Chapter 11: Introduction to Organic Chemistry – Hydrocarbons and Functional Groups

Organic Compounds and Hydrocarbons

Definitions and Classification

Organic chemistry is the study of compounds containing carbon, typically bonded to hydrogen, oxygen, nitrogen, and other elements. Hydrocarbons are the simplest organic compounds, consisting only of carbon and hydrogen.

• Organic compound: A compound primarily composed of carbon and hydrogen, often with other elements.

• Hydrocarbon: A compound containing only carbon and hydrogen atoms.

• Alkane: A saturated hydrocarbon with only single bonds between carbon atoms.

• Cycloalkane: An alkane whose carbon atoms form a ring structure.

• Expanded structural formula: Shows all atoms and bonds in a molecule.

• Condensed structural formula: Groups atoms together to simplify the representation, e.g., CH3CH2CH3.

Example: Methane (CH4) is the simplest alkane.

Shapes and Naming of Alkanes

Molecular Geometry and Nomenclature

Alkanes have a tetrahedral geometry around each carbon atom due to sp3 hybridization. The carbon atoms in alkanes are arranged in a zigzag chain because of the tetrahedral bond angles (109.5°).

• Methane shape: Tetrahedral, with bond angles of 109.5°.

• Naming prefixes: Used for alkanes with more than one carbon atom: meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-, non-, dec-.

• Condensed structural formula: For example, butane: CH3CH2CH2CH3.

Example: The formula for pentane is CH3CH2CH2CH2CH3.

Branches, Isomers, and Substituents

Structural Variations in Alkanes

Alkanes can have branches or substituents, leading to structural isomers—compounds with the same molecular formula but different structures.

• Branch/Substituent: A group attached to the main carbon chain.

• Structural isomer: Molecules with the same formula but different connectivity.

• Branched alkane: An alkane with one or more substituents.

• Haloalkane: An alkane with a halogen substituent (e.g., Cl, Br).

• Alkyl group: A group derived from an alkane by removing one hydrogen (e.g., methyl, ethyl).

Example: Isobutane and n-butane are structural isomers.

IUPAC Naming and Structural Formulas

Systematic Naming and Drawing

The International Union of Pure and Applied Chemistry (IUPAC) provides rules for naming alkanes and cycloalkanes. The name reflects the longest carbon chain and the position of substituents.

• IUPAC name: Systematic name based on structure.

• Geometric formula for cycloalkane: Shows ring structure, e.g., cyclopentane.

Example: 2-methylpropane is a branched alkane.

Combustion of Alkanes

Chemical Reactions and Equations

Alkanes undergo complete combustion in the presence of oxygen to produce carbon dioxide and water.

• Balanced chemical equation: For methane combustion:

$\mathrm{CH_4 + 2\ O_2 \rightarrow CO_2 + 2\ H_2O}$

Example: Combustion of propane:

$\mathrm{C_3H_8 + 5\ O_2 \rightarrow 3\ CO_2 + 4\ H_2O}$

Properties of Alkanes

Physical and Chemical Characteristics

Alkanes are generally nonpolar, insoluble in water, and less dense than water. Their physical state at room temperature depends on chain length.

• Solubility: Insoluble in water.

• Physical state: Short-chain alkanes are gases; longer chains are liquids or solids.

• Density: Alkanes float on water.

Example: Hexane is a liquid at room temperature.

Functional Groups and Classes of Organic Compounds

Key Functional Groups

Functional groups are specific groups of atoms within molecules that determine the chemical properties of the compound. Recognizing functional groups is essential for classifying organic compounds.

• Alkene: Contains a carbon-carbon double bond.

• Alkyne: Contains a carbon-carbon triple bond.

• Aromatic compound: Contains a benzene ring.

• Alcohol: Contains a hydroxyl group (-OH).

• Thiols: Contains a sulfhydryl group (-SH).

• Ether: Contains an oxygen atom between two alkyl groups (R-O-R').

• Carbonyl group: Contains a C=O bond.

• Aldehyde: Carbonyl group at the end of a chain.

• Ketone: Carbonyl group within a chain.

• Carboxylic acid: Contains a carboxyl group (-COOH).

• Ester: Contains a -COOR group.

• Amine: Contains a nitrogen atom.

• Amide: Contains a carbonyl group bonded to nitrogen.

Example: Ethanol is an alcohol; acetone is a ketone.

Classification by Functional Groups

Identifying Compound Classes

Organic molecules are classified based on their functional groups. This classification helps predict their chemical behavior.

• Alkanes: Only single bonds.

• Alkenes: At least one double bond.

• Alkynes: At least one triple bond.

• Aldehydes: Terminal carbonyl group.

• Carboxylic acids: Carboxyl group.

• Esters: Carboxylate group.

• Amides: Carboxyl group bonded to nitrogen.

Example: Butanoic acid is a carboxylic acid.

Saturated and Unsaturated Hydrocarbons

Definitions and Properties

Saturated hydrocarbons contain only single bonds, while unsaturated hydrocarbons have double or triple bonds.

• Saturated hydrocarbon: Alkane; all carbon-carbon bonds are single.

• Unsaturated hydrocarbon: Alkene or alkyne; contains double or triple bonds.

• Cis-trans isomers: Isomers with different spatial arrangements around a double bond.

• Aromatic compounds: Compounds with benzene rings.

Example: Ethene (ethylene) is an unsaturated hydrocarbon.

Identification and Naming of Hydrocarbons

Structural Formulas and IUPAC Names

Hydrocarbons can be identified and named based on their structural formulas and functional groups.

• Alkanes, alkenes, cycloalkenes, alkynes, aromatic compounds: Identified by their bonding and ring structures.

• IUPAC naming: Systematic approach for naming based on structure and substituents.

• Condensed structural formula: For example, cyclohexene: C6H10.

Example: Benzene is an aromatic compound.

Cis-Trans Isomerism in Alkenes

Geometric Isomers

Alkenes can exhibit cis-trans isomerism due to restricted rotation around the double bond.

• Cis isomer: Substituents on the same side of the double bond.

• Trans isomer: Substituents on opposite sides.

Example: 2-butene exists as cis-2-butene and trans-2-butene.

Addition and Hydration Reactions

Reactions of Unsaturated Hydrocarbons

Alkenes and alkynes undergo addition reactions, where atoms are added to the carbon-carbon multiple bond. Hydration is a specific addition of water.

• Addition: Atoms or groups are added across a double or triple bond.

• Hydration: Addition of water (H2O) to a double bond, forming an alcohol.

Example: Hydration of ethene:

$\mathrm{CH_2=CH_2 + H_2O \rightarrow CH_3CH_2OH}$

Summary Table: Functional Groups and Compound Classes

Classification of Organic Compounds

Functional Group	Class of Compound	Example
None (single bonds)	Alkane	Propane
C=C (double bond)	Alkene	Ethene
C≡C (triple bond)	Alkyne	Acetylene
Benzene ring	Aromatic compound	Benzene
-OH	Alcohol	Ethanol
-SH	Thiol	Methanethiol
R-O-R'	Ether	Diethyl ether
C=O (end)	Aldehyde	Formaldehyde
C=O (middle)	Ketone	Acetone
-COOH	Carboxylic acid	Acetic acid
-COOR	Ester	Methyl acetate
-NH2	Amine	Methylamine
-CONH2	Amide	Acetamide

Additional info: Table entries inferred for completeness based on standard organic chemistry functional groups.