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Chapter 22: Organic Chemistry – Structure, Properties, and Nomenclature of Hydrocarbons

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Organic Chemistry: Introduction

Definition and Scope

Organic chemistry is the branch of chemistry that studies compounds containing carbon, typically in combination with hydrogen, nitrogen, oxygen, and sulfur. It excludes certain simple carbon compounds such as carbon dioxide (CO2), carbon monoxide (CO), carbonates, and carbides.

  • Organic compounds are the basis of life and include a vast array of molecules, from simple hydrocarbons to complex biomolecules.

  • The unique bonding properties of carbon allow for millions of different organic compounds.

Why Carbon Is Unique

Bonding and Structure

  • Carbon can form up to four covalent bonds, allowing for a variety of structures: chains, rings, and branches.

  • Carbon-carbon bonds are strong and stable, enabling the formation of large, complex molecules.

  • Carbon atoms can catenate (form chains), and can form single, double, or triple bonds.

  • Carbon can form both open-chain (aliphatic) and ring (cyclic) structures.

Hydrocarbons

Definition and Classification

Hydrocarbons are organic compounds composed exclusively of carbon and hydrogen. They are classified as either aliphatic or aromatic.

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

  • Aromatic hydrocarbons: Contain benzene rings.

  • Hydrocarbons are generally insoluble in water due to their nonpolar nature.

Types of Hydrocarbons

Type of Hydrocarbon

Type of Bonds

Generic Formula

Example

Alkanes

All single

CnH2n+2

Methane (CH4), Ethane (C2H6)

Alkenes

One (or more) double

CnH2n

Ethene (C2H4)

Alkynes

One (or more) triple

CnH2n-2

Ethyne (C2H2)

Formulas and Models

Types of Formulas

  • Molecular formula: Shows the number and type of atoms (e.g., C4H10).

  • Structural formula: Shows how atoms are bonded.

  • Condensed structural formula: Groups hydrogens with their attached carbon.

  • Carbon skeleton (line) formula: Shows only carbon-carbon bonds as lines.

  • Models (ball-and-stick, space-filling) provide three-dimensional representations.

Isomerism

Structural Isomers

  • Compounds with the same molecular formula but different connectivity of atoms.

  • Example: Butane and isobutane (C4H10).

Stereoisomers

  • Same connectivity, different spatial arrangement.

  • Optical isomers (enantiomers): Nonsuperimposable mirror images, often due to a chiral center (carbon with four different substituents).

Chirality

  • A molecule is chiral if it has a nonsuperimposable mirror image.

  • Chiral centers are typically carbons bonded to four different groups.

Optical Activity

  • Enantiomers rotate plane-polarized light in opposite directions:

    • Dextrorotatory (d-): Rotates light to the right.

    • Levorotatory (l-): Rotates light to the left.

  • A racemic mixture contains equal amounts of both enantiomers and does not rotate plane-polarized light.

Saturated Hydrocarbons: Alkanes

Properties and General Formula

  • All C–C single bonds; saturated with hydrogens.

  • General formula for chain alkanes:

  • Ring alkanes have fewer hydrogens than chains with the same number of carbons.

Physical Properties

  • Boiling and melting points increase with molar mass.

  • Nonpolar; main intermolecular forces are London dispersion forces.

  • Less dense than water.

Table: Physical Properties of n-Alkanes

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

Uses of Hydrocarbons

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

Naming Alkanes: IUPAC System

  • Find the longest continuous carbon chain (base name ends in -ane).

  • Identify and name substituents (alkyl groups).

  • Number the chain from the end nearest a substituent.

  • List substituents alphabetically, using prefixes (di-, tri-) for multiples.

Common Prefixes for Number of Carbons

Number of Carbons

Prefix

1

meth-

2

eth-

3

prop-

4

but-

5

pent-

6

hex-

7

hept-

8

oct-

9

non-

10

dec-

Common Alkyl Groups

Condensed Structural Formula

Name

-CH3

Methyl

-CH2CH3

Ethyl

-CH2CH2CH3

Propyl

-CH2CH2CH2CH3

Butyl

-CH(CH3)2

Isopropyl

-CH2CH(CH3)2

Isobutyl

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