Introduction to Organic Chemistry: Alkanes and Their Nomenclature

Introduction to Organic Chemistry

Overview of Organic Compounds

Organic chemistry is the study of compounds primarily composed of carbon and hydrogen, often containing other elements such as oxygen, nitrogen, sulfur, and halogens. Organic compounds are the foundation of all living organisms and many synthetic materials.

• Hydrocarbons are compounds containing only carbon and hydrogen.

• Hydrocarbons are classified as alkanes, alkenes, alkynes, and aromatic compounds based on the types of bonds and structures present.

Alkanes

General Formula and Structure

Alkanes are saturated hydrocarbons, meaning they contain only single bonds between carbon atoms. The general formula for a straight-chain alkane is:

$\mathrm{C}_n\mathrm{H}_{2n+2}$

• n = number of carbon atoms

• Each carbon forms four single covalent bonds (tetrahedral geometry).

• Alkanes are also called paraffins.

Naming Alkanes (IUPAC System)

The names of alkanes are determined by the International Union of Pure and Applied Chemistry (IUPAC) system. The root name indicates the number of carbons in the longest continuous chain, and the suffix -ane is used.

• 1 carbon: Methane ($\mathrm{CH}_4$)

• 2 carbons: Ethane ($\mathrm{C}_2\mathrm{H}_6$)

• 3 carbons: Propane ($\mathrm{C}_3\mathrm{H}_8$)

• 4 carbons: Butane ($\mathrm{C}_4\mathrm{H}_{10}$)

• 5 carbons: Pentane ($\mathrm{C}_5\mathrm{H}_{12}$)

• 6 carbons: Hexane ($\mathrm{C}_6\mathrm{H}_{14}$)

• 7 carbons: Heptane ($\mathrm{C}_7\mathrm{H}_{16}$)

• 8 carbons: Octane ($\mathrm{C}_8\mathrm{H}_{18}$)

• 9 carbons: Nonane ($\mathrm{C}_9\mathrm{H}_{20}$)

• 10 carbons: Decane ($\mathrm{C}_{10}\mathrm{H}_{22}$)

Isomerism in Alkanes

Isomers are compounds with the same molecular formula but different structural arrangements. In alkanes, constitutional (structural) isomers differ in the connectivity of their atoms.

• Isomerism begins with butane ($\mathrm{C}_4\mathrm{H}_{10}$), which has two isomers: n-butane and isobutane (methylpropane).

• Isomers have distinct physical and chemical properties.

Alkyl Groups

An alkyl group is derived from an alkane by removing one hydrogen atom. Alkyl groups are named by replacing the -ane ending with -yl.

Alkyl Group Structure	Name
–CH3	Methyl
–CH2CH3	Ethyl
–CH2CH2CH3	Propyl
–CH2CH2CH2CH3	Butyl
–CH2CH2CH2CH2CH3	Pentyl

Naming Branched Alkanes

IUPAC Rules for Naming Alkanes with Substituents

When naming branched alkanes, follow these steps:

1. Identify the longest continuous chain of carbon atoms (parent chain) and name it as an alkane.

2. Number the carbon atoms in the parent chain from the end nearest a substituent (branch).

3. Name and locate each substituent (alkyl group) as a prefix to the parent name, listing them in alphabetical order. Use numbers to indicate their positions.

Example: 2-Methylbutane

• Longest chain: 4 carbons (butane)

• Methyl group attached to carbon 2

• Name: 2-methylbutane

Example: 2-Methylpropane

• Longest chain: 3 carbons (propane)

• Methyl group attached to carbon 2

• Name: 2-methylpropane

Example: 4-Ethyl-2-methylhexane

• Longest chain: 6 carbons (hexane)

• Substituents: ethyl group at carbon 4, methyl group at carbon 2

• List substituents alphabetically: 4-ethyl, 2-methyl

• Name: 4-ethyl-2-methylhexane

Summary Table: Steps for Naming Alkanes

Step	Description
1	Find the longest carbon chain and name it as an alkane.
2	Number the chain from the end nearest a substituent.
3	Name and locate each substituent; list in alphabetical order as prefixes.

Practice Problem: Determining Number of Carbons

Given an alkane with 14 hydrogen atoms, determine the number of carbon atoms:

Use the general formula: $\mathrm{C}_n\mathrm{H}_{2n+2}$

Set $2n + 2 = 14$

$2n = 14 - 2 = 12$

$n = 12 / 2 = 6$

Therefore, the alkane is hexane ($\mathrm{C}_6\mathrm{H}_{14}$).

Key Terms

• Alkane: Saturated hydrocarbon with only single bonds.

• Isomer: Compounds with the same molecular formula but different structures.

• Alkyl group: An alkane missing one hydrogen, acts as a substituent.

• Substituent: An atom or group of atoms attached to the main carbon chain.