Why are alkenes more reactive than alkanes?

Alkenes are more reactive than alkanes primarily due to the presence of the carbon-carbon double bond. This double bond consists of a strong sigma ($sigma$) bond and a weaker pi ($pi$) bond. The $pi$-bond has exposed electron density above and below the plane of the molecule, making it a region of high electron density. This electron-rich nature allows the alkene to act as a nucleophile, readily attacking electron-deficient species (electrophiles). The $pi$-bond is also weaker than the $sigma$-bond, making it easier to break and form new, stronger $sigma$-bonds in addition reactions. Alkanes, having only strong $sigma$-bonds, are much less reactive.

What is Markovnikov's Rule and why is it important for alkenes?

Markovnikov's Rule states that in the electrophilic addition of an unsymmetrical reagent (like H-X or H-OH) to an unsymmetrical alkene, the positive part of the reagent (usually hydrogen) adds to the carbon atom of the double bond that already has a greater number of hydrogen atoms. Conversely, the negative part adds to the carbon with fewer hydrogen atoms. This rule is crucial because it helps predict the major product of such reactions, which proceeds via the formation of the most stable carbocation intermediate (tertiary > secondary > primary). Understanding this rule is essential for predicting reaction outcomes in NEET.

How does cis-trans isomerism affect the physical properties of alkenes?

Cis-trans isomerism significantly impacts physical properties. Cis isomers, due to their bent structure, often possess a net dipole moment, leading to stronger dipole-dipole interactions and consequently higher boiling points compared to their more symmetrical trans counterparts, which often have zero net dipole moment. However, trans isomers, being more symmetrical, can pack more efficiently into a crystal lattice in the solid state. This better packing leads to stronger intermolecular forces in the solid, resulting in higher melting points for trans isomers compared to cis isomers.

What is the Baeyer's test and what does it indicate?

Baeyer's test uses cold, dilute, alkaline potassium permanganate ($ ext{KMnO}_4$) solution. When an alkene is treated with this reagent, the purple color of the permanganate solution disappears, and a brown precipitate of manganese dioxide ($ ext{MnO}_2$) is formed. This color change indicates the presence of unsaturation (carbon-carbon double or triple bonds). The reaction itself is a syn-dihydroxylation, where two hydroxyl groups are added to the same face of the double bond, forming a vicinal diol. It's a classic qualitative test for alkenes.

What is the peroxide effect in alkene reactions?

The peroxide effect, also known as the Kharasch effect, refers to the anti-Markovnikov addition of hydrogen bromide (HBr) to unsymmetrical alkenes in the presence of organic peroxides. Unlike the ionic Markovnikov addition, this reaction proceeds via a free radical mechanism. The peroxide initiates the formation of a bromine radical, which then adds to the alkene carbon that results in the formation of the more stable carbon radical. This leads to the bromine adding to the carbon with more hydrogen atoms. It is important to remember that this effect is specific to HBr and does not occur with HCl or HI.

Why are alkenes insoluble in water but soluble in organic solvents?

Alkenes are nonpolar or very weakly polar molecules. The carbon-carbon double bond itself is nonpolar, and while C-H bonds have a slight polarity, the overall molecular geometry often leads to cancellation of these small bond moments. Water, on the other hand, is a highly polar solvent, capable of forming strong hydrogen bonds. According to the 'like dissolves like' principle, nonpolar substances do not dissolve in polar solvents because they cannot form strong enough interactions to overcome the strong intermolecular forces (like hydrogen bonds) within the polar solvent. Conversely, alkenes readily dissolve in nonpolar organic solvents like benzene or ether, as both are nonpolar and can form weak but effective London dispersion forces with each other.

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Version 1Updated 22 Mar 2026

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The physical properties of alkenes, like their physical state, melting and boiling points, density, and solubility, are primarily influenced by their molecular mass, molecular geometry, and the absence of strong intermolecular forces such as hydrogen bonding. Chemically, alkenes are characterized by the presence of a carbon-carbon double bond, which contains a $pi$ -bond. This $pi$ -bond is a region…

Quick Summary

Alkenes are unsaturated hydrocarbons featuring at least one carbon-carbon double bond. Their physical properties are largely dictated by molecular size and geometry. Smaller alkenes (C2-C4) are gases, C5-C17 are liquids, and larger ones are solids.

