Organic Reaction Mechanisms Notes

Reactions in organic chemistry involve the movement of electrons, leading to the breaking and making of bonds.
Covalent bonds are formed through the sharing of electrons (module one).
Diagrams represent reaction mechanisms, showing electron movement with arrows.
An arrow indicates a pair of electrons moving from the tail to the head.
Electron movement can break bonds, resulting in charged ions.
Heterolytic bond cleavage: One atom receives both electrons from the bond, creating different charges on the resulting ions.

Reactions occur at specific parts of molecules due to their vulnerability.
Highly electronegative atoms (halogens like O, N, and F) attract electrons (module one).
Electronegativity: The ability of an atom to attract electrons towards itself.
Electronegative atoms bonded to less electronegative atoms create weak bonds, making them reactive.
The electronegative atom becomes slightly negatively charged, while the other atom becomes slightly positively charged.

A good leaving group can accept both electrons from a bond, leaving as a negative ion and forming a positive ion (carbocation).
Carbocation: A carbon atom with a positive charge.
The carbocation is susceptible to nucleophilic attack.
SN1 reactions occur in two steps. "SN1" can be remembered as "one thing happening at a time."

Electrons are pulled away from the bond towards the leaving group (X).
The bond breaks, forming a carbocation (+C) and a negative leaving group (-X).
The carbocation reacts with a nucleophile (Nu), forming a new bond and stabilizing the charge.

The leaving group's departure is slower than nucleophilic attack.
Reaction rate depends on the concentration of reactants in the slow step (step 1).
Rate Law: rate = k[reactant] where k is the rate constant. The rate is only affected by the rate constant; the amount of nucleophile does not affect the speed.
Half-broken bond: Transition state.
Carbocation: Reaction intermediate (formed and used up during the reaction).
Transition state symbol:
eq (an equal sign with a straight line through it).

A nucleophile gets close to the slightly positive carbon, stabilizing it and helping the leaving group to leave (even if it's a poor leaving group).
The nucleophile approaches from the opposite side as the leaving group, causing the leaving group to depart.
Products are the same as in SN1: a nucleophile attached to the carbon and the leaving group.
SN2 involves both nucleophile attack and leaving group departure in one step.
Transition state: Nucleophile forming a partial bond with the carbon, while the leaving group still has a partial bond.
The transition state has an overall negative charge.

Experimental determination involves increasing the concentration of the nucleophile.
If the reaction rate increases, it's SN2.
If the reaction rate doesn't change, it's SN1 (zero order with respect to the nucleophile).

SN2 reactions: Invert the configuration if the reacting carbon is chiral, making it stereospecific.
The nucleophile attacks from the opposite side of the leaving group due to charge repulsion.
The nucleophile ends up on the opposite side of the leaving group.
SN1 reactions: No stereospecificity.
The leaving group leaves, forming a carbocation intermediate.
The nucleophile can attack from either side (left or right), resulting in a 50/50 mix of R and S isomers.
In SN2, if the starting molecule is in the R configuration, the product will be in the S configuration, and vice versa.

Steric hindrance: The amount of blockage around the carbon.
Bulky alkyl groups around the carbon prevent nucleophile access, slowing down SN2 reactions.
SN1 reactions are favored with bulky groups because the leaving group leaves first, creating space for nucleophilic attack.

Alkyl groups can partially donate electrons to the central carbon, reducing the positive charge and weakening the bond to the leaving group.
This favors SN1 reactions.

A good leaving group (weak base, large molecule/atom) favors SN1 reactions.
Large atoms have valence shells far from the nucleus, forming longer, weaker bonds with carbon.
Examples: Bromine, iodine (good leaving groups); nitrogen, oxygen, fluorine (poor leaving groups).
A poor leaving group favors SN2 reactions.

SN2 reactions require a good nucleophile to compete with the leaving group.
SN1 reactions don't depend on nucleophile strength because the leaving group leaves before the nucleophile attacks.

Polar solvents, especially those forming hydrogen bonds, stabilize ions and favor SN1 reactions.
Water molecules (polar solvent) attract the leaving group and reduce the positive charge on the carbon.
Hydrogen bonding of polar solvents to nucleophiles reduces their reactivity, making SN2 less likely.

An extra step is needed to deprotonate the molecule and regain neutrality by donating electrons from the N-H bond to the nitrogen, releasing an H+.

Promote leaving group departure by forming a bond with the hydrogen, causing electrons to be lost from the carbon to leaving group bond which floats off floating off with a positive carbocation left behind.