Question Video: Determining the Structure of an Alcohol from the Product of Oxidation | Nagwa Question Video: Determining the Structure of an Alcohol from the Product of Oxidation | Nagwa

Question Video: Determining the Structure of an Alcohol from the Product of Oxidation Chemistry • Third Year of Secondary School

The following product results from the oxidation of an alcohol. Was the alcohol primary, secondary, or tertiary? Which of the following alcohols could the reactant be? [A] Alcohol A [B] Alcohol B [C] Alcohol C [D] Alcohol D

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Video Transcript

The following product results from the oxidation of an alcohol. Was the alcohol primary, secondary, or tertiary? Which of the following alcohols could the reactant be?

Let’s remove the second question for now to give us some space. Before we discuss the reaction mentioned in the question, let’s take a look at the difference between primary, secondary, and tertiary alcohols. An alcohol is a molecule that contains a hydroxy group. In a primary alcohol, the hydroxy group is bonded to a carbon atom that has one alkyl substituent. In a secondary alcohol, the hydroxy group is bonded to a carbon atom that has two alkyl substituents. And in a tertiary alcohol, the hydroxy group is bonded to a carbon atom that has three alkyl substituents.

We are told in the question that an alcohol undergoes oxidation to form the product shown. When an alcohol is mixed with an oxidizing agent, represented here by a capital O in brackets, an oxidation reaction can occur. Over the course of this reaction, the hydroxy group and the carbon atom bonded to the hydroxy group will each lose a hydrogen atom. To account for this loss of bonds, a new carbon-oxygen double bond will be formed. The products of this reaction are a molecule which contains a carbonyl group, a carbon atom double bonded to an oxygen atom, and water. The type of carbonyl-containing molecule formed depends on the structure of the original alcohol and the reaction conditions used.

Let’s take a look at the oxidation of a primary alcohol. The hydroxy group and the carbon atom bonded to the hydroxy group will each lose a hydrogen atom, and a new carbon-oxygen double bond will be formed. This produces a molecule in which an oxygen atom is double bonded to a carbon atom that has one alkyl substituent and one hydrogen substituent. This structure is an aldehyde. Now, let’s take a look at the oxidation of a secondary alcohol. The two hydrogen atoms are lost, and a new carbon- oxygen double bond is formed. This produces a product in which an oxygen atom is double bonded to a carbon atom that has two alkyl substituents. This structure is called a ketone.

Now, let’s look at the oxidation of tertiary alcohols. The hydroxy group has a hydrogen atom that could be lost during oxidation. However, the carbon atom bonded to the hydroxy group does not have a hydrogen substituent. Thus, tertiary alcohols do not undergo oxidation, so we know that tertiary cannot be the answer to this question. Let’s have a look at the product provided in the question. We see that this molecule has an oxygen atom double bonded to a carbon atom that has two alkyl substituents. This is the structure of a ketone. As primary alcohols oxidize to form aldehydes and secondary alcohols oxidize to form ketones, this product must be the result of the oxidation of a secondary alcohol.

Now, let’s return to the second question.

Which of the following alcohols could the reactant be?

We’ve already established that during the oxidation of a secondary alcohol, the hydroxy group and the carbon atom bonded to the hydroxy group will each lose a hydrogen atom, and a new oxygen-carbon double bond will be formed, producing a ketone. In this question, we know the structure of the ketone that is formed, and we want to know the structure of the original alcohol. To work backwards from the ketone to the alcohol, we can undo what occurred during oxidation. During oxidation, an oxygen-carbon double bond was formed. This means that in the original alcohol, these two atoms were single bonded together. During oxidation, the hydroxy group lost a hydrogen atom.

To undo this, we need to reattach a hydrogen atom to the oxygen atom. The carbon atom bonded to the hydroxy group also lost a hydrogen atom during oxidation. To undo this, we need to bond a hydrogen atom to the carbon atom bonded to the hydroxy group, so this is the original structure of the alcohol. Looking at the answer choices, we can see that the structure that we drew matches the structure of answer choice (D). Thus, the alcohol that could be the reactant is (D).

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