Question

Write the mechanism of dehydration of ethanol.

Hint:

At 443 K, ethanol undergoes dehydration to form ethene. At 413 K, it forms diethyl ether instead. Temperature controls the product.

Answer 1

Dehydration of ethanol occurs in the presence of concentrated sulphuric acid (H$_2$SO$_4$) at 443–463 K. The mechanism involves the following steps: Step 1: Protonation of ethanol.
Ethanol gets protonated by H$^+$ from conc. H$_2$SO$_4$: \[ CH_3CH_2OH + H^+ \;\longrightarrow\; CH_3CH_2OH_2^+ \] Step 2: Formation of carbocation.
The protonated alcohol loses a water molecule to form an ethyl carbocation: \[ CH_3CH_2OH_2^+ \;\longrightarrow\; CH_3CH_2^+ + H_2O \] Step 3: Deprotonation to form alkene.
The ethyl carbocation loses a proton (H$^+$) to give ethene: \[ CH_3CH_2^+ \;\longrightarrow\; CH_2=CH_2 + H^+ \] Overall reaction.
\[ CH_3CH_2OH \;\xrightarrow{conc. H_2SO_4, \, 443-463K}\; CH_2=CH_2 + H_2O \] Conclusion:
The dehydration of ethanol is an
acid-catalysed elimination reaction (E1 mechanism), leading to the formation of ethene.