Which of the following organic compounds is very soluble in water?

Analysis of the Question

The question asks us to identify which of the given organic compounds is highly soluble in water. Solubility in water for organic compounds is generally determined by the ability of the molecule to form hydrogen bonds with water molecules.

Concept Explanation

1. Hydrogen Bonding: For an organic compound to be very soluble in water, it must be polar and capable of forming hydrogen bonds with water. Small carboxylic acids, alcohols, and amines are typically soluble because their polar functional groups (like -OH or -COOH) interact strongly with water.
2. Hydrocarbons: Compounds like alkanes, alkenes (e.g., $C_2H_4$), and alkynes (e.g., $C_2H_2$) are non-polar. Because they cannot form hydrogen bonds with water, they are generally insoluble in water.
3. Esters: While esters (like $CH_3COOC_2H_5$) have some polarity, they cannot form hydrogen bonds with themselves and have limited solubility in water compared to carboxylic acids.
4. Carboxylic Acids: Ethanoic acid ($CH_3COOH$) has a polar carboxyl group (-COOH). The small size of the molecule allows the polar end to interact effectively with water, making it miscible (very soluble) in water.

Step-by-Step Solution

• Option A ($CH_3COOH$): This is Ethanoic acid. It is a polar molecule that forms strong hydrogen bonds with water. It is completely miscible in water.
• Option B ($C_2H_2$): This is Ethyne. It is a non-polar hydrocarbon and is only sparingly soluble in water.
• Option C ($C_2H_4$): This is Ethene. Like ethyne, it is a non-polar hydrocarbon and is insoluble in water.
• Option D ($CH_3COOC_2H_5$): This is Ethyl ethanoate (an ester). While it has some solubility, it is significantly less soluble than the corresponding carboxylic acid because it lacks the hydroxyl (-OH) group necessary for strong hydrogen bonding.

Comparing these, $CH_3COOH$ is the most soluble due to its ability to form extensive hydrogen bonds.

Conclusion

The correct option is A.

Exam Tip

When you see questions about solubility in organic chemistry, look for the functional group. If you see -OH (alcohols) or -COOH (carboxylic acids), those molecules are usually water-soluble, especially if the carbon chain is short (1–4 carbons). Hydrocarbons (compounds with only Carbon and Hydrogen) are almost always insoluble in water!