Question

Which of the following reactions takes places when tertiary butanol passed over heated copper at 573 K ?

• A. Dehydrogenation reaction
• B. Dehydration reaction
• C. Oxidation reaction
• D. Addition reaction

Hint:

Reaction of alcohols with heated Cu at 573 K (300 $^\circ$C):
Primary alcohols $\rightarrow$ Aldehydes (Dehydrogenation)
Secondary alcohols $\rightarrow$ Ketones (Dehydrogenation)
Tertiary alcohols $\rightarrow$ Alkenes (Dehydration)
Tertiary alcohols lack an $\alpha$-hydrogen on the carbon bearing the -OH group, so they cannot be dehydrogenated to a carbonyl compound by this method. They undergo elimination of water (dehydration).

Answer 1

The Correct Option is B

The reaction of alcohols over heated copper at $573 \text{ K}$ (which is $300 ^\circ\text{C}$) is a standard method for dehydrogenation or dehydration, depending on the type of alcohol. \begin{itemize} \item Primary alcohols (RCH$_2$OH): Undergo dehydrogenation (loss of H$_2$) to form aldehydes (RCHO). Example: CH$_3$CH$_2$OH $\xrightarrow{\text{Cu, 573K}}$ CH$_3$CHO + H$_2$. \item Secondary alcohols (R$_2$CHOH): Undergo dehydrogenation to form ketones (R$_2$C=O). Example: (CH$_3$)$_2$CHOH $\xrightarrow{\text{Cu, 573K}}$ (CH$_3$)$_2$C=O + H$_2$. \item Tertiary alcohols (R$_3$COH): Tertiary alcohols do not have a hydrogen atom attached to the carbon atom bearing the -OH group (the $\alpha$-carbon). Therefore, they cannot undergo dehydrogenation in the same way as primary and secondary alcohols to form a carbonyl compound. Instead, tertiary alcohols tend to undergo dehydration (loss of H$_2$O) when passed over heated copper at this temperature, forming alkenes. \end{itemize} Tertiary butanol (t-butanol) is (CH$_3$)$_3$C-OH. Structure: CH$_3$ $|$ CH$_3$-C-OH $|$ CH$_3$ The carbon atom attached to the -OH group has no hydrogen atoms directly bonded to it. When t-butanol is passed over heated copper at $573 \text{ K}$, it undergoes dehydration to form 2-methylpropene (isobutylene). (CH$_3$)$_3$C-OH $\xrightarrow{\text{Cu, 573K}}$ (CH$_3$)$_2$C=CH$_2$ + H$_2$O. This is a dehydration reaction. \[ \boxed{\text{Dehydration reaction}} \]