Question

\(\mathrm{Br}-\left(\mathrm{CH}_{2}\right)_{4}-\mathrm{COOH} \frac{\text { (1) } \mathrm{OH}^{-}}{\text {(2) } \mathrm{H}^{+}} ?\)

Short answer

The final product is 5-hydroxypentanoic acid.

Step-by-step solution

Identify the Reaction

Examine the given compound, which is 5-bromopentanoic acid: \( \mathrm{Br}-(\mathrm{CH}_2)_4-\mathrm{COOH} \). The problem specifies a two-step reaction: First with \( \mathrm{OH}^{-} \), followed by \( \mathrm{H}^+ \). These reagents indicate a substitution reaction followed by neutralization.

Substitution with Hydroxide Ion

In the first step, the \( \mathrm{OH}^{-} \) ion will attack the electrophilic carbon (C1) attached to the bromine atom. This results in a nucleophilic substitution (\( \mathrm{S_N2} \) reaction), effectively replacing the bromine atom with a hydroxyl group. The intermediate product is \( \mathrm{HO}-(\mathrm{CH}_2)_4-\mathrm{COOH} \).

Protonation

In the second step, the protonation involves the addition of \( \mathrm{H}^+ \) to neutral parts of the alcohol formed in the previous step; however, since all are normally protonated at this stage because they are part of a carboxylic acid, no significant change occurs. Thus, maintaining the stability of intermediate, keeping it as is: \( \mathrm{HO}-(\mathrm{CH}_2)_4-\mathrm{COOH} \), which is an alcohol, namely, pentanoic acid containing two hydroxyl groups, or simply a form of diol.

Key concepts

Nucleophilic Substitution

In organic chemistry, nucleophilic substitution is a fundamental type of reaction. This process involves the replacement of one group or atom in a molecule with a nucleophile. A nucleophile is a chemical species that donates an electron pair to form a chemical bond. This process is common in synthetic chemistry for transforming organic compounds. When we look at the reaction using 5-bromopentanoic acid, we observe a nucleophilic substitution reaction taking place. The bromine atom attached to the carbon chain acts as the leaving group. It is replaced by the hydroxide ion, which serves as the nucleophile in this scenario.

• The nucleophile: Hydroxide ion (\( \mathrm{OH}^{-} \))
• The leaving group: Bromine atom (\( \mathrm{Br} \))

This type of nucleophilic substitution is typically referred to as \( \mathrm{S_N2} \) reaction. In \( \mathrm{S_N2} \), or bimolecular nucleophilic substitution, the reaction involves a single, concerted step where the nucleophile attacks the electrophilic carbon and the leaving group exits simultaneously. This leads to the formation of a new compound with a direct inversion of configuration, though note, for cyclic molecules, this may affect the stereochemistry.

Carboxylic Acids

Carboxylic acids are organic compounds that feature a carboxyl group (\( \mathrm{-COOH} \)). This functionality is responsible for the acid properties in these molecules. In terms of structure, the carboxyl group comprises a carbonyl (\( \mathrm{C=O} \)) and a hydroxyl group (\( \mathrm{-OH} \)) attached to the same carbon. This arrangement is what gives carboxylic acids their unique chemical behavior.Carboxylic acids are prevalent not just in laboratory settings but also in many natural and industrial contexts. Their acidity emerges from the ability of the \( \mathrm{-COOH} \) group to release a proton \((\mathrm{H}^+)\), forming a carboxylate anion \( \mathrm{-COO}^- \).With 5-bromopentanoic acid, the carboxylic acid group remains intact throughout both steps of the reaction. During initial nucleophilic substitution, the carboxylic group remains unchanged. Subsequently, in the protonation step, although it appears the \( \mathrm{H}^+ \) is added, it does not significantly alter the carboxylate structure due to its existing protonated state.

Hydroxyl Group

The hydroxyl group (\( \mathrm{-OH} \)) is an important functional group in organic chemistry. It represents alcohol functionalities when attached to carbon atoms. This group forms a fundamental part of many organic molecules, contributing notably to their physical and chemical properties. Hydroxyl groups are polar, meaning they have positive and negative ends. This polarity arises from the difference in electronegativity between oxygen and hydrogen. Consequently, molecules with hydroxyl groups generally have higher boiling points due to hydrogen bonding capabilities.In 5-bromopentanoic acid undergoing nucleophilic substitution, the hydroxyl group is introduced when the \( \mathrm{OH}^{-} \) ion replaces the bromine atom. This change results in the formation of 5-hydroxypentanoic acid, which carries this new \( \mathrm{-OH} \) in place of the original bromine. Ultimately, no new \( \mathrm{-OH} \) groups are added in subsequent reaction steps, but the appearance of this group signifies an important transformation in the substrate's structure. The presence of multiple hydroxyl groups in a molecule can further define its reactivity and solubility, essential considerations in synthetic and analytical chemistry.