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Alkenes: bonding and reactivity

Alkenes: bonding and reactivity

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Explainer Video

Tutor: Alisha

Summary

Alkenes: bonding and reactivity

In a nutshell

Alkenes are unsaturated hydrocarbons. They consist of both a sigma bond and a pi pond. The presence of the pi-bond makes them quite reactive, serving as a good starting material to produce other organic molecules.



Alkenes

Alkenes are classed as unsaturated hydrocarbons as they contain carbon-to-carbon double bonds (C=CC=CC=C​). They have the general formula CnH2n.C_nH_{2n}.Cn​H2n​.​ 


​Here are some simple alkenes:

NAME

 MOLECULAR FORMULA 

Ethene
​C2H4C_2H_4C2​H4​​​
Propene
​C3H6C_3H_6C3​H6​​​
Butene
​C4H8C_4H_8C4​H8​​​
Cyclopentene
​C5H8C_5H_{8}C5​H8​​​



Bonding

The overlap of atomic orbitals and the sharing of electrons between two different atoms leads to the formation of a covalent bond.
1.
s orbitals
2.
Overlap
3.
​σ\sigmaσ- bond​


Sigma bonds (σ−bond)(\sigma-bond)(σ−bond) are formed by the overlap of s orbitals between two atoms. There is a high electron density in this region giving rise to a strong electrostatic attraction between the nuclei and the shared pair of electrons. This results in a high bond enthalpy. 
1.
p orbitals
2.
Overlap
3.
​π\piπ- bond​


A double bond consists of a sigma bond and a pi bond (π−bond)(\pi-bond)(π−bond). Pi bonds are formed by the sideways overlap of adjacent p-orbitals, above and below the plane of the atoms. The electrons are less localised than those in the sigma bond. There is a weaker force of attraction between the π\piπ electrons and nuclei, therefore a lower bond enthalpy compared to sigma bonds. 



Reactivity of alkenes

The low bond enthalpy and high electron density in the pi bond of alkenes makes the C=CC=CC=C bonds reactive and thus prone to an attack by electrophiles. The reactivity of alkenes make them a good starting material to produce other organic molecules.

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Learn with Basics

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Combustion, oxidation and thermal decomposition

Unit 1

Combustion, oxidation and thermal decomposition

Hydrocarbons: alkanes and alkenes

Unit 2

Hydrocarbons: alkanes and alkenes

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Alkenes: bonding and reactivity

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Alkenes: bonding and reactivity

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FAQs - Frequently Asked Questions

What makes alkenes reactive?

The low bond enthalpy and high electron density in the pi bond of alkenes makes the C=C bonds reactive and thus prone to an attack by electrophiles.

How are pi (π−) bonds formed?

Pi (π−) bonds are formed by the sideways overlap of adjacent p-orbitals, above and below the plane of the atoms.

How are sigma (σ−) bonds formed?

Sigma (σ−) bonds are formed by the overlap of s orbitals between two atoms.

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