Exam 3 Flashcards
Sequential reaction
- first reaction (to completion)
- Second reaction (no interaction with the first reaction)
Substitution Rxn
Replacing one group on a carbon with another
Addition Rxn
combining reactants to form new products (typically one pi bond is split to form two new sigma bonds)
Elimination Rxn
A Hydrogen and a Leaving Group are removed and replaced by a pi bond
Rearrangement Rxn
a reaction that changes orientation
Concerted
All bonds are broken and formed simultaneously - no intermediates with formal charges
Step-wise
bonds are broken and formed in specific steps creating intermediates with formal charges that can move
Homolydic cleavage
each atom gets one electron, typically seen in a free radical halogenation reaction
Heterolydic Cleavage
electrons travel together
Free energy equation
∆G= ∆H - (Tx∆S)
∆H = BFE - BDE
∆H is positive
Energy is absorbed - Endothermic
∆H is negative
Energy is released - exothermic
stronger bonds…
requires more energy to break
Energy Diagram
one peak represents a concerted reaction, multiple peaks represent each step in the reaction
intermediate stability
Carbon with the charge:
primary < secondary < tertiary < benzylic < Allylic (adjacent to a double bond) < Tertiary allylic
Nucleophilic addition
nucleophile uses a lone pair of e- to add itself to the molecule and push off another atom
Leaving Group
atom or group comfortable with a negative charge
Proton Transfer
addition of a hydrogen
Pi bond formation
Transfer of a non bonded lone pair to move the positive charge
Free radical halogenation
*Stereochemistry for chirol centers
1. homolytic cleavage of a halogen, one removes a hydrogen
2. a second homolytic cleavage of a halogen to bond with the radical carbon intermediate
SN2 Reaction
- Concerted substitution
- Backside attack
Factors to consider:
1. Electrophile
methyl > primary > secondary, no tertiary addition
2. Leaving group
I > Br > Cl > F
3. Nucleophile
Must have an available lone pair
4. Solvent
Polar aprotic - no O-H Bonds
DMSO, HMPA, Acetone
SN1 Reaction
- Stepwise
- Leaving groups leaves on its own first
- Racemic
- Cation rearrangement for best stability (Hydride/Methyl shift)
Factors to consider:
1. Electrophile
tertiary > secondary > primary > Methyl
2. Leaving group
I > Br > Cl > F
3. Nucleophile
Weak - Compounds with lone pairs but are neutral
4. Solvent
Polar protic - an O-H Bond helps with cation stability
Elimination Reaction - E1
- Stepwise
- Hydride shift
- double bond with more substituents is preferable
- Leaving group leaves on its own
- Trans double bond is more stable
Factors to consider:
1. Electrophile
tertiary > secondary > primary > Methyl
2. Leaving group
I > Br > Cl > F
3. Solvent
Polar protic - stabilizes the carbocation intermediate and one of its lone pairs are able to remove the hydrogen to form the double bond
4. Environment
When heat is applied E1 is favored over SN1
Alcohol (OH) Leaving group
- poor leaving group
- can be activated with either an acid (SN1/E1) or with TsCl and pyr (SN1, SN2, E1, E2)
Activation with Acid (SN1, E1)
- a hydrogen is taken from the acid to make the OH an H2O
- H2O is a good leaving group, so it does
- Hydrogens can shift if needed
- the H2O in solution can remove the second H which leaves its electrons to the electrophile
Activation with TsCl and Pyr (SN1, SN2, E1, E2)
- TsCl turns the OH into OTs which is a good leaving group, akin to Br
- from there it can do any of the reactions
Elimination reaction - E2
- Concerted
- Strong Base - no positive charges allowed
- small bases -> less hydrogens
-Bulky Bases -> more hydrogens - stereoselective: leaving group and hydrogen must be in the same plane, preferably anti planer
Factors to consider:
1. Electrophile
tertiary > secondary > primary > Methyl
2. Leaving group
I > Br > Cl > F
3. Nucleophile
strong base - any salt with an organic compound (NaOCH3, ect.)
4. Solvent
not important but typically Conj. Acid of the base
E2 Elimination in rings
Normal ring:
- Trans diaxial formation (one up one down)
Bridged system:
- Syn elimination only, both down
