Physical and Chemical Properties — Core Principles

Core Principles

Alkanes are saturated hydrocarbons with the general formula $C_nH_{2n+2}$, featuring only strong, non-polar C-C and C-H single bonds. Their physical properties are governed by weak van der Waals forces.

As chain length increases, boiling points, melting points, and density generally increase. Branching, however, decreases boiling points due to reduced surface area for intermolecular interactions. Alkanes are non-polar, making them insoluble in water but soluble in non-polar organic solvents.

Chemically, alkanes are relatively unreactive ('paraffins'). Their key reactions include complete combustion (producing $CO_2$ and $H_2O$) and incomplete combustion ($CO$ or $C$), both highly exothermic.

Halogenation occurs via a free radical substitution mechanism under UV light or heat, showing selectivity for tertiary > secondary > primary hydrogen atoms. Pyrolysis (cracking) breaks larger alkanes into smaller alkanes and alkenes at high temperatures, vital for the petroleum industry.

Isomerisation converts straight-chain alkanes to branched ones using $AlCl_3/HCl$ catalyst, improving fuel quality. Aromatization converts higher alkanes to aromatic compounds. These properties define their utility as fuels, solvents, and chemical feedstocks.

Often confused with

Side-by-side differences the NEET paper likes to test.

Physical and Chemical Properties vs Branched Alkanes

Aspect	Physical and Chemical Properties	Branched Alkanes
Boiling Point	Higher (due to larger surface area for van der Waals forces)	Lower (due to reduced surface area and more spherical shape)
Melting Point	Generally lower than highly symmetrical branched alkanes, but increases with chain length	Can be higher for highly symmetrical branched alkanes due to efficient crystal packing, but generally increases with chain length
Density	Increases with chain length, generally slightly higher for same carbon number due to tighter packing	Increases with chain length, generally slightly lower for same carbon number due to less efficient packing
Fuel Quality (Octane Number)	Lower (tend to cause knocking in engines)	Higher (burn more smoothly, less knocking)

Straight-chain alkanes, also known as n-alkanes, typically exhibit higher boiling points compared to their branched isomers of the same carbon count. This is attributed to their extended shape, which allows for greater surface area contact between molecules, leading to stronger intermolecular van der Waals forces.

Conversely, branched alkanes, being more compact and spherical, have reduced surface area, resulting in weaker intermolecular forces and thus lower boiling points. While melting points generally increase with chain length for both, highly symmetrical branched alkanes can sometimes have higher melting points due to more efficient crystal lattice packing.

Branched alkanes are also preferred in gasoline due to their higher octane number, indicating smoother combustion.

Why it is tested: For NEET, understanding the comparative physical properties, especially boiling points, between straight-chain and branched alkanes is crucial. Questions often test the ability to rank isomers based on their boiling points. The concept of octane number and why branched alkanes are superior fuels is also relevant, linking chemical properties to practical applications.