OChem Reaction Types Flashcards
Describe the SN1 reaction
- 2 step carbocationization
- results in racemic products
- step 1 is RDS
- step 2 is planar sp2 hybridized
Energy diagram has 2 humps
rate law is -d(sub)/dt= k(sub)
first order obs kinetics
Describe the SN2 reaction
- must show TS
- back side attack
- concerted mechanism
- inverted stereochemistry (stereospecific)
energy diagram is 1 hump
rate law is -d(sub)/dt= k(sub)(nucleophile)
second order obs kinetics
Describe the E1 reaction
- 2 step carbocationization
- second step planar (180 degree)
- products are major minor or traces
- stereoselective
- indicator: 1.0 mol loc concentration base, alcohol (OH)
Energy diagram has 2 humps
rate law is -d(sub)/dt= k(sub)
first order obs kinetics
Describe the E2 reaction
- 1 concerted step
- anti-periplanar
- stereoselective
- products major minor or traces
- show TS!
- potential Newman Projections needed
- indicators: strogn bases (better than OH), alkoxide (O-), large concentration bases
energy diagram is 1 hump
rate law is -d(sub)/dt= k(sub)(nucleophile)
second order obs kinetics
What is difference between stereospecific and stereoselective?
- stereospecific mechanism specifies the stereochemical outcome of a given reactant
- stereoselective reaction selects products from those made available by the same, non-specific mechanism acting on a given reactant– reesulting in different amounts of each product (major minor or traces)
What are the steps in determining the mechanism: SN1, SN2, E1, or E2?
- look at LG
- if not one, no rxn
- will not be F, acetate, or second row element (except S) - substrate substituion structure (1, 2, 3 degree carbon)– number of things attached that are not H
- 1 degree= no SN1 or E1
- 2 degree= all posibilities kept
- 3 degree= no SN2 bc slowed from sterics - Nucleophile strength
- strong base = E2
- mediocre base= not E2 unless in excess amount
- bad bases no elimination (must be SN1 or SN2) - draw products (major, minor, traces)
- consider solvent
- protic speeds up unimolecular processes
- polar aprotic speeds up bimolecular processes - consider sterics
- bulky will slow down SN2
Protic solvents ________ all reactions
decrease
Polar aprotic solvents ________ all reactions
increase
how good are these leaving groups?
I-, Br-, Cl-, OTs-, OCF3-, OMs-, H2O
great!
how good are these leaving groups?
F-, CH3COO-, NH3
mediocre
how good are these leaving groups?
OH-, OCH3, NH2-
BAD
what are these solvents?
acetic acid, H2O, ROH, NH3
polar protic solvents (have at least 1 hydrogen that is connected directly to a particular electronegative atom)
what are these solvents?
DMF, nitromethane, acetone, aceonitrile, DMSO, ethyl acetate
Polal aprotic (a solvent that lacks an acidic proton/hydrogen and is polar)
what are these solvents?
alkanes, cycloalkanes, toluene, benzene, ether
Non-polar solvents (composed of atoms that have small differences in electronegativity)
* will never affect solvent rate
how good are these nucleophiles?
I-, HS-, RS-
very good!
