Question

Starting with 1 -propanol, 2 -hydroxypropanoate can be prepared by treating with the reagents, in the order (a) \(\mathrm{Cl}_{2} / \mathrm{P}, \mathrm{KMnO}_{4}\) and aqueous \(\mathrm{KOH}\) (b) \(\mathrm{Cl}_{2} / \mathrm{P}, \mathrm{KMnO}_{4}\) and alcoholic \(\mathrm{KOH}\) (c) \(\mathrm{KMnO}_{4}, \mathrm{Cl}_{2} / \mathrm{P}\) and aqueous \(\mathrm{KOH}\) (d) \(\mathrm{KMnO}_{4}, \mathrm{Cl}_{2} / \mathrm{P}\) and alcoholic \(\mathrm{KOH}\)

Short answer

The correct order of reagents for this conversion is: first, \(\mathrm{KMnO}_{4}\); second, \(\mathrm{Cl}_{2} / \mathrm{P}\); and third, aqueous \(\mathrm{KOH}\). This sequence of reactions initially oxidizes 1-propanol to propanoic acid, followed by conversion to potassium propanoate, and finally, reaction with HCl to yield 2-hydroxypropanoate.

Step-by-step solution

Identify the structure of 1-propanol

1-propanol is an alcohol with the chemical formula \(\mathrm{C}_{3}\mathrm{H}_{8}\mathrm{O}\). It has the following structure: H H H | | | H--C-C-C--OH | | | H H H

Understand the properties and reactions of the given reagents

(a) \(\mathrm{Cl}_{2} / \mathrm{P}\): This reagent is involved in the conversion of alcohols to alkyl halides. (b) \(\mathrm{KMnO}_{4}\): Potassium permanganate is a strong oxidizing agent. It can be used to oxidize primary alcohols to carboxylic acids and secondary alcohols to ketones. (c) Aqueous \(\mathrm{KOH}\): Aqueous potassium hydroxide is a strong base and can be used for hydrolysis of esters to produce carboxylic acids and alcohols. (d) Alcoholic \(\mathrm{KOH}\): Alcoholic potassium hydroxide is also a strong base, but it is used for elimination reactions, such as converting alkyl halides to alkenes.

Evaluate each sequence of reagents for the desired conversion

(a) \(\mathrm{Cl}_{2} / \mathrm{P}, \mathrm{KMnO}_{4}\) and aqueous \(\mathrm{KOH}\): First, 1-propanol would be converted into propyl chloride by Cl2/P. Then, KMnO4 would oxidize propyl chloride into propanoic acid, but not 2-hydroxypropanoate. Aqueous KOH would have no effect in this case. (b) \(\mathrm{Cl}_{2} / \mathrm{P}, \mathrm{KMnO}_{4}\) and alcoholic \(\mathrm{KOH}\): As in (a), Cl2/P and KMnO4 would result in propanoic acid, not 2-hydroxypropanoate. Alcoholic KOH wouldn't have any effect either. (c) \(\mathrm{KMnO}_{4}, \mathrm{Cl}_{2} / \mathrm{P}\), and aqueous \(\mathrm{KOH}\): First, KMnO4 would oxidize 1-propanol into propanoic acid. Then, Cl2/P would have no effect on propanoic acid. Finally, aqueous KOH would convert propanoic acid to potassium propanoate. A further reaction with HCl would yield 2-hydroxypropanoate, so this is the correct sequence. (d) \(\mathrm{KMnO}_{4}, \mathrm{Cl}_{2} / \mathrm{P}\), and alcoholic \(\mathrm{KOH}\): As in (c), KMnO4 would oxidize 1-propanol into propanoic acid, but neither Cl2/P nor alcoholic KOH would produce the desired product. The correct sequence is (c) \(\mathrm{KMnO}_{4}, \mathrm{Cl}_{2} / \mathrm{P}\), and aqueous \(\mathrm{KOH}\).

Key concepts

Reagent Properties

Understanding the properties of reagents is fundamental in organic chemistry. Reagents are substances or compounds that are added to a system to bring about a chemical reaction or to test if a reaction occurs.

• **Chlorine and Phosphorus (\( \mathrm{Cl}_{2} / \mathrm{P} \)):** This combination is effective in transforming alcohols into alkyl halides. Alkyl halides are organic compounds that contain halogen atoms. This transformation is often used to prepare molecules for further reactions, such as nucleophilic substitution or elimination. In our specific case, this reagent is not necessary for producing the desired 2-hydroxypropanoate.
• **Potassium Permanganate (\( \mathrm{KMnO}_{4} \)):** This is a powerful oxidizing agent used in many chemical reactions. In an oxidation reaction, it can convert primary alcohols into carboxylic acids. It is important to remember its high reactivity, making it ideal for oxidizing a variety of functional groups.
• **Aqueous Potassium Hydroxide (\( \mathrm{KOH} \)):** Known for being a strong base, it is excellent for hydrolysis reactions, like transforming esters into acids and alcohols. This makes it versatile and useful for completing sequences of transformations.
• **Alcoholic Potassium Hydroxide (\( \mathrm{KOH} \)):** Differing from its aqueous counterpart, this reagent is better for elimination reactions, turning alkyl halides into alkenes. Despite its usefulness, it doesn't aid in producing 2-hydroxypropanoate here.

Alcohol Oxidation

Alcohol oxidation is a crucial reaction in organic chemistry, transforming alcohols into other functional groups. This process involves the increase of the oxidation state of the alcohol.

• **Mechanism and Process:** In the case of 1-propanol, when used with a potent oxidizing agent like potassium permanganate (\( \mathrm{KMnO}_{4} \)), the alcohol is oxidized to an aldehyde, and further oxidation leads to the formation of a carboxylic acid.
• **Practical Applications:** Understanding alcohol oxidation is vital in organic synthesis. It allows chemists to perform desired transformations by selecting appropriate conditions and reagents, such as in the successive transformation from 1-propanol to propanoic acid.
• **Selectivity and Control:** Controlling the oxidation process can prevent over-oxidation to undesired products. With \( \mathrm{KMnO}_{4} \), the process is robust and consistent, which is why it was chosen in this chemical sequence.

Chemical Reactions Sequence

Chemical reaction sequences form the backbone of synthesis in organic chemistry. These are sets of consecutive reactions that use various reagents sequentially to arrive at a target molecule from a starting material.

• **Understanding the Pathway:** Starting with a primary alcohol, 1-propanol, the use of \( \mathrm{KMnO}_{4} \) enables the initial transformation into propanoic acid. Subsequent use of aqueous \( \mathrm{KOH} \) converts it into potassium propanoate, with the sequence optionally ending with an acid to yield the desired 2-hydroxypropanoate.
• **Importance of Sequence Choice:** Choosing the appropriate sequence of reagents can critically influence the outcome of the synthesis. Incorrect reagent selection or sequence often leads to unwanted side products or failure to acquire the desired compound.
• **Common Pitfalls:** Mistakes often involve using reagents out of order, which can lead to stable intermediates forming that hinder further reactions. Here, the correct path is crucial to achieving the target molecule.
• **Optimization of Reaction Conditions:** Conditions such as temperature, solvent, and concentration can significantly affect each step of the sequence. For instance, replacing aqueous \( \mathrm{KOH} \) with an alcoholic one in this context would not suffice to reach the product.