Melting and boiling points generally increase with molecular mass but decrease with branching. Cis-trans isomerism causes cis isomers to have slightly higher boiling points due to dipole moments, while trans isomers often have higher melting points due to better crystal packing.

Alkenes are nonpolar, making them insoluble in water but soluble in organic solvents, and they are less dense than water.

Chemically, the electron-rich $\pi$ -bond makes alkenes highly reactive, primarily undergoing electrophilic addition reactions. Key reactions include hydrogenation (addition of $\text{H}_2$ to form alkanes), halogenation (addition of $\text{X}_2$ to form dihalides), hydrohalogenation (addition of $\text{HX}$ to form alkyl halides, following Markovnikov's rule, or anti-Markovnikov with HBr/peroxides), and hydration (addition of $\text{H}_2\text{O}$ to form alcohols, following Markovnikov's rule).

They also undergo oxidation reactions like Baeyer's test (cold, dilute $\text{KMnO}_4$ for diols) and oxidative cleavage (hot $\text{KMnO}_4$ or ozonolysis for aldehydes, ketones, or carboxylic acids).

Alkenes can also polymerize and undergo combustion.

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Key Concepts

Markovnikov's Rule Application

This rule is fundamental for predicting the regioselectivity of electrophilic addition reactions to…

Ozonolysis for Structure Elucidation

Ozonolysis is a powerful analytical tool used to determine the position of the double bond in an unknown…

Distinguishing Cis and Trans Isomers

While cis and trans isomers have the same molecular formula, their distinct geometries lead to differences in…

Physical State: —

\text{C}_{2-4}

gases,

\text{C}_{5-17}

liquids,

\text{C}_{18+}

solids.

Boiling Point: —

\uparrow

with MW,

\downarrow

with branching. Cis > Trans (due to dipole).

Melting Point: —

\uparrow

with MW. Trans > Cis (due to packing).

Solubility: — Insoluble in

\text{H}_2\text{O}

, soluble in organic solvents.

Density: — Less dense than

\text{H}_2\text{O}

Electrophilic Addition: — Characteristic reaction.

- Hydrogenation: $\text{R-CH=CH-R'} + \text{H}_2 \xrightarrow{\text{Ni/Pt/Pd}} \text{R-CH}_2\text{-CH}_2\text{-R'}$ (Syn-addition) - Halogenation: $\text{R-CH=CH-R'} + \text{X}_2 \xrightarrow{\text{CCl}_4} \text{R-CH(X)-CH(X)-R'}$ (Anti-addition) - Hydrohalogenation: $\text{R-CH=CH}_2 + \text{HX} \rightarrow \text{Markovnikov product}$ - Anti-Markovnikov (HBr/Peroxide): $\text{R-CH=CH}_2 + \text{HBr} \xrightarrow{\text{Peroxide}} \text{R-CH}_2\text{-CH}_2\text{Br}$ - Hydration: $\text{R-CH=CH}_2 + \text{H}_2\text{O} \xrightarrow{\text{H}^+} \text{Markovnikov alcohol}$

Oxidation:

- Baeyer's Test: Cold, dil, alk $\text{KMnO}_4 \rightarrow$ Vicinal diol (Syn-dihydroxylation, purple to brown ppt) - Ozonolysis: $\text{R}_2\text{C=CR}_2 \xrightarrow{\text{1. O}_3 \text{ 2. Zn/H}_2\text{O}} \text{R}_2\text{C=O} + \text{O=CR}_2$ - **Hot $\text{KMnO}_4$ :** Cleavage to ketones/carboxylic acids/ $\text{CO}_2$ .

Polymerization: — Addition polymerization to form long chains.

Combustion: —

\text{C}_n\text{H}_{2n} + \frac{3n}{2}\text{O}_2 \rightarrow n\text{CO}_2 + n\text{H}_2\text{O}

Markovnikov's Rule: Hydrogen to the Rich (more H's). Anti-Markovnikov: Bromine to the Rich (more H's, with Peroxides).

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