Introduction to Hydrocarbons and Alkanes: Structure, Nomenclature, and Isomerism

Hydrocarbons and Organic Compounds

Definition and Prevalence

Hydrocarbons are compounds composed only of carbon (C) and hydrogen (H). Organic compounds may also contain halogens such as fluorine (F), chlorine (Cl), bromine (Br), or iodine (I). Organic compounds are ubiquitous in daily life, making up over 90% of compounds around us, including food, medications, fuels, plastics, soaps, and detergents.

• Hydrocarbons: Only C and H

• Organic compounds: May also include F, Cl, Br, I

• Examples: Ethanol (C2H6O)

Functional Groups (FGs)

Functional groups are groups of atoms within molecules that have similar chemical and physical properties. They define the reactivity and properties of organic molecules.

General Properties of Organic Compounds

• Relatively low melting and boiling points (typically < 250°C) due to covalent bonding

• Not soluble in water

• Flammable

• Less dense than water (e.g., oil spills float on water)

Representations of Carbon Compounds

Bonding and Models

Each carbon atom has four valence electrons and forms four bonds. Organic molecules can be represented in several ways:

• Expanded structural formula: Shows all atoms and bonds explicitly

• Condensed structural formula: Groups atoms together (e.g., CH3CH2CH3)

• Line-angle formula: Each vertex and line end represents a carbon; hydrogens are implied

• 3D models: Space-filling, ball-and-stick, wedge-dash (shows 3D orientation)

Alkanes

Definition and Structure

Alkanes are saturated hydrocarbons, meaning they contain only single bonds between carbon atoms. They are the simplest type of hydrocarbon.

• General formula: CnH2n+2

• All bonds are single (sigma) bonds

Naming Alkanes

Alkanes are named using a prefix indicating the number of carbons, followed by the suffix "-ane".

# of Carbons	Prefix
1	meth-
2	eth-
3	prop-
4	but-
5	pent-
6	hex-
7	hept-
8	oct-

Structural Representations

• Condensed: CH3CH2CH2CH3

• Line-angle: Each line represents a bond; each vertex is a carbon

• Expanded: Shows all atoms and bonds

Example: Hexane can be drawn in expanded, condensed, or line-angle form.

Isomerism in Alkanes

Structural (constitutional) isomers are compounds with the same molecular formula but different connectivity of atoms.

• Start with the straight-chain structure

• Shorten the chain by one carbon and attach it elsewhere

• Check for duplicate structures by numbering the carbon chain

• Repeat until all unique isomers are found

Example: C5H12 (pentane) has three structural isomers.

Cycloalkanes

Definition and Nomenclature

Cycloalkanes are hydrocarbons arranged in a ring (cyclic) structure. They have two fewer hydrogens than their straight-chain counterparts.

Structure	# of Carbons	Name
Triangle	3	cyclopropane
Square	4	cyclobutane
Pentagon	5	cyclopentane
Hexagon	6	cyclohexane
Heptagon	7	cycloheptane
Octagon	8	cyclooctane

Alkanes with Substituents

Substituents and Branches

A substituent is a branch attached to the main carbon chain. Alkyl groups are common substituents, named by replacing the "-ane" ending with "-yl".

• R–CH3: methyl

• R–CH2CH3: ethyl

• R–CH2CH2CH3: propyl

• R–CH2CH2CH2CH3: butyl

• R–CH2CH2CH2CH2CH3: pentyl

After two carbons, alkyl groups can have structural isomers (e.g., isopropyl, sec-butyl, tert-butyl, isobutyl).

Halogen Substituents

Halogen atoms can replace hydrogens in alkanes, and are named by changing the ending to "-o":

• R–F: fluoro

• R–Cl: chloro

• R–Br: bromo

• R–I: iodo

Properties and Uses of Alkanes

• Solvents

• Fuels: Propane and butane are gases used in heating; octane is a component of gasoline

• C1–C4: gases at room temperature (e.g., propane)

• C5–C8: liquids at room temperature (e.g., gasoline)

• C9–C17: more viscous liquids (e.g., kerosene, jet fuel)

• C18 and above: solids (e.g., waxes)

Alkenes and Alkynes

Definition and Structure

• Alkenes: Hydrocarbons with at least one carbon-carbon double bond (C=C)

• Alkynes: Hydrocarbons with at least one carbon-carbon triple bond (C≡C)

• Both are unsaturated hydrocarbons (do not have the maximum number of hydrogens)

Example: Ethene (C2H4), Ethyne (C2H2)

Cis-Trans Isomerism in Alkenes

• Cis: Atoms/groups with the highest atomic number on the same side of the double bond

• Trans: Atoms/groups with the highest atomic number on opposite sides

• Double bonds cannot rotate, leading to geometric (cis/trans) isomerism

• Must have two different groups on each carbon of the double bond to exhibit cis/trans isomerism

Geometry

• Alkenes: Planar geometry around the double bond

• Alkynes: Linear geometry around the triple bond

Summary Table: Hydrocarbon Types

Type	Bonding	General Formula	Saturation
Alkane	Single bonds	CnH2n+2	Saturated
Alkene	At least one double bond	CnH2n	Unsaturated
Alkyne	At least one triple bond	CnH2n-2	Unsaturated
Cycloalkane	Single bonds, ring	CnH2n	Saturated

Additional info: These notes provide foundational knowledge for understanding organic molecules, their structures, and basic nomenclature, which are essential for further study in GOB Chemistry.