Organic Compounds and Alkanes

Hydrocarbons: Organic compounds consisting solely of carbon (C) and hydrogen (H) atoms.
- Aliphatic Hydrocarbons: Non-cyclic, straight-chain or branched hydrocarbons. Subdivided into:
- Acyclic: Include alkanes (C-C single bonds), alkenes (C=C double bonds), alkynes (C≡C triple bonds), and alkadienes.
- Alicyclic: Hydrocarbons where carbon atoms are arranged in a ring (cyclic). Includes cycloalkanes, cycloalkenes, and cycloalkadienes.
- Aromatic Hydrocarbons: Compounds like benzene and alkylbenzenes, characterized by stability due to conjugated pi electron systems, following Huckel's rule, which states that a compound is aromatic if it contains (4n+2) pi electrons, where n is a non-negative integer.

Alkanes: A specific class of hydrocarbons characterized by:
- Formula: Alkanes have the general molecular formula $CnH{2n+2}$ .
- Structure: Composed solely of single C-C and C-H bonds (e.g., includes methane (CH₄)). Generally inert due to strong sigma (σ) bonds, making them saturated with the maximum hydrogen atoms.
- Cycloalkanes: Alkanes with carbon atoms arranged in a ring. General formula is $CnH{2n}$ .

Isomerism occurs when two or more different structures exist for the same molecular formula. These variations are called isomers.
- Example: Starting from butane (C₄H₁₀), two isomers are possible: n-butane and isobutane (2-methylpropane).
- For pentane (C₅H₁₂) and beyond, multiple isomers exist with the same formula but different structural arrangements.

IUPAC Naming Conventions:
- For straight-chain alkanes, identify the longest continuous carbon chain as the parent structure. Here is a table of names with corresponding carbon atoms and condensed structural formulae:
- Methane: $C1H4$ (CH₄)
- Ethane: $C2H6$ (CH₃CH₃)
- Propane: $C3H8$ (CH₃CH₂CH₃)
- Butane: $C4H{10}$ (CH₃(CH₂)₂CH₃)
- Pentane: $C5H{12}$ (CH₃(CH₂)₃CH₃)
- Hexane: $C6H{14}$ (CH₃(CH₂)₄CH₃)
- Heptane: $C7H{16}$ (CH₃(CH₂)₅CH₃)
- Octane: $C8H{18}$ (CH₃(CH₂)₆CH₃)
- Nonane: $C9H{20}$ (CH₃(CH₂)₇CH₃)
- Decane: $C{10}H{22}$ (CH₃(CH₂)₈CH₃)
Branched-Chain Alkanes:
- To name branched-chain alkanes, follow these steps:
1. Identify the longest chain for the base name.
2. Identify the substituent (branching) groups, which are modifications of alkanes, with structure $CnH{2n+1}$ . Recognizable alkyl groups include:
 - Methyl: $C1H3$ (CH₃)
 - Ethyl: $C2H5$ (C₂H₅)
3. Number the carbon atoms in the main chain from the end nearest to a substituent group. E.g., 2-Methyl-2-methylbutane.
4. If there are multiple substituents of the same type, use prefixes to denote amounts:
 - 2: di-
 - 3: tri-
 - 4: tetra-
 - 5: penta-
 - 6: hexa-
5. When different substituents are present, list them in alphabetical order. The numerical positions do not affect this ordering.

Alkanes primarily undergo combustion and halogenation (free radical substitution).

Hydrogenation: Converting alkenes or alkynes into alkanes by adding hydrogen, typically using catalysts like nickel (Ni) at high temperatures.
- Reaction Example: CnH{2n} + H2 ightarrow CnH_{2n+2}
Decarboxylation: Heating sodium ethanoate or sodium benzoate with soda lime (NaOH + CaO) to produce alkanes.
- Chemical Reaction: ext{CH}3 ext{COONa(s)} ightarrow ext{CH}4 + ext{Na}2 ext{CO}3
Heating aluminium carbide with hydrochloric acid (HCl) can also generate methane.
- Reaction: ext{Al}4 ext{C}3 + 12 ext{HCl}
  ightarrow 3 ext{CH}4 + 4 ext{AlCl}3

Alkanes are relatively stable due to:
- Strong C-H and C-C bonds.
- Similar electronegativities of carbon and hydrogen result in weak polarization of C-H bonds, which makes them generally resistant to attacks from charged species.
- Reactions often require the introduction of radical mechanisms or conditions (e.g., UV light, heat, catalysts).

Homolytic Fission (Homolysis): Each bonding atom retains one electron. This creates free radicals which are highly reactive. High temperatures, equal electronegativities, and UV light favor this kind of cleavage.
Heterolytic Fission (Heterolysis): An uneven distribution of electrons leads to ions. It prefers conditions where atoms involved have significant electronegativity differences, often facilitated in polar solvents.

Combustion in excess oxygen produces carbon dioxide and water, releasing significant heat, termed complete combustion.
- General Equation: CnH{2n+2} + (3n + 1)O2 ightarrow nCO2 + (n + 1)H_2O

In the presence of UV light, alkanes react with halogens (e.g., Cl₂, Br₂) to produce haloalkanes and hydrogen halides.
- Key Reaction: ext{RH} + ext{X}_2
  ightarrow ext{RX} + ext{HX}
- Multiple substitution reactions of methane with chlorine can lead to various chlorinated products depending on the ratio of reactants.

Alkyl radicals are classified based on the number of carbon substituents attached to the carbon bearing the unpaired electron:
- Methyl Radical (no alkyl group attached) < Primary Radical (one alkyl group) < Secondary Radical (two alkyl groups) < Tertiary Radical (three alkyl groups).
Alkyl groups can stabilize radicals by donating electron density through an electron-donating effect, resulting in a positive inductive effect on adjacent groups.

Initiation: Homolytic fission of a halogen molecule forms free radicals.
Propagation: Repeated steps where free radicals react with alkanes to produce haloalkanes and new radicals.
Termination: Reaction steps where two radicals combine to form stable products, ceasing radical generation.