Organic Chemistry: Structure, Properties, and Nomenclature of Hydrocarbons and Functional Groups

Organic Chemistry Overview

Definition and Scope

Organic chemistry is the branch of chemistry that studies compounds containing carbon, typically bonded to hydrogen, nitrogen, oxygen, and sulfur. The unique bonding capabilities of carbon allow for millions of different organic compounds, which are central to biological, industrial, and environmental processes.

• Organic compounds exclude CO, CO2, carbonates, and carbides.

• Most common odors and fragrances are caused by organic molecules.

Unique Properties of Carbon

Bonding and Structure

Carbon atoms can form strong, stable bonds with up to four other atoms, including other carbon atoms. This allows for a vast array of molecular structures, including chains, rings, and multiple bonds.

• Catenation: Carbon atoms can link together to form long chains.

• Bond Types: Carbon can form single, double, or triple bonds.

• Ring Structures: Carbon atoms can form cyclic compounds.

Hydrocarbons

Classification

Hydrocarbons are compounds containing only carbon and hydrogen. They are classified as aliphatic (alkanes, alkenes, alkynes) or aromatic (contain benzene rings).

• Aliphatic hydrocarbons: Can be saturated (alkanes) or unsaturated (alkenes, alkynes).

• Aromatic hydrocarbons: Contain benzene rings.

Types of Hydrocarbons

Type of Hydrocarbon	Type of Bonds	Generic Formula	Example
Alkanes	All single	CnH2n+2	Ethane
Alkenes	One or more double	CnH2n	Ethene
Alkynes	One or more triple	CnH2n-2	Ethyne

Structural Formulas and Isomerism

Structural Formulas

Structural formulas show how atoms are bonded in a molecule. Condensed structural formulas group hydrogens with their attached carbons, while line formulas show only carbon-carbon bonds as lines.

Isomerism

Isomers are compounds with the same molecular formula but different structures.

• Structural isomers: Differ in the connectivity of atoms.

• Stereoisomers: Same connectivity, different spatial arrangement.

• Optical isomers (enantiomers): Nonsuperimposable mirror images, often with chiral centers.

Physical Properties of Hydrocarbons

Boiling Points and Density

Boiling and melting points of hydrocarbons increase with molar mass. Hydrocarbons are nonpolar and less dense than water, with dispersion forces as the main intermolecular attraction.

n-Alkane	Boiling Point (°C)
Methane	-161.5
Ethane	-88.6
Propane	-42.1
n-Butane	-0.5
n-Pentane	36.0
n-Hexane	68.7
n-Heptane	98.5
n-Octane	125.6

Naming Hydrocarbons: IUPAC System

Alkanes

The IUPAC system names alkanes based on the longest continuous carbon chain. Branches are named as alkyl groups and numbered to give the lowest possible numbers to substituents.

• Prefixes for base names: meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-, non-, dec-

• Substituent prefixes: di-, tri-, tetra- (for multiple identical groups)

Number of Carbon Atoms	Prefix
1	meth-
2	eth-
3	prop-
4	but-
5	pent-
6	hex-
7	hept-
8	oct-
9	non-
10	dec-

Examples of Alkane Nomenclature

Example: 3-ethylpentane

• Longest chain: pentane (5 carbons)

• Substituent: ethyl group at carbon 3

• Name: 3-ethylpentane

Alkenes and Alkynes

Structure and Nomenclature

Alkenes contain at least one C=C double bond; alkynes contain at least one C≡C triple bond. The base chain must include the multiple bond, and numbering starts from the end closest to the bond.

• Alkene formula: CnH2n

• Alkyne formula: CnH2n-2

• Base name ends in -ene (alkenes) or -yne (alkynes)

Isomerism in Alkenes: Cis–Trans Isomerism

Geometric Isomers

Rotation around a double bond is restricted, leading to cis–trans isomerism. Cis isomers have substituents on the same side; trans isomers have them on opposite sides.

Hydrocarbon Reactions

Combustion

Combustion of hydrocarbons is highly exothermic and produces CO2 and H2O.

• Example equation: $\mathrm{CH_3CH_2CH_3(g) + 5\ O_2(g) \rightarrow 3\ CO_2(g) + 4\ H_2O(g)}$

Halogen Substitution (Alkanes)

Alkanes can undergo substitution reactions with halogens, replacing hydrogen atoms with halogen atoms.

• Initiated by heat or UV light

• Example: $\mathrm{CH_4 + Cl_2 \rightarrow CH_3Cl + HCl}$

Addition Reactions (Alkenes and Alkynes)

Alkenes and alkynes undergo addition reactions, where molecules add across the multiple bond.

• Hydrogenation: Addition of H2 to convert unsaturated to saturated hydrocarbons.

• Halogenation: Addition of X2 (halogen).

• Hydrohalogenation: Addition of HX, following Markovnikov's rule.

Aromatic Hydrocarbons

Benzene and Its Derivatives

Aromatic hydrocarbons contain benzene rings. Benzene is a resonance hybrid and does not behave like alkenes in addition reactions.

• Substitution reactions replace hydrogen atoms with other groups.

• Benzene as a substituent is called a phenyl group.

• Disubstituted derivatives use ortho-, meta-, para- prefixes for relative positions.

Functional Groups in Organic Chemistry

Alcohols

Alcohols contain the R—OH group. Naming involves numbering the main chain from the end closest to —OH and using the -ol suffix.

• Common examples: ethanol, methanol, isopropyl alcohol

Alcohol Reactions

• Substitution: Alcohol reacts with acid to form halogenated hydrocarbon and water.

• Dehydration: Removal of water to form alkenes.

• Oxidation: Alcohols oxidize to aldehydes, then to carboxylic acids.

• Reaction with active metals: Forms alkoxide and hydrogen gas.

Aldehydes and Ketones

Contain the carbonyl group (C=O). Aldehydes have at least one hydrogen attached to the carbonyl carbon; ketones have two R groups.

• Naming: Aldehydes use -al suffix; ketones use -one suffix.

• Reactions: Addition across C=O, reduction to alcohols.

Carboxylic Acids and Esters

Carboxylic acids contain the RCOOH group and are named with the -oic acid suffix. Esters are formed by condensation reactions between carboxylic acids and alcohols, named with the -oate suffix.

• Carboxylic acids: Sour taste, weak acids (e.g., acetic acid, citric acid).

• Esters: Sweet odor, used in fragrances and flavorings.

Ethers and Amines

Ethers have the general formula ROR and are named by listing the alkyl groups followed by 'ether.' Amines contain nitrogen and are named by listing alkyl groups attached to N, followed by -amine.

• Ethers: Common solvent, low boiling point.

• Amines: Organic bases, often have strong odors.

Amine Reactions

• React with acids to form ammonium salts.

• React with carboxylic acids to form amides via condensation.

Summary Table: Major Hydrocarbon Uses

Number of Carbon Atoms	State	Major Uses
1–4	Gas	Heating fuel, cooking fuel
5–7	Low-boiling liquids	Solvents, gasoline
6–18	Liquids	Gasoline
12–24	Liquids	Jet fuel, portable-stove fuel
18–50	High-boiling liquids	Diesel fuel, lubricants, heating oil
50+	Solids	Petroleum jelly, paraffin wax

Additional info: This guide covers the essential concepts of organic chemistry relevant to general chemistry, including structure, nomenclature, isomerism, physical properties, and reactions of hydrocarbons and major functional groups.