Problem 95 - Chemistry The Central Science In Si Units

Question

Identify the Lewis acid and Lewis base among the reactants in each of the following reactions: (a) \(\mathrm{Fe}\left(\mathrm{ClO}_{4}\right)_{3}(s)+6 \mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons\) $$ \left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}(a q)+3 \mathrm{ClO}_{4}^{-}(a q) $$ (b) \(\mathrm{CN}^{-}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons \mathrm{HCN}(a q)+\mathrm{OH}^{-}(a q)\) (c) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N}(g)+\mathrm{BF}_{3}(g) \rightleftharpoons\left(\mathrm{CH}_{3}\right)_{3} \mathrm{NBF}_{3}(s)\) (d) \(\mathrm{HIO}(l q)+\mathrm{NH}_{2}^{-}(l q) \rightleftharpoons \mathrm{NH}_{3}(l q)+\mathrm{IO}^{-}(l q)\) (lq denotes liquid ammonia as solvent)

Step-by-Step Solution

Answer

(a) Lewis acid: Fe³⁺; Lewis base: H₂O. (b) Lewis acid: H₂O; Lewis base: CN⁻. (c) Lewis acid: BF₃; Lewis base: (CH₃)₃N. (d) Lewis acid: HIO; Lewis base: NH₂⁻.

Step 1: Define Lewis Acid and Lewis Base

A Lewis acid is a chemical species that can accept an electron pair, while a Lewis base is a species that can donate an electron pair. This definition generalizes the Brønsted-Lowry concept of acids and bases by focusing on the transfer of electron pairs instead of protons.

Step 2: Identify Lewis Acid and Base in Reaction (a)

In the reaction \( \mathrm{Fe}\left(\mathrm{ClO}_{4}\right)_{3}(s) + 6 \mathrm{H}_{2}\mathrm{O}(l) \rightleftharpoons \left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}(a q) + 3\mathrm{ClO}_{4}^{-}(a q) \), \( \mathrm{Fe}^{3+} \) acts as the Lewis acid because it accepts electron pairs from water molecules to form \( \left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+} \). The water molecules, \( \mathrm{H}_{2}\mathrm{O} \), act as Lewis bases as they donate electron pairs to \( \mathrm{Fe}^{3+} \).

Step 3: Identify Lewis Acid and Base in Reaction (b)

For the reaction \( \mathrm{CN}^{-}(a q) + \mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons \mathrm{HCN}(a q) + \mathrm{OH}^{-}(a q) \), The Lewis base is \( \mathrm{CN}^{-} \) as it donates an electron pair to form \( \mathrm{HCN} \), while \( \mathrm{H}_{2} \mathrm{O} \) acts as the Lewis acid as it accepts the electron pair from \( \mathrm{CN}^{-} \).

Step 4: Identify Lewis Acid and Base in Reaction (c)

In \( \left(\mathrm{CH}_{3}\right)_{3}\mathrm{~N}(g) + \mathrm{BF}_{3}(g) \rightleftharpoons \left(\mathrm{CH}_{3}\right)_{3}\mathrm{NBF}_{3}(s) \), \( \mathrm{BF}_{3} \) functions as the Lewis acid because it accepts an electron pair from \( \left(\mathrm{CH}_{3}\right)_{3}\mathrm{~N} \). Here, \( \left(\mathrm{CH}_{3}\right)_{3}\mathrm{~N} \) is the Lewis base because it donates an electron pair to bind with \( \mathrm{BF}_{3} \).

Step 5: Identify Lewis Acid and Base in Reaction (d)

For \( \mathrm{HIO}(l q) + \mathrm{NH}_{2}^{-}(l q) \rightleftharpoons \mathrm{NH}_{3}(l q) + \mathrm{IO}^{-}(l q) \), \( \mathrm{HIO} \) acts as the Lewis acid because it accepts an electron pair from \( \mathrm{NH}_{2}^{-} \), which acts as the Lewis base, donating an electron pair to \( \mathrm{HIO} \).

Key Concepts

Electron Pair Acceptance

Electron pair acceptance is a concept within Lewis acid-base chemistry. It describes how a chemical species known as a Lewis acid accepts an electron pair from another species, a Lewis base, during a reaction. The ability to accept electron pairs differentiates Lewis acids from other types of acids.

A characteristic example of a Lewis acid is the molecule \( ext{BF}_3\). This molecule has an empty orbital that can accommodate an electron pair from a Lewis base. Because \( ext{BF}_3\) is electron-deficient, it readily accepts an electron pair, making it a potent Lewis acid. In the reaction \(( ext{CH}_3)_3 ext{N} + ext{BF}_3 ightarrow ( ext{CH}_3)_3 ext{NBF}_3\), \( ext{BF}_3\) acts as the electron pair acceptor, thus engaging in electron pair acceptance.

Lewis acids are prevalent in chemistry, ranging from metal ions, like \( ext{Fe}^{3+}\), to non-metal compounds. Identifying Lewis acids in reactions is fundamental in understanding how molecules interact at an electronic level. This knowledge is crucial for predicting the outcomes of reactions and synthesizing complex compounds.

Electron Pair Donation

Electron pair donation is the complementary process to electron pair acceptance and is exhibited by Lewis bases. A Lewis base donates a pair of electrons to bond with a Lewis acid. This donation occurs because the Lewis base has a pair of electrons that are not involved in bonding, making them available for donation.

In the reaction \( ( ext{CH}_3)_3 ext{N} + ext{BF}_3 ightleftharpoons ( ext{CH}_3)_3 ext{NBF}_3\), \( ( ext{CH}_3)_3 ext{N} \) serves as the Lewis base. It has a lone pair on the nitrogen atom, which readily donates to form a bond with \( ext{BF}_3\). This donation enables the formation of a stable adduct.

Common Lewis bases include water molecules \( ( ext{H}_2 ext{O}) \) and ions such as \( ext{CN}^{-} \). Recognizing Lewis bases in reactions helps in understanding the role these species play in chemical processes, and it is integral to designing reactions for both academic study and industrial application.

Acid-Base Reactions

Acid-base reactions in the Lewis framework involve the interaction between a Lewis acid and a Lewis base, resulting in the formation of a new compound. Unlike classical acid-base reactions that involve proton transfer, Lewis acid-base reactions are centered around electron pair exchanges.

In the reaction \( ext{CN}^{-} + ext{H}_2 ext{O} ightleftharpoons ext{HCN} + ext{OH}^{-}\), \( ext{CN}^{-}\) acts as a Lewis base donating its electron pair to \( ext{H}_2 ext{O},\) which acts as a Lewis acid by accepting the electron pair. This electron pair transfer results in the formation of \( ext{HCN}\) and \( ext{OH}^{-}\).

Understanding Lewis acid-base reactions expands the horizon of chemical interactions beyond traditional definitions, offering a wider approach to observing reactions. Employing this concept in complex chemical systems can enhance the predictability and manipulation of reactions, crucial for advancements in chemical synthesis and pharmaceuticals.