These reaction mechanisms are quite long to discuss. Here is a brief discussion about the topic.
SN2 means Nucleophilic substitution reaction bimolecular. Meaning the reaction rate depends on the concentration of the 1) reactant (substrate) and 2) the nucleophile
SN1 means nucleophilic substitution unimolecular. The reaction rate depends only on the concentration of the reactant and not on the nucleophile.
E2 elimination reaction bimolecular same principle as SN2 only difference is that this is ELIMINATION.
E1 - elimination reaction unimolecular same principle as SN1 only difference is that this is ELIMINATION
SN2 and E2 reactions involve strong nucleophiles; SN1 adn E1 reactions involve weak nucleophiles. Nucleophiles are substances that love positive charges. (phile-"love"). How do you differentiate between a strong and weak nucleophile?
A strong nucleophile is can be the conjugate base of an acid or a substance that has a negative charge
Weak Nucleophile Strong Nucleophile
Alkyl halides (RX) usually undergo these substitution and elimination reactions.
SN2 - the nucleophile attacks the backside of the carbon atom. Product formed is of inverse configuration of that of the reactant. SN2 is a one step reaction.
SN1- a two step reaction. Leaving group (X) halide leaves formina a carbocation intermediate. Nucleophile then attacks the carbocation. Two products formed: one with inverse configuration as to that of the reactant and the other is with the same config of the reactant.
In the substitution reactions, the halideis substituted by the nucleophile.
RX + OH - ----> (SN2) ROH + X-
RX + HOH -----> (SN1) ROH + H+ + X-
E2 reaction - one step reaction. E1 - two step reaction.
ELIMINATION REACTIONS PRODUCE ALKENES.
ClCH2CH3 + OH- ---> (E2) CH2=CH2
the halide leaves forming an unstable carbocation. A hydrogen is taken by the nucleophile. Hydrogen comes from the beta carbon with fewest hydrogen.
ClCH2CH3 + HOH ---> (E1) CH2=CH2
To sum it uo, if you are given an equation and asked to determine the type of reaction mechanism involved follow the steps below:
DIFFERENTIATE SUBSTITUTION AND ELIMINATION REACTIONS
DIFFERENTIATE FOR SN2/E2 and SN1/E1
Now you know if the reaction is SUBSTITUTION or ELIMINATION. Look at the reactant side. You will see two reactants. One is the hydrocarbon (most commonly the alkyl halide) and the other is the nucleophile.
Is the nucleophile carrying a negative charge?
YES, the nucleophile has a negative charge.The answer is either SN2 or E2
Is the product an alkene?
YES - E2
NO - SN2
NO, the nucleophile does not carry a negative charge. The answer may be SN1 or E1
Is the product an alkene?
YES - E1
NO - SN1
Relative reactivities of alkyl halides
SN2 methyl halide primary>secondary> tertiary
*Tertiary RX have steric hindrance which gives the nucleophile difficulty to attack the alpha carbon
SN1, E2, and E1 : tertiary>secondary, primary> methyl halide
*THese reactions favor the formation of the most stable carbocation.