• 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
  

• Like any carbonyl compound, its stereochemistry makes it susceptible to nucleophiles.
• On the MCAT, look for carboxylic acid to behave as an acid or as the substrate in  a nucleophilic substitution reaction 
• It is a very strong organic acid, adn the conjugate base is stabilized by resonance. Electron withdrawing groups on the alpha carbon help to further stabilize the conjugate base and thus increase the acidity of the corresponding carboxylic acid.
• Carboxylic acids are able to make strong double hydrogen bonds to form a dimer, which in turn significantly increases the boiling point of carboxylic acids by effectively doubling the molecular weight of the molecules leaving the liquid phase.
• The double bonds in unsaturated carboxylic acids impede the crystal lattice and lower melting point.
• carboxylic acids with 4 or less carbons: miscible with water; with 5 or more: increasingly less soluble in water; 10 or more: insoluble in water.
• carboxylic acids are soluble in most nonpolar solvents because the dimer form allows the carboxylic acid to solvate without disrupting the hydrogen bonds of the dimer.
  

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. 

 

 

Aldol (ald from aldehyde and ol from alcholol) condensation is all about alpha-hydrogen activity and the susceptibility of carbonyl carbons to a nucleophile. This reaction is catalyzed by an acid or a base and it occurs when one aldehyde reacts with another, when keton reacts with another, or when an aldehyde reacts with a ketone. I'll break down the reaction step by step:

1. (Base catalyzed reaction) alpha-hydrogen is abstracted by the base resulting in an enolate ion.
2. The enolate ion acts as a nucleophile and attacks the carbony carbon to form an alkoxide ion.
3. Alkoxide ion removes a proton from water to complete the aldol.

(f.y.i. Alkoxide ion is a stronger base than a hydroxide ion becuase it has an electron donating alkyl group attached to the oxygen, thus increasing it s negative charge.)

The resulting aldol molecule is unstable and is easily dehydrated by heat or a base to become an enal. The enal is stabilized by its conjugated double bonds.

Although this reaction is not as simple as it looks, it is easy to memorize if you keep in mind the acidity of the alpha-hydrogen and the planar configuration of the carbonyl, which makes it susceptible to nucleophilic attack.

This is a  must know reaction for the MCAT!!