Question

Sequence of reagents required to convert m-bromoaniline to benzoic acid is
(dry ether = పొడి ఈథర్)

• A. \( \text{NaNO}_2 | \text{HCl}, 273 - 278 \text{ K}; \text{CH}_3\text{CH}_2\text{OH}; \text{KCN}; \text{H}_3\text{O}^+ \)
• B. \( \text{NaNO}_2 | \text{HCl}, 273 - 278 \text{ K}; \text{Mg} | \text{dry ether}; \text{CO}_2 | \text{dry ether}; \text{H}_3\text{O}^+ \)
• C. \( \text{NaNO}_2 | \text{HCl}, 298 \text{ K}; \text{CuCN}; \text{H}_3\text{O}^+ \)
• D. \( \text{NaNO}_2 | \text{HCl}, 273 \text{ K}; \text{H}_2\text{O}; \text{KCN}; \text{H}_3\text{O}^+ \)

Answer 1

The Correct Option is B

The goal is to convert m-bromoaniline to benzoic acid. This transformation requires two key changes:
1. The amino group (-\( \text{NH}_2 \)) needs to be converted into a carboxylic acid group (-\( \text{COOH} \)).
2. The bromine atom needs to be handled appropriately (either removed or used in the formation of the carboxylic acid).
Let's analyze the steps involved in a feasible synthesis route and then check the options. Strategy: A common strategy to convert an aryl amine to a carboxylic acid is via the diazonium salt, followed by either Sandmeyer reaction to introduce a cyano group (which is then hydrolyzed to carboxylic acid) or by removing the amino group and forming a Grignard reagent from an existing halogen, which then reacts with \( \text{CO}_2 \). Step 1: Diazotization of m-bromoaniline
The amino group (-\( \text{NH}_2 \)) is converted into a diazonium salt ( \( \text{-N}_2^+ \text{Cl}^- \) ). This reaction requires sodium nitrite (\( \text{NaNO}_2 \)) and hydrochloric acid (\( \text{HCl} \)) at a very low temperature (0-5 \(^{\circ}\)C or 273-278 K) to prevent decomposition of the diazonium salt. \[ \text{m-bromoaniline} \xrightarrow{\text{NaNO}_2, \text{HCl}, 273-278 \text{ K}} \text{m-bromobenzenediazonium chloride} \] Step 2: Removal of the Diazonium Group (replacement with Hydrogen)
To proceed with a Grignard reaction, we need an aryl halide. The diazonium group needs to be replaced by a hydrogen atom, leaving the bromine intact. This can be achieved by treating the diazonium salt with ethanol (\( \text{CH}_3\text{CH}_2\text{OH} \)) or hypophosphorous acid (\( \text{H}_3\text{PO}_2 \)). \[ \text{m-bromobenzenediazonium chloride} \xrightarrow{\text{CH}_3\text{CH}_2\text{OH}} \text{bromobenzene} \] Now we have bromobenzene. Step 3: Formation of Grignard Reagent
Bromobenzene is an aryl halide, which can be converted into a Grignard reagent by reacting it with magnesium metal in dry ether. \[ \text{bromobenzene} \xrightarrow{\text{Mg} | \text{dry ether}} \text{phenylmagnesium bromide} (\text{C}_6\text{H}_5\text{MgBr}) \] Step 4: Reaction with Carbon Dioxide
The Grignard reagent (phenylmagnesium bromide) reacts with carbon dioxide ( \( \text{CO}_2 \) ) to form a magnesium carboxylate intermediate. This reaction is typically carried out in dry ether. \[ \text{C}_6\text{H}_5\text{MgBr} \xrightarrow{\text{CO}_2 | \text{dry ether}} \text{C}_6\text{H}_5\text{COOMgBr} \] Step 5: Acidic Hydrolysis
The magnesium carboxylate salt is then hydrolyzed with an acidic solution ( \( \text{H}_3\text{O}^+ \) ) to yield the final carboxylic acid, benzoic acid. \[ \text{C}_6\text{H}_5\text{COOMgBr} \xrightarrow{\text{H}_3\text{O}^+} \text{C}_6\text{H}_5\text{COOH} (\text{benzoic acid}) \] Evaluation of Options: \begin{itemize} \item Option (1): \( \text{NaNO}_2 | \text{HCl}, 273 - 278 \text{ K}; \text{CH}_3\text{CH}_2\text{OH}; \text{KCN}; \text{H}_3\text{O}^+ \) \begin{itemize} \item The first two steps would convert m-bromoaniline to bromobenzene. \item However, \( \text{KCN} \) cannot perform a nucleophilic substitution on bromobenzene to replace bromine with a cyano group under typical conditions. The Sandmeyer reaction with \( \text{CuCN/KCN} \) works on diazonium salts, not aryl halides. So, this sequence would fail to introduce the carbon atom needed for the carboxylic acid. \end{itemize} \item Option (2): \( \text{NaNO}_2 | \text{HCl}, 273 - 278 \text{ K}; \text{Mg} | \text{dry ether}; \text{CO}_2 | \text{dry ether}; \text{H}_3\text{O}^+ \) \begin{itemize} \item This sequence precisely follows the steps outlined above: 1. Diazotization ( \( \text{NaNO}_2 | \text{HCl}, 273-278 \text{ K} \) ) to form m-bromobenzenediazonium chloride. 2. Replacement of diazonium group with H using ethanol ( \( \text{CH}_3\text{CH}_2\text{OH} \) ), leading to bromobenzene. (Note: The option lists \( \text{CH}_3\text{CH}_2\text{OH} \) directly after the diazotization step, implying its role in reducing the diazonium group). 3. Formation of Grignard reagent ( \( \text{Mg} | \text{dry ether} \) ) from bromobenzene. 4. Reaction with \( \text{CO}_2 \) ( \( \text{CO}_2 | \text{dry ether} \) ). 5. Acidic workup ( \( \text{H}_3\text{O}^+ \) ). \item This sequence successfully leads to benzoic acid. \end{itemize} \item Option (3): \( \text{NaNO}_2 | \text{HCl}, 298 \text{ K}; \text{CuCN}; \text{H}_3\text{O}^+ \) \begin{itemize} \item The diazotization temperature (298 K or 25 \(^{\circ}\)C) is too high. Aromatic diazonium salts are unstable at room temperature and would decompose, primarily forming m-bromophenol. \item Even if the diazonium salt somehow survived, reacting m-bromobenzenediazonium chloride with \( \text{CuCN} \) would replace the diazonium group with a cyano group, yielding m-bromobenzonitrile. Subsequent hydrolysis with \( \text{H}_3\text{O}^+ \) would give m-bromobenzoic acid, not benzoic acid. \end{itemize} \item Option (4): \( \text{NaNO}_2 | \text{HCl}, 273 \text{ K}; \text{H}_2\text{O}; \text{KCN}; \text{H}_3\text{O}^+ \) \begin{itemize} \item Diazotization occurs correctly at 273 K. \item Reaction with \( \text{H}_2\text{O} \) (without \( \text{CH}_3\text{CH}_2\text{OH} \)) would convert the diazonium group to a hydroxyl group, forming m-bromophenol. \item \( \text{KCN} \) would not then convert m-bromophenol to a cyanophenyl derivative or benzoic acid. \end{itemize} \end{itemize} Therefore, Option (2) provides the correct sequence of reagents for the desired conversion.