Alkanes: Structure, Nomenclature, and Properties (General Chemistry Study Notes)

Chapter 11: Introduction to Organic Chemistry

11.1 Organic Compounds

Organic compounds are chemical substances containing carbon atoms, typically bonded to hydrogen, oxygen, nitrogen, and other elements. The study of organic chemistry focuses on the structure, properties, and reactions of these compounds.

• Organic compounds are primarily composed of carbon and hydrogen.

• They form the basis of all living organisms and many synthetic materials.

• Hydrocarbons are the simplest organic compounds, consisting only of carbon and hydrogen.

11.2 Alkanes

Alkanes are a class of hydrocarbons characterized by single bonds between carbon atoms. They are saturated hydrocarbons, meaning they contain the maximum number of hydrogen atoms possible for their carbon skeleton.

• General formula: $C_nH_{2n+2}$, where n is the number of carbon atoms.

• Saturated hydrocarbons: Only single bonds (C–C and C–H).

• Example: Ethane ($C_2H_6$)

Example Calculation:

• If an alkane has 14 hydrogen atoms, solve for n in $2n+2=14$:

• $2n+2=14$

• $2n=12$

• $n=6$ (hexane)

Application: Used in fuels, lubricants, and as starting materials in chemical synthesis.

11.3 Alkanes with Substituents

Alkanes can have branches or substituent groups, leading to structural isomers. These isomers have the same molecular formula but different connectivity of atoms.

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

• Constitutional/Structural isomers: Differ in how atoms are connected.

• Example: Butane ($CH_3CH_2CH_2CH_3$) and methylpropane ($CH_3CH(CH_3)CH_3$) both have $C_4H_{10}$.

11.4 Properties of Alkanes

Physical properties of alkanes, such as melting point, boiling point, and density, increase with molecular weight (chain length).

• As the number of carbon atoms increases, so do melting and boiling points.

• Alkanes are generally nonpolar and insoluble in water.

• They are less dense than water.

Ranking Example: For alkanes $C_2H_6$, $CH_4$, $C_3H_8$, $C_4H_{10}$, the boiling point increases with chain length: $C_4H_{10} > C_3H_8 > C_2H_6 > CH_4$.

Alkane Formulas Used in Organic Chemistry

• Molecular formula: Lists the number and type of atoms (e.g., $C_2H_6$).

• Structural formula: Shows each atom and bond (e.g., for ethane: H–C–C–H).

• Condensed formula: Groups atoms to show connectivity (e.g., $CH_3CH_2CH_3$).

• Line-angle formula: Represents carbon skeleton with lines; each vertex or end is a carbon atom.

Names of Alkanes

Alkanes are named according to the IUPAC system, using prefixes to indicate the number of carbon atoms.

Name	# Carbons	Molecular Formula
Methane	1	CH4
Ethane	2	CH3CH3
Propane	3	CH3CH2CH3
Butane	4	CH3CH2CH2CH3
Pentane	5	CH3CH2CH2CH2CH3
Hexane	6	CH3CH2CH2CH2CH2CH3
Heptane	7	CH3CH2CH2CH2CH2CH2CH3
Octane	8	CH3CH2CH2CH2CH2CH2CH2CH3
Nonane	9	CH3CH2CH2CH2CH2CH2CH2CH2CH3
Decane	10	CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3

Physical Properties of Alkanes

Alkane	Molecular Formula	Condensed Formula	Melting Point (°C)	Boiling Point (°C)
Methane	CH4	CH4	-182.5	-161.5
Ethane	C2H6	CH3CH3	-183.3	-88.6
Propane	C3H8	CH3CH2CH3	-187.7	-42.1
Butane	C4H10	CH3CH2CH2CH3	-138.3	0.0
Pentane	C5H12	CH3CH2CH2CH2CH3	-129.7	36.1
Hexane	C6H14	CH3CH2CH2CH2CH2CH3	-95.3	68.7
Heptane	C7H16	CH3CH2CH2CH2CH2CH2CH3	-90.6	98.4
Octane	C8H18	CH3CH2CH2CH2CH2CH2CH2CH3	-56.8	125.7
Nonane	C9H20	CH3CH2CH2CH2CH2CH2CH2CH2CH3	-53.6	150.8
Decane	C10H22	CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3	-29.7	174.0

Additional info: As molecular weight increases, melting and boiling points and density increase.

Isomers

Isomers are molecules with the same molecular formula but different structures. In alkanes, isomerism begins with four or more carbon atoms.

• Branch or substituent: A side group attached to the main carbon chain.

• Constitutional/structural isomers: Differ in connectivity of atoms.

• Example: Butane and methylpropane ($C_4H_{10}$)

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

Guide to Naming Alkanes (IUPAC System)

The IUPAC system provides rules for naming alkanes, including those with branches (substituents).

1. Identify the longest continuous carbon chain and name it as the parent alkane.

2. Number the carbon atoms in the chain starting from the end nearest a substituent.

3. Name and locate each substituent (branch), listing them in alphabetical order as prefixes to the parent name.

Example: 2-methylbutane (longest chain: butane, methyl group at carbon 2)

Multiple Substituents: Use prefixes di-, tri-, tetra- for identical substituents. Indicate positions with numbers separated by commas.

• Example: 2,3-dimethylpentane (methyl groups at carbons 2 and 3 on a pentane chain)

Summary Table: Naming and Formulas of Alkanes

Name	Molecular Formula	Condensed Formula	Melting Point (°C)	Boiling Point (°C)
Methane	CH4	CH4	-182.5	-161.5
Ethane	C2H6	CH3CH3	-183.3	-88.6
Propane	C3H8	CH3CH2CH3	-187.7	-42.1
Butane	C4H10	CH3CH2CH2CH3	-138.3	0.0
Pentane	C5H12	CH3CH2CH2CH2CH3	-129.7	36.1

Practice Problems

• Problem 1: An alkane has 14 hydrogen atoms. How many carbon atoms does it have?

• Solution: Use $C_nH_{2n+2}$, set $2n+2=14$, solve for $n=6$ (hexane).

• Problem 2: An alkane with 11 carbon atoms. How many hydrogen atoms?

• Solution: $C_{11}H_{2(11)+2}=C_{11}H_{24}$

Key Terms

• Alkane: Saturated hydrocarbon with only single bonds.

• Isomer: Same molecular formula, different structure.

• Alkyl group: Alkane minus one hydrogen atom, forms branches.

• Molecular formula: Shows number and type of atoms.

• Structural formula: Shows connectivity of atoms.

• Condensed formula: Groups atoms to show structure.

• Line-angle formula: Simplified representation of carbon skeleton.