Problem 5 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Which one of the following statements is not correct? (a) Alcohols are weaker acids than water (b) Acid strength of alcohols decreases in the following \(\mathrm{RCH}_{2} \mathrm{OH}>\mathrm{R}_{2} \mathrm{CHOH}>\mathrm{R}_{3} \mathrm{COH}\) (c) Carbon-oxygen bond length in methanol, \(\mathrm{CH}_{3} \mathrm{OH}\) is shorter than that of C - O bond length in phenol. (d) The bond angle in methanol is \(108.9^{\circ}\). COC

Step-by-Step Solution

Answer

Statement (c) is not correct.

Step 1: Analyze Statement (a)

Alcohols are generally weaker acids than water because the alkoxide ion ( ext{RO}^{-}) formed after deprotonation is less stable compared to the hydroxide ion ( ext{OH}^{-}) due to the alkyl group. This statement is correct.

Step 2: Analyze Statement (b)

The acidity of alcohols decreases as more alkyl groups are added because these groups stabilize the positive charge on the carbon, making the cation less stable after deprotonation. Thus, primary alcohols ( ext{RCH}_2 ext{OH}) are usually more acidic than secondary ( ext{R}_2 ext{CHOH}) and tertiary ( ext{R}_3 ext{COH}) alcohols. This statement is correct.

Step 3: Analyze Statement (c)

In phenol, the carbon-oxygen bond has partial double bond character due to resonance, making it shorter than the carbon-oxygen bond in methanol, which is a single bond. Therefore, the statement that methanol has a shorter ext{C-O} bond than phenol is not correct.

Step 4: Analyze Statement (d)

The bond angle in methanol is due to the ext{sp}^3 hybridization around the carbon atom, and typical tetrahedral bond angles are around 109.5^{ ext{o}}. Thus, a bond angle of 108.9^{ ext{o}} is plausible and is a typical deviation due to the lone pairs on the oxygen. This statement is correct.

Key Concepts

Alcohol Structure

Alcohols are organic compounds characterized by the presence of a hydroxyl group \(\text{(-OH)}\) attached to a carbon atom. This structural feature significantly influences their physical and chemical properties. Generally, alcohols are categorized based on the number of carbon atoms bonded to the carbon that bears the hydroxyl group. These categories include:

• Primary Alcohols: Contain the hydroxyl group attached to a carbon atom bonded to only one other carbon (\(\text{RCH}_2\text{OH}\)).
• Secondary Alcohols: Feature the hydroxyl group attached to a carbon atom connected to two other carbon atoms (\(\text{R}_2\text{CHOH}\)).
• Tertiary Alcohols: Have the hydroxyl group on a carbon bonded to three other carbons (\(\text{R}_3\text{COH}\)).

The structure of alcohols is pivotal in determining their acidity. Primary alcohols are more acidic because of less steric hindrance and reduced electron-releasing effects from additional carbon groups.
These structural aspects inherently affect the stability of the alkoxide ion \(\text{RO}^-\). Typically, as the branching increases from primary to tertiary, alcohols become less acidic. Understanding this structural dependence is key to grasping the properties and reactions of alcohols.

Carbon-oxygen bond

The carbon-oxygen bond in alcohols is formed by the overlap of the sp\(^3\) hybridized orbitals from carbon and the oxygen atom. In methanol (\(\text{CH}_3\text{OH}\)), this bond is a single bond, and its length is a measure of the strength and stability of the connections between these atoms.
The bond length in methanol differs from that in phenol due to resonance effects in phenol. In phenol, the carbon-oxygen bond possesses partial double bond character because of resonance with the aromatic ring. This gives phenol a shorter bond length compared to the longer single bond of methanol.
This discrepancy highlights the influence of molecular structure and resonance on bond lengths. In methanol, the absence of resonance effects results in a typical single bond length for the C-O bond. However, understanding these differences is essential in predicting reactivity and interactions in various chemical environments.

Bond angle in methanol

The bond angle in methanol (\(\text{CH}_3\text{OH}\)) results from its molecular geometry and hybridization. The central carbon atom in methanol is sp\(^3\) hybridized, indicative of a tetrahedral geometry.
Ideal tetrahedral bond angles are approximately 109.5\(^\circ\). However, in methanol, the bond angle is slightly less, at 108.9\(^\circ\). This small deviation arises due to the presence of the lone pairs on the oxygen atom. Lone pairs exert greater repulsion than bonded pairs, compressing the bond angle slightly.
Understanding these bond angles is important as they affect the spatial arrangement of atoms and impact the overall dipole moment and intermolecular interactions. This knowledge aids in comprehending physical properties and reactive behavior of methanol in diverse chemical reactions.