Questions & explanations
1. Which of the following statements is correct about the behavior of alcohols in Lucas test and Victor Meyer test?
- Primary alcohols give immediate turbidity with Lucas reagent and a blood red color in Victor Meyer test.
- Tertiary alcohols give immediate turbidity with Lucas reagent and no color in Victor Meyer test.
- Secondary alcohols give immediate turbidity with Lucas reagent and a blue color in Victor Meyer test.
- Primary alcohols give no turbidity with Lucas reagent and a blue color in Victor Meyer test.
Answer: Tertiary alcohols give immediate turbidity with Lucas reagent and no color in Victor Meyer test.
In the Lucas test, tertiary alcohols react immediately with Lucas reagent (ZnCl₂ + conc. HCl) to form an insoluble alkyl chloride, causing immediate turbidity. Secondary alcohols react more slowly (within 5–10 minutes), while primary alcohols do not react at room temperature. In the Victor Meyer test, primary alcohols give a blood red color, secondary alcohols give a blue color, and tertiary alcohols produce no color because they lack an α-hydrogen necessary for the color reaction. Therefore, the statement that tertiary alcohols give immediate turbidity in the Lucas test and no color in the Victor Meyer test is correct.
2. Which reagent is used to convert ethanol to ethyl bromide?
- Br₂ (aq)
- Br₂/CCl₄
- PBr₃
- NaBr/H₂SO₄
Answer: PBr₃
Ethanol reacts with phosphorus tribromide (PBr₃) to give ethyl bromide (bromoethane) via cleavage of the C–O bond, replacing the –OH group with –Br. Br₂ (aq) or Br₂/CCl₄ typically adds to alkenes or reacts with phenols, not with alcohols. NaBr/H₂SO₄ can also give ethyl bromide but requires heating and is more commonly used for primary alcohols; however, PBr₃ is a standard reagent for converting alcohols to alkyl bromides under mild conditions.
3. Which of the following alcohols is most acidic?
- Ethanol
- 2-Methylpropan-2-ol
- Phenol
- Cyclohexanol
Answer: Phenol
Acidity of alcohols depends on the stability of the conjugate base (alkoxide). Phenol is significantly more acidic than aliphatic alcohols because the phenoxide ion is stabilized by resonance delocalization of the negative charge into the aromatic ring. Ethanol, 2-methylpropan-2-ol (tertiary), and cyclohexanol are aliphatic alcohols with comparable low acidity; phenol's pKa is about 10, while aliphatic alcohols have pKa around 16.
4. What is the IUPAC name and classification of the alcohol with the structure CH₃CH₂CH(OH)CH₃?
- Butan-1-ol; primary alcohol
- Butan-2-ol; secondary alcohol
- 2-Methylpropan-1-ol; primary alcohol
- Butan-2-ol; tertiary alcohol
Answer: Butan-2-ol; secondary alcohol
The longest carbon chain containing the OH group is four carbons (butane). The OH is attached to the second carbon, so the name is butan-2-ol. The carbon bearing the OH is attached to two other carbons, making it a secondary (2°) alcohol. Butan-1-ol would have OH on terminal carbon (primary). 2-Methylpropan-1-ol is a different isomer. Tertiary alcohol requires three alkyl groups on the OH carbon.
5. Dehydration of 2-methylbutan-2-ol with concentrated H₂SO₄ gives which alkene as the major product?
- 2-Methylbut-1-ene
- 3-Methylbut-1-ene
- 2-Methylbut-2-ene
- Pent-1-ene
Answer: 2-Methylbut-2-ene
2-Methylbutan-2-ol is a tertiary alcohol. Dehydration follows Zaitsev’s rule, forming the more substituted alkene as the major product. The possible alkenes are 2-methylbut-1-ene and 2-methylbut-2-ene. 2-Methylbut-2-ene is more substituted (trisubstituted) and is the major product. 3-Methylbut-1-ene is not formed because it would require rearrangement. Pent-1-ene is from a different structure.
6. Which carbonyl compound, when treated with excess CH₃MgBr followed by hydrolysis, yields a tertiary alcohol?
- Methanal
- Ethanal
- Propanone
- Butanal
Answer: Propanone
Grignard reagent (CH₃MgBr) adds to carbonyl compounds. With formaldehyde (methanal) it gives a primary alcohol after hydrolysis. With other aldehydes (ethanal, butanal) it gives secondary alcohols. With a ketone (propanone), the addition yields a tertiary alcohol because the carbonyl carbon is already bonded to two alkyl groups, and adding the Grignard’s alkyl group gives a tertiary alcohol.
7. Which of the following reactions gives a primary alcohol as the major product?
- Acid-catalyzed hydration of propene
- Reduction of propanal with NaBH₄
- Reduction of propanone with LiAlH₄
- Hydrolysis of 2-bromopropane with aqueous KOH
Answer: Reduction of propanal with NaBH₄
Reduction of an aldehyde (propanal) gives a primary alcohol (propan-1-ol). Acid-catalyzed hydration of propene follows Markovnikov’s rule, giving a secondary alcohol (propan-2-ol). Reduction of a ketone (propanone) yields a secondary alcohol (propan-2-ol). Hydrolysis of 2-bromopropane (a secondary haloalkane) gives a mixture or mainly isopropyl alcohol (secondary alcohol).
8. Oxidation of a secondary alcohol with acidified K₂Cr₂O₇ yields:
- A carboxylic acid
- A ketone
- An aldehyde
- An alkene
Answer: A ketone
Oxidation of secondary alcohols with strong oxidizing agents like acidified K₂Cr₂O₇ gives ketones. Further oxidation to carboxylic acids does not occur because cleavage of C–C bonds is required. Primary alcohols oxidize to aldehydes then to carboxylic acids. Tertiary alcohols do not oxidize under mild conditions. Alkenes are products of dehydration, not oxidation.
9. Which of the following alcohols has the highest boiling point due to hydrogen bonding?
- Methanol
- Ethanol
- Propan-1-ol
- Butan-1-ol
Answer: Butan-1-ol
Boiling points of alcohols increase with increasing molecular mass and chain length due to stronger van der Waals forces, even though hydrogen bonding is similar per OH group. Among the given primary alcohols, butan-1-ol has the largest molar mass and longest carbon chain, resulting in the highest boiling point.