Hydrocarbons - Alkanes (Preparation and Reactions)

Alkanes are saturated open-chain hydrocarbons containing carbon-carbon single bonds with the general formula $C_nH_{2n+2}$.

In alkanes, every carbon atom is $sp^3$ hybridized, forming four sigma bonds in a tetrahedral geometry with a bond angle of $109.5^\circ$.

Preparation via Sabatier-Senderens Reaction: Hydrogenation of alkenes or alkynes using catalysts like $Ni$, $Pt$, or $Pd$ at specific temperatures.

Wurtz Reaction: Treatment of alkyl halides with sodium metal in dry ether to produce higher alkanes with an even number of carbon atoms ($2RX + 2Na \xrightarrow{\text{dry ether}} R-R + 2NaX$).

Decarboxylation: Heating sodium salts of carboxylic acids with soda lime ($NaOH$ and $CaO$ in $3:1$ ratio) to yield an alkane with one less carbon atom than the parent salt.

Kolbe's Electrolytic Method: Electrolysis of an aqueous solution of sodium or potassium salt of a carboxylic acid gives alkanes at the anode.

Substitution Reactions: Alkanes undergo free radical substitution, most notably halogenation in the presence of $UV$ light or heat ($h\nu$).

Combustion: Alkanes burn in oxygen to produce $CO_2$, $H_2O$, and a large amount of heat ($"\Delta H"$ is negative).

Isomerization: $n$-Alkanes heated with anhydrous $AlCl_3$ and $HCl$ gas rearrange to form branched-chain isomers.

Aromatization: Alkanes with six or more carbon atoms heated to $773 K$ at $10-20$ atm over oxides of $V$, $Cr$, or $Mo$ get dehydrogenated and cyclized to benzene or its derivatives.