Which of the following reactions is most likely to result in a noticeable change in odor due to the formation of a new functional group?

• A. Oxidation of a primary alcohol ($R_1-\mathrm{CH}-\mathrm{OH}$) to a carboxylic acid ($R_1-\mathrm{COOH}$)
• B. Addition of bromine ($\mathrm{Br}{2}_{}$) to an alkene ($C=C$) to form a dibromoalkane ($C-C-\mathrm{Br}-\mathrm{Br}$)
• C. Conversion of an alkane ($C-C$) to a cycloalkane ($◯$)
• D. Hydrogenation of an alkene ($C=C$) to an alkane ($C-C$)

Answer: (A)

Verified step by step guidance

1. Identify the functional groups involved in each reaction and consider how their chemical nature affects odor. Functional groups like carboxylic acids often have strong, distinctive odors, while alkanes and cycloalkanes are generally odorless or have very mild odors.
2. Examine the oxidation of a primary alcohol (R\_1-CH2-OH) to a carboxylic acid (R\_1-COOH). This reaction introduces a highly polar and often strongly odorous functional group (carboxylic acid), which is likely to cause a noticeable change in smell.
3. Consider the addition of bromine (Br2) to an alkene (C=C) to form a dibromoalkane (C-C-Br-Br). This reaction changes the molecule but typically does not produce a functional group known for a strong odor, so the odor change may be minimal or absent.
4. Look at the conversion of an alkane (C-C) to a cycloalkane (a ring structure). Both are hydrocarbons with similar nonpolar characteristics and generally weak odors, so this change is unlikely to produce a noticeable odor difference.
5. Analyze the hydrogenation of an alkene (C=C) to an alkane (C-C). This reaction saturates the double bond but does not introduce a new functional group with a distinct odor, so the odor change is expected to be minimal.