This question asks about the reagent(s) that can be used for the dehydration of alcohols to form alkenes, as shown in the reaction diagram.

Concept Explanation: Dehydration of Alcohols

Dehydration is an elimination reaction where a molecule of water is removed from an alcohol to form an alkene. This reaction requires an acid catalyst and heat.

The general reaction is:

$R - CH 2 - CH 2 - OH \overset{[H +], Δ}{\to} R - CH = CH 2 + H 2 O$

Step-by-Step Analysis of Options:

Step 1: Evaluate conc. H₂SO₄, Δ

Concentrated sulfuric acid is a strong acid and a classic reagent for alcohol dehydration. It protonates the hydroxyl group, making it a better leaving group, facilitating elimination. This is a standard method.

Step 2: Evaluate Al₂O₃, Δ

Alumina (Al₂O₃) is a solid catalyst used for the vapor-phase dehydration of alcohols. It provides an acidic surface that promotes the elimination of water without the need for a strong liquid acid, making it useful in industrial processes.

Step 3: Evaluate ThO₂, Δ

Thoria (ThO₂) is another solid catalyst, similar to alumina, used for the dehydration of alcohols in the vapor phase. It is an effective catalyst for this elimination reaction.

Step 4: Final Conclusion

All three listed reagents—conc. H₂SO₄, Al₂O₃, and ThO₂—are well-established catalysts for the dehydration of alcohols to alkenes under heating conditions. Therefore, the correct answer is "All".

Related Topics & Formulae

Key Theory: The dehydration of alcohols follows Zaitsev's rule, which states that the more substituted alkene (the more stable alkene) is the major product. The reaction mechanism for acid-catalyzed dehydration is E1 for tertiary alcohols and can be E2 for primary alcohols under strong acid conditions.

General Reaction Formula:

$Alcohol \overset{Acid catalyst, Δ}{\to} Alkene + H 2 O$

Other Related Reagents: Phosphoric acid (H₃PO₄) is also commonly used instead of H₂SO₄ to avoid oxidation side reactions.