Substitution

Substitution Reaction  By: Sarah and Sadie





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= Definition = Substitution Reaction is consider a functional group in which a particular compound is replaced by another compound. In Organic chemistry the Electrophilic and Nucleophilic substitutions are crucial.

= Introduction = Substitution Reaction has several main types of reacting. The three main types of substitution reaction are Electrophilic, Nucleophilic, and Radical. These three specific types will be discussed later on. These reaction types vary depending on the reagent. Knowing reaction types can help you determine the outcome of a product.

//Here is a small, simple example of Substitution Reaction:// **C6H6 + HNO3 (heat and H2SO4 as [|catalyst]) forms C6H5NO2 + H2O **    //Here is another simple example of Substitution Reaction:// **CH4 + Cl2 --> CH3Cl + HCl **  **A hydrogen from the CH4 was replaced by one Cl.**

= Nucleophilic Substitution = This substitution occurs when a reagent is considered a nucleophile. (Nucleophiles) react with [|aliphatic] subtrates in a nucleophilic aliphatic substitution reaction. The electron-rich nucleophile will bond or attack the positive or partially positive charge of an atom that is attached to the [|leaving group]. The positive or partially positive atoms are referred to as electrophiles.

//Here is a simple generic form of Nucleophilic Subsitution://


 * Nuc: + R-LG → R-Nuc + LG: R-Nuc **
 * Explanation: When the nucleophile (nuc) attacks the substrate (r-lg) which forms a new bond and in turn leaves the leaving group to detach from the electron pair. The substrate is left neutral or positively charged.**

//This is a simple example of Nucleophilic Substitution://


 * R-Br + OH− → R-OH + Br− **

Reactions Mechanisms
Two diffferent mechanisms are responsible for these substituions:


 * [|Uni-molecular]Nucleophilic Substitution, more commonly known as SN1(One step)
 * [|Bi-molecular] Nucleophilic Substitution, more commonly known as SN2(More than one step)
 * "SN" stands for "Nucleophilic Substitution"

SN1 reaction mechanism
This specific reaction involves a [|carbocation] intermediate. It is commonly seen as secondary or tertiary alkyl halides and secondary and tertiary alcohols. This is most commonly known is '//dissociation mechanism'//.

SN2 reaction mechanism
A lone pair from the nucleophile attacks the electron [|deficient] core and becomes bonded to it.This leaves out the leaving group. This is often known as '//interchange mechanism'//.
 * Explanation: You begin with hydroxide ion and bromoethane. As the hydroxide ion moves to the attack the bromoethane, the bromo gets pushed out. This leaves ethanol and a bromide ion.**

Radical
This type of substitution reaction involves [|free radicals] which are the [|reactive intermediate]. This usually involves two steps; sometimes three.

= Electrophilic Substitution = This substitution occurs when an electrophile displaces a group within a compound. Usually, but not all the time, it gets displaced with a hydrogen atom.

= Electrophilic aromatic substitution: = Electrophilic [|aromatic] substitutions are [|chemical reactions]. This occurs within [|aromatic]compounds. An aromatic ring usually loses its hydrogen ( it can be something else..) and is replaced with an electrophile. The most important reactions of this type that take place are [|aromatic nitration], [|aromatic halogenation] , [|aromatic sulfonation] and acylation and alkylating [|Friedel-Crafts reactions].

= Electrophilic aliphatic substitution = In Electrophilic [|aliphatic] substitution an electrophile displaces the functional group.

An example:

Reaction Mechanisms

 * SE1 reaction mechanism(Unimolecular)
 * SE2 reaction mechanism(Bimolecular)
 * Stands for "Substitution Electrophile"
 * Attacking of the electrophile

SE1 reaction mechanism
In this reaction the substrate [|ionizes] into a [|carbanion] and a positively charged organic residue. The carbanion then recombines with the electrophile.

SE2 reaction mechanism
This is most commonly seen within the benzene ring nucleus. The electrons become[| delocalized] and the electrophile can then attack.

= Video = media type="youtube" key="MvWQOYLm1f4?fs=1" height="344" width="425"

= Resources =

Video
"Video". Substitution reactions Animation. youtube.com. Nov 8th 2010. []

Images
"SN2 Reaction picture" Reaction Mechanism. wikipedia.com. Nov 9th 2010. "Aliphatic picture". __Nitrosation.__ wikipedia.com. Nov 9th 2010. [] "Moving Image". __Substitution.__ Nov 9th 2010. []

Information
"Substitution Reaction". Wikipedia.com. Nov 3rd 2010. [] "Small Example". __Question__. answerbag.com. Nov 4th 2010. [] "Electrophilic Substitution". wikipedia.com. Nov 8th 2010. [] "Nucleophilic Substitution". Nucleophilic Substitution. wikipedia.com. Nov 9th 2010. [] Raymond E. Davis, Regina Frey, Mickey Sarquis, Jerry L. Sarquis. "Modern Chemistry". Texas : Holt, Rinehart, and Winston. 2006.Text. "Se2 and Se1 reaction mechanisms". __Substitution Reaction Mechanisms.__ transtutor.com. Nov 9th 2010. [] "Radical". __Radical Substitution__. wikipedia.com. Nov 9th 2010. []