Question

Can their reaction with hydrogen distinguish between an alkene and a cycloalkane containing the same number of carbon atoms?

Short answer

Explain your answer. Yes, the reaction with hydrogen can distinguish between an alkene and a cycloalkane containing the same number of carbon atoms. This is because alkenes undergo hydrogenation, where the carbon-carbon double bond is reduced to a single bond and hydrogen atoms are added to the molecule. Conversely, cycloalkanes do not react with hydrogen under normal conditions, as they are saturated hydrocarbons without carbon-carbon double bonds to reduce.

Step-by-step solution

Understanding the structure of alkenes and cycloalkanes

Alkenes are hydrocarbons with at least one carbon-carbon double bond. Cycloalkanes are hydrocarbons with a ring structure, where all of the carbon atoms are connected by single bonds.

Reaction of Alkenes with Hydrogen

Alkenes react with hydrogen in a process called hydrogenation. In this reaction, the carbon-carbon double bond is reduced to a single bond, and a hydrogen atom is added to each of the two carbon atoms that were previously double-bonded. This results in the formation of an alkane. The general reaction can be represented as: R_2C=CR_2 + H_2 → R_2CH-CH_2R_2

Reaction of Cycloalkanes with Hydrogen

Cycloalkanes, on the other hand, do not react with hydrogen under normal conditions, as they have no carbon-carbon double bonds to reduce. They are considered saturated hydrocarbons since all of the carbon atoms in the ring are connected by single bonds.

Comparing the reactions

Comparing the reactions of alkenes and cycloalkanes with hydrogen, we can see that there is a distinct difference between the two. Alkenes undergo hydrogenation, whereas cycloalkanes do not react with hydrogen under normal conditions.

Conclusion

The reaction with hydrogen can, indeed, distinguish between an alkene and a cycloalkane containing the same number of carbon atoms. Alkenes undergo hydrogenation, converting the carbon-carbon double bond to a single bond and adding hydrogen atoms to the molecule. Cycloalkanes do not react with hydrogen in normal conditions, as they are saturated hydrocarbons with no carbon-carbon double bonds to reduce.

Key concepts

Alkenes

Alkenes are a fascinating group of hydrocarbons. They are characterized by the presence of at least one carbon-carbon double bond. This double bond is what makes alkenes unique and gives them their distinct chemical properties.
Because of the double bond, alkenes are considered unsaturated hydrocarbons. This means they can undergo addition reactions, where atoms or groups are added to the carbon atoms forming the double bond.

One of the most common reactions of alkenes is **hydrogenation**. In hydrogenation, the double bond is converted to a single bond. This happens when hydrogen molecules (H₂) add to the carbon atoms, resulting in the formation of an alkane.
Through hydrogenation, alkenes turn into more stable, saturated hydrocarbons. This process is widely used in the food industry, such as in the conversion of vegetable oils to margarine.

Cycloalkanes

Cycloalkanes are another type of hydrocarbon, notable for their ring structure. Each carbon in the ring is connected by single covalent bonds, making them quite stable.
Unlike alkenes, cycloalkanes do not contain double bonds. As a result, they do not participate in hydrogenation reactions. This is because they have no sites for adding hydrogen atoms under normal conditions.

This stability makes cycloalkanes an important class of saturated hydrocarbons. They serve as excellent examples of how the shape and structure of molecules affect their reactivity. Cycloalkanes usually only engage in reactions that involve breaking the carbon ring, which requires more energy and specific conditions.

Saturated Hydrocarbons

Saturated hydrocarbons are compounds where all carbon atoms form single bonds with hydrogen or other carbon atoms. This saturation means they are fully "saturated" with hydrogen. There are no double or triple bonds in these molecules.
The lack of reactive sites makes saturated hydrocarbons generally stable and less chemically reactive compared to their unsaturated counterparts. This includes both cycloalkanes and alkanes.

In terms of chemical behavior, this stability means that saturated hydrocarbons do not easily undergo addition reactions like hydrogenation. Instead, they are more likely to engage in substitution reactions or combustion. This characteristic explains their limited interaction with hydrogen, as seen in the behavior of cycloalkanes.