Problem 97 - Chemistry The Central Science

Question

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

Step-by-Step Solution

Answer

(a) Lewis acid: Fe(ClO4)3; 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: Identify electron pair donor and acceptor

In this reaction, water molecules (H2O) donate an electron pair to the Fe3+ ion to form Fe(H2O)6(3+). Thus: - H2O acts as the Lewis base because it donates the electron pair. - Fe(ClO4)3 acts as the Lewis acid because it accepts the electron pair. (b) \(\mathrm{CN}^{-}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons \mathrm{HCN}(a q)+\mathrm{OH}^{-}(a q)\)

Step 1: Identify electron pair donor and acceptor

In this reaction, the CN- ion donates an electron pair to the H2O molecule to form HCN. Thus: - CN- acts as the Lewis base because it donates the electron pair. - H2O acts as the Lewis acid because it accepts the electron pair. (c) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N}(g)+\mathrm{BF}_{3}(g) \rightleftharpoons\left(\mathrm{CH}_{3}\right)_{3} \mathrm{NBF}_{3}(s)\)

Step 1: Identify electron pair donor and acceptor

In this reaction, the (CH3)3N molecule donates an electron pair to the BF3 molecule to form (CH3)3NBF3. Thus: - (CH3)3N acts as the Lewis base because it donates the electron pair. - BF3 acts as the Lewis acid because it accepts the electron pair. (d) \(\mathrm{HIO}(l q)+\mathrm{NH}_{2}^{-}(l q) \rightleftharpoons \mathrm{NH}_{3}(l q)+\mathrm{IO}^{-}(l q)\)

Step 1: Identify electron pair donor and acceptor

In this reaction, the NH2- ion donates an electron pair to the HIO molecule to form NH3 and IO-. Thus: - NH2- acts as the Lewis base because it donates the electron pair. - HIO acts as the Lewis acid because it accepts the electron pair.

Key Concepts

Electron Pair Donor

In a Lewis acid-base reaction, the electron pair donor plays a crucial role. This is the species that provides a pair of electrons to form a chemical bond. Essentially, the electron pair donor acts as the source of the electrons in the reaction.
This entity is termed the Lewis base. Whenever you see a molecule offering up an electron pair, you're looking at an electron pair donor. Commonly, these are molecules or ions that have lone pairs of electrons readily available.

In our reactions, here are the electron pair donors (Lewis bases):

• In reaction (a), water \(\text{H}_2\text{O}\) offers up its lone pair to the iron ion.
• In reaction (b), the cyanide ion \(\text{CN}^-\) donates its electron pair.
• In reaction (c), trimethylamine \(\text{(CH}_3\text{)}_3\text{N}\) is the donor.
• In reaction (d), the amide ion \(\text{NH}_2^-\) gives its electrons to the iodous acid.

Identifying the electron pair donor is essential for understanding the direction in which a reaction will proceed.

Electron Pair Acceptor

The electron pair acceptor in a Lewis acid-base reaction is the counterpart to the electron pair donor. This is the species that receives the electron pair, allowing a new bond to form. Unlike the donor, which has electrons to give, the acceptor seeks the shared pair to complete its electron configuration.
This recipient is known as the Lewis acid. Acids are typically electron-deficient and are more than willing to accept electrons from a donor.

Let's highlight the electron pair acceptors (Lewis acids) in our reactions:

• In reaction (a), the iron(III) \(\text{Fe}^{3+}\) ion accepts electrons from the water.
• In reaction (b), water \(\text{H}_2\text{O}\) accepts the electron pair from the cyanide ion.
• In reaction (c), boron trifluoride \(\text{BF}_3\) accepts the electrons from trimethylamine.
• In reaction (d), iodous acid \(\text{HIO}\) is the electron acceptor from the amide ion.

Think of electron pair acceptors like guests at a party—they're there to enjoy what others are offering.

Lewis Acid

A Lewis acid is a chemical species that acts as an electron pair acceptor. It is characterized by its ability to form a chemical bond by accepting a pair of electrons. In many cases, Lewis acids have vacant orbitals or are positively charged ions, which naturally attract electrons.
These characteristics mean that Lewis acids are often catalysts in chemical reactions, functioning to speed up reactions by facilitating the transfer of electron pairs.

In the given reactions, the Lewis acids identified are:

• In reaction (a), \(\text{Fe(ClO}_4\text{)}_3\) is the Lewis acid.
• In reaction (b), traditionally thought of as a base, here \(\text{H}_2\text{O}\) acts as the Lewis acid.
• In reaction (c), the classic Lewis acid \(\text{BF}_3\) takes center stage.
• In reaction (d), \(\text{HIO}\) acts as the Lewis acid.

Recognizing the Lewis acid is fundamental to understanding the mechanics of many reactions in which electron transfer is key.

Lewis Base

A Lewis base is a chemical entity that acts as an electron pair donor in a reaction. This property distinguishes the Lewis base as a vital part of many chemical processes, as it provides the electrons necessary for bond formation.
Lewis bases typically have lone pair electrons, which are ready to be shared, making them ideal for this role. In practice, they can be neutral molecules or anions with extra electron density.

Here are the Lewis bases in the sample reactions:

• In reaction (a), \(\text{H}_2\text{O}\) donates electrons to \(\text{Fe}^{3+}\).
• In reaction (b), \(\text{CN}^-\) serves as the donor.
• In reaction (c), \(\text{(CH}_3\text{)}_3\text{N}\) offers up its electron pair.
• In reaction (d), the \(\text{NH}_2^-\) ion donates its electron pair to \(\text{HIO}\).

The ability to donate electron pairs makes Lewis bases indispensable in the formation of chemical bonds.