Problem 1 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following has lowest solubility in water? (a) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHOH}\) (b) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{COH}\) (c) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\) (d) \(\mathrm{MeOH}\)

Step-by-Step Solution

Answer

(b) \((\text{CH}_3)_3\text{COH}\) has the lowest solubility in water.

Step 1: Identify Functional Groups

All given compounds are alcohols, meaning they have an -OH group, which is generally hydrophilic and can form hydrogen bonds with water.

Step 2: Consider the Size of Alkyl Groups

The solubility of alcohols in water decreases as the size of the alkyl group increases because larger groups are more hydrophobic. Compare the alkyl chains: (a) Dimethylmethanol (3 carbons): \((\text{CH}_3)_2\text{CHOH}\)(b) Trimethylmethanol (4 carbons): \((\text{CH}_3)_3\text{COH}\) (c) Ethanol (2 carbons): \(\text{C}_2\text{H}_5\text{OH}\) (d) Methanol (1 carbon): \(\text{MeOH}\)

Step 3: Rank the Compounds by Alkyl Size

Use the size ranking to predict solubility: (d) Methanol (1 carbon) is the smallest and most soluble. (c) Ethanol (2 carbons) is next in size. (a) Dimethylmethanol (3 carbons) follows. (b) Trimethylmethanol (4 carbons) is the largest.

Step 4: Conclusion Based on Trends

Larger alkyl groups reduce solubility. Therefore, the compound with the largest alkyl group, (b) \((\text{CH}_3)_3\text{COH}\), will have the lowest solubility in water.

Key Concepts

Functional Groups

In the context of alcohols, the term 'functional group' primarily refers to the presence of the hydroxyl group (-OH). This is a defining feature of alcohols, making them capable of forming hydrogen bonds, which are crucial for water solubility. The -OH group is hydrophilic, meaning it has a strong affinity for water. When alcohols dissolve in water, the -OH group interacts with water molecules, enhancing solubility.

• Alcohols have one or more hydroxyl (-OH) groups.
• The -OH group is polar, allowing it to form hydrogen bonds.
• Strong interaction between -OH (of alcohol) and water makes these alcohols hydrophilic.

However, the solubility is not solely dependent on this group; the surrounding hydrocarbon (alkyl group) can influence the overall solubility. The presence of a -OH group in the molecule makes the alcohol potential candidates for hydrogen bonding, significantly improving its interaction with water molecules.

Hydrogen Bonding

Hydrogen bonding is critical in understanding the solubility of alcohols in water. It is a type of chemical attraction where a hydrogen atom, which is covalently bonded to a more electronegative atom such as oxygen, forms a weak bond with another electronegative atom. This concept is crucial for explaining how alcohols dissolve in water.

• Hydrogen bonds form between the hydrogen in -OH and oxygen in water.
• These bonds are weaker than covalent bonds but strong enough to affect solubility.
• They increase the interaction between alcohol and water.

The strength and presence of hydrogen bonds play a role in the degree of solubility. For instance, methanol, with its single carbon atom, has less steric hindrance, allowing more access to its -OH group for hydrogen bonding compared to bulkier alcohols like trimethylmethanol.

Alkyl Groups

Alkyl groups are hydrocarbon chains or branches attached to the functional group in organic molecules. In alcohols, these alkyl groups can range in size and complexity, affecting the solubility of the alcohol in water. The general rule is: as the size of the alkyl group increases, the overall solubility in water decreases because the larger alkyl groups are more hydrophobic.

• Alkyl groups are composed of carbon and hydrogen.
• They can be linear, branched, or cyclic.
• Greater size means more hydrophobic interactions.

For example, in methanol (\(\text{MeOH}\)), the alkyl group is a single methyl group. This small size allows methanol to interact well with water. Conversely, trimethylmethanol has four carbons in its alkyl group (\(\text{CH}_3)_3 ext{COH}\)), offering less solubility due to increased hydrophobic characteristics. This concept is pivotal in predicting and understanding the water solubility of various alcohols.