Hydrocarbons · Lesson 1 of 14

Hydrocarbon Families and Alkane Structures

“See how carbon skeletons distinguish hydrocarbon families and generate alkane isomers.”

Learning Objectives

• Classify open-chain, cyclic, saturated, unsaturated, and aromatic hydrocarbons. • Build the alkane general formula from carbon valence. • Distinguish different carbon skeletons from different drawings of one skeleton. • Identify primary, secondary, tertiary, and quaternary carbon atoms.

Hydrocarbons around us

A hydrocarbon contains only carbon and hydrogen. Changing the number of carbons, their connections, or their bonds changes the substance. A small gaseous fuel, a liquid solvent, and a solid wax can all belong to this broad family. The useful starting question is therefore not simply which elements are present, but how their atoms are joined.

Petroleum supplies mixtures separated into fractions such as petrol, kerosene, and diesel. Natural gas is rich in methane; compressing it gives CNG, while cooling it into liquid form gives LNG. LPG contains readily liquefied gases, chiefly propane and butanes. Coal gas is obtained by heating coal without an adequate supply of air. Hydrocarbons also provide starting materials for plastics, dyes, and medicines, and some serve as solvents. Their usefulness as fuels does not make their combustion pollution-free.

Family	Carbon framework and bonding	Example
Alkanes	Open chains with only single bonds	Methane, ethane
Cycloalkanes	Rings with only single bonds	Cyclohexane
Alkenes	At least one carbon–carbon double bond	Ethene
Alkynes	At least one carbon–carbon triple bond	Ethyne
Aromatic hydrocarbons	Special cyclic delocalised electron systems	Benzene, naphthalene

Saturated means that the carbon skeleton has only single bonds. Unsaturated hydrocarbons contain carbon–carbon multiple bonds. A ring by itself does not make a compound aromatic: cyclohexane is a saturated ring. Benzene has a different electronic arrangement, which will be developed later. Model-building with four-connection carbon pieces and one-connection hydrogen pieces helps reveal these distinctions.

Building the alkane series

Methane has one carbon bonded to four hydrogens. Replacing one hydrogen by a methyl group adds one carbon and a net two hydrogens. Repeating this operation gives ethane, propane, and the higher members. A homologous series is a family in which neighbouring members differ by one methylene unit and share a characteristic structural pattern.

Open-chain saturated hydrocarbonsLaTeX

C_{n} H_{2 n + 2} (n \geq 1)

Here n is the number of carbon atoms. This formula applies to an acyclic saturated hydrocarbon; closing a ring removes two hydrogens.

The formula can also be derived by counting bonds. For $n$ carbons in one open connected skeleton there are $n - 1$ carbon–carbon bonds. The carbons supply $4 n$ valence connections, and those carbon–carbon bonds use $2 (n - 1)$ of them. The remaining $4 n - 2 (n - 1) = 2 n + 2$ connections end in hydrogen atoms. Branching changes connections but not this count.

Each carbon in an alkane is approximately tetrahedral and $s p^{3}$ hybridised. In methane the H–C–H angle is about $109. 5^{\circ}$ . Typical textbook values are about $154 pm$ for a carbon–carbon single bond and $112 pm$ for a carbon–hydrogen bond. An extended chain is a sequence of tetrahedral centres, not a straight line of collinear atoms.

A tetrahedral carbon centre— The solid wedge points towards you and the dashed bond points away; the four bonds occupy three-dimensional space.

One formula can describe different skeletons

The first three alkanes each have only one carbon skeleton. With four carbons, however, a continuous chain and a branched skeleton become possible. Butane and 2-methylpropane both have ten hydrogens, yet are different compounds with different physical properties. This is structural isomerism; when the carbon skeleton changes, the particular type is chain isomerism.

Finding all hexane skeletons

Problem
Find the chain isomers of $C_{6} H_{14}$ .

1.Begin with a six-carbon chain: hexane. Shorten the longest chain to five carbons and attach one methyl branch. Distinct placements give 2-methylpentane and 3-methylpentane; reversing the drawing creates no new compounds.
2.With a four-carbon longest chain, attach two methyl groups. The distinct valid arrangements are 2,2-dimethylbutane and 2,3-dimethylbutane.
3.These are five isomers in all. A proposed shorter parent chain must be checked carefully because tracing through a branch often reveals a longer chain already counted.

Pentane similarly has three skeletons: pentane, 2-methylbutane, and 2,2-dimethylpropane. Isomer counts grow quickly: hexane has five, heptane nine, and decane seventy-five. Rotating a drawing on paper, bending a chain, or numbering it backwards does not by itself create a new skeleton.

Reading the degree of a carbon

Problem
Classify every carbon in $C H_{3} C H_{2} C (C H_{3})_{2} C H_{2} CH (C H_{3})_{2}$ .

1.Count carbon neighbours, not hydrogen neighbours. The central carbon written as C has four carbon neighbours and is quaternary. The CH group has three carbon neighbours and is tertiary.
2.The two CH2 groups each have two carbon neighbours and are secondary. The five CH3 groups each have one carbon neighbour and are primary.
3.The corresponding hydrogen counts per carbon are three for a primary carbon here, two for a secondary carbon, one for the tertiary carbon, and zero for the quaternary carbon.

Common mistake

Methane has no carbon neighbour, so the usual neighbour-count definition does not assign it as a primary carbon. It is a special case; do not use the number of hydrogen atoms alone to define carbon degree. Aromatic also does not simply mean pleasant-smelling.

Quick check

Which substance is a hydrocarbon?

Quick check

Which feature distinguishes a cycloalkane from an open-chain alkane?

Quick check

What is the formula of an acyclic saturated hydrocarbon with seven carbons?

Quick check

How many carbon neighbours does a tertiary carbon have?

Quick check

Which change necessarily creates a chain isomer?

Practice Problems

Classify methane, cyclohexane, ethene, ethyne, and benzene by framework and bonding.
Derive the hydrogen count of an open-chain saturated hydrocarbon with $n$ carbons.
Draw both $C_{4} H_{10}$ skeletons and all three $C_{5} H_{12}$ skeletons.
Draw the five $C_{6} H_{14}$ skeletons and explain how you avoided duplicates.
Use a tetrahedral model to explain why a carbon chain drawn as a zigzag is not a sequence of collinear atoms.
Explain how hydrocarbons function both as fuels and as chemical starting materials.

Key Takeaways

• Hydrocarbons contain only carbon and hydrogen; framework and bonding identify their family. • Acyclic saturated hydrocarbons follow $C_{n} H_{2 n + 2}$ . • Chain isomers have different carbon connectivity, not merely differently oriented drawings. • Carbon degree counts directly bonded carbon neighbours.

Start of chapter

Next · Lesson 2

Naming Branched Alkanes

Hydrocarbons around us

Building the alkane series

One formula can describe different skeletons

Quiz

Practice Problems

Key Takeaways