Question

Draw and name all the isomers of (a) \(\mathrm{C}_{6} \mathrm{H}_{14}\) (b) \(\mathrm{C}_{4} \mathrm{H}_{8} ;\) (c) \(\mathrm{C}_{4} \mathrm{H}_{6}\). [Hint: Do not forget double bonds, rings, and combinations of these.].

Short answer

Isomers of \(C_{6}H_{14}\) are: Hexane, 2-Methylpentane, 3-Methylpentane, 2-2Dimethylbutane, and 2-3Dimethylbutane. Isomers of \(C_{4}H_{8}\) are: But-1-ene, But-2-ene, 2-Methylprop-1-ene, and Cyclobutane. Isomers of \(C_{4}H_{6}\) are: But-1-yne, But-2-yne, 2-Butene, and Cyclobutene.

Step-by-step solution

Identifying Isomers of \(C_{6}H_{14}\)

Isomers for this hydrocarbon will all be chain and position isomers because \(C_{6}H_{14}\) has the general formula of an alkane (CnH2n+2), and alkanes contain only single bonds. The possible isomers are: Hexane, 2-Methylpentane, 3-Methylpentane, 2-2Dimethylbutane, and 2-3Dimethylbutane.

Identifying Isomers of \(C_{4}H_{8}\)

This molecular formula fits the general formula for alkenes (CnH2n), so the isomers can exist as chain, position, and functional isomers. The possible isomers include: But-1-ene, But-2-ene, 2-Methylprop-1-ene, and Cyclobutane.

Identifying Isomers of \(C_{4}H_{6}\)

This molecular formula fits the general formula for alkynes (CnH2n-2) and cycloalkenes. The isomers can exist as chain, positional, and functional-isomers. The possible isomers are: But-1-yne, But-2-yne, 2-Butene, and Cyclobutene.

Key concepts

Structural Isomers

Structural isomers are fascinating aspects of chemistry. They have the same molecular formula but different structures. This means they contain the same number and type of atoms but connected differently.
Imagine having a set of Lego blocks; you can rearrange them to build distinct shapes without changing the pieces. This is exactly what happens in structural isomers. For instance, in the case of the molecules mentioned, hexane and its isomers have six carbon atoms and fourteen hydrogen atoms, but their structural arrangements vary.
Structural isomers can vary significantly in their physical and chemical properties, despite sharing a molecular formula.

• Chain isomers: These differ in the carbon chain structure. The atoms are connected in different sequences.
• Position isomers: These are formed when groups of atoms are positioned differently on a carbon chain.
• Functional isomers: Involve different functional groups being created with the same atoms.

Alkanes

Alkanes are simple hydrocarbon compounds consisting entirely of carbon and hydrogen atoms, connected by single bonds. These compounds follow the general formula \(\text{C}_n\text{H}_{2n+2}\). Alkanes are often seen as the most basic of organic compounds.
Alkanes are commonly referred to as saturated hydrocarbons because they have the maximum possible number of hydrogen atoms per carbon atom. They are categorized by:

• Straight-chain alkanes: These have carbon atoms connected linearly.
• Branched-chain alkanes: Where the carbon atoms form a tree-like structure instead of a straight line.

Alkanes tend to be quite stable and inert. A notable aspect of alkanes is that, due to their single bonds, they have fewer isomers compared to other hydrocarbons.

Alkenes

Alkenes are hydrocarbons that contain at least one carbon-carbon double bond, following the formula \(\text{C}_n\text{H}_{2n}\). This double bond is key to their chemistry and greatly affects their reactivity compared to alkanes.
The presence of a double bond introduces structural variations, allowing for:

• Chain isomers: Where molecular chains are organized differently.
• Positional isomers: With varying positions of the double bond within the carbon chain.

Alkenes are unsaturated hydrocarbons since they do not contain the maximum number of hydrogen atoms possible. This unsaturation gives them certain reactive properties, making them essential in chemical synthesis and the production of polymers like plastics.

Alkynes

Alkynes are hydrocarbons with one or more carbon-carbon triple bonds, defined by the molecular formula \(\text{C}_n\text{H}_{2n-2}\). The presence of a triple bond bestows unique characteristics and reactivity upon alkynes.
Triple bonds result in significant structural rigidity and a linear geometry around the bond. Variations in alkynes lead to:

• Chain isomers: Different arrangements of the carbon skeleton.
• Positional isomers: Changes in the location of the triple bond along the carbon backbone.

Alkynes are highly reactive, more so than alkenes, and are unsaturated like alkenes. Their reactivity can be employed in creating complex chemicals and various organic reactions.