Question

Acid-catalysed hydration of alkenes except ethene leads to the formation of (1) primary alcohol (2) secondary or tertiary alcohol (3) mixture of primary and secondary alcobols (4) mixture of secondary and tertiary alcohols

Short answer

Secondary or tertiary alcohol.

Step-by-step solution

Understanding the Reaction

In acid-catalysed hydration, an alkene reacts with water (H₂O) in the presence of an acid catalyst to form an alcohol. The reaction follows Markovnikov's rule.

Markovnikov's Rule

Markovnikov's rule states that during the addition of a protic acid (HX) to an unsymmetrical alkene, the hydrogen atom (H) attaches to the carbon with more hydrogen atoms, and the substituent (OH) attaches to the carbon with fewer hydrogen atoms.

Forming Secondary and Tertiary Alcohols

Considering Markovnikov's rule, the addition of water (H₂O) to an alkene results in the hydroxyl group (OH) attaching to the more substituted carbon atom. Therefore, secondary or tertiary alcohols are typically formed during the hydration of alkenes (except ethene).

Conclusion

As ethene does not apply in this context, the result of the acid-catalysed hydration of other alkenes is the formation of secondary or tertiary alcohols.

Key concepts

Markovnikov's Rule

Markovnikov's rule is super important in organic chemistry. It predicts the outcome of electrophilic addition reactions. Specifically, it helps us understand where atoms go during these reactions.

When you add a molecule like H₂O to an unsymmetrical alkene, it breaks up into H and OH groups. According to Markovnikov's rule, the hydrogen atom (H) will attach to the carbon that already has more hydrogen atoms.

On the other hand, the hydroxyl group (OH) will attach to the carbon that has fewer hydrogen atoms.

• This often results in the formation of the most stable (more substituted) product.
• The reason is that more substituted carbons stabilize the developing carbocation intermediate better.

Markovnikov's rule makes predictions easy. For example, if you take propene (CH₃-CH=CH₂) and add water (H₂O), the H will attach to the end with more hydrogens (CH₂ part), and the OH will go to the middle carbon (CH part).

Secondary Alcohol Formation

Secondary alcohols form when the hydroxyl group (OH) attaches to a secondary carbon atom. A secondary carbon has two other carbon atoms connected to it.

Let's take a simple unsymmetrical alkene like propene (CH₃-CH=CH₂) again. Adding water (H₂O) to it will result in the following:

• The hydrogen (H) from H₂O attaches to the CH₂ end of the alkene.
• The hydroxyl group (OH) from H₂O attaches to the central CH carbon.

The result is 2-propanol (CH₃-CHOH-CH₃), which is a secondary alcohol.

Why does this happen? It’s because the secondary carbon is more substituted. It stabilizes any intermediates that form during the reaction. So, in most cases, you'll end up with secondary alcohols.

Tertiary Alcohol Formation

A tertiary alcohol is formed when the hydroxyl group (OH) attaches to a tertiary carbon atom. This carbon is bonded to three other carbon atoms.

Consider the alkene 2-methylpropene (CH₂=C(CH₃)₂). When you hydrate this molecule, here’s what happens:

• The H from H₂O attaches to one of the CH₂ parts.
• The OH from H₂O attaches to the central carbon, which is bonded to three other carbon atoms (CH part).

The product is tert-butanol (C(CH₃)₃OH), a tertiary alcohol.

Tertiary carbons are even more stable due to the presence of three alkyl groups. This added stability makes tertiary alcohols a common product in some alkene hydration reactions.

Just remember, tertiary alcohol formation follows similar rules: the more substituted the carbon where OH will go, the more stable the resulting alcohol.