Question

Draw a structure to represent the principal product of each of the following reactions: (a) 1 -pentanol \(+\) dichromate ion (acid solution) (b) butyric acid + ethanol (acid solution) (c) 2 -methyl-1-butene \(+\mathrm{HBr}\)

Short answer

The principal products of the reactions are: (a) Pentanal, (b) Ethyl butanoate, (c) 2-bromo-2-methylbutane.

Step-by-step solution

Oxidation of 1-Pentanol

Addition of dichromate ion to 1-pentanol under acidic conditions results in the oxidation of the alcohol to a carbonyl compound. Depending on the conditions, this can either lead to an aldehyde or a carboxylic acid. In this case, the aldehyde or carbonyl compound formed is pentanal.

Esterification of Butyric acid and Ethanol

Butyric acid and ethanol react under acidic conditions to form an ester via an esterification reaction. The acid part comes from the butyric acid which contributes a 4 carbon chain, and the alcohol part comes from the ethanol which contributes a 2 carbon chain. The ester linkage is a carbonyl group adjacent to an ether linkage. The resultant ester is ethyl butanoate.

Addition of HBr to 2-Methyl-1-Butene

Hydrobromic acid (HBr) adds to the carbon-carbon double bond of 2-methyl-1-butene. In this reaction, the more stable carbocation intermediate is the one where the positive charge is on the more substituted carbon. Thus, the bromine prefers to attach at the tertiary carbon end (3rd carbon). The result is 2-bromo-2-methylbutane.

Key concepts

Oxidation of Alcohols

The oxidation of alcohols is a fundamental reaction in organic chemistry where alcohols are transformed into carbonyl compounds. This process typically involves the use of strong oxidizing agents like the dichromate ion (Cr₂O₇²⁻) in acidic solutions.

In the case of primary alcohols such as 1-pentanol, oxidation initially leads to the formation of an aldehyde. Here, 1-pentanol is oxidized to form pentanal, a five-carbon aldehyde. The transformation occurs through the removal of hydrogen atoms and the formation of a double bond between carbon and oxygen, resulting in the carbonyl group (C=O).

Further oxidation can continue under more vigorous conditions, potentially converting the aldehyde to a carboxylic acid, although this depends highly on the reaction environment. Primary alcohols generally offer more versatility in oxidation pathways compared to secondary and tertiary alcohols.

Key points to remember about the oxidation of alcohols include:

• Oxidation involves the increase in the number of bonds to oxygen.
• Dichromate ion in acid typically serves as a powerful oxidizing agent.
• Primary alcohols can be oxidized to either aldehydes or carboxylic acids.

Esterification

Esterification is a reaction that forms esters, involving the reaction between a carboxylic acid and an alcohol in the presence of an acid catalyst, usually sulfuric acid.

In the esterification process described here, butyric acid reacts with ethanol, resulting in the formation of the ester ethyl butanoate. This ester is a compound characterized by the presence of a carbonyl group adjacent to an ether linkage (RC(=O)OR').

During this reaction, the hydroxyl group (-OH) from the carboxylic acid combines with a hydrogen atom from the alcohol, forming a molecule of water and forming a new bond called an ester linkage.

This reaction is equilibrium, meaning it can go forwards to form the ester or backwards to form the acid and alcohol. To push the equilibrium towards ester formation, one can:

• Use excess alcohol to drive the reaction forward.
• Remove water as it is formed to shift the equilibrium.
• Utilize a strong acid catalyst to increase reaction speed and yield.

Hydrohalogenation

Hydrohalogenation is a reaction where hydrogen halides such as HBr add across the double bond of alkenes, resulting in the formation of an alkyl halide. This reaction proceeds via a carbocation intermediate and adheres to Markovnikov's rule.

In the specific case of 2-methyl-1-butene reacting with HBr, the double bond opens up, and the hydrogen atom adds to the less substituted carbon (forming the more stable carbocation) while the bromine atom attaches to the more substituted carbon. Consequently, this reaction produces 2-bromo-2-methylbutane.

Markovnikov's rule underlines the concept that, in hydrohalogenation, the hydrogen atom from the hydrogen halide will preferentially bind to the carbon with the greater number of hydrogen atoms, thus stabilizing the more substituted carbon cation. Here are some key things to remember:

• Hydrohalogenation proceeds via a carbocation intermediate.
• Markovnikov's rule predicts the regioselectivity of the addition.
• This reaction is commonly used to add halogens across double bonds in alkenes.