Which one of the following does not lose water even in conc. $H_2SO_4$?
Understanding Dehydration with Concentrated Sulfuric Acid
Concentrated sulfuric acid ($H_2SO_4$) is known for its strong dehydrating properties. It removes elements of water ($H_2O$) from various compounds.
Analyzing Chemical Reactions with Conc. $H_2SO_4$
We examine how each substance reacts:
- Ethanol ($C_2H_5OH$): Concentrated $H_2SO_4$ effectively removes a water molecule from ethanol, leading to the formation of ethene ($C_2H_4$) and water. This is a classic dehydration reaction. $C_2H_5OH \xrightarrow{Conc. H_2SO_4, heat} C_2H_4 + H_2O$ Thus, ethanol loses water.
- Formic Acid ($HCOOH$): Formic acid reacts with concentrated $H_2SO_4$ via decomposition, yielding carbon monoxide ($CO$) and water ($H_2O$). $HCOOH \xrightarrow{Conc. H_2SO_4} CO + H_2O$ In this case, water is a product of decomposition, not simply removed from the intact molecule. The sulfuric acid absorbs the water formed. This specific reaction type is distinguished from simple dehydration where the substance itself loses water to become a less hydrated form.
- Acetic Acid ($CH_3COOH$): Acetic acid typically undergoes other reactions with concentrated $H_2SO_4$, such as sulfonation or protonation, rather than direct dehydration to form an anhydride under these conditions.
- Benzene ($C_6H_6$): Benzene reacts with concentrated $H_2SO_4$ primarily through electrophilic aromatic substitution, specifically sulfonation, forming benzenesulfonic acid ($C_6H_5SO_3H$). Water is also a product of this reaction, but the molecule does not lose water as its primary transformation.
Identifying Resistance to Water Loss
Based on the analysis:
- Ethanol clearly loses water via dehydration.
- Benzene undergoes sulfonation, not dehydration.
- Acetic acid reacts differently, not via simple dehydration.
- Formic acid decomposes, producing water which is absorbed by $H_2SO_4$. While water is formed, the process is decomposition rather than the molecule 'losing water' to form a stable, dehydrated counterpart like an alkene. In the context of typical dehydration reactions that conc. $H_2SO_4$ causes (like on alcohols or sugars), formic acid behaves differently and is often considered resistant to simple dehydration.
Therefore, formic acid is the substance among the options that fits the description of not losing water in the conventional dehydration sense.
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