Structure, Conformations & Physical Properties

Conformations of Alkanes (Ethane & $$n$$-Butane):

• $$\text{Concept: Newman and Sawhorse projections represent spatial arrangements along C--C bond axis.}$$

• $$\text{Stability Order (n-Butane): Anti > Gauche > Partially Eclipsed > Fully Eclipsed}$$

• $$\text{Dihedral Angles } (\theta): \text{Anti } (180^\circ), \, \text{Gauche } (60^\circ), \, \text{Eclipsed } (0^\circ)$$

• $$\text{Exception: Gauche conformation of Ethylene Glycol is more stable than Anti due to intramolecular H-bonding.}$$

Physical Properties:

• $$\begin{aligned} \text{Boiling Point Concept: BP } \propto \text{Molar Mass} \propto \frac{1}{\text{Branching}}\end{aligned}$$ Decreases with branching due to reduced surface area

• $$\text{Formula (BP Order): } n\text{-pentane} > \text{isopentane} > \text{neopentane}$$

• Melting Point Concept: Depends on crystal packing symmetry. Even-carbon alkanes have higher MP than odd ones.

• $$\text{Formula (MP Order): } n\text{-hexane} > n\text{-pentane}$$