Problem 7 - An Introduction to Physical Science

Question

Give the general molecular formulas for alkanes, cycloalkanes, alkenes, and alkynes. Write the structural formula for one example of each of these classes.

Step-by-Step Solution

Answer

Alkanes: \( C_nH_{2n+2} \); Cycloalkanes: \( C_nH_{2n} \); Alkenes: \( C_nH_{2n} \); Alkynes: \( C_nH_{2n-2} \). Examples include methane, cyclohexane, ethene, and acetylene.

Step 1: Identifying the General Formula for Alkanes

Alkanes are hydrocarbons with single bonds between carbon atoms. They follow the general formula \( C_nH_{2n+2} \). An example is methane, \( CH_4 \), where \( n = 1 \). Its structural formula is a single carbon atom bonded to four hydrogen atoms.

Step 2: Determining the Formula for Cycloalkanes

Cycloalkanes are saturated hydrocarbons with carbon atoms arranged in a ring and have single bonds. Their general formula is \( C_nH_{2n} \). A simple example is cyclohexane, \( C_6H_{12} \), with a hexagonal ring of six carbon atoms each bonded to two hydrogens.

Step 3: Finding the General Formula for Alkenes

Alkenes are hydrocarbons that contain at least one carbon-carbon double bond. They have the general formula \( C_nH_{2n} \). Ethene, \( C_2H_4 \), is an alkene with a double bond between the two carbon atoms. Its structural formula is \( H_2C=CH_2 \).

Step 4: Identifying the Formula for Alkynes

Alkynes are hydrocarbons with at least one carbon-carbon triple bond. Their general formula is \( C_nH_{2n-2} \). An example is ethyne, also known as acetylene, \( C_2H_2 \), which features a carbon-carbon triple bond, and its structural formula is \( HC\equiv CH \).

Key Concepts

Alkanes

Alkanes are the simplest type of hydrocarbons, composed entirely of carbon and hydrogen atoms. These molecules have only single covalent bonds between carbon atoms, making them saturated hydrocarbons. The general formula for alkanes is \( C_nH_{2n+2} \), where \( n \) is the number of carbon atoms.

Alkanes can vary from the simplest, methane \( (CH_4) \) with one carbon atom, to larger chains that include pentane \( (C_5H_{12}) \).

• These compounds are typically less reactive due to the stable nature of single bonds.
• They are commonly found in natural gas and petroleum.
• Alkanes are used as fuels, such as butane in lighters.

Alkenes

Alkenes are hydrocarbons characterized by the presence of at least one carbon-carbon double bond. This feature makes them unsaturated, allowing them to undergo addition reactions more easily than alkanes. The general formula for alkenes is \( C_nH_{2n} \).

A well-known example of an alkene is ethene, \( C_2H_4 \), more commonly referred to as ethylene.

• Alkenes are used extensively in the production of plastics, such as polyethene.
• The double bond is key in reactions like polymerization and hydrogenation.
• They contribute to fruit ripening processes and have importance in plant hormones.

Alkynes

Alkynes are a fascinating group of hydrocarbons that contain at least one carbon-carbon triple bond, making them highly reactive. The presence of this triple bond renders them unsaturated hydrocarbons. The general formula for alkynes is \( C_nH_{2n-2} \).

Acetylene \( (C_2H_2) \) is a common alkyne, known for its use in cutting and welding torches because of its high flame temperature.

• Alkynes can participate in addition reactions, similar to alkenes.
• They are significant in organic synthesis and are used to produce various chemicals.
• These compounds have linear structures due to the triple bond.

Cycloalkanes

Cycloalkanes are unique among hydrocarbons due to their ring-like structure, composed of carbon atoms connected in a circular fashion. These compounds have all single bonds, making them saturated like alkanes but with a ring instead of a linear arrangement. The general formula is \( C_nH_{2n} \).

Cyclohexane \( (C_6H_{12}) \) is a common example of a cycloalkane, renowned for its application as a solvent in the chemical industry.

• Cycloalkanes have a variety of uses, from solvents to intermediates in chemical synthesis.
• Their rings can vary in size, from small rings with three carbon atoms to larger ones.
• They are used in the manufacturing of rubber and other complex chemical architectures.