Question

Vapours of isomeric butyl alcohols are passed over copper at \(573 \mathrm{~K}\). The products obtained from normal, sec, iso and tertiary butyl alcohols respectively are (a) aldehyde, ketone, aldehyde and ketone, (b) aldehyde, ketone, ketone and aldehyde (c) aldehyde, ketone, aldehyde and alkene (d) ketone, aldehyde, aldehyde and alkene

Short answer

Answer: The products of the catalytic dehydration of the four isomeric butyl alcohols are an aldehyde, a ketone, another aldehyde, and an alkene.

Step-by-step solution

Identify the isomeric butyl alcohols

The four isomeric butyl alcohols are: 1. Normal butyl alcohol (n-butanol): \(\mathrm{CH_3CH_2CH_2CH_2OH}\) 2. Secondary butyl alcohol (sec-butanol): \(\mathrm{CH_3CH_2CH(OH)CH_3}\) 3. Isobutyl alcohol (iso-butanol): \(\mathrm{(CH_3)_2CHCH_2OH}\) 4. Tertiary butyl alcohol (t-butanol): \(\mathrm{(CH_3)_3COH}\)

Dehydration of alcohols over copper

When alcohols are passed over copper at \(573\mathrm{~K}\), they undergo a dehydration reaction, which involves the elimination of water molecule. Depending on the alcohol structure and its position of the hydroxyl group, the products formed can vary between aldehydes, ketones, and alkenes.

Determine the product for each isomeric alcohol

For each isomeric butyl alcohol, let's determine the product: 1. n-butanol: Dehydration of n-butanol over copper leads to the formation of butanal, which is an aldehyde. The reaction can be represented as: \(\mathrm{CH_3CH_2CH_2CH_2OH \rightarrow CH_3CH_2CH_2CHO + H_2O}\) 2. sec-butanol: Dehydration of sec-butanol over copper leads to the formation of butanone, which is a ketone. The reaction can be represented as: \(\mathrm{CH_3CH_2CH(OH)CH_3 \rightarrow CH_3CH_2COCH_3 + H_2O}\) 3. iso-butanol: Dehydration of iso-butanol over copper leads to the formation of isobutanal, which is an aldehyde. The reaction can be represented as: \(\mathrm{(CH_3)_2CHCH_2OH \rightarrow (CH_3)_2CHCHO + H_2O}\) 4. t-butanol: Dehydration of t-butanol over copper leads to the formation of isobutene, which is an alkene. The reaction can be represented as: \(\mathrm{(CH_3)_3COH \rightarrow (CH_3)_2C=CH_2 + H_2O}\)

Match the products with the options

Comparing the results from Step 3 with the provided options, we can conclude that the correct answer is: Option (c) aldehyde, ketone, aldehyde, and alkene

Key concepts

Isomeric Butyl Alcohols

Understanding the properties of isomeric butyl alcohols is crucial when predicting the reaction outcomes on dehydration. Isomers are molecules that share the same molecular formula but differ in the arrangement of atoms. With butyl alcohols, there are four key isomers each with unique structural configurations that impact how they react when undergoing dehydration.

Normal butyl alcohol (n-butanol) contains a straight chain of four carbon atoms with the hydroxyl group at an end. Secondary butyl alcohol (sec-butanol) has the hydroxyl group on the second carbon, making it secondary. Isobutyl alcohol (iso-butanol), the hydroxyl group is located on an end carbon of a branched chain. Finally, tertiary butyl alcohol (t-butanol) has its hydroxyl group attached to a carbon that is connected to three other carbon atoms, making it tertiary.

Reaction Over Copper

The reaction of alcohols over copper plays a pivotal role in the chemistry of dehydration. Copper acts as a catalyst at high temperatures, specifically at 573 K, to facilitate the elimination of a water molecule. This dehydration process is influenced by the structure of the alcohol and the placement of its hydroxyl group. The reactions need to be handled with care, employing precise temperature control to achieve the desired outcome. Various alcohols would produce different types of compounds—aldehydes, ketones, or alkenes—when passed over copper due to the unique manner in which the dehydration takes place.

It is essential to note that primary alcohols typically form aldehydes, secondary alcohols form ketones, while tertiary alcohols tend to form alkenes through a slightly different mechanism involving the rearrangement of molecules.

Formation of Aldehydes, Ketones, and Alkenes

The end products of alcohol dehydration—aldehydes, ketones, and alkenes—depend on the type of isomeric alcohol and its dehydration mechanism. For primary alcohols, such as n-butanol, the removal of a hydrogen and hydroxyl group from adjacent carbon atoms results in the formation of an aldehyde. In contrast, secondary alcohols, like sec-butanol, yield ketones due to the central location of their hydroxyl group, which after dehydration leaves a carbonyl group flanked by two carbon atoms.

Meanwhile, tertiary alcohols, like t-butanol, skip the aldehyde or ketone formation altogether, leading instead to alkenes as the result of a double bond forming between carbon atoms when dehydration occurs.

This specificity in the outcome based on the molecular structure is a key factor that students must understand to anticipate the products of alcohol dehydration reactions accurately.