Question

Identify the Lewis acid and the Lewis base in each reaction. (a) \(\mathrm{I}_{2}(\mathrm{~s})+\mathrm{I}^{-}(\mathrm{aq}) \longrightarrow \mathrm{I}_{3}^{-}(\mathrm{aq})\) (b) \(\mathrm{SO}_{2}(\mathrm{~g})+\mathrm{BF}_{3}(\mathrm{~g}) \longrightarrow \mathrm{O}_{2} \mathrm{SBF}_{3}(\mathrm{~s})\) (c) \(\mathrm{Au}^{+}(\mathrm{aq})+2 \mathrm{CN}^{-}(\mathrm{aq}) \longrightarrow\left[\mathrm{Au}(\mathrm{CN})_{2}\right]^{-}(\mathrm{aq})\) (d) \(\mathrm{CO}_{2}(\mathrm{~g})+\mathrm{H}_{2} \mathrm{O}(\ell) \longrightarrow \mathrm{H}_{2} \mathrm{CO}_{3}(\mathrm{aq})\)

Short answer

(a) \(\mathrm{I}_{2}\) is the Lewis acid, \(\mathrm{I}^{-}\) is the Lewis base. (b) \(\mathrm{BF}_{3}\) is the Lewis acid, \(\mathrm{SO}_{2}\) is the Lewis base. (c) \(\mathrm{Au}^{+}\) is the Lewis acid, \(\mathrm{CN}^{-}\) is the Lewis base. (d) \(\mathrm{CO}_{2}\) is the Lewis acid, \(\mathrm{H}_{2}\mathrm{O}\) is the Lewis base.

Step-by-step solution

Understanding Lewis Acids and Bases

A Lewis acid is a species that accepts an electron pair, while a Lewis base is a species that donates an electron pair.

Analyzing Reaction (a)

In the reaction \(\mathrm{I}_{2}(\mathrm{~s})+\mathrm{I}^{-}(\mathrm{aq}) \longrightarrow \mathrm{I}_{3}^{-}(\mathrm{aq})\), the \(\mathrm{I}_{2}\) molecule accepts an electron pair from the \(\mathrm{I}^{-}\) ion, thus acting as the Lewis acid. The \(\mathrm{I}^{-}\) ion donates an electron pair, making it the Lewis base.

Analyzing Reaction (b)

In the reaction \(\mathrm{SO}_{2}(\mathrm{~g})+\mathrm{BF}_{3}(\mathrm{~g}) \longrightarrow \mathrm{O}_{2} \mathrm{SBF}_{3}(\mathrm{~s})\), \(\mathrm{BF}_{3}\) is known for its electron-deficient boron atom, making it the Lewis acid. \(\mathrm{SO}_{2}\) donates an electron pair to \(\mathrm{BF}_{3}\), so it acts as the Lewis base.

Analyzing Reaction (c)

In the reaction \(\mathrm{Au}^{+}(\mathrm{aq})+2 \mathrm{CN}^{-}(\mathrm{aq}) \longrightarrow\left[\mathrm{Au}(\mathrm{CN})_{2}\right]^{-}(\mathrm{aq})\), the \(\mathrm{Au}^{+}\) ion accepts electron pairs from the two \(\mathrm{CN}^{-}\) ions, functioning as the Lewis acid. The \(\mathrm{CN}^{-}\) ions donate electron pairs, making them the Lewis bases.

Analyzing Reaction (d)

In the reaction \(\mathrm{CO}_{2}(\mathrm{~g})+\mathrm{H}_{2} \mathrm{O}(\ell) \longrightarrow \mathrm{H}_{2} \mathrm{CO}_{3}(\mathrm{aq})\), \(\mathrm{CO}_{2}\) acts as the Lewis acid by accepting electron pairs from water molecules \(\mathrm{H}_{2} \mathrm{O}\), which donates electron pairs and serves as the Lewis base.

Key concepts

Electron Pair Donation

In the context of Lewis acid-base chemistry, electron pair donation is a fundamental concept. A Lewis base is defined as a chemical species that donates an electron pair to a Lewis acid. This process forms a coordinate covalent bond between the two. The donor of the electron pair is typically a molecule or ion that has extra electrons in its outer shell.

• Example: Consider iodide ion \((I^-)\) in the reaction with diatomic iodine \(I_2\). The \(I^-\) acts as the Lewis base, donating an electron pair to \(I_2\), leading to the formation of \(I_3^-\).
• Other common examples include \(OH^-\) (hydroxide ion), \(CN^-\) (cyanide ion), and NH₃ (ammonia).

Recognizing electron pair donation helps in identifying the species acting as Lewis bases in chemical reactions.
This step is crucial for understanding molecular interactions and reactivity.

Electron Pair Acceptance

Electron pair acceptance is the counterpart to electron pair donation in Lewis acid-base interactions. A Lewis acid is a species that accepts an electron pair, which allows it to form a new chemical bond. This acceptance is often due to a deficiency in the electron configuration of the molecule or ion.

• Example: Boron trifluoride \((BF_3)\) is well-known for being an electron-deficient compound. In its reaction with sulfur dioxide \(SO_2\), \(BF_3\) acts as the Lewis acid by accepting an electron pair from \(SO_2\).
• Metals with positive charges, such as gold \((Au^+)\), are also typical Lewis acids. They accept electron pairs and participate in complex formation, as seen with two cyanide ions supplying electrons to \(Au^+\).

This concept plays a pivotal role in understanding how molecules interact and form more complex structures.

Chemical Reactions

Chemical reactions involving Lewis acids and bases are pivotal in many industrial and biological processes. These reactions are a type of chemical transformation characterized by the transfer of electron pairs from bases to acids, leading to new products.

• They often involve complexation reactions where the electron-deficient species (Lewis acid) forms a bond with the electron-rich species (Lewis base).
• Example: In the triiodide \(I_3^-\) formation, \(I_2\) interacts with \(I^-\), resulting in the electron-rich iodide ion transferring electrons to the electron-poor iodine molecule.

The dynamics of these reactions can vary, but the concept of electron pair sharing is a constant, unifying theme.
They are critical in catalysis, stabilization of charged species, and as intermediates in organic synthesis.

Acid-Base Chemistry

Acid-base chemistry is a branch of chemistry that studies the properties and behavior of acids and bases. While traditional definitions categorize acids and bases by their ability to donate or accept protons, the Lewis definition broadens this view by focusing on the exchange of electron pairs.

• In the Lewis framework, acids and bases are not limited to hydrogen exchange. Instead, their identity is about electron pair interactions.
• Example: The formation of carbonic acid \(H_2CO_3\) can be viewed as a Lewis acid-base interaction. Carbon dioxide \((CO_2)\) acts as the Lewis acid by accepting an electron pair from water \((H_2O)\).

Understanding acid-base chemistry in terms of Lewis interactions allows deeper insight into many chemical processes and equilibria.
It expands the toolkit for explaining reactions that don't involve hydrogen or hydroxide ions, making it incredibly versatile.