Question

Give a line drawing and molecular formula for: (a) A linear alkane with four carbon atoms (b) An alkene with six carbon atoms in a ring and one double bond (c) A linear alkyne with six carbon atoms and two triple bonds

Short answer

(a) Butane: \( C_4H_{10} \). (b) Cyclohexene: \( C_6H_{10} \). (c) Hexadiyne: \( C_6H_2 \).

Step-by-step solution

Identify the Linear Alkane

A linear alkane with four carbon atoms is known as butane. Alkanes are saturated hydrocarbons with the general formula \( C_nH_{2n+2} \). For butane \( (n=4) \), the formula becomes \( C_4H_{2(4)+2} = C_4H_{10} \). In a line drawing, butane is represented as a straight chain of four carbon atoms, each connected by single bonds: \( \text{—} \text{—} \text{—} \text{—} \).

Identify the Cycloalkene

An alkene with six carbon atoms in a ring and one double bond is known as cyclohexene. Alkenes have at least one carbon-carbon double bond, and cycloalkenes are cyclic hydrocarbons. The molecular formula for cyclohexene is \( C_6H_{10} \). In line drawing, cyclohexene is represented as a hexagonal ring with one double bond between two of the carbon atoms.

Identify the Linear Alkyne

A linear alkyne with six carbon atoms and two triple bonds is called hexadiyne. Alkynes contain carbon-carbon triple bonds and their general formula is \( C_nH_{2n-2} \). For two triple bonds, the formula is adjusted for the saturation decline: \( C_6H_2 \). The linear structure involves only two hydrogen atoms and alternating triple bonds: \( \equiv \text{—} \equiv \).

Key concepts

Alkanes

Alkanes are the simplest form of hydrocarbons and are known as saturated hydrocarbons. This means they have single carbon-carbon bonds and get their "saturated" characteristic from having the maximum number of hydrogen atoms attached.
The general formula for alkanes is \( C_nH_{2n+2} \). This formula helps predict the number of hydrogen atoms based on the carbon atoms present.
For example, in the case of a linear alkane with four carbon atoms, known as butane, we see this structure: \( C_4H_{10} \). This formula indicates that there are four carbon atoms connected in a straight chain, each bonded to two or three hydrogens, depending on its position within the chain.
Alkanes are known for their ability to resist reactions, making them relatively inert under normal conditions. They often serve as the starting point in organic reactions, where more complex chemicals are derived.

• Saturated hydrocarbons with only single bonds.
• General formula: \( C_nH_{2n+2} \).
• Straight chain structure.

Alkenes

Alkenes are hydrocarbons that contain at least one carbon-carbon double bond, making them unsaturated hydrocarbons. The presence of a double bond gives them different chemical properties compared to alkanes, like increased reactivity.
When looking at an alkene with six carbon atoms in a ring and one double bond, such as cyclohexene, its molecular formula is \( C_6H_{10} \). This indicates six carbon atoms in a cyclic structure with one of the pairs forming a double bond.
The introduction of a double bond reduces the number of hydrogen atoms by 2 compared to its alkane counterpart. This change is significant as it introduces the potential for various chemical reactions, notably addition reactions where the double bond opens up to accommodate new atoms.

• Contain one or more carbon-carbon double bonds.
• General formula varies but differs from alkanes due to double bonds.
• Often found in cyclic compounds like cycloalkenes.

Alkynes

Alkynes are characterized by the presence of carbon-carbon triple bonds, making them the most reactive among the three types of hydrocarbons we're discussing. Like alkenes, alkynes are also classified as unsaturated, as their structure prevents them from holding the maximum number of hydrogen atoms.
The general formula for alkynes is \( C_nH_{2n-2} \). They share a linear or branched structure, with triple bonds altering their hydrogen count. When a linear alkyne with six carbon atoms and two triple bonds is intended, such as hexadiyne, it presents a unique case: \( C_6H_2 \).
This drastic reduction in hydrogen atoms is due to the constraints of forming multiple triple bonds. These structures exhibit unique reactivities, leading to substitution and addition reactions — essential for creating more complex organic molecules.

• Contain triple carbon-carbon bonds.
• General formula: \( C_nH_{2n-2} \).
• Known for their high reactivity and unique reactions.