Q.Assertion: Addition of water to but-1-ene in acidic medium yields butan-1-ol.
Reason: Addition of water in acidic medium proceeds through the formation of primary carbocation.

Concept: Electrophilic Addition Reactions -- acid-catalysed hydration of an unsymmetrical alkene follows Markovnikov's rule: the more stable carbocation forms, and water attacks there.

Reasoning:

1. But-1-ene (CH2=CH-CH2-CH3) is protonated at C1 (the terminal, less-substituted carbon), placing the positive charge at C2 -- a secondary carbocation, not a primary one.
2. Water then attacks this secondary carbocation at C2, and after loss of a proton, the product is butan-2-ol, not butan-1-ol. …

Understanding the reaction

Acid-catalysed hydration of an alkene is a classic electrophilic addition, and its regiochemistry is governed entirely by carbocation stability (Markovnikov's rule), not by which carbon is 'easier to reach'.

Step 1: Protonation

But-1-ene is CH3-CH2-CH=CH2 (double bond between C1 and C2, C1 being the terminal =CH2). Under acid catalysis, H+ adds to C1 -- the carbon that already carries more hydrogens -- because this places the resulting positive charge on C2, which is flanked by an ethyl group and a methyl group:

CH3-CH2-CH=CH2 + H+ -> CH3-CH2-CH(+)-CH3

This is a secondary carbocation. A primary carbocation (positive charge on the terminal carbon) would be far less stable and does not form preferentially here.

Step 2: Water attacks the carbocation

Water attacks the more stable secondary carbocation at C2. After loss of a proton, the product is:

CH3-CH2-CH(OH)-CH3 = butan-2-ol

Step 3: Evaluating the assertion and the reason

• Assertion ('yields butan-1-ol'): false -- the real product is butan-2-ol. …

Concept: Markovnikov’s Rule & Carbocation Stability in Electrophilic Addition

Method: Stability-Based Carbocation Analysis

Step 1: Identify the reaction type

The addition of water ($H_2O$) to an alkene in acidic medium is acid-catalysed hydration — an electrophilic addition reaction.

Step 2: Determine the intermediate carbocation

• But-1-ene: $CH_3CH_2CH=CH_2$
• Protonation of the double bond can occur at two positions:

Step 3: Apply Markovnikov’s rule

The reaction proceeds via the more stable carbocation — the secondary one.

Thus, the major product is butan-2-ol, not butan-1-ol.

Step 4: Evaluate the Assertion

“Addition of water to but-1-ene in acidic medium yields butan-1-ol.”

This is wrong — the major product is butan-2-ol.

Step 5: Evaluate the Reason

“Addition of water in acidic medium proceeds through the formation of primary carbocation.” …

Common Mistakes & How to Avoid Them

Mistake 1: Assuming the product is butan-1-ol (primary alcohol)

Why students make this mistake:

They see "addition of water" and think the —OH group simply attaches to the first carbon (C-1) of but-1-ene, giving butan-1-ol.

The correct understanding:

In acidic medium, the reaction follows Markovnikov's rule. The proton (H⁺) adds to the less substituted carbon (C-1), forming a secondary carbocation at C-2 (not a primary one). Water then attacks this secondary carbocation, yielding butan-2-ol (a secondary alcohol).

How to avoid:

• Always check the carbocation stability order: tertiary > secondary > primary > methyl.
• For unsymmetrical alkenes, the proton goes to the carbon with more hydrogens, so the carbocation forms on the more substituted carbon.
• Key result: But-1-ene + H₂O/H⁺ → butan-2-ol, not butan-1-ol.

Mistake 2: Believing the reason statement is correct

Why students make this mistake:

They think "primary carbocation" sounds plausible because the proton adds to C-1 (which is primary).

The correct understanding:

The intermediate is actually a secondary carbocation (at C-2), not a primary one. A primary carbocation is highly unstable and does not form under these conditions.

How to avoid:

• Memorise the stability trend and apply it to every electrophilic addition.
• If a primary carbocation would form, the reaction will rearrange to a more stable carbocation (e.g., via hydride shift).
• Key result: The reason statement is wrong — the intermediate is secondary, not primary.

Mistake 3: Choosing option (A) or (B)

Why students make this mistake:

They think both statements are correct, or they think the reason explains the assertion even if the product is wrong.

The correct understanding:

• Assertion is wrong (product is butan-2-ol).
• Reason is wrong (intermediate is secondary carbocation).
• Assertion: wrong (product is butan-2-ol).
• Reason: wrong (intermediate is secondary carbocation). So neither statement is strictly correct. But the given options don't have "both wrong". Let's re-read the options:

(D) Assertion is wrong statement but reason is correct statement. …