Problem 80 - Chemistry and Chemical Reactivity

Question

Draw the structure and name the product formed if the following alcohols are oxidized. Assume an excess of the oxidizing agent is used. If the alcohol is not expected to react with a chemical oxidizing agent, write NR (no reaction). (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) (b) 2-butanol (c) 2 -methyl-2-propanol (d) 2 -methyl-1-propanol

Step-by-Step Solution

Answer

(a) Butanoic acid; (b) 2-butanone; (c) NR; (d) 2-methylpropanoic acid.

Step 1: Identify the type of alcohol (a)

The compound \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2}\mathrm{OH} \) is a primary alcohol. Primary alcohols are oxidized to form aldehydes and can further be oxidized to form carboxylic acids if excess oxidizing agent is present.

Step 2: Determine the structure and name after oxidation (a)

Upon oxidation, \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2}\mathrm{OH} \) first forms butanal, and with further oxidation, forms butanoic acid. The final product, assuming excess oxidizing agent, is butanoic acid (\( \mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}_2\mathrm{COOH} \)).

Step 3: Identify the type of alcohol (b)

2-butanol is a secondary alcohol. Secondary alcohols are oxidized to ketones.

Step 4: Determine the structure and name after oxidation (b)

2-butanol is oxidized to form 2-butanone. The structure is \( \mathrm{CH}_3\mathrm{COCH}_2\mathrm{CH}_3 \).

Step 5: Identify the type of alcohol (c)

2-methyl-2-propanol is a tertiary alcohol. Tertiary alcohols generally do not undergo oxidation with common chemical oxidizing agents, so NR (no reaction) occurs in this case.

Step 6: Identify the type of alcohol (d)

2-methyl-1-propanol is a primary alcohol. Upon oxidation, it will form an aldehyde and can further oxidize to a carboxylic acid.

Step 7: Determine the structure and name after oxidation (d)

2-methyl-1-propanol first forms 2-methylpropanal upon oxidation and can further be oxidized to 2-methylpropanoic acid with excess oxidizing agent. The final product is 2-methylpropanoic acid (\( \mathrm{(CH_3)_2CHCOOH} \)).

Key Concepts

Primary Alcohol

Primary alcohols are one of the simplest types of alcohols in organic chemistry. They consist of a carbon atom bonded to the hydroxyl group (-OH) and two hydrogen atoms. This configuration makes them highly reactive to oxidation reactions.
When you oxidize a primary alcohol, the first product formed is an aldehyde. If the reaction environment has an excess of oxidizing agents, the aldehyde can further oxidize to form a carboxylic acid.
For example, the primary alcohol

• butanol (\[ \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2}\mathrm{OH}\]) oxidizes to butanal initially.
• With more oxidizing agent, it becomes butanoic acid (\[\mathrm{CH}_3i \mathrm{CH}_2\\mathrm{CH}_2\\mathrm{COOH}\]).
Understanding this helps in predicting the outcome of oxidation reactions for primary alcohols.

Secondary Alcohol

Secondary alcohols are generally oxidized to ketones. Unlike primary alcohols, secondary alcohols have a carbon atom bonded to two other carbon atoms and one hydroxyl group. This makes them more resistant to further oxidation beyond the ketone stage.
Consider 2-butanol as an example. It is a secondary alcohol and can be oxidized to form 2-butanone.

• The chemical representation of 2-butanone is \[\mathrm{CH}_3\mathrm{COCH}_2\mathrm{CH}_3\].
  

This oxidation process is typical and does not result in the formation of carboxylic acids, making secondary alcohols unique in their oxidation behavior. This is due to the lack of a hydrogen atom on the alpha carbon, which is necessary for further oxidation.

Tertiary Alcohol

Tertiary alcohols have three carbon groups attached to the carbon that holds the hydroxyl group. They are the most resistant to oxidation among alcohol types.
For example, 2-methyl-2-propanol is a classic example of a tertiary alcohol. It does not undergo oxidation with common oxidizing agents like primary and secondary alcohols do. This is due to the lack of hydrogen atoms on the carbon with the hydroxyl group necessary for the oxidation reaction.
In chemical reactions or studies where oxidation is needed, tertiary alcohols are generally noted with "no reaction" (NR). Their stability against oxidation is an essential consideration in synthetic chemistry and various industrial processes.

Oxidizing Agents

Oxidizing agents play a crucial role in the oxidation of alcohols. They accept electrons from the substance being oxidized, facilitating various chemical transformations.
Common oxidizing agents include:

• PCC (pyridinium chlorochromate), which gently oxidizes primary alcohols to aldehydes without further oxidizing them to carboxylic acids.
• Potassium dichromate \(\left( \mathrm{K}_2\mathrm{Cr}_2\mathrm{O}_7 \right)\)and sulfuric acid,which are used for stronger oxidations to form carboxylic acids from primary alcohols or ketones from secondary alcohols.

When using an excess of strong oxidizing agents, reactions can proceed to full oxidation products like carboxylic acids. Choosing the right oxidizing agent is crucial for achieving the desired level of oxidation.