Questions & explanations
1. During the Hell-Volhard-Zelinsky reaction, propanoic acid is treated with Br₂ in the presence of a catalytic amount of PBr₃. The major product formed is:
- 2-bromopropanoic acid
- 3-bromopropanoic acid
- 1-bromopropanoic acid
- 2,2-dibromopropanoic acid
Answer: 2-bromopropanoic acid
The Hell-Volhard-Zelinsky (HVZ) reaction involves α-bromination of carboxylic acids. In propanoic acid (CH₃CH₂COOH), the α-carbon is the carbon adjacent to the carboxyl group. Using Br₂ and a catalytic amount of PBr₃ selectively replaces an α-hydrogen with bromine, giving 2-bromopropanoic acid (CH₃CHBrCOOH). The other options are wrong: 3-bromopropanoic acid would require substitution at the β-carbon, which does not occur; 1-bromopropanoic acid is not a valid structure (the carboxyl carbon cannot be brominated by HVZ); and 2,2-dibromopropanoic acid would require further bromination under more forcing conditions, but with one equivalent of Br₂ the major product is the monobrominated acid.
2. Which of the following substituted acetic acids is the strongest acid?
- CH3COOH
- ClCH2COOH
- Cl2CHCOOH
- Cl3CCOOH
Answer: Cl3CCOOH
Electron-withdrawing substituents like chlorine increase the acidity of carboxylic acids by stabilizing the carboxylate ion through the inductive effect. The more chlorine atoms present, the stronger the inductive withdrawal of electrons, and thus the stronger the acid. Trichloroacetic acid (Cl3CCOOH) has three chlorine atoms, making it the strongest among the given options.
3. Reduction of propanoic acid (CH3CH2COOH) with LiAlH4 yields which product?
- 1-Propanol
- Propane
- Propionaldehyde
- Propene
Answer: 1-Propanol
Lithium aluminium hydride (LiAlH4) is a strong reducing agent that reduces carboxylic acids directly to primary alcohols. Propanoic acid (C3) is reduced to 1‑propanol (CH3CH2CH2OH). Propane would require decarboxylation, propionaldehyde is an intermediate in partial reduction but not the final product with excess LiAlH4, and propene is an alkene from dehydration.
4. What is the IUPAC name of the compound CH3CH2CH2COOH?
- Butanoic acid
- Propanoic acid
- Pentanoic acid
- Butanone
Answer: Butanoic acid
The compound has a four-carbon straight chain (butane) with a carboxylic acid group (-COOH) at the end. According to IUPAC nomenclature, the suffix '-oic acid' replaces the final 'e' of the alkane name, hence butanoic acid is correct. Propanoic acid has three carbons, pentanoic acid has five, and butanone is a ketone, not a carboxylic acid.
5. Treatment of acetic acid with thionyl chloride (SOCl2) yields which product?
- Acetyl chloride
- Acetic anhydride
- Ethyl acetate
- Acetamide
Answer: Acetyl chloride
Thionyl chloride (SOCl2) converts carboxylic acids into acyl chlorides (acid chlorides). The reaction replaces the –OH group of the carboxylic acid with –Cl, giving acetyl chloride (CH3COCl). Acetic anhydride would require dehydration, ethyl acetate is an ester, and acetamide is an amide. Therefore, acetyl chloride is the correct product.
6. Arrange the following compounds in decreasing order of acidity: ethanol, phenol, acetic acid.
- Acetic acid > Phenol > Ethanol
- Phenol > Acetic acid > Ethanol
- Ethanol > Phenol > Acetic acid
- Acetic acid > Ethanol > Phenol
Answer: Acetic acid > Phenol > Ethanol
Acetic acid is a stronger acid than phenol because the carboxylate ion is resonance-stabilized over two oxygen atoms, while phenoxide ion has resonance only over the aromatic ring. Ethanol is the weakest because the alkoxide ion is not stabilized by resonance. Hence the correct order is acetic acid > phenol > ethanol.
7. What is the primary reason carboxylic acids are stronger acids than alcohols?
- Resonance stabilization of the carboxylate ion
- Higher electronegativity of oxygen in the carbonyl group
- Hydrogen bonding between carboxylic acid molecules
- Stronger C–H bond in the carboxylic acid
Answer: Resonance stabilization of the carboxylate ion
Carboxylic acids dissociate to form a carboxylate ion (RCOO−) in which the negative charge is delocalized over two oxygen atoms via resonance. This delocalization stabilizes the conjugate base, making the acid stronger. Alcohols lack such resonance stabilization of their alkoxide ions, hence they are weaker acids.
8. Acid-catalyzed hydrolysis of ethyl acetate (CH3COOC2H5) yields which pair of products?
- Acetic acid and ethanol
- Acetic acid and methanol
- Formic acid and ethanol
- Propionic acid and methanol
Answer: Acetic acid and ethanol
Ethyl acetate is an ester. Under acidic hydrolysis, the ester bond is cleaved to give the parent carboxylic acid (acetic acid) and the alcohol (ethanol). Methanol is not formed because the alcohol part of ethyl acetate is ethanol. Formic acid and propionic acid are not relevant to this ester.
9. Hydrolysis of acetonitrile (CH3CN) in acidic medium produces which carboxylic acid?
- Acetic acid
- Formic acid
- Propionic acid
- Butyric acid
Answer: Acetic acid
Acetonitrile (CH3CN) on hydrolysis (addition of water in the presence of an acid catalyst) gives acetic acid (CH3COOH). The -CN group is converted to -COOH with the same number of carbon atoms. Formic acid has one carbon, propionic acid three, and butyric acid four, so they are incorrect.