Key Points

Hydrocarbons are organic compounds containing only carbon and hydrogen. They are classified as saturated (alkanes), unsaturated (alkenes and alkynes), and aromatic hydrocarbons based on the types of carbon-carbon bonds.

Alkanes are saturated hydrocarbons containing only carbon-carbon single bonds. Their general formula is $C_nH_{2n+2}$. They are also known as paraffins because of their low reactivity.

Due to free rotation about the C-C single bond, ethane exists in infinite conformations. The most stable is the staggered conformation, while the least stable is the eclipsed conformation due to torsional strain.

Alkyl halides react with sodium metal in dry ether to form higher alkanes with an even number of carbon atoms. The reaction is: $2R-X + 2Na \xrightarrow{\text{dry ether}} R-R + 2NaX$.

Alkenes are unsaturated hydrocarbons with at least one carbon-carbon double bond (C=C) and have the general formula $C_nH_{2n}$. The double bond consists of one strong sigma ($\sigma$) bond and one weaker pi ($\pi$) bond.

Alkenes show geometrical isomerism due to restricted rotation around the C=C double bond. The cis-isomer has identical groups on the same side, while the trans-isomer has them on opposite sides.

During the addition of an unsymmetrical reagent (like HBr) to an unsymmetrical alkene, the negative part of the addendum attaches to the carbon atom that has fewer hydrogen atoms. The reaction proceeds via a more stable carbocation.

In the presence of a peroxide, the addition of HBr to an unsymmetrical alkene occurs contrary to Markovnikov's rule. The reaction proceeds via a free-radical mechanism, and the Br atom attaches to the carbon with more hydrogen atoms.

Alkenes react with ozone ($O_3$) followed by hydrolysis with $Zn-H_2O$ to cleave the double bond, forming aldehydes or ketones. This reaction is used to determine the position of the double bond in an alkene.

Alkynes are unsaturated hydrocarbons with at least one carbon-carbon triple bond (C≡C) and have the general formula $C_nH_{2n-2}$. The triple bond consists of one sigma ($\sigma$) bond and two pi ($\pi$) bonds.

Hydrogen atoms attached to a triply bonded carbon are acidic due to the high electronegativity of the sp-hybridized carbon atom. Terminal alkynes react with strong bases like sodamide ($NaNH_2$) to form acetylides.

Ethyne undergoes two types of polymerization. Linear polymerization forms polyacetylene, a conducting polymer, while cyclic polymerization in a red hot iron tube at 873 K produces benzene ($C_6H_6$).

Aromatic hydrocarbons, or arenes, are cyclic compounds with special stability. Benzene ($C_6H_6$) is the simplest example, characterized by a planar ring structure with delocalized pi electrons.

For a compound to be aromatic, it must be cyclic, planar, have a continuous ring of p-orbitals, and contain a total of $(4n+2) \pi$ electrons, where n is a non-negative integer (0, 1, 2, ...).

Benzene is characterized by electrophilic substitution reactions where an electrophile ($E^+$) replaces a hydrogen atom. Key examples include nitration, halogenation, sulphonation, and Friedel-Crafts reactions.

Friedel-Crafts alkylation introduces an alkyl group onto the benzene ring using an alkyl halide and a Lewis acid like $AlCl_3$. Acylation introduces an acyl group using an acyl halide or anhydride.

A substituent already present on a benzene ring directs the position of the next incoming electrophile. Groups are classified as either ortho, para-directing or meta-directing.

Activating groups (e.g., $-OH, -NH_2, -CH_3$) increase electron density at ortho and para positions, directing substitution there. Deactivating groups (e.g., $-NO_2, -CN, -COOH$) withdraw electron density, making the meta position the site of substitution.