Problem 117 - Chemistry The Molecular Nature of Matter

Question

When ethanol is heated in the presence of an acid catalyst, ethene and water form; an elimination reaction occurs. When butan-2-ol is heated under similar conditions, two alkenes form (although not in equal quantities). Write the structures and IUPAC names of these two alkenes.

Step-by-Step Solution

Answer

The two alkenes that form are but-2-ene and 1-butene.

Step 1: Understand the Type of Reaction

Recognize that heating butan-2-ol with an acid catalyst is a dehydration reaction, an elimination reaction where water is removed to form an alkene.

Step 2: Draw the Structure of Butan-2-ol

Draw the structure of butan-2-ol, a four-carbon alcohol with the OH group on the second carbon.

Step 3: Determine the Possible Alkenes

Identify the possible alkenes through elimination. Removing water from butan-2-ol can create two different alkenes depending on which adjacent hydrogen is removed.

Step 4: Draw the First Alkene

Remove water from the second carbon and the adjacent hydrogen from the first carbon to form the more substituted alkene, but-2-ene.

Step 5: Name the First Alkene

Name the first alkene by identifying the longest carbon chain containing the double bond and numbering the chain so the double bond starts from the end nearest to it: but-2-ene.

Step 6: Draw the Second Alkene

Remove water from the second carbon and the adjacent hydrogen from the third carbon to form the less substituted alkene, 1-butene.

Step 7: Name the Second Alkene

The second alkene would be named by finding the longest carbon chain containing the double bond, which would be but-1-ene or simply 1-butene.

Key Concepts

Dehydration Reaction

In organic chemistry, a dehydration reaction is a type of elimination reaction where water is removed from a molecule. This process typically requires an alcohol and an acid catalyst to proceed. For example, when butan-2-ol is heated with an acid catalyst, the reaction involves the removal of an -OH group from the alcohol and a hydrogen atom from a neighboring carbon atom.

In butan-2-ol's case, the -OH group is on the second carbon, so the removal could occur from either the first or third carbon to create different alkenes. This reaction is not only essential for organic synthesis but also plays a crucial role in the biochemistry of living organisms, such as the metabolic pathways that generate energy.

Alkene Formation

Alkene formation is a common result of dehydration reactions in organic molecules. Alkenes are hydrocarbons that contain a carbon-carbon double bond, and their general formula is \( C_{n}H_{2n} \). The position and number of double bonds in alkenes can significantly affect their chemical properties and reactivity.

The process of forming alkenes can involve the removal of a water molecule (a -OH group and a hydrogen atom), typically from an alcohol. When butan-2-ol undergoes dehydration, one possible product is but-2-ene, which has a double bond between the second and third carbons. Alternatively, the reaction could result in 1-butene, with a double bond between the first two carbons. These variations are a result of which adjacent hydrogen is removed during the reaction.

IUPAC Nomenclature

The IUPAC nomenclature system is a standardized method of naming chemical compounds, ensuring that each compound has a unique and universally understood name. In naming alkenes, the longest continuous chain of carbons containing the double bond is identified. The chain is numbered in a way that the carbons in the double bond receive the lowest possible numbers.

For instance, but-2-ene gets its name because the longest chain has four carbons (hence 'but'), and the double bond starts on the second carbon. Alternatively, if the double bond starts at the first carbon, the compound is named 1-butene. This naming system also considers substituents on the carbon chain, but these are not present in the simple alkenes formed from butan-2-ol's dehydration. Accurate IUPAC naming is crucial for clear communication in the scientific community.