Hydrocarbons I: Structure, Properties, and Reaction Mechanisms

Hydrocarbons: Classification and Structure

Definition and Classes of Hydrocarbons

Hydrocarbons are organic compounds composed exclusively of carbon and hydrogen atoms. They are fundamental to organic chemistry and are classified based on the types of bonds between carbon atoms.

• Saturated hydrocarbons: Only single C–C bonds (alkanes).

• Unsaturated hydrocarbons: Contain one or more multiple C–C bonds (alkenes, alkynes, aromatics).

Example: Methane (CH4) is the simplest hydrocarbon.

Drawing Chemical Structures

Chemical structures can be represented in several ways, each providing different levels of detail:

• Molecular formula: Shows the number of each atom (e.g., C2H6).

• Structural formula: Displays all atoms and bonds explicitly.

• Condensed structural formula: Groups atoms to simplify the structure (e.g., CH3CH3).

• Bond-lined (skeletal) formula: Omits carbon and hydrogen atoms; each vertex represents a carbon atom.

Example: For 2,2-dimethylpentane:

• Molecular formula: C7H16

• Condensed: (CH3)3C(CH2)2CH3

• Skeletal: Zig-zag lines with branches for methyl groups

Alkanes, Alkenes, and Alkynes

Alkanes (CnH2n+2)

Alkanes are acyclic, saturated hydrocarbons forming a homologous series. Each member differs by a –CH2– group.

• Straight-chain alkanes: Carbons form a continuous, unbranched chain.

• General formula: $C_nH_{2n+2}$

• Example: Methane (CH4), Ethane (CH3CH3)

Physical Properties of Straight-Chain Alkanes

Number of Carbons	Formula	Melting Point (°C)	Boiling Point (°C)
1	CH4	-183	-162
2	CH3CH3	-172	-89
3	CH3CH2CH3	-188	-42
4	CH3(CH2)2CH3	-138	0
5	CH3(CH2)3CH3	-130	36
6	CH3(CH2)4CH3	-95	69
7	CH3(CH2)5CH3	-91	98
8	CH3(CH2)6CH3	-57	126
9	CH3(CH2)7CH3	-51	151
10	CH3(CH2)8CH3	-30	174

Alkenes (CnH2n)

Alkenes are unsaturated hydrocarbons with at least one carbon-carbon double bond. The double bond consists of one sigma (σ) and one pi (π) bond.

• General formula: $C_nH_{2n}$

• Example: Ethylene (C2H4)

• Structure: Trigonal planar geometry around double bond

Natural Occurrence: Ethene acts as a plant hormone; other alkenes contribute to flavors and fragrances (e.g., citronellol, limonene).

Alkynes (CnH2n-2)

Alkynes are hydrocarbons containing a carbon-carbon triple bond, which is shorter and stronger than single or double bonds.

• General formula (acyclic): $C_nH_{2n-2}$

• General formula (cyclic): $C_nH_{2n-4}$

• Bond length: 1.20 Å (alkyne), 1.33 Å (alkene), 1.54 Å (alkane)

• Example: Acetylene (H–C≡C–H)

Application: Acetylene is used as a fuel due to its high energy release upon combustion.

Structures and Physical Properties of Hydrocarbons

Solubility and States

Hydrocarbons are nonpolar and dissolve in nonpolar solvents. Their physical state depends on chain length:

• 1–4 carbons: gases

• 5–20 carbons: liquids

• >20 carbons: waxy solids

Intermolecular Forces and Boiling/Melting Points

The strength of van der Waals forces (London dispersion) determines melting and boiling points. These forces increase with:

• Longer chain length (more electrons)

• Larger surface area (straight-chain vs. branched)

Example: n-butane (straight-chain) has a higher boiling point than isobutane (branched).

Physical Properties Table

Type	Intermolecular Forces	Solubility	Melting/Boiling Points
Alkanes	Weak van der Waals	Organic solvents	Low, increases with chain length
Alkenes	Weak van der Waals, slight dipole	Organic solvents	Low, increases with chain length
Alkynes	Weak van der Waals, slight dipole	Organic solvents	Low, increases with chain length

Straight-Chain vs. Branched-Chain Hydrocarbons

• Straight-chain: higher surface area, stronger van der Waals forces, higher boiling point

• Branched-chain: lower surface area, weaker van der Waals forces, lower boiling point

Example: n-butane (B.P. 0°C) vs. isobutane (B.P. –12°C)

Reaction Profiles and Mechanisms

Reaction Profile

A reaction profile (reaction coordinate diagram) shows the change in energy as a chemical reaction progresses. It illustrates bond breaking and formation, and the energy barrier (activation energy).

Example equation:

• Combustion: $(C_6H_{12}O_6)_n + 6nO_2 \rightarrow 6nCO_2 + 6nH_2O + \text{energy}$

Nucleophiles and Electrophiles

• Nucleophiles: Species attracted to positive centers; donate electrons. Examples: $:\!OH$, $R-:\!OH$, $:\!NH_3$

• Electrophiles: Species attracted to negative centers; accept electrons. Examples: $H_3O^+$, $Cl_2$, $Br_2$

Reaction Mechanisms

Mechanisms describe the stepwise process of bond breaking and formation, tracking electron movement.

• Polar mechanisms: Involve nucleophile and electrophile; electron pairs are transferred.

• Radical mechanisms: Involve species with unpaired electrons; single electrons are transferred.

Example (Polar): $H_2O + H^+ \rightarrow H_3O^+$

Example (Radical): $Br\cdot + CH_3\cdot \rightarrow Br-CH_3$

Curly Arrows in Mechanisms

• Double-headed curly arrow: Shows movement of electron pairs (used in polar mechanisms).

• Single-headed curly arrow: Shows movement of single electrons (used in radical mechanisms).

Usage:

• Arrow starts at electron source (lone pair or bond) and points to electron destination (atom or bond).

Bond Fission: Homolytic and Heterolytic Cleavage

Homolytic Fission

Bond breaks so each fragment retains one electron, forming radicals. Common in nonpolar molecules and under conditions like light or heat.

• Example: $Cl_2 \xrightarrow{\text{light}} Cl\cdot + Cl\cdot$

Heterolytic Fission

Bond breaks so one fragment retains both electrons, forming ions (carbocation and carbanion). Common in polar molecules.

• Example: $CH_3Cl \rightarrow CH_3^+ + Cl^-$

Summary Table: Nucleophile, Electrophile, Radical

Species	Type
Br–	Nucleophile
AlCl3	Electrophile
CH3OH	Nucleophile
NO2*	Radical
Br*	Radical

References

• Carey, F.A. (2008) Organic Chemistry, 7th ed. McGraw Hill

• McMurry, J. (2008) Organic Chemistry, 7th ed. Thomson Brooks Cole

• Bruice, P.Y. (2017) Organic Chemistry, 8th ed. Prentice Hall International

• Brown, W.H., Poon, T. (2016) Introduction to Organic Chemistry, 6th ed. Wiley