Introduction to Organic Chemistry: Alkanes and Nomenclature

Introduction to Organic Chemistry

Definition and Scope

Organic chemistry is the branch of chemistry that studies compounds primarily composed of carbon and hydrogen, often with other elements such as oxygen, nitrogen, sulfur, and halogens. Organic compounds are distinguished from inorganic compounds by their structure, bonding, and properties.

• Organic compounds: Compounds with carbon as the principal element, typically containing C-H bonds.

• Inorganic compounds: Compounds that do not primarily contain carbon-hydrogen frameworks.

Classification of Organic Compounds

Hydrocarbons and Derivatives

Organic compounds are broadly classified into hydrocarbons (containing only carbon and hydrogen) and their derivatives (containing additional elements).

• Hydrocarbons: Only carbon and hydrogen atoms.

• Hydrocarbon derivatives: Contain other elements such as O, N, S, or halogens.

Hydrocarbons

Saturated and Unsaturated Hydrocarbons

Hydrocarbons are divided into saturated and unsaturated types based on the types of bonds between carbon atoms.

• Saturated hydrocarbons (Alkanes): Only single bonds between carbon atoms. General formula: $C_nH_{2n+2}$

• Unsaturated hydrocarbons: Contain one or more double (alkenes) or triple (alkynes) bonds.

Bonding and Structure in Organic Compounds

Bonding Patterns

Carbon is tetravalent, forming four covalent bonds. The geometry around carbon atoms depends on the type of bonding:

• Tetrahedral (sp3): Four single bonds, bond angles of 109.5° (e.g., methane, ethane).

• Trigonal planar (sp2): Double bonds, bond angles of 120° (e.g., ethene).

• Linear (sp): Triple bonds, bond angles of 180° (e.g., ethyne).

Functional Groups

Definition and Importance

A functional group is a specific group of atoms within a molecule that imparts characteristic chemical properties and reactivity. Organic molecules are grouped into families based on their functional groups.

• Functional groups determine the chemical behavior of organic molecules.

• Examples include hydroxyl (-OH), carbonyl (C=O), carboxyl (-COOH), amino (-NH2), and halides (-Cl, -Br, etc.).

Physical and Chemical Properties: Organic vs. Inorganic Compounds

Comparison Table

Organic and inorganic compounds differ in their bonding, physical properties, and solubility.

Property	Organic (Covalent)	Inorganic (Ionic)
Boiling/Melting Points	Lower	Higher
Intermolecular Forces	Weaker (London, dipole, H-bonding)	Stronger (ionic)
Solubility in Water	Mostly insoluble	Mostly soluble
Electrical Conductivity	Nonelectrolytes (do not conduct)	Electrolytes (conduct)
Physical State	Often liquids/gases	Often solids

Drawing Organic Structures

Types of Structural Representations

Organic molecules can be represented in several ways to convey different levels of detail:

• Expanded (structural) formula: Shows all atoms and bonds explicitly.

• Condensed formula: Groups atoms together, omitting some bonds.

• Line-angle (skeletal) formula: Each vertex or line end represents a carbon; hydrogens on carbons are implied.

Nomenclature of Alkanes

IUPAC System

The International Union of Pure and Applied Chemistry (IUPAC) provides systematic rules for naming organic compounds. The name of an alkane consists of three parts: prefix (substituents), parent (number of carbons), and suffix (family).

• Identify the longest continuous carbon chain (parent chain).

• Number the chain to give substituents the lowest possible numbers.

• Name and number substituents (branches), using prefixes (di-, tri-, etc.) for multiples.

• List substituents alphabetically.

Greek Roots for Number of Carbons

The parent name is based on the number of carbons in the main chain:

Number of Carbons	Root	Example
1	meth-	methane
2	eth-	ethane
3	prop-	propane
4	but-	butane
5	pent-	pentane
6	hex-	hexane
7	hept-	heptane
8	oct-	octane
9	non-	nonane
10	dec-	decane

Isomerism in Alkanes

Structural (Constitutional) Isomers

Isomers are compounds with the same molecular formula but different structures. In alkanes, structural isomers differ in the connectivity of their carbon atoms (straight-chain vs. branched).

• Straight-chain isomer: All carbons in a single continuous chain.

• Branched-chain isomer: At least one carbon is attached as a branch.

Cycloalkanes

Structure and Nomenclature

Cycloalkanes are saturated hydrocarbons with carbon atoms arranged in a ring. The prefix "cyclo-" is used in their names (e.g., cyclohexane).

• General formula: $C_nH_{2n}$

• Number the ring to give substituents the lowest possible numbers.

Summary Table: Alkanes

Molecular Formula	IUPAC Prefix	IUPAC Name	Structural Formula
CH4	meth-	methane	CH4
C2H6	eth-	ethane	CH3CH3
C3H8	prop-	propane	CH3CH2CH3
C4H10	but-	butane	CH3CH2CH2CH3
C5H12	pent-	pentane	CH3(CH2)3CH3
C6H14	hex-	hexane	CH3(CH2)4CH3
C7H16	hept-	heptane	CH3(CH2)5CH3
C8H18	oct-	octane	CH3(CH2)6CH3
C9H20	non-	nonane	CH3(CH2)7CH3
C10H22	dec-	decane	CH3(CH2)8CH3

Key Equations

• General formula for acyclic alkanes: $C_nH_{2n+2}$

• General formula for cycloalkanes: $C_nH_{2n}$

Additional info:

• Organic chemistry is foundational for understanding biological molecules, pharmaceuticals, and materials science.

• Practice in drawing and naming organic molecules is essential for mastery.