Question

For which value of \(n\) compound, \(\mathrm{HO}_{2} \mathrm{C}-\left(\mathrm{CH}_{2}\right)_{\mathrm{n}}-\mathrm{CO}_{2} \mathrm{H}\) undergo only dehydration on heating? (A) \(n=1\) (B) \(n=2\) (C) \(n=3\) (D) \(n=4\)

Short answer

The smallest value of 'n' for which the given compound undergoes only dehydration on heating without any intramolecular cyclization is \(n=3\). Therefore, the correct answer is (C) \(n=3\).

Step-by-step solution

Write the general formula for the given compound

Write the general formula for the compound as \(\mathrm{HO}_{2}\mathrm{C}-\left(\mathrm{CH}_{2}\right)_{\mathrm{n}}-\mathrm{CO}_{2}\mathrm{H}\), where 'n' is the number of CH2 units.

Identify the functional groups and dehydration sites

The given compound has a carboxylic acid group (COOH) at both ends. Dehydration can occur when the hydrogen atom of the -OH group from one COOH group and the oxygen atom from the -COOH group of the other COOH group are removed.

Understand the mechanism of dehydration

When the compound undergoes dehydration, a water molecule will be removed, and a new bond will form between the carbon atoms of the two carboxylic groups. This results in the formation of an anhydride with a general formula as follows: \(\mathrm{O=C(O)}-\left(\mathrm{CH}_{2}\right)_{\mathrm{n}+1}-\mathrm{C}(O)=\mathrm{O}\). For the given compound to undergo only dehydration (i.e., no other reaction occurs), there should be no possibility of intramolecular cyclization.

Determine the value of 'n' for which dehydration occurs without intramolecular cyclization

If the value of 'n' is too small, cyclization could occur when the new bond is formed between the carbon atoms of the two carboxylic groups. However, if 'n' is large enough, the chain will be too long to allow for cyclization. (A) If \(n=1\), the compound would be too small, and intramolecular cyclization could occur. (B) If \(n=2\), cyclization could still occur due to the proximity of the two carboxylic groups. (C) If \(n=3\), the chain would be long enough to prevent intramolecular cyclization during dehydration. (D) If \(n=4\), the chain is even longer, and thus dehydration could still occur without cyclization.

Choose the correct answer

Since we are looking for the smallest value of 'n' for which the given compound undergoes only dehydration on heating, the correct answer is (C) \(n=3\).

Key concepts

Dehydration Reaction

In the realm of organic chemistry, a dehydration reaction involves the removal of water (H₂O) from a molecule. This process typically occurs in compounds containing hydroxyl groups (–OH), like carboxylic acids. When a dehydration reaction takes place, the hydroxyl group loses a hydrogen ion (H⁺), while another part of the molecule provides the oxygen atom. The escaping water molecule is a sign of the formation of new bonds within the compound.

A classic example can be seen when carboxylic acids undergo dehydration. The reaction usually leads to the creation of an anhydride by forming a bond between carbon atoms of two carboxylic acid groups. Understanding the mechanism of dehydration is crucial for recognizing how molecular structures can transform during chemical reactions.

Carboxylic Acid

Carboxylic acids have the functional group –COOH and are prevalent in organic chemistry. The structure comprises a carbonyl group (C=O) attached to a hydroxyl group (–OH). This combination gives carboxylic acids their acidic properties. The presence of both a donor (hydroxyl) and an acceptor (carbonyl) allows these compounds to engage in diverse reactions, including dehydration.

During heating, carboxylic acids can lose a water molecule, especially when two such groups are present. This results in the formation of anhydrides. Attribute their reactivity to the dual functionality within the molecule, allowing both intra- and intermolecular transformations.

• Highly polar due to the –COOH group
• Can form hydrogen bonds, affecting their physical properties
• Important in biological and synthetic pathways

Intramolecular Reactions

Intramolecular reactions occur within a single molecule, involving the reorganization of atoms or groups in such a way that new structures are formed. These reactions are driven by the proximity of reactive groups, which can lead to cyclization or the formation of rings. Intramolecular reactions differ from intermolecular reactions where two separate molecules react.

The possibility of a carboxylic acid undergoing an intramolecular reaction depends on the number of –CH₂– units within the molecule. A smaller number of these units increases the chances of cyclization, as the functional groups are close enough to interact directly. However, as demonstrated in the solution, by increasing the 'n' value (number of –CH₂– groups), the likelihood of cyclization decreases, allowing for other reactions like dehydration to occur smoothly without ring formation.