Question

(a) Explain why the reaction of \(\mathrm{SF}_{4}\) with \(\mathrm{BF}_{3}\) yields \(\left[\mathrm{SF}_{3}\right]^{+},\) whereas the reaction with \(\mathrm{CsF}\) gives \(\mathrm{Cs}\left[\mathrm{SF}_{5}\right]\) (b) Suggest how \(\mathrm{SF}_{4}\) might react with a carboxylic acid, \(\mathrm{RCO}_{2} \mathrm{H}\)

Short answer

(a) SF4 reacts with BF3 to form \([ ext{SF}_3]^+\) by donating a fluoride ion, while it forms \( ext{Cs[SF}_5]\) with CsF by accepting a fluoride. (b) SF4 could convert carboxylic acids to acyl fluorides by replacing -OH with F.

Step-by-step solution

Analyze SF4 with BF3

In the reaction between SF4 and BF3, SF4 acts as a Lewis base, and BF3 acts as a Lewis acid because BF3 is electron-deficient and can accept an electron pair. SF4 donates an electron pair to BF3, forming a complex where SF4 transfers a fluoride ion to BF3. This process results in the formation of the ionic compound \([ ext{SF}_3]^+\), where SF4 has lost a fluoride ion.

Analyze SF4 with CsF

In the reaction between SF4 and CsF, CsF provides a fluoride ion due to its ionic nature (Cs+ and F-). The fluoride ion (F-) attaches to SF4, creating the more fluorinated species \( ext{SF}_5^-\). The cesium ion (Cs+) associates with the \( ext{SF}_5^-\) ion, resulting in the formation of the ionic compound \( ext{Cs[SF}_5]\).

Predict SF4 Reaction with RCO2H

SF4 is a good fluorinating agent, so when it reacts with a carboxylic acid like RCO2H, it likely replaces the hydroxyl group (-OH) with a fluorine atom. This may yield an acyl fluoride (RC(O)F) when RCO2H is treated with SF4. The by-products of this reaction can include SOF2 or other sulfur-fluorine species.

Key concepts

Lewis Acid-Base Reactions

Lewis acid-base reactions are a fundamental concept in inorganic chemistry where an electron pair is transferred. These reactions involve a Lewis acid, which is an electron pair acceptor, and a Lewis base, which is an electron pair donor.
In the reaction between \( \text{SF}_4 \) and \( \text{BF}_3 \), sulfur tetrafluoride acts as a Lewis base. This is because it can donate an electron pair to \( \text{BF}_3 \), which is an electron-deficient species thus functioning as a Lewis acid.
The interaction occurs due to the empty p-orbital in \( \text{BF}_3 \) that readily accepts the electron pair from \( \text{SF}_4 \), which ultimately results in the formation of the complex \( [\text{SF}_3]^+ \).

• This is due to \( \text{BF}_3 \) receiving a fluoride ion from \( \text{SF}_4 \)
• It demonstrates a classic Lewis acid-base interaction, where electron pair sharing or transfer defines the process

Fluorination Reactions

Fluorination reactions involve replacing one atom or group in a molecule with a fluorine atom. These reactions are significant in various chemical processes, especially in introducing strong bonds due to the high electronegativity of fluorine.
When \( \text{SF}_4 \) reacts with \( \text{CsF} \), the fluorine from \( \text{CsF} \) replaces a substituent on \( \text{SF}_4 \), forming \( \text{Cs[SF}_5] \).
In this reaction, \( \text{CsF} \) acts as a source of fluoride ions.

• The fluoride ion from \( \text{CsF} \) competently adds to \( \text{SF}_4 \)
• This results in the formation of an ionic compound \( \text{Cs[SF}_5] \), where the cesium ion associates with the resulting fluoride-rich moiety
• Fluorination increases the stability, often making the resultant compound more chemically robust

Acyl Fluoride Formation

Acyl fluoride formation is an important transformation in organic and inorganic chemistry, where a hydroxyl group is replaced by a fluorine atom in carboxylic acids. This transformation is facilitated by fluorinating agents like \( \text{SF}_4 \).
When \( \text{SF}_4 \) reacts with a carboxylic acid ( \( \text{RCO}_2\text{H} \)), it effectively substitutes the hydroxyl group for a fluorine atom. This yields an acyl fluoride \( \text{RC(O)F} \).

• The replacement occurs through \( \text{SF}_4 \) providing a fluorine atom
• Common by-products of this reaction include \( \text{SOF}_2 \) and possibly other sulfur-fluorine compounds
• This reaction pathway exemplifies a practical application where modifying organic acids into their acyl fluoride forms enhances their reactivity for subsequent transformations