Read the passage carefully and answer the questions based on the passage:
Alcohols are versatile compounds as they can act as both nucleophile and electrophiles. They undergo reactions based on cleavage of O-H and C-O bonds. The cleavage of O-H bond reflects the acidic character of alcohols. However they are weaker acids than water, phenol and carboxylic acids. Alkoxide ion formed by loss of proton from the OH group of alcohols react with water to form the corresponding conjugate acid-base pair. They act as Bronsted base due to presence of unshared electron pairs on oxygen.Both alcohols and phenol react with carboxylic acids, acid chlorides and anhydrides to form esters. The introduction of acetyl group in alcohols or phenol is known as acetylation. Alcohols react with hydrogen halides to form alkyl halides which form the basis of Lucas test. They also undergo acid catalyzed dehydration to form alkenes. Alcohols can be easily oxidized with variety of oxidizing agents. The nature of the product depends on the type of oxidizing agent and the nature of alcohol.
Identify the most acidic alcohol among the following:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Butanol
An alcohol acts as an acid by losing the proton of its O–H group to form an alkoxide ion, \( \ce{RO^-} \). Alkyl groups are electron releasing (+I effect): they push electron density towards the oxygen, which makes the O–H bond harder to break and destabilises the alkoxide ion. The more (and the more branched) the alkyl groups around the carbon carrying –OH, the weaker the acid:
\( \displaystyle \text{primary} \gt \text{secondary} \gt \text{tertiary} \)
Butanol (butan-1-ol, \( \ce{CH3CH2CH2CH2OH} \)) is a primary alcohol with a straight chain, so the electron release towards the oxygen is the smallest of the four and its alkoxide ion is also the easiest to solvate. Butanol is therefore the most acidic alcohol here.
Why the other options are incorrect
Option (2) Isobutanol — Incorrect. 2-Methylpropan-1-ol, \( \ce{(CH3)2CH-CH2OH} \), is also primary, but the branched isopropyl group releases more electron density and crowds the alkoxide ion, so it is a slightly weaker acid than straight-chain butanol.
Option (3) Tertiary butanol — Incorrect. In 2-methylpropan-2-ol, \( \ce{(CH3)3C-OH} \), three methyl groups sit on the carbon bearing –OH; this tertiary alcohol is the weakest acid of the four.
Option (4) Isopropanol — Incorrect. Propan-2-ol, \( \ce{(CH3)2CH-OH} \), is a secondary alcohol, so it is less acidic than a primary alcohol.
NCERT (2026-27 edition):
Acidity of Alcohols – “The acidic character of alcohols is due to the polar nature of O–H bond. An electron-releasing group (–CH3, –C2H5) increases electron density on oxygen tending to decrease the polarity of O-H bond. This decreases the acid strength.”