Hydrocarbons — Core Principles

Core Principles

Hydrocarbons are organic compounds made exclusively of carbon and hydrogen atoms. They are categorized into saturated (alkanes, containing only C-C single bonds, general formula $C_nH_{2n+2}$) and unsaturated (alkenes with C=C double bonds, $C_nH_{2n}$, and alkynes with C≡C triple bonds, $C_nH_{2n-2}$).

Alkanes are relatively unreactive, undergoing free radical substitution (e.g., halogenation) and combustion. Alkenes and alkynes are more reactive due to their pi bonds, primarily undergoing electrophilic addition reactions.

Key reactions for alkenes include hydrogenation, halogenation, hydrohalogenation (Markovnikov's rule, peroxide effect for HBr), hydration, and ozonolysis. Alkynes also undergo similar additions, and terminal alkynes exhibit acidity due to sp-hybridized carbons.

Aromatic hydrocarbons, like benzene, are cyclic, planar, and follow Huckel's rule ($4n+2$ pi electrons), exhibiting special stability. Their characteristic reactions are electrophilic substitution (nitration, halogenation, sulfonation, Friedel-Crafts reactions).

Understanding these classifications, reactions, and associated rules (Markovnikov, Zaitsev, Huckel) is fundamental for NEET aspirants.

Important Differences

vs Alkanes, Alkenes, and Alkynes

Aspect	This Topic	Alkanes, Alkenes, and Alkynes
Bonding	Alkanes: Only C-C single bonds	Alkenes: At least one C=C double bond
General Formula	$C_nH_{2n+2}$	$C_nH_{2n}$
Hybridization of C-C multiple bond carbons	$sp^3$	$sp^2$
Reactivity	Least reactive (saturated)	More reactive (unsaturated, due to pi bond)
Characteristic Reactions	Free radical substitution, combustion	Electrophilic addition, oxidation, polymerization
Test for Unsaturation	No reaction with bromine water or Baeyer's reagent	Decolorizes bromine water and Baeyer's reagent
Acidity	Non-acidic	Non-acidic

Alkanes are saturated hydrocarbons with only single bonds, making them relatively inert and undergoing substitution reactions. Alkenes and alkynes are unsaturated, possessing double and triple bonds respectively, which are sites of high electron density. This makes them significantly more reactive, primarily undergoing addition reactions. Alkynes, particularly terminal ones, also exhibit weak acidic character due to the high s-character of their sp-hybridized carbons, a property not found in alkanes or alkenes. These differences in bonding and reactivity are crucial for distinguishing between these fundamental classes of hydrocarbons.