Question

What products would you obtain from reaction of pentan-1-ol with the following reagents? (a) \(\mathrm{PBr}_{3}\) (b) \(\mathrm{SOCl}_{2}\) (c) \(\mathrm{CrO}_{3}, \mathrm{H}_{2} \mathrm{O}, \mathrm{H}_{2} \mathrm{SO}_{4}\) (d) Dess-Martin reagent

Short answer

(a) 1-bromopentane; (b) 1-chloropentane; (c) pentanoic acid; (d) pentanal.

Step-by-step solution

Reaction with PBr3

When pentan-1-ol reacts with \( \mathrm{PBr}_3 \), the hydroxyl group (-OH) is replaced by a bromine atom. This reaction converts the alcohol (ROH) into an alkyl bromide (RBr). For pentan-1-ol, this means the product will be 1-bromopentane.

Reaction with SOCl2

Pentan-1-ol reacting with \( \mathrm{SOCl}_2 \) results in the substitution of the -OH group with a chlorine atom. This reaction forms an alkyl chloride from the alcohol. Thus, the product from this reaction is 1-chloropentane.

Reaction with CrO3, H2O, and H2SO4

\( \mathrm{CrO}_3 \) in the presence of \( \mathrm{H}_2\mathrm{O} \) and \( \mathrm{H}_2\mathrm{SO}_4 \) oxidizes primary alcohols such as pentan-1-ol to carboxylic acids. In this reaction, pentan-1-ol is oxidized to pentanoic acid.

Reaction with Dess-Martin Reagent

The Dess-Martin reagent is used to oxidize primary alcohols to aldehydes. In this case, pentan-1-ol is oxidized to pentanal.

Key concepts

Alcohol to Alkyl Halide Conversion

In organic chemistry, converting an alcohol into an alkyl halide involves substituting the hydroxyl group (-OH) with a halogen atom. This transformation is pivotal when preparing alkyl halides, compounds serving as intermediates in further organic synthesis.
One common method for achieving this conversion involves using phosphorus tribromide (\(\mathrm{PBr}_3\)) or thionyl chloride (\(\mathrm{SOCl}_2\)).

• When pentan-1-ol reacts with \(\mathrm{PBr}_3\), the -OH group is replaced by a bromine atom, resulting in 1-bromopentane. This reaction is typically performed under acidic conditions to enhance the reactivity.
• Similarly, the reaction of pentan-1-ol with \(\mathrm{SOCl}_2\) substitutes the -OH group with chlorine, forming 1-chloropentane. This conversion is often carried out with a base such as pyridine to neutralize the generated hydrogen chloride.

Understanding such conversions aids in the functional group interconversion, facilitating the synthesis of complex molecules in organic chemistry.

Oxidation of Primary Alcohols

Oxidation reactions are integral to organic synthesis, especially in transforming primary alcohols into more oxidized functional groups like aldehydes and carboxylic acids. The choice of reagent determines the oxidation product achieved.

• Using \(\mathrm{CrO}_3\) with water \(\mathrm{H}_2\mathrm{O}\) and sulfuric acid \(\mathrm{H}_2\mathrm{SO}_4\), primary alcohols are fully oxidized to carboxylic acids. For instance, pentan-1-ol is oxidized to pentanoic acid. This process involves a change from an alcohol group to a carboxylic acid through an intermediate aldehyde stage.
• The Dess-Martin reagent provides a milder alternative, oxidizing primary alcohols to aldehydes without progressing to carboxylic acids. Hence, pentan-1-ol becomes pentanal under these conditions. This method is favored for its selectivity and mild reaction conditions, which preserve sensitive molecular structures.

Both methods are vital tools in synthetic chemistry, allowing precise control over chemical transformations.

Organic Synthesis Reagents

Reagents play vital roles in controlling the direction and extent of a chemical reaction within organic synthesis. The choice of reagent is crucial as it determines both the product and the efficiency of the reaction.

• Phosphorus tribromide (\(\mathrm{PBr}_3\)) is an effective reagent for converting alcohols to alkyl bromides, as seen in the transformation of pentan-1-ol to 1-bromopentane. It acts by substituting the -OH group with bromine.
• Thionyl chloride (\(\mathrm{SOCl}_2\)) allows for a similar transformation, converting alcohols to alkyl chlorides. This is demonstrated in the synthesis of 1-chloropentane from pentan-1-ol, with the aid of a base for neutralization.
• Cro3 in aqueous and acidic environments is a potent oxidizing agent that can push primary alcohols towards carboxylic acids.
• The Dess-Martin Periodinane offers a refined approach to achieve selective oxidations, turning primary alcohols into aldehydes efficiently, as exemplified in converting pentan-1-ol to pentanal.

These reagents highlight the diverse approaches available in organic synthesis for effective manipulation of molecule structures.