Study Guide: Hydrocarbons, Nomenclature, and Reactions (CHEM 132, Chapter 12)

Hydrocarbons: Structure, Classification, and Properties

Types of Hydrocarbons

Hydrocarbons are organic compounds composed exclusively of carbon and hydrogen atoms. They are classified based on the types of bonds between carbon atoms and their structural features.

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

• Alkenes (Unsaturated Hydrocarbons): Contain at least one carbon-carbon double bond. General formula: $C_nH_{2n}$.

• Alkynes (Unsaturated Hydrocarbons): Contain at least one carbon-carbon triple bond. General formula: $C_nH_{2n-2}$.

• Arenes (Aromatic Hydrocarbons): Contain conjugated ring systems, such as benzene ($C_6H_6$).

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

Bonding Rules

• Each carbon atom forms four covalent bonds (tetra-valency).

• Hydrogen forms one covalent bond.

• Double and triple bonds count as two and three shared pairs, respectively.

Molecular and Structural Formulas

• Molecular formula: Shows the number and type of atoms (e.g., $C_2H_6$ for ethane).

• Structural formula: Shows the arrangement of atoms and bonds (e.g., $CH_3CH_3$ for ethane).

Isomerism in Hydrocarbons

Constitutional (Structural) Isomers

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

• Constitutional isomers: Differ in the connectivity of their atoms.

• Conformational isomers: Differ by rotation around single bonds.

• Configurational isomers: Cannot be interconverted by rotation around single bonds (e.g., cis/trans isomers).

Nomenclature of Hydrocarbons

IUPAC Naming Rules

The International Union of Pure and Applied Chemistry (IUPAC) system provides standardized rules for naming organic compounds.

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

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

• Name and number side chains (alkyl groups) and halogen substituents (e.g., chloro-, bromo-).

• Use prefixes (di-, tri-, tetra-) for multiple identical substituents.

• Combine substituent names and numbers with the parent name, using dashes and commas appropriately.

Prefixes for Carbon Chain Lengths:

• 1: meth-

• 2: eth-

• 3: prop-

• 4: but-

• 5: pent-

• 6: hex-

• 7: hept-

• 8: oct-

• 9: non-

• 10: dec-

Drawing and Naming Structures

• Given a molecular formula, draw all possible constitutional isomers and name them using IUPAC rules.

• Given an IUPAC name, draw the corresponding structure.

Physical Properties of Hydrocarbons

Polarity and Physical Properties

• Hydrocarbons are generally nonpolar due to similar electronegativities of C and H.

• Physical properties such as melting point and boiling point depend on:

  • Length of the hydrocarbon chain (longer chains = higher boiling/melting points).

  • Branching (more branching = lower boiling point).

  • Presence of multiple bonds (double/triple bonds can lower melting/boiling points compared to alkanes of similar size).

Chemical Reactions of Hydrocarbons

Reactions of Alkanes

• Halogenation: Substitution reaction where a hydrogen atom is replaced by a halogen (e.g., Cl, Br).

Example:

• $CH_4 + Cl_2 \rightarrow CH_3Cl + HCl$

Reactions of Alkenes

• Hydrogenation: Addition of $H_2$ across the double bond to form an alkane.

• Halogenation: Addition of $X_2$ (e.g., $Cl_2$, $Br_2$) across the double bond.

• Hydrohalogenation: Addition of $HX$ (e.g., $HCl$, $HBr$) across the double bond.

• Hydration: Addition of $H_2O$ (in the presence of acid) to form an alcohol.

Markovnikov's Rule: In the addition of $HX$ to an unsymmetrical alkene, the hydrogen atom attaches to the carbon with more hydrogen atoms already attached, and the halide attaches to the carbon with fewer hydrogens.

Example:

• $CH_2=CH_2 + HBr \rightarrow CH_3CH_2Br$

Summary Table: Reactions of Hydrocarbons

Hydrocarbon Type	Reaction	General Equation	Product
Alkane	Halogenation	$RH + X_2 \rightarrow RX + HX$	Haloalkane
Alkene	Hydrogenation	$RCH=CHR' + H_2 \rightarrow RCH_2CH_2R'$	Alkane
Alkene	Halogenation	$RCH=CHR' + X_2 \rightarrow RCHX-CHXR'$	Dihaloalkane
Alkene	Hydrohalogenation	$RCH=CHR' + HX \rightarrow RCHX-CH_2R'$	Haloalkane
Alkene	Hydration	$RCH=CHR' + H_2O \rightarrow RCHOH-CH_2R'$	Alcohol

Counting and Prefixes

Numbering and Naming Chains

• Use prefixes for one through ten to indicate the number of carbons in the main chain or substituents.

• Number the chain to give the lowest possible numbers to substituents and multiple bonds.

Application: Drawing and Naming Isomers

• Given a molecular formula, draw all possible constitutional isomers and name them using IUPAC rules.

• Given an IUPAC name, draw the corresponding structure.

Summary

• Understand the classification, nomenclature, and reactions of alkanes, alkenes, alkynes, and arenes.

• Be able to draw and name isomers, predict physical properties, and write reaction equations for hydrocarbons.

• Apply IUPAC rules and Markovnikov's Rule in naming and predicting products of reactions.