Problem 2 - Chemistry: Principles and Reactions

Question

Classify each of the following hydrocarbons as alkanes, alkenes, or alkynes. (a) \(\mathrm{C}_{12} \mathrm{H}_{24}\) (b) \(\mathrm{C}_{7} \mathrm{H}_{12}\) (c) \(\mathrm{C}_{13} \mathrm{H}_{28}\)

Step-by-Step Solution

Answer

Question: Classify the following hydrocarbons as alkanes, alkenes, or alkynes: (a) \(\mathrm{C}_{12} \mathrm{H}_{24}\), (b) \(\mathrm{C}_{7} \mathrm{H}_{12}\), and (c) \(\mathrm{C}_{13} \mathrm{H}_{28}\). Answer: (a) \(\mathrm{C}_{12} \mathrm{H}_{24}\) is an alkene, (b) \(\mathrm{C}_{7} \mathrm{H}_{12}\) is an alkyne, and (c) \(\mathrm{C}_{13} \mathrm{H}_{28}\) is an alkane.

Step 1: Classifying Hydrocarbon (a) \(\mathrm{C}_{12} \mathrm{H}_{24}\)

First, we'll compare the given hydrocarbon to the general formulas: 1. Alkane: \(\mathrm{C}_{12}\mathrm{H}_{2(12)+2}=\mathrm{C}_{12}\mathrm{H}_{26}\) 2. Alkene: \(\mathrm{C}_{12}\mathrm{H}_{2(12)}=\mathrm{C}_{12}\mathrm{H}_{24}\) 3. Alkyne: \(\mathrm{C}_{12}\mathrm{H}_{2(12)-2}=\mathrm{C}_{12}\mathrm{H}_{22}\) Since the given hydrocarbon matches the alkene formula, we can classify \(\mathrm{C}_{12} \mathrm{H}_{24}\) as an alkene.

Step 2: Classifying Hydrocarbon (b) \(\mathrm{C}_{7} \mathrm{H}_{12}\)

Next, we'll compare this hydrocarbon to the general formulas: 1. Alkane: \(\mathrm{C}_{7}\mathrm{H}_{2(7)+2}=\mathrm{C}_{7}\mathrm{H}_{16}\) 2. Alkene: \(\mathrm{C}_{7}\mathrm{H}_{2(7)}=\mathrm{C}_{7}\mathrm{H}_{14}\) 3. Alkyne: \(\mathrm{C}_{7}\mathrm{H}_{2(7)-2}=\mathrm{C}_{7}\mathrm{H}_{12}\) Since the given hydrocarbon matches the alkyne formula, we can classify \(\mathrm{C}_{7} \mathrm{H}_{12}\) as an alkyne.

Step 3: Classifying Hydrocarbon (c) \(\mathrm{C}_{13} \mathrm{H}_{28}\)

Finally, we'll compare this hydrocarbon to the general formulas: 1. Alkane: \(\mathrm{C}_{13}\mathrm{H}_{2(13)+2}=\mathrm{C}_{13}\mathrm{H}_{28}\) 2. Alkene: \(\mathrm{C}_{13}\mathrm{H}_{2(13)}=\mathrm{C}_{13}\mathrm{H}_{26}\) 3. Alkyne: \(\mathrm{C}_{13}\mathrm{H}_{2(13)-2}=\mathrm{C}_{13}\mathrm{H}_{24}\) Since the given hydrocarbon matches the alkane formula, we can classify \(\mathrm{C}_{13} \mathrm{H}_{28}\) as an alkane.

Key Concepts

Alkanes

Alkanes are the simplest type of hydrocarbons. They only contain single bonds between the carbon atoms and are sometimes referred to as saturated hydrocarbons. This means that each carbon atom is bonded to as many hydrogen atoms as possible. The general formula is \[\text{Alkane: } \mathrm{C}_{n}\mathrm{H}_{2n+2}.\] This formula helps to determine if a given hydrocarbon is an alkane by comparing the number of hydrogen atoms. For example, in the exercise, hydrocarbon (c) \(\mathrm{C}_{13} \mathrm{H}_{28}\) fits this formula, making it an alkane. One characteristic of alkanes is that they are relatively unreactive due to the strength and stability of the carbon-hydrogen bonds and carbon-carbon single bonds.
However, they can undergo combustion reactions, where they react with oxygen to produce carbon dioxide and water. Alkanes are often used as fuels because of this easy combustibility.
They are usually found in natural gas and petroleum products.

Alkenes

Alkenes are a type of hydrocarbons that include at least one carbon-carbon double bond, making them unsaturated. This double bond changes their chemical properties and reactivity compared to alkanes. The general formula for alkenes is \\[\text{Alkene: } \mathrm{C}_{n}\mathrm{H}_{2n}.\] In the exercise above, hydrocarbon (a) \(\mathrm{C}_{12} \mathrm{H}_{24}\) matches this formula, classifying it as an alkene. The presence of the double bond allows alkenes to participate in a variety of chemical reactions termed as addition reactions.
One common type of reaction is the addition of hydrogen in a process called hydrogenation, which converts alkenes into alkanes. Alkenes are important in the industry as they serve as starting materials for a wide range of chemicals and polymers, such as polyethylene.
They are also more reactive than alkanes due to the electron-rich double bond, which can easily be broken and involved in further chemical transformations.

Alkynes

Alkynes are hydrocarbons that incorporate at least one carbon-carbon triple bond. This triple bond creates a highly unsaturated and more reactive structure compared to alkanes and alkenes. The general formula for alkynes is \[\text{Alkyne: } \mathrm{C}_{n}\mathrm{H}_{2n-2}.\] In the given exercise, hydrocarbon (b) \(\mathrm{C}_{7} \mathrm{H}_{12}\) fits this alkyne formula. The triple bonds give alkynes different properties, including higher boiling points and reactivity.
Alkynes can undergo various chemical reactions, including addition reactions similar to alkenes, where molecules like chlorine or bromine add across the triple bond. Because of the triple bonds, alkynes are particularly useful in organic synthesis and serve as building blocks for more complex structures.
An example of a well-known alkyne is acetylene, which is used in welding due to its high heat production when combusted.