The correct answer is Option (2) → (A), (C) and (D) only
Detailed Analysis of Statements
- (A) The formation of ether may involve an $S_N2$ or $S_N1$ reaction: This is correct. While the dehydration of primary alcohols (like ethanol) typically follows the $S_N2$ pathway, secondary or tertiary alcohols (though less efficient) tend to follow the $S_N1$ pathway via a carbocation intermediate.
- (B) The method is successful with secondary and tertiary alcohols: This is incorrect. This method is primarily successful for primary alcohols. In the case of secondary and tertiary alcohols, the elimination reaction (dehydration to form an alkene) competes strongly with substitution. Steric hindrance in tertiary alcohols makes the $S_N2$ attack of another alcohol molecule nearly impossible, leading almost exclusively to alkenes.
- (C) The reaction requires protic acids: This is correct. The reaction is catalyzed by concentrated protic acids such as $H_2SO_4$ or $H_3PO_4$. The acid is necessary to protonate the $-OH$ group, converting it into a good leaving group ($-H_2O^+$).
- (D) The reaction is carried out at low temperature: This is correct (relative to alkene formation). Temperature control is critical. For example, the dehydration of ethanol:
- At 413 K (140°C), the major product is diethyl ether (substitution).
- At 443 K (170°C), the major product is ethene (elimination).
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