ALKANE
ALKENE
ALKYNE
The Aliphatics
World C — Carbon Chemistry
34.A — ALKANES (THE PARAFFINS)
CₙH₂ₙ₊₂
R34.01 Saturated Hydrocarbons
- General Formula: $C_nH_{2n+2}$.
- Only C-C and C-H single bonds ($sp^3$ hybridized, tetrahedral).
- "Paraffin" means "little affinity" - they are relatively unreactive.
☀️ UV
CH₄
R34.02 Substitution Reactions
- Since all bonds are full, adding atoms requires swapping.
- Halogenation (e.g., $CH_4 + Cl_2 \rightarrow CH_3Cl + HCl$).
- Requires UV light or heat to break the halogen bond.
R34.03 Combustion
- Excellent fuels (Methane = Natural Gas, Octane = Petrol).
- Burn cleanly with a blue flame if oxygen is sufficient.
- Highly exothermic: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + Heat$.
C₁-C₄
C₅-C₁₇
C₁₈+
R34.04 Physical State vs Chain Length
- $C_1 - C_4$: Gases (Methane, Butane).
- $C_5 - C_{17}$: Liquids (Pentane, Octane).
- $C_{18+}$: Waxy solids (Paraffin wax).
- Boiling point increases with chain length (stronger Van der Waals forces).
34.B — ALKENES (THE OLEFINS)
CₙH₂ₙ
R34.05 Unsaturated (Double Bond)
- General Formula: $C_nH_{2n}$.
- Contains at least one C=C double bond ($sp^2$ hybridized, trigonal planar).
- The exposed $\pi$ (pi) electron cloud makes them highly reactive.
R34.06 Addition Reactions
- The weak $\pi$ bond breaks, creating two new bonding sites.
- Halogenation (Adding $Br_2$ decolorizes orange bromine water).
- Hydration (Adding $H_2O$ with acid catalyst makes Alcohol).
Ni/H₂
→
→
R34.07 Hydrogenation (Oils to Fats)
- Adding $H_2$ across the double bond converts an alkene to an alkane.
- Requires a Nickel (Ni) or Palladium (Pd) catalyst.
- Used industrially to turn liquid vegetable oils into solid margarine.
R34.08 Polymerization
- Thousands of alkene molecules (monomers) link together.
- The double bonds open up to form long continuous single-bonded chains.
- Ethene $\rightarrow$ Polyethene (Polythene plastic).
34.C — ALKYNES (THE ACETYLENES)
CₙH₂ₙ₋₂
R34.09 Highly Unsaturated
- General Formula: $C_nH_{2n-2}$.
- Contains at least one $C\equiv C$ triple bond ($sp$ hybridized, linear).
- Two $\pi$ bonds make them the most reactive aliphatics.
R34.10 Oxy-Acetylene Flame
- Ethyne (Acetylene) burns in pure oxygen to produce extremely high temperatures ($>3000^\circ C$).
- Used heavily in industrial metal welding and cutting.
R34.11 Acidic Hydrogen
- Terminal alkynes (triple bond at the end of the chain) have weakly acidic hydrogens.
- The 50% *s-character* of the $sp$ orbital holds electrons tightly, allowing the $H^+$ to leave.
- Can react with strong bases to form acetylide salts.
sp³ (109.5°)
sp² (120°)
sp (180°)
R34.12 Geometry Summary
- Alkanes: Tetrahedral, flexible rotation.
- Alkenes: Planar, rigid double bond (allows cis/trans isomerism).
- Alkynes: Linear, rigid triple bond.