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Composed by: Alec and Cory



Introduction

An addition reaction, in organic chemistry, is one in which two parts of a molecule are added to an unsaturated molecule, increasing the saturation of the molecule. In simpler terms, two or more molecules combine to form a larger molecule. Hydrogenation is a common type of addition reaction where hydrogen atoms are added to an unsaturated molecule. Addition reactions can only occur with chemical compounds that have multiply-bonded atoms, such as carbon-carbon double bonds (alkenes) or triple bonds (alkynes). Among alkenes and alkynes, molecules consisting of carbonyl groups can also partake in an addition reaction.


Reaction Examples

In organic chemistry, an addition reaction can be illustrated when a simple molecule gets added across a carbon-carbon double bond in an unsaturated compound. An addition reaction only occurs with unsaturated compounds containing double or triple bonds. They can either be the radical addition of a halide to an alkene, electrophilic or nucleophilic. Below is the general formulas for each of the mentioned types of addition reactions. From top to bottom, electrophilic, nucleophilic, and the addition of a halide to an alkene are represented:

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Halide to an Alkene

One example of an addition reaction is the radical addition of a halide to an alkene. Below shows an addition reaction between chlorine (Cl2) and ethene (CH2=CH2 ) which produces 1,2-dichloroethane (ClCH2(CH2)Cl).


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Electrophilic Addition Reaction

An electrophilic reaction is an addition reaction where a double bond is broken and two new bonds are formed. Here is an example of a electrophilic addition reaction:

Hydrochloric acid reacts with an alkene, ethene, to produce 1-chloroethane:
HCl + CH2:CH2 → CH3CH2Cl

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For an animation of a different electrophilic addition reaction, refer to the video under the Reaction Mechanism Examples heading.

Nucleophilic Addition Reaction

A nucleophilic addition reaction occurs when a double bond is removed by the creation of two new covalent bonds with the addition of a nucleophile.

An example of nucleophilic addition is the addition of hydrogen cyanide across the carbonyl bond in aldehydes to form cyanohydrins. Here, ethanal reacts with hydrogen cyanide to create 2-hydroxypropanenitrile:



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A different nucleophilic addition reaction can be shown in the following animation video:







Reaction Mechanism Examples


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In this reaction mechanism, ethene is reacting with a bromine molecule. Electrons are leaving the double bond from the ethene and moving to one of the bromine atoms. The electron leaves the bromine-bromine bond and moves to the other bromine atom. The bromine atom then bonds to the original ethene that had lost electrons from the double bond. The remaining bromine atom then bonds to the original ethene that had lost electrons from the double bond which then creates 1,2-dibromoethane.Below is a video of an animation of the reaction shown above:









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In this reaction mechanism, ethene is reacting with hydrobromic acid. Electrons are moving from the double bond to the hydrogen atom. The electron leaves the hydrogen-bromine bond and moves to the bromine atom. The hydrogen then is bonded with the original ethene that had lost electrons from the double bond. The bromine atom then bonds with the original ethene that had lost electrons from the double bond as well. This produces 1-bromoethane.



References


Websites:
Darling, DAvid. World's of David Darling. Web. 5 Nov. 2010. <www.daviddarling.info/encyclopedia/.../addition_reaction.html >.

TalkTalk. 2010. Web. 5 Nov. 2010. < http://www.talktalk.co.uk/reference/encyclopaedia/hutchinson/m0026436.html>.

N.p., n.d. Web. 9 Nov 2010. <http://science.jrank.org/pages/11819/addition-reaction.html>.

http://en.wikipedia.org/wiki/Nucleophilic_addition

http://en.wikipedia.org/wiki/Electrophilic_addition

http://en.wikipedia.org/wiki/Addition_reaction


http://www.chemguide.co.uk/mechanisms/nucadd/hcn.html




Images:
Structural Representation of ethene and chlorine Addition Reaction: http://wpcontent.answcdn.com/wikipedia/commons/a/aa/Chlorine_and_etene_addition.png
Atomic Symbol Representation of ethene and chlorine Addition Reaction: http://content.tutorvista.com/chemistry_11/content/us/class11chemistry/chapter17/images/img162.gif
Ethene Reaction Diagram with bromine molecule: http://en.wikipedia.org/wiki/Halogen_addition_reaction
Ethene Reaction Diagram with hydrobromic acid: http://schools-wikipedia.org/wp/a/Alkene.htm
Ethene Reaction with hydrochloric acid: http://www.chemguide.co.uk/mechanisms/eladd/ethhclmech.GIF
Ethanal and hydrogen cyanide Reaction: http://www.chemguide.co.uk/mechanisms/nucadd/hcn.html
General Formulas for Types: http://en.wikipedia.org/wiki/Addition_reaction


Video:
Electrophilic Reaction: http://www.youtube.com/watch?v=-t3yEoYcSdI
Nucleophilic Reaction: http://www.youtube.com/watch?v=c95pJW4BIiI