Question

The reagent that may be used to separate cyclohexanone and 2,6 -dimethylcyclohexanone is (A)NaHSO ${}_3$ (B) ${\mathrm{NaHSO}}_4$ (C) ${\mathrm{NaHCO}}_3$ (D) ${\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{NHNH}}_2$

Short answer

The correct reagent for separating cyclohexanone and 2,6-dimethylcyclohexanone is (A) NaHSO${}_3$ (sodium bisulfite) as it can react selectively with ketone carbonyl groups to form bisulfite adducts, which can be separated by crystallization.

Step-by-step solution

Identify the functional groups of the given compounds

Both cyclohexanone and 2,6-dimethylcyclohexanone have only a ketone (C=O) functional group. Cyclohexanone is a simple cyclic ketone, while 2,6-dimethylcyclohexanone has two methyl groups on the ring as substituents.

Examine the reagents and their possible reactions

Now let's analyze each given reagent against the ketone functional group: (A) NaHSO${}_3$ - Sodium bisulfite can react with aldehyde or ketone carbonyl groups to form a bisulfite adduct. This reaction is reversible and can be used to separate ketones from non-ketones by crystallization. (B) NaHSO${}_4$ - Sodium bisulfate is a strong acid, but it does not have a specific reaction with ketone carbonyl groups that can be used for the separation of ketones. (C) NaHCO${}_3$ - Sodium bicarbonate acts as a weak base but does not react with ketones. It can only neutralize acidic functional groups, so it isn't suitable for the separation of ketones. (D) C${}_6$H${}_5$NHNH${}_2$ - Phenylhydrazine is a reagent that reacts with aldehydes and ketones to form phenylhydrazones. However, it isn't used for the separation of ketones based on their structure.

Choose the correct reagent for separating the two ketones

Among the given reagents, only sodium bisulfite (NaHSO${}_3$) can react selectively with ketone carbonyl groups and form bisulfite adducts, which can be separated by crystallization. Thus, the answer to the exercise is: (A) NaHSO${}_3$.

Key concepts

Ketone Functional Group

The ketone functional group is a pivotal aspect of organic chemistry. It is characterized by a carbon atom doubly bonded to an oxygen atom, known as a carbonyl group, denoted as C=O. This group is what primarily distinguishes ketones from other organic compounds. The carbonyl carbon in ketones is flanked by two other carbon atoms, making it a non-terminal functional group.

It’s crucial to note that ketones differ from aldehydes in that aldehydes have at least one hydrogen atom attached to the carbonyl carbon. In the context of cyclohexanone and 2,6-dimethylcyclohexanone, the ketone group is embedded in a cyclic structure.

The presence of this functional group in cyclohexanone makes it an ideal candidate for reactions that target carbonyl components. Changes to the ketone functional group can dramatically alter the chemical properties and reactivity of the compound, which is why understanding its nature is vital for organic reaction planning.

Sodium Bisulfite Reaction

Sodium bisulfite ($ext{NaHSO}_3$) is a versatile reagent in organic chemistry, particularly known for its ability to react with aldehydes and certain types of ketones. This reaction involves the formation of a bisulfite adduct. The ketone or aldehyde carbonyl group (C=O) interacts with sodium bisulfite, leading to the addition of hydrogen sulfite (HSO₃−) to the carbonyl carbon.

The reaction is useful for separating ketones due to the reversible nature of the bisulfite addition. Upon formation, these adducts can often crystallize out of solution, providing a means to physically separate components in a mixture.

The process is particularly effective when there is a difference in the reactivity or steric hindrance between different ketones. Sodium bisulfite is favored in many applications because it allows for an extraction process that can be easily reversed by simply washing with acid, which decomposes the bisulfite adduct back into the free ketone and sodium salt.

Organic Chemistry Reagents

Organic chemistry reagents are substances or compounds used to bring about chemical reactions in organic compounds. These can include acids, bases, oxidizing agents, reducing agents, and more specialized reagents like sodium bisulfite.

Each reagent is chosen for its ability to target specific functional groups or reaction pathways. For example, phenylhydrazine is known for forming solid derivatives with ketones but is not necessarily used for separating ketones, contrary to sodium bisulfite.

In selecting reagents for a particular reaction, chemists consider factors such as reactivity, selectivity, and the nature of the solvent and substrates involved. Understanding these reagents' properties and behaviors helps in designing efficient synthetic routes and achieving desired reaction outcomes with precision.