Hydrocarbons

Introduction and Classification

Hydrocarbons are binary organic compounds composed entirely of carbon and hydrogen. They are broadly classified into two main categories: aliphatic (open-chain) and aromatic (containing a benzene ring). Aliphatic hydrocarbons are further divided into saturated hydrocarbons (alkanes) containing single covalent bonds, and unsaturated hydrocarbons (alkenes and alkynes) containing double or triple bonds. For board exams, you must be able to distinguish between these classes using general formulas: alkanes ($C_n2n+2$), alkenes ($C_nH_{2n}$), and alkynes ($C_nH_{2n-2}$).

Alkanes (Paraffins) and Alkenes (Olefins)

Alkanes are known as paraffins due to their low chemical reactivity, which is attributed to strong $\sigma$ (sigma) bonds and non-polar characteristics. Their most important chemical reactions are substitution reactions (such as halogenation in the presence of sunlight) and combustion. Conversely, alkenes contain a weak $\pi$ (pi) bond alongside a $\sigma$ bond, making them much more reactive unsaturated hydrocarbons. Alkenes typically undergo addition reactions (such as hydrogenation, halogenation, and hydrohalogenation) and oxidation with potassium permanganate ($KMnO_4$), which serves as a classic laboratory test for unsaturation (Baeyer's test).

Preparation Methods and Sources

Board exams frequently test specific preparation methods for these compounds. Alkanes are commonly prepared by the catalytic hydrogenation of alkenes/alkynes or the reduction of alkyl halides. Alkenes are generally prepared via the dehydration of alcohols (using concentrated $H_2SO_4$ as a dehydrating agent) or the dehydrohalogenation of alkyl halides in alcoholic KOH. Additionally, you should memorize the industrial sources of hydrocarbons, primarily petroleum (crude oil), natural gas, and coal, along with the fractional distillation process used to separate petroleum into useful fractions like petrol, kerosene, and diesel.