Problem 5 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following orders of acid strength if correct? (a) \(\mathrm{RCOOH}>\mathrm{HC} \equiv \mathrm{CH}>\mathrm{HOH}>\mathrm{ROH}\) (b) \(\mathrm{RCOOH}>\mathrm{ROH}>\mathrm{HOH}>\mathrm{HC} \equiv \mathrm{CH}\) (c) \(\mathrm{RCOOH}>\mathrm{HOH}>\mathrm{ROH}>\mathrm{HC} \equiv \mathrm{CH}\) (d) \(\mathrm{RCOOH}>\mathrm{HOH}>\mathrm{HC} \equiv \mathrm{CH}>\mathrm{ROH}\)

Step-by-Step Solution

Answer

Option (c)  ext{RCOOH}> ext{HOH}> ext{ROH}> ext{HC} ext{CH} is the correct order of acid strength.

Step 1: Understanding Acidic Strength

Acidic strength refers to the tendency of a compound to donate a proton. Typically, strong acids dissociate completely in water, while weak acids do not.

Step 2: Evaluating Carboxylic Acids

Carboxylic acids ( ext{RCOOH}) are generally stronger acids than alcohols ( ext{ROH}) and water due to the resonance stability of their conjugate base (carboxylate ion). Hence,  ext{RCOOH} comes first in the order.

Step 3: Comparing Water and Alcohols

Water (HOH) is typically a weaker acid than carboxylic acids but stronger than alcohols (ROH) because the hydroxide ion (from water) is less stable than the alkoxide ion (from alcohol). So,  ext{HOH} should follow after  ext{RCOOH}.

Step 4: Assessing Alkynes

Terminal alkynes (like  ext{HC}  CH) have acidic hydrogen at the terminal position and form acetylide ions, which are not very stable; hence, they are weaker acids compared to both water and alcohols.

Step 5: Choosing the Correct Order

Thus, the correct order considering the typical strengths is:  ext{RCOOH} > ext{HOH} > ext{ROH} > ext{HC}  ext{CH}, which corresponds to option (c).

Key Concepts

Carboxylic Acids

Carboxylic acids are organic compounds characterized by the presence of a carboxyl group \((─COOH)\). This group is highly influential in determining the acidic nature of the molecule. The strength of carboxylic acids as acids lies in the stability of their conjugate base, known as the carboxylate ion (RCOO⁻). The carboxylate ion is resonance-stabilized, which means the negative charge is spread over two oxygen atoms, rather than being localized on one. This delocalization of charge makes the carboxylate ion quite stable. Thus, carboxylic acids easily release their hydrogen ion (H⁺), leading to significant acidity. This is what makes them stronger acids compared to other organic compounds such as alcohols.

Alcohols

Alcohols have an \(─OH\) group, also known as a hydroxyl group, attached to a carbon atom. This group is polar and capable of forming hydrogen bonds. While the \(─OH\) group can donate a hydrogen ion, the resulting conjugate base, known as the alkoxide ion (RO⁻), lacks the resonance stabilization seen in carboxylates. Without this stabilization, the negative charge on the alkoxide ion is concentrated on one oxygen atom, making it less stable than a carboxylate ion. As a result, alcohols are weaker acids than carboxylic acids. Generally, in the hierarchy of acidity, alcohols are less acidic than both carboxylic acids and water but stronger than terminal alkynes.

Water

Water, with its chemical formula \(H_2O\), is a unique and universal solvent that exhibits slight acidity. When water gives up a proton, it forms the hydroxide ion (OH⁻). This ion, while less stable than the carboxylate ion, is more stable than the alkoxide ions from alcohols. This slight superiority in stability gives water a marginally higher acidic strength than alcohols. Moreover, the ability of water to perform autoionization allows some dissociation into \(H^+\) and \(OH^-\) ions, contributing to its mild acidity. Therefore, in a comparative acidic strength scenario, water earns its position higher than alcohols but remains weaker than carboxylic acids.

Terminal Alkynes

Terminal alkynes are hydrocarbons with a carbon-carbon triple bond \(C≡C\) and an acidic hydrogen at one end of the molecule. This acidic hydrogen can be released to form an acetylide ion. However, the resulting ion is not resonance-stabilized, which makes it less stable and hence, less enthusiastic to part with the hydrogen ion compared to other molecules. Due to this instability of the acetylide ion, the acidic strength of terminal alkynes is lesser than water or alcohols. Thus, terminal alkynes often sit at the bottom of the list when we order substances by acid strength, emphasizing their relative weakness among the mentioned compounds.