Comprehensive Study Notes on Alkanes and Hydrocarbons

General Formula: Alkanes follow the homologous series formula of $C_nH_{2n+2}$ .
Hybridization: In alkanes, all carbon atoms exhibit $sp^3$ hybridization.
Molecular Geometry: The geometry around each carbon atom is tetrahedral.
Chemical Bonding: Alkanes consist of only sigma ( $\sigma$ ) bonds, specifically carbon-carbon ( $C-C$ ) and carbon-hydrogen ( $C-H$ ) bonds.
Saturations: They are classified as saturated hydrocarbons because they contain the maximum number of hydrogen atoms per carbon atom with no double or triple bonds.
Polarity: Alkanes are non-polar molecules due to the similar electronegativities of carbon and hydrogen.
Reactivity: They are relatively inert (unreactive) under normal conditions. Their characteristic chemical behavior is to undergo free radical substitution reactions.

Types of Isomerism: Alkanes exhibit chain isomerism only.
Structural Requirements: Isomerism in the alkane series begins with butane ( $C_4H_{10}$ ).
Butane ( $C_4H_{10}$ ) Isomers: There are two structural isomers for butane:
- n-butane: A straight-chain alkane.
- Isobutane: Also known as 2-methylpropane, which features a branched chain.
Pentane ( $C_5H_{12}$ ) Isomers: There are 3 distinct chain isomers.
Hexane ( $C_6H_{14}$ ) Isomers: There are 5 distinct chain isomers.

Physical State at Room Temperature:
- $C_1$ to $C_4$ : These lower alkanes exist as gases.
- $C_5$ to $C_{17}$ : These intermediate alkanes exist as liquids.
- $C_{18}$ and above: These higher alkanes exist as waxy solids.
Boiling Point (B.P.) Trends:
- The boiling point increases as the molecular mass increases due to stronger London dispersion forces.
- Branching in the carbon chain decreases the boiling point because it reduces the surface area for intermolecular contact.
Solubility:
- Alkanes are insoluble in water (polar solvent).
- Alkanes are soluble in non-polar solvents (e.g., ether, benzene).

Paraffin: Alkanes are also known as paraffins. The term is derived from the Latin "parum affinis," meaning "little affinity," which characterizes their low chemical reactivity.
Combustion: Alkanes undergo combustion reactions that are highly exothermic. As a result, they are primarily used as fuels.
Free Radical Substitution: This is the characteristic reaction type for alkanes, where a hydrogen atom is replaced by another atom (typically a halogen) via a radical mechanism.
Cracking and Reforming: These are essential industrial processes used to convert long-chain alkanes into lighter, more useful fuels and aromatic hydrocarbons.

Hydrogenation of Alkenes and Alkynes:
- This process is known as catalytic hydrogenation.
- Reagents: Hydrogen gas ( $H_2$ ) in the presence of a metal catalyst such as Nickel ( $Ni$ ), Platinum ( $Pt$ ), or Palladium ( $Pd$ ).
- Reaction for Alkenes: $R-CH=CH-R' + H_2 \xrightarrow{Ni/Pt/Pd} R-CH_2-CH_2-R'$
- Reaction for Alkynes: $R-C\equiv C-R' + 2H_2 \xrightarrow{Ni/Pt/Pd} R-CH_2-CH_2-R'$
Other Noted Preparation Precursors:
- The transcript indicates preparation methods involving Alkyl Halides ( $R-X$ ) and Sodium salts of carboxylic acids ( $R-COONa$ ), such as the Wurtz reaction ( $2R-X + 2Na \rightarrow R-R + 2NaX$ ) and decarboxylation, though specific detailed steps for these were abbreviated in the source text.