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Substitution Reactions of Alkyl Halides

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Kacie Minner

on 30 April 2014

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Transcript of Substitution Reactions of Alkyl Halides

Substitution Reactions of Alkyl Halides

Kaitlyn Corder
Tiffany Haley
Kacie Minner
Jena Murray

What is a Substitution Reaction?
A substitution reaction is a type of chemical reaction where an atom or functional group of a molecule is replaced by another atom or functional group.
The fundamental substitution reactions involve alkyl halides.

Substitution Mechanisms
There are two mechanisms through which substitution reactions can occur. Sn1 and Sn2; each involves reaction of a nuleophile and an electrophile.
What Factors Effect A Substitution Reaction
Leaving Groups
Structure of Alkyl Halide
Comparison of Sn1 Sn2 Reactions
Alkyl Halides
An alkyl halide are halogens with alkly groups attached to the halogens. Halogens include F, Cl, Br, and I and correspond to X.
Why are alkyl halides a good introduction to substitution reactions?
Alky halides have good leaving groups, because the halide is easily displaced. Once learning substitution reactions with alkyl halides it is easier to to do more complex reactions.
A Sn2 reaction is a one-step concerted reaction.
The nucleophile attacks the backside of the electrophile that holds the leaving group and displaces it.
The rate of reaction depends upon the concentration of the nucleophile and the alkyl halide.
A Sn1 reaction is a two step reaction that takes place through a carbocation intermediate.
Due to Carbocation intermediate, both hydrogen and methyl shifts can occur resulting in rearrangement.
The rate of the reaction depends on the concentration of the alkyl halide.
Factors Affecting a Sn1 Reaction
Structure of Alkyl Halide
Leaving Group
The SN1 tends to proceed with weak nucleophiles – generally neutral compounds such as solvents like CH3OH, H2O, CH3CH2OH, and so on.
How do you know which will happen?
Factors Affecting a Sn2 Reaction
Structure of Alkyl Halide
The rate of SN2 reactions are affected by the structure of the alkyl halide.
As the alky groups attached to the halide become larger or more numerous the rate of the reaction with the nucleophile decreases.

Leaving Group
Stronger bases = stronger nucleophiles
Better nucleophile = faster reaction
Good leaving group reduces the barrier to a reaction.
Stable anions that are weak bases are usually good leaving groups and can delocalize charge
The weaker the base the better it is as a leaving group.
The SN2 reaction is favored by polar aprotic solvents – these are solvents such as acetone, DMSO, acetonitrile, or DMF that are polar enough to dissolve the substrate and nucleophile but do not participate in hydrogen bonding with the nucleophile.
An SN1 reaction speeds up with a good leaving group. This is because the leaving group is involved in the rate-determining step. A good leaving group wants to leave so it breaks the C-Leaving Group bond faster. Once the bond breaks, the carbocation is formed and the faster the carbocation is formed, the faster the nucleophile can come in and the faster the reaction will be completed.
The more stable the carbocation, the quicker the reaction.
Sn1 reactions prefer tertiary substrates. The following is the order of reactivity – 3> 2 > 1 > methyl groups
1. The concentration of the nucleophile
2. The reactivity of the nucleophile
3. The solvent in which the reaction is carried out
A Sn1 reaction is favored by a polar protic solvent. Sometimes in an SN1 reaction the solvent acts as the nucleophile. This is called a solvolysis reaction . The polarity and the ability of the solvent to stabilize the intermediate carbocation is very important because it helps the reaction to occur faster.
In polar protic solvents the nucleophilicity increases going down the periodic table.
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