Substitution Reaction

• Substitution reactions occurs wehn one functional group replaces another; Sn1 and Sn2
• An Sn1 reaction has 2 steps and has a rate that is dependent on only one of the reactants
• Formation of the carbocation; this is the rate-determining step.
• The second step happens very quickly; the nucleohphile attacks the carbocation
• In an Sn1 reaction the leaving group (the group being replaced) simply breaks away on its own to leave a carbocation behind

•   If the cabocation carbon began and ended an Sn1 reaction as a chiral carbone, both enantiomer would be produced; the intermediate carbocation is planar and the nucelophile is able to attack it from either sides.
• Rearrangement may occur if the carbocation can rearrange to a more stable form
• Elimination (E1 reaction) often accompnaies Sn1 reactions because the nucleophile may act as a base to abstract a proton from the carbocation, forming a C-C double bond.

• Sn2 reactions occur in a single step; nucleophile attacks the intact substrate from behind the leaving group and knocks the leaving group free while bonding to the substrate.
  
• The rate is dependent on the concentration of the nucleophile and the substrate.
• If the carbon were chiral, the relative configuration wold be changed but the absolute configuration might or might not be changed.
• Tertiary carbon would sterically hinder the nucleophile in this reaction; Sn2 reactions don't typically occur with tertiary substrates.
  
•  Rate decreases from methyl to secondary substrates
• If the nucleophile is a strong base and the substrate too hindered, an elimination (E2 reaction) may occur
• In an E2 reaction, the nucleophile acts as a base abstracting a proton and, in the same step, the leaving group leaves the substrate forming a C-C doubl bond.
• Bulky nucleophiles also hinder Sn2 reactions