Alkenes — Core Principles

Core Principles

Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond ($C=C$). Their general formula is $C_nH_{2n}$. Each carbon in the double bond is $sp^2$ hybridized, resulting in a trigonal planar geometry and restricted rotation, which gives rise to cis-trans isomerism.

The double bond consists of a strong sigma ($\sigma$) bond and a weaker, more exposed pi ($\pi$) bond, making alkenes electron-rich and highly reactive towards electrophiles. \n\nNomenclature involves finding the longest chain containing the double bond, numbering to give the double bond the lowest locant, and changing the '-ane' suffix to '-ene'.

\n\nAlkenes are prepared by elimination reactions like dehydration of alcohols and dehydrohalogenation of alkyl halides (often following Saytzeff's rule), or by partial hydrogenation of alkynes. \n\nTheir characteristic reactions are electrophilic additions, including hydrogenation, halogenation (anti-addition), hydrohalogenation (Markovnikov's rule, or anti-Markovnikov with HBr/peroxides), and hydration.

They also undergo oxidation (Baeyer's test, ozonolysis) and polymerization. These reactions are fundamental to their industrial applications, such as the production of plastics like polyethylene and polypropylene, and their role in natural processes like fruit ripening.

Often confused with

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

Alkenes vs Alkanes and Alkynes

Aspect	Alkenes	Alkanes and Alkynes
General Formula (acyclic, one bond)	Alkanes: $C_nH_{2n+2}$	Alkenes: $C_nH_{2n}$
Type of C-C Bonds	Only single bonds	At least one double bond
Hybridization of C-C bond carbons	$sp^3$	$sp^2$
Geometry around C-C bond carbons	Tetrahedral	Trigonal planar
Characteristic Reactions	Free radical substitution	Electrophilic addition
Degree of Unsaturation	Saturated (0)	Unsaturated (1)
Test for Unsaturation	No reaction with $Br_2/CCl_4$ or Baeyer's reagent	Decolorizes $Br_2/CCl_4$ and Baeyer's reagent

Alkanes are saturated hydrocarbons with only single bonds, exhibiting $sp^3$ hybridization and tetrahedral geometry, primarily undergoing free radical substitution. Alkenes, with at least one double bond, are unsaturated, feature $sp^2$ hybridization and trigonal planar geometry, and are characterized by electrophilic addition reactions.

Alkynes, containing at least one triple bond, are even more unsaturated, possess $sp$ hybridization and linear geometry, and also undergo electrophilic addition, often twice across the triple bond. The degree of unsaturation dictates their reactivity and the types of reactions they readily undergo, with alkenes and alkynes being much more reactive than alkanes towards electrophiles.

Why it is tested: For NEET, understanding these fundamental differences is crucial for predicting reaction products, identifying unknown compounds through chemical tests, and grasping the underlying principles of organic reactivity. Questions often involve distinguishing between these hydrocarbon types based on their reactions or structural features.