Chapter 11: Introduction to Organic Chemistry – Hydrocarbons, Isomerism, and Aromatic Compounds

Chapter 11: Introduction to Organic Chemistry

11.1 Organic Compounds

Organic compounds are chemical substances containing carbon atoms bonded to hydrogen, oxygen, nitrogen, or other elements. They form the basis of organic chemistry and are essential for life and industrial applications.

• Definition: Organic compounds are primarily made of carbon and hydrogen, often with other elements.

• Examples: Methane (CH4), Ethanol (C2H5OH), Benzene (C6H6).

• Applications: Fuels, plastics, pharmaceuticals, and biological molecules.

11.2 Alkanes

Alkanes are saturated hydrocarbons containing only single bonds between carbon atoms. They follow the general formula $C_nH_{2n+2}$.

• Structure: Straight or branched chains of carbon atoms.

• Naming: Use the root name based on the longest continuous carbon chain, with the suffix -ane.

• Example: Pentane (C5H12).

11.3 Alkanes with Substituents

Substituted alkanes contain one or more groups (substituents) attached to the main carbon chain. The IUPAC system is used for systematic naming.

• Substituents: Groups such as methyl (–CH3), bromo (–Br), chloro (–Cl), etc.

• Naming Rules:

  1. Identify the longest carbon chain (parent chain).

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

  3. Name and number each substituent; list them alphabetically.

  4. Combine substituent names and numbers with the parent chain name.

• Example: 2-bromo-3-methylpentane

  • Parent chain: pentane (5 carbons)

  • Substituents: bromo at position 2, methyl at position 3

• Example: 3,4-dimethyl-2-hexene

  • Parent chain: hexene (6 carbons, double bond at position 2)

  • Substituents: methyl groups at positions 3 and 4

• Example: 5-chloro-2-methyl-3-octene

  • Parent chain: octene (8 carbons, double bond at position 3)

  • Substituents: chloro at position 5, methyl at position 2

11.4 Properties of Alkanes

Alkanes are generally nonpolar, insoluble in water, and less dense than water. They are relatively unreactive but can undergo combustion and substitution reactions.

• Physical Properties: Low boiling and melting points compared to other organic compounds.

• Chemical Properties: Combustion to produce CO2 and H2O.

11.5 Alkenes and Alkynes

Alkenes and alkynes are unsaturated hydrocarbons containing double and triple bonds, respectively. Their general formulas are $C_nH_{2n}$ for alkenes and $C_nH_{2n-2}$ for alkynes.

• Alkenes: Contain at least one carbon-carbon double bond (-ene suffix).

• Alkynes: Contain at least one carbon-carbon triple bond (-yne suffix).

• Example: 2-hexene (double bond between carbons 2 and 3).

11.6 Cycloalkanes

Cycloalkanes are cyclic alkanes with the formula $C_nH_{2n}$. They have two fewer hydrogen atoms than their open-chain counterparts.

• Structure: Carbon atoms arranged in a ring.

• Naming: Use the prefix cyclo- before the alkane name.

• Examples:

  • Cyclopropane (3C), Cyclobutane (4C), Cyclopentane (5C), Cyclohexane (6C)

Type of Formula	Cyclopropane	Cyclobutane	Cyclohexane
Structural	Triangle	Square	Hexagon
Line	Triangle	Square	Hexagon

• Substituted Cycloalkanes: Name substituents in alphabetical order, use lowest possible numbers.

• Examples:

  • Methylcyclopentane (methyl group on cyclopentane)

  • Chlorocycloheptane (chloro group on cycloheptane)

  • Ethylcyclopropane (ethyl group on cyclopropane)

11.7 Geometric (Cis-Trans) Isomers

Geometric isomers are a type of stereoisomerism found in alkenes due to restricted rotation around the double bond. Cis-trans isomerism occurs when each carbon of the double bond has two different groups attached.

• Cis Isomer: Similar groups are on the same side of the double bond.

• Trans Isomer: Similar groups are on opposite sides of the double bond.

• Requirement: Both carbons of the C=C must have two different groups attached for cis-trans isomerism.

• Example:

  • cis-1,2-Dichloroethene vs. trans-1,2-Dichloroethene

Compound	Cis Isomer	Trans Isomer	Possible?
CHCl=CHCl	Yes	Yes	Yes
CH2=CHCH3	No	No	No

11.8 Reactions of Hydrocarbons

Hydrocarbons undergo various chemical reactions, including combustion, substitution, addition, and polymerization.

• Combustion: Hydrocarbons react with oxygen to produce carbon dioxide and water.

• Substitution: Replacement of hydrogen atoms with other atoms (common in alkanes).

• Addition: Atoms are added to double or triple bonds (common in alkenes and alkynes).

11.9 Aromatic Compounds

Aromatic compounds contain one or more benzene rings. Benzene is the simplest aromatic hydrocarbon, with the formula $C_6H_6$.

• Benzene Structure:

  • Six carbon atoms joined in a planar hexagonal ring.

  • Each carbon is bonded to one hydrogen atom.

  • Delocalized electrons create stability and unique reactivity.

• Applications: Found in dyes, drugs, plastics, and explosives.

Compound	Formula	Structure
Benzene	C6H6	Hexagonal ring

Summary Table: Hydrocarbon Types

Type	General Formula	Bond Type	Example
Alkane	$C_nH_{2n+2}$	Single	Ethane (C2H6)
Alkene	$C_nH_{2n}$	Double	Ethene (C2H4)
Alkyne	$C_nH_{2n-2}$	Triple	Ethyne (C2H2)
Cycloalkane	$C_nH_{2n}$	Single (cyclic)	Cyclohexane (C6H12)
Aromatic	Variable	Delocalized (ring)	Benzene (C6H6)

Key Equations

• Alkane formula: $C_nH_{2n+2}$

• Alkene formula: $C_nH_{2n}$

• Alkyne formula: $C_nH_{2n-2}$

• Benzene formula: $C_6H_6$

Additional info:

• These notes cover the foundational aspects of organic chemistry relevant to a general chemistry course, focusing on hydrocarbon classification, nomenclature, isomerism, and aromaticity.

• Lab activities may include molecular modeling of organic compounds to visualize structures and isomerism.