OChem Reaction Types

Question 1

Describe the SN1 reaction

Answer 1

• 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

Question 2

Describe the SN2 reaction

Answer 2

• 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

Question 3

Describe the E1 reaction

Answer 3

• 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

Question 4

Describe the E2 reaction

Answer 4

• 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

Question 5

What is difference between stereospecific and stereoselective?

Answer 5

• 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)

Question 6

What are the steps in determining the mechanism: SN1, SN2, E1, or E2?

Answer 6

1. look at LG
   - if not one, no rxn
   - will not be F, acetate, or second row element (except S)
2. 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
3. Nucleophile strength
   - strong base = E2
   - mediocre base= not E2 unless in excess amount
   - bad bases no elimination (must be SN1 or SN2)
4. draw products (major, minor, traces)
5. consider solvent
   - protic speeds up unimolecular processes
   - polar aprotic speeds up bimolecular processes
6. consider sterics
   - bulky will slow down SN2

Question 7

Protic solvents ________ all reactions

Answer 7

decrease

Question 8

Polar aprotic solvents ________ all reactions

Answer 8

increase

Question 9

how good are these leaving groups?

I-, Br-, Cl-, OTs-, OCF3-, OMs-, H2O

Answer 9

great!

Question 10

how good are these leaving groups?

F-, CH3COO-, NH3

Answer 10

mediocre

Question 11

how good are these leaving groups?

OH-, OCH3, NH2-

Answer 11

BAD

Question 12

what are these solvents?

acetic acid, H2O, ROH, NH3

Answer 12

polar protic solvents (have at least 1 hydrogen that is connected directly to a particular electronegative atom)

Question 13

what are these solvents?

DMF, nitromethane, acetone, aceonitrile, DMSO, ethyl acetate

Answer 13

Polal aprotic (a solvent that lacks an acidic proton/hydrogen and is polar)

Question 14

what are these solvents?

alkanes, cycloalkanes, toluene, benzene, ether

Answer 14

Non-polar solvents (composed of atoms that have small differences in electronegativity)
* will never affect solvent rate

Question 15

how good are these nucleophiles?

I-, HS-, RS-

Answer 15

very good!

Question 16

how good are these nucleophiles?

Br-, HO-, RO-, CN-, N3-

Answer 16

good nucleophiles

Question 17

how good are these nucleophiles?

NH3, Cl-, F-, RCO2-

Answer 17

fair nucleophiles

Question 18

how good are these nucleophiles?

H2O, ROH

Answer 18

weak nucleophiles

Question 19

how good are these nucleophiles?

RCO2H

Answer 19

very weak nucleophiles

Question 20

how good are these bases?

NH2-, H-, HC(triple bond)C-

Answer 20

very strong!

Question 21

how good are these bases?

RO-, HO-

Answer 21

strong bases

Question 22

how good are these bases?

RNH2, Na2CO3

Answer 22

medicocre bases

Question 23

how good are these bases?

NH3, CN-, RCOO-, Cl-, BR-, I-, NaHCO3

Answer 23

weak bases

Question 24

what does a 1st degree substrate substitution indicate?

Answer 24

• if strong base, SN2 is major and E2 is minor (but E2 is major if base is sterically hindered)
• if weak base, only SN2

Question 25

what does a 2nd degree substrate substitution indicate?

Answer 25

* if strong base E2 is major and SN2 is minor (SN2 will not occur if there is a sterically hindered nucleophile) * if polar protic, SN1 or E1 * if polar aprotic, SN2

Question 26

what does a 3rd degree substrate substitution indicate?

Answer 26

* if strong base, E2 * if weak base, SN1 or E1

Question 27

what is the preferred substrate subsitiution of SN1, E1, and E2?

Answer 27

3 degree > 2 > 1 > methyl

Question 28

what is the preferred substrate subsitiution of SN2?

Answer 28

methyl >>1degree>> 2 > 3