Explain the mechanism of acid-catalyzed dehydration of ethanol to ethene with all the steps involved. Also, discuss the acidic nature of phenols compared to alcohols.

Q: Explain the mechanism of acid-catalyzed dehydration of ethanol to ethene with all the steps involved. Also, discuss the acidic nature of phenols compared to alcohols.

Mechanism of Acid-Catalyzed Dehydration of Ethanol \nThe dehydration of ethanol to ethene in the presence of concentrated sulphuric acid ($H2SO4$) at 443 K takes place in three distinct steps: Step 1: Protonation of ethanol: Ethanol undergoes fast protonation by an electrophilic attack of the hydronium ion ($H3O^+$) contributed by the acid to form protonated ethanol (ethyloxonium ion). This makes the oxygen atom electron-deficient and sets a good leaving group. Equation: $CH3-CH2-OH + H^+ \rightleftharpoons CH3-CH2-OH2^+$ Step 2: Formation of carbocation: The protonated ethanol molecule loses a water molecule ($H2O$) to form a primary carbocation ($CH3-CH2^+$). This is the slowest and rate-determining step of the overall reaction mechanism. Equation: $CH3-CH2-OH2^+ \rightarrow CH3-CH2^+ + H2O$ Step 3: Elimination of a proton: The carbocation loses a proton ($H^+$) to a base (like water) to form ethene. The proton is regained to regenerate the acid catalyst. Equation: $CH3-CH2^+ \rightarrow CH2=CH2 + H^+$ --- Comparison of Acidic Nature of Phenols and Alcohols Acidity of Alcohols: Alcohols are very weak acids. When an alcohol loses a proton, it forms an alkoxide ion ($R-O^-$). The alkyl group ($R-$) has an electron-releasing (+I effect) nature, which intensifies the negative charge on the oxygen atom, destabilizing the alkoxide ion and making it difficult for alcohols to release protons. Acidity of Phenols: Phenols are considerably more acidic than alcohols and water. When phenol loses a proton, it forms a phenoxide ion ($C6H5O^-$). The phenoxide ion is stabilized by resonance because the negative charge is delocalized over the benzene ring. Furthermore, unionized phenol does not exhibit resonance stabilization to the same extent as the phenoxide ion. Consequently, the equilibrium shifts towards the formation of the phenoxide ion, making phenols much stronger acids than aliphatic alcohols.

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Mechanism of Acid-Catalyzed Dehydration of Ethanol 
The dehydration of ethanol to ethene in the presence of concentrated sulphuric acid ($H2SO4$) at 443 K takes place in three distinct steps: Step 1: Protonation of ethanol: Ethanol undergoes fast protonation by an electrophilic attack of the hydronium ion ($H3O^+$) contributed by the acid to form protonated ethanol (ethyloxonium ion). This makes the oxygen atom electron-deficient and sets a good leaving group. Equation: $CH3-CH2-OH + H^+ ightleftharpoons CH3-CH2-OH2^+$ Step 2: Formation of carbocation: The protonated ethanol molecule loses a water molecule ($H2O$) to form a primary carbocation ($CH3-CH2^+$). This is the slowest and rate-determining step of the overall reaction mechanism. Equation: $CH3-CH2-OH2^+ ightarrow CH3-CH2^+ + H2O$ Step 3: Elimination of a proton: The carbocation loses a proton ($H^+$) to a base (like water) to form ethene. The proton is regained to regenerate the acid catalyst. Equation: $CH3-CH2^+ ightarrow CH2=CH2 + H^+$ --- Comparison of Acidic Nature of Phenols and Alcohols Acidity of Alcohols: Alcohols are very weak acids. When an alcohol loses a proton, it forms an alkoxide ion ($R-O^-$). The alkyl group ($R-$) has an electron-releasing (+I effect) nature, which intensifies the negative charge on the oxygen atom, destabilizing the alkoxide ion and making it difficult for alcohols to release protons. Acidity of Phenols: Phenols are considerably more acidic than alcohols and water. When phenol loses a proton, it forms a phenoxide ion ($C6H5O^-$). The phenoxide ion is stabilized by resonance because the negative charge is delocalized over the benzene ring. Furthermore, unionized phenol does not exhibit resonance stabilization to the same extent as the phenoxide ion. Consequently, the equilibrium shifts towards the formation of the phenoxide ion, making phenols much stronger acids than aliphatic alcohols.

NCERT · Class 12 · Chemistry · Alcohols, Phenols and EthersExplain the mechanism of acid-catalyzed dehydration of ethanol to ethene with all the steps involved. Also, discuss the acidic nature of phenols compared to alcohols.

Mechanism of Acid-Catalyzed Dehydration of Ethanol

Comparison of Acidic Nature of Phenols and Alcohols