Alkanes Knowledge-Based Study

Study notes

1. Alkanes are saturated hydrocarbons composed entirely of carbon and hydrogen atoms connected exclusively by single covalent bonds. In an open-chain (acyclic) alkane, each carbon atom uses $sp^3$ hybrid orbitals to form four sigma bonds oriented toward the vertices of a regular tetrahedron, giving bond angles of approximately 109.5 degrees. Because each carbon atom is bonded to the maximum possible number of hydrogen atoms, these compounds are termed saturated.

   All acyclic alkanes conform to the general molecular formula:

   $C_n H_{2n+2}$

   where n is an integer representing the number of carbon atoms. The simplest members of this family include methane ($CH_4$), ethane ($C_2H_6$), propane ($C_3H_8$), and butane ($C_4H_{10}$). Alkanes constitute a homologous series, meaning that each successive member differs from the preceding one by a constant $-CH_2-$ (methylene) group and exhibits a regular gradation in physical properties.

   Systematic naming of alkanes is established by the International Union of Pure and Applied Chemistry (IUPAC) using a sequential set of rules:

   1. Locate the parent chain: Identify the longest continuous chain of carbon atoms. The parent name consists of a Greek or Latin root indicating chain length (meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-, non-, dec-) followed by the suffix -ane.
   2. Number the parent chain: Number the carbon atoms consecutively starting from the end closer to the first branching point, ensuring substituents receive the lowest possible locants.
   3. Identify and name substituents: Saturated hydrocarbon branches are called alkyl groups and are named by replacing the suffix -ane with -yl (for example, methyl for $-CH_3$ and ethyl for $-CH_2CH_3$).
   4. Combine prefixes and assemble the name: When identical substituents appear, designate their frequency with multiplying prefixes (di-, tri-, tetra-). List different substituents in alphabetical order, disregarding multiplying prefixes. Separate numbers from letters with hyphens, and separate adjacent numbers with commas.

   The physical behavior of alkanes is dictated by their nonpolar covalent nature and weak intermolecular attractions:

   • Intermolecular Forces: Because the electronegativity difference between carbon and hydrogen is minimal, alkane molecules are essentially nonpolar. The only forces holding alkane molecules together in the liquid and solid states are weak London dispersion forces.
   • Boiling Point Progression: Boiling points increase systematically with increasing molecular size and surface area because larger electron clouds are more polarizable, leading to stronger dispersion forces. Under standard conditions, unbranched alkanes containing 1 to 4 carbons are gases, 5 to 17 carbons are liquids, and 18 or more carbons are waxy solids.
   • Influence of Branching: Among constitutional isomers sharing the same molecular formula, branched isomers exhibit lower boiling points than their unbranched counterparts. Branching makes molecules more spherical and compact, reducing the available surface area for intermolecular contact and weakening dispersion forces.
   • Solubility and Density: Alkanes are hydrophobic and virtually insoluble in water, but they readily dissolve in nonpolar organic solvents. All liquid and solid alkanes have densities substantially lower than that of water (approximately 0.6 to 0.8 g/mL) and therefore float on water.

   Alkanes were historically known as paraffins (derived from the Latin words meaning little affinity) because their strong, nonpolar carbon-carbon ($C-C$) and carbon-hydrogen ($C-H$) single bonds make them largely unreactive toward typical laboratory reagents such as acids, bases, and oxidizing agents at room temperature. However, they undergo major reactions under energetic conditions:

   • Combustion: In the presence of excess oxygen, alkanes burn vigorously via an exothermic reaction, releasing substantial thermal energy along with carbon dioxide and water vapor:

   $CH_4 + 2 O_2 \rightarrow CO_2 + 2 H_2O + \text{heat}$

   If the oxygen supply is limited, incomplete combustion occurs, producing toxic carbon monoxide ($CO$) or particulate carbon (soot).

   • Free-Radical Halogenation: In the presence of heat or ultraviolet (UV) light, alkanes react with halogens (principally chlorine and bromine) to form haloalkanes and hydrogen halides. This substitution reaction proceeds through a radical chain mechanism consisting of three distinct stages: initiation (homolytic bond cleavage to form halogen radicals), propagation (generation of alkyl radicals and subsequent product formation), and termination (combination of two radicals to form a stable molecule).