Unit 4 - Organic Chemistry: Chapter 24: Alkanes

Alkanes

Introduction to Alkanes

• Alkanes are the simplest family of hydrocarbons. Hydrocarbons contain only carbon and hydrogen.
• They are primarily obtained from the fractional distillation of crude oil.
• Alkanes contain only carbon-carbon and carbon-hydrogen single bonds.
• Many alkanes are used as fuels (e.g., methane in natural gas, ethane and propane in liquefied petroleum gases).
• Key Point: Alkanes are classified as saturated hydrocarbons because they contain only C-C single bonds and have the maximum possible number of hydrogen atoms for a given number of carbon atoms.

Learning Objectives

• Know the general formula for alkanes.
• Explain why alkanes are classified as saturated hydrocarbons.
• Understand how to draw structural and displayed formulae for alkanes with up to five carbon atoms and name the unbranched-chain isomers.
• Describe the reactions of alkanes with halogens in the presence of ultraviolet radiation, limited to mono-substitution.

Table 24.1: Some Alkanes

Isomers of Alkanes

• Isomers are compounds with the same molecular formula but different structural formulae.
• Alkanes in Table 24.1 are straight-chain or unbranched-chain alkanes.
• Butane and pentane have branched-chain isomers (Figure 24.2).

Figure 24.2: Branched Chain Isomers of $C<em>4H</em>{10}$ and $C<em>5H</em>{12}$

• Isomers of Butane ($C<em>4H</em>{10}$):
  • butane
  • 2-methylpropane
• Isomers of Pentane ($C<em>5H</em>{12}$):
  • pentane
  • 2-methylbutane
  • 2,2-dimethylpropane

Homologous Series

• Alkanes form a homologous series.
• A homologous series is a series of compounds that:
  • have the same functional group
  • have similar chemical properties
  • show a trend (gradation) in physical properties
  • can be described by the same general formula
  • differ from the next by a -$CH_2$- unit.
• Alkanes are the simplest homologous series and do not really have a functional group, as they just contain single C-C and C-H bonds.
• The general formula for alkanes is $C<em>nH</em>{2n+2}$, where n is the number of carbon atoms.
  • Example: Propane ($C<em>3H</em>8$) has 3 carbons, so it has $(2 \times 3) + 2 = 8$ hydrogens.
• Each member of a homologous series differs by a -$CH_2$- unit (Figure 24.3).

MEMBERS OF A HOMOLOGOUS SERIES SHOW A TREND (GRADATION) IN PHYSICAL PROPERTIES

• The first four alkanes are gases at room temperature (-25°C).
• Other alkanes are liquids at room temperature.
• Solids start to appear at about $C<em>{18}H</em>{38}$.
• Boiling points increase in a regular way as the molecules become bigger (Figure 24.4).
• As molecules get larger, intermolecular forces of attraction increase, requiring more energy to break these attractions, hence the increase in boiling points.

MEMBERS OF A HOMOLOGOUS SERIES HAVE SIMILAR CHEMICAL PROPERTIES

• Chemical properties depend on functional groups and bonding within the molecules.
• Alkanes have only carbon-carbon single bonds and carbon-hydrogen bonds, so they behave similarly.
• Alkanes are fairly unreactive organic compounds because C-C and C-H bonds are strong.
• Alkanes are not inert in the sense that they don't react with anything, like neon for example, but they are not very reactive for organic compounds and only really undergo two reactions.

Two Reactions of Alkanes

Combustion

• All alkanes burn in air or oxygen.
• Complete combustion (sufficient oxygen) produces carbon dioxide and water.
  • Example: $CH<em>4(g) + 2O</em>2(g) \rightarrow CO<em>2(g) + 2H</em>2O(l)$
  • Example: $2C<em>2H</em>6(g) + 7O<em>2(g) \rightarrow 4CO</em>2(g) + 6H_2O(l)$
• Incomplete combustion (insufficient oxygen) produces carbon monoxide or carbon (soot) instead of carbon dioxide.
  • Example: $2C<em>2H</em>6(g) + 5O<em>2(g) \rightarrow 4CO(g) + 6H</em>2O(l)$
  • Example: $2C<em>2H</em>6(g) + 3O<em>2(g) \rightarrow 4C(s) + 6H</em>2O(l)$
• Important Note: Hydrogen gas ($H<em>2$) is not formed in combustion reactions; water ($H</em>2O$) is always produced.

Substitution

• Alkanes react with halogens in the presence of ultraviolet radiation (UV light).
• A hydrogen atom in the alkane is replaced by a halogen atom.
• This is a substitution reaction because one atom is replaced by another.
• UV light splits halogen molecules into atoms (free radicals), which are very reactive and react with alkanes.
• Example: Methane reacts with bromine in sunlight to form bromomethane and hydrogen bromide.
  • $CH<em>4(g) + Br</em>2(g) \rightarrow CH_3Br(g) + HBr(g)$
• With excess bromine, multi-substitution can occur, forming a mixture of $CH<em>2Br</em>2$, $CHBr<em>3$, and $CBr</em>4$, plus HBr.
• The hydrogen chloride gas (HCl(g)) formed in the reaction must dissolve in water and dissociate to form H+ ions to form hydrochloric acid.
• Mono-substitution occurs when only one hydrogen atom is replaced by a halogen atom.
• When propane reacts with bromine, two structural isomers can form: 1-bromopropane and 2-bromopropane (Figure 24.6).

Products

• The products formed from substitution reactions of alkanes with halogens are called halogenoalkanes.
• They are named according to the format x-haloalkane, where x indicates the position of the halogen atom in the longest carbon chain.
• Example: When bromine reacts with propane, we get 1-bromopropane and 2-bromopropane.