCl}_{3} \rightleftharpoons \mathrm{FeCl}_{5}\left(\right.\) or \(\left.\mathrm{Cl}^{+} \mathrm{FeCl}_{4}^{-}\right)\) (2) \(\mathrm{C}_{6} \mathrm{H}_{6}+\mathrm{Cl}^{+} \mathrm{FeCl}_{4}^{-} \rightleftharpoons \mathrm{C}_{6} \mathrm{H}_{6} \mathrm{Cl}^{+}+\mathrm{FeCl}_{4}^{-}\) (3) \(\mathrm{C}_{6} \mathrm{H}_{6} \mathrm{Cl}^{+} \rightleftharpoons \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Cl}+\mathrm{H}^{+}\) (4) \(\mathrm{H}^{+}+\mathrm{FeCl}_{4}^{-} \rightleftharpoons \mathrm{HCl}+\mathrm{FeCl}_{3}\) (a) Which of the step(s) is (are) Lewis acid-base reactions? (b) Identify the Lewis acids and bases in each of those steps.

Short answer

Steps 1, 2, and 4 are Lewis acid-base reactions. In Step 1: \(\mathrm{FeCl}_{3}\) is the Lewis acid and \(\mathrm{Cl}_{2}\) the Lewis base. In Step 2: \(\mathrm{Cl}^{+}\) is the Lewis acid and \(\mathrm{C}_{6} \mathrm{H}_{6}\) the Lewis base. In Step 4: \(\mathrm{H}^{+}\) is the Lewis acid and \(\mathrm{FeCl}_{4}^{-}\) the Lewis base.

Step-by-step solution

Understand Lewis Acids and Bases

Lewis acids are chemical species that accept an electron pair, while Lewis bases are chemical species that donate an electron pair. In the context of this problem, identify where electron pairs are donated and accepted.

Analyze the First Step

In the first step: \(\mathrm{Cl}_{2} + \mathrm{FeCl}_{3} \rightleftharpoons \mathrm{FeCl}_{5}\left(\right.\) or \(\left.\mathrm{Cl}^{+} \mathrm{FeCl}_{4}^{-}\right)\). Here, \(\mathrm{FeCl}_{3}\) accepts an electron pair from \(\mathrm{Cl}_{2}\), making \(\mathrm{FeCl}_{3}\) a Lewis acid and \(\mathrm{Cl}_{2}\) a Lewis base.

Analyze the Second Step

In the second step: \(\mathrm{C}_{6} \mathrm{H}_{6} + \mathrm{Cl}^{+} \mathrm{FeCl}_{4}^{-} \rightleftharpoons \mathrm{C}_{6} \mathrm{H}_{6} \mathrm{Cl}^{+} + \mathrm{FeCl}_{4}^{-}\). The \(\mathrm{Cl}^{+}\) accepts an electron pair from \(\mathrm{C}_{6} \mathrm{H}_{6}\), making \(\mathrm{Cl}^{+}\) a Lewis acid and \(\mathrm{C}_{6} \mathrm{H}_{6}\) a Lewis base.

Analyze the Third Step

In the third step: \(\mathrm{C}_{6} \mathrm{H}_{6} \mathrm{Cl}^{+} \rightleftharpoons \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Cl} + \mathrm{H}^{+}\). Here, there is no donation or acceptance of electron pairs as a single event, so it's not a Lewis acid-base reaction.

Analyze the Fourth Step

In the fourth step: \(\mathrm{H}^{+} + \mathrm{FeCl}_{4}^{-} \rightleftharpoons \mathrm{HCl} + \mathrm{FeCl}_{3}\). Here, \(\mathrm{H}^{+}\) accepts an electron pair from \(\mathrm{FeCl}_{4}^{-}\), making \(\mathrm{H}^{+}\) a Lewis acid and \(\mathrm{FeCl}_{4}^{-}\) a Lewis base.

Summarize Lewis Acid-Base Steps

The Lewis acid-base reactions are Steps 1, 2, and 4. In Step 1, \(\mathrm{FeCl}_{3}\) is the Lewis acid and \(\mathrm{Cl}_{2}\) is the Lewis base. In Step 2, \(\mathrm{Cl}^{+}\) is the Lewis acid and \(\mathrm{C}_{6} \mathrm{H}_{6}\) is the Lewis base. In Step 4, \(\mathrm{H}^{+}\) is the Lewis acid and \(\mathrm{FeCl}_{4}^{-}\) is the Lewis base.

Key concepts

Lewis Acid

A Lewis acid is a chemical species that can accept an electron pair.
This is a key concept in understanding many chemical reactions.
In the given exercise, \(\text{FeCl}_3\) in Step 1 functions as a Lewis acid.
It accepts an electron pair from \(\text{Cl}_2\), helping initiate the reaction.
Another example is the \(\text{Cl}^+\) ion in Step 2, which accepts an electron pair from benzene (\(\text{C}_6\text{H}_6\)).
This makes \(\text{Cl}^+\) a Lewis acid as well. The ability to accept electron pairs makes these species able to drive forward certain reaction mechanisms efficiently.

Recognizing Lewis acids in reactions helps in predicting how substances will interact. In organic chemistry and industrial applications, understanding these interactions is essential.

Lewis Base

A Lewis base is a species that donates an electron pair.
This is the counterpart to a Lewis acid in chemical reactions.
In the example provided, \(\text{Cl}_2\) acts as a Lewis base in Step 1 by donating an electron pair to \(\text{FeCl}_3\).
Additionally, benzene (\