Alkanes are simple hydrocarbons with single bonds between carbon atoms. They're the building blocks of organic chemistry, forming a series where each member differs by one CH₂ group.

Despite their simplicity, alkanes have interesting properties. Their boiling points increase with molecular weight, and straight-chain alkanes boil at higher temperatures than their branched counterparts. This is all due to intermolecular forces.

Properties of Alkanes

Introduction to Alkanes

• Alkanes are a class of organic compounds (hydrocarbons) consisting of only carbon and hydrogen atoms
• They are characterized by single bonds between carbon atoms, making them saturated hydrocarbons
• Alkanes form a homologous series, with each member differing by one CH₂ group

Chemical reactivity of alkanes

• Alkanes are relatively unreactive compared to other organic compounds due to the strength and stability of C-C and C-H bonds
• Alkanes undergo combustion reactions with oxygen produces carbon dioxide and water in a highly exothermic reaction ($CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$)
• Alkanes react with halogens, such as chlorine, under UV light undergoes free radical halogenation where hydrogen atoms are substituted by chlorine atoms ($CH_4 + Cl_2 \xrightarrow{UV} CH_3Cl + HCl$)
  • Further chlorination can occur, replacing more hydrogen atoms (dichloromethane, chloroform, carbon tetrachloride)

Alkane melting and boiling points

• Boiling points and melting points of alkanes increase with increasing molecular weight due to increased strength of intermolecular forces, primarily London dispersion forces
  • London dispersion forces are weak attractive forces between temporary dipoles
  • Larger molecules have more electrons and a greater surface area, resulting in stronger London dispersion forces (pentane vs decane)
• As the number of carbon atoms in an alkane increases, the boiling point and melting point also increase (methane has a lower boiling point than ethane)
  • Methane (CH₄): boiling point -161.5°C, melting point -182.5°C
  • Ethane (C₂H₆): boiling point -88.6°C, melting point -183.3°C

Straight-chain vs branched alkanes

• Straight-chain alkanes have higher boiling points than branched alkanes with the same number of carbon atoms due to differences in molecular structure and intermolecular forces
• Straight-chain alkanes have a more linear structure allows for greater surface area contact between molecules resulting in stronger London dispersion forces (n-hexane)
• Branched alkanes have a more compact structure reduces the surface area available for intermolecular interactions resulting in weaker London dispersion forces (2-methylpentane)
• n-Pentane (straight-chain, 5 carbons) has a higher boiling point ($36.1^\circ C$) than isopentane (branched, 5 carbons) ($27.7^\circ C$)
• n-Hexane (straight-chain, 6 carbons): boiling point 68.7°C
• 2-Methylpentane (branched, 6 carbons): boiling point 60.3°C

Structural Variations of Alkanes

• Alkanes can exist in different conformations, which are spatial arrangements of atoms that can be interconverted by rotation around single bonds
• Isomers are compounds with the same molecular formula but different structural arrangements
  • Structural isomers have different connectivity of atoms
  • Stereoisomers have the same connectivity but different spatial arrangements