• 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
  

1. Most of the time on the MCAT and aldehyde or keton will be acting either as the substrate in nucleophilic addition or as a Bronsted-Lowry acid by donating one of its alpha-hydrogens
2. A carbon that is attached to a carbonyl carbon is in the alpha position and is called an alpha carbon; the next carbon is beta carbon and so on down the Greek alphabet.
3. alpha carbon anions are stabilized by resonance. This anion is called an enolate (usually alpha carbon anions are very strong bases and unstable)
   *en from alkene and ol from alcohol 
   *enolate ion is the conjugate base of ketone and aldehyde
   
4. Both aldehydes and ketones are less acidic than alcohols; any electron withdrawing groups attached to the alpha carbon or the carbonyl tend to stabilize the conjugate base and thus increase acidity.
5. Due to the properties of the alpha-hydrogen (hydrogen attached to the alpha carbon) and carbony, ketones and aldehydes exist at room temperature as enol tautomers.