exam 3 - Sn1/Sn2/E1/E2 rxns Flashcards
what are the characteristics of a good leaving group?
- conjugate bases of strong acids (Cl-, Br-, I-)
- able to stabilize the negative charge upon departure
- bonded to an sp3 C
- bigger atoms are better leaving groups
how do you turn an alcohol into a good leaving group (besides protonation)?
- use sulfonate ester and strong base (like pyridine)
- mechanism: Cl leaves sulfonate ester and S grabs OH on molecule, the H then deprotonates and a strong base picks it up
what is a nucleophile
- donates e- pairs to form new bonds
- electron rich, negatively charged or neutral with lone pairs
- ex: H2O, OH-, Cl-, NH3
- acts as a lewis base
electrophile
- accepts an e- pair to form a new bond
- electron deficient, partial or fully positive charge
- H+, NO2+, carbocations
- acts as a lewis acid
what is the general mechanism for substitution rxns?
one sigma bond broken (departure of LG), one sigma bond formed (nucleophile attaches to electrophile)
what is the mechanism/rate dependent on in an Sn1 rxn?
- loss of LG forms C+
- nucleophile attacks the C+
- leads to racemic mix of products: if nucleophile attacks from side opposite to LG, inversion; if same side, retention
- unimolecular (RDS involves 1 molecule/SM)
- rate is dependent only on substrate concentration
what is the mechanism/rate dependent on in an Sn2 rxn?
1.nucleophile attacks from opposite side (backside)of LG while LG leaves
- with presence of chiral center, invert stereocenter @ Alpha carbon
- produces single product
- bimolecular (RDS involves 2 molecules)
- rate of reaction is dependent on concentration of nucleophile and substrate
what is the general mechanism of elimination rxns?
two sigma bonds broken (one from LG and one from Beta hydrogen) and one Pi bond formed (C=C)
what are the steps of an E1 rxn? what is the rate dependent on?
- LG leaves, forming a C+
- Beta hydrogen is removed
- 2 step rxn
- proceeds through C+
- results in a C=C between Alpha and Beta carbons
- unimolecular (RDS involves 1 SM that forms the C+)
- rate depends on concentration of substrate
what must be true for Sn1/E1 rxns to occur?
the C+ intermediate must be stable.
what do strong nucleophiles and strong bases favor?
strong nucleophiles favor substitution, and strong bases (especially strong hindered bases like tert-butoxide) favor elimination
what is the mechanism of an E2 rxn? what is the rate dependent on?
- LG and Beta hydrogen are removed simultaneously
- Remove the Beta hydrogen that’s anti to the LG (use Newman projections –> rotate if not anti at first)
- one step rxn, bimolecular
- rate depends on concentration of substrate and base
what needs to be present for elimination rxns to occur?
Beta hydrogens
define heteroatom substituted
a carbon bonded to atoms other than C or O; becomes more electrophilic
define allylic
an allylic carbon is located next to a C=C and gains reactivity because the adjacent pi system can stabilize positive charges
define benzylic
a benzylic carbon is directly attached to a benzene ring; the ring’s resonance stabilizes positive charge
define propargylic
a propargylic carbon is one carbon away from a C-C triple bond
which type of reactions are the fastest? why?
- bimolecular
- they don’t have to go through a C+ intermediate
classifications of lewis base- category a
- good e- donor, weak base
- anion, conjugate acid pKa < 15, uncharged sp3 N/S/P
- degree of Alpha carbon: 1- Sn2, 2- Sn2, 3- no Sn2, no E2
- ex: Cl-, Br-, I-, N3-, CN-, H2S-, R3P, R3N, RSH, HS-, RS-
classifications of lewis base- category b
- good e- donor, moderate base
- anion, conjugate acid pKa ~ 15-30
- degree of Alpha carbon: 1- Sn2, 2- E2 (if no B-H, Sn2), 3- E2
- ex: OH-, H3CO-, H3CH2CO-
classifications of lewis base- category c
- good e- donor, strong base
- conjugate acid pKa >30, hindered conjugate acid pKa > 10
- degree of Alpha carbon: 1- E2 (no B-H, Sn2), 2- E2, 3- E2
- DBU, H:-, tBuO-
classifying lewis base- category d
- poor e- donor, weak base
- non-resonance stabilized C+: 1 and 2 degree Alpha carbon- no Sn/E, 3 degree- Sn1/E1
- resonance stabilized C+: Sn1/E1
- H2O, R-OH
describe the effects of steric hindrance on the Alpha carbon on rxn rates and possibilities
- degree of substitution of Alpha carbon once LG leaves: 0-2 (not sterically encumbered), 3 (sterically encumbered)
- if Alpha carbon is sterically encumbered, Sn2 is impossible
- rxn rates: Sn1/E1- higher degree of substitution at Alpha carbon increases rxn rates due to stability, Sn2- lower degree of substitution favors faster rxns due to less steric hindrance, E2- more substituted Alpha carbons favor faster rxns due to the formation of more stable alkenes
describe the different types of solvents
- polar protic solvents: can donate a proton (has H that could be acidic)
- polar aprotic solvents: can’t donate proton (acetone, DMSO, DMF)
when are polar protic solvents needed?
- in Sn1/E1 rxns
- they are things like water, alcohols, things that can H bond (H on N, O, F)
describe the effects of solvents and rxn rates
- polar protic solvents: slow down Sn2/E2 rxns because they form a solvent shell around the nucleophile/lewis base
- solvent traps the nucleophile and slows the rxn
when are polar aprotic solvents needed?
- in Sn2/E2 rxns
- they are things like acetone (O=C(CH3)2), DMF (HC=ON(CH3)2), and DMSO (O=S(CH3)2)
- can’t use polar protic solvents because they form a shell of solvent around nucleophile/base
define bimolecular, which reactions are biomolecular, what the rxn rate depends on, and what their energy diagrams look like
- two reactants interact in a single step
- Sn2/E2 (1 step involving two things)
- Sn2 rate depends on concentration of nucleophile and electrophile, E2 rate depends on concentration of substrate and base
- both energy diagrams are a single, smooth curve
what degree of substitution allows Sn2 to occur?
1-2
what degree of substitution allows Sn1 to occur?
2-3
giveaways for Sn1
- LG/potentially good LG
- weak nucleophile (H2O, R-OH, CH3COOH (acetic acid), H3C-OH (methanol), AgNO3, acid)
- 2-3 degree substituted C+
- stable C+
- acid as reactant/product
giveaways for E1
- B-hydrogens
- stable C+ (3 degree substituted)
- weak base as reagent
giveaways for Sn2
- strong nucleophile (CN, negatively charged, large, low EN)
- 1-2 degree substituted alpha carbon
giveaways for E2
- nuclephile reagent is a strong base (negatively charged N and O: (CH3)2CO-, DIPEA, other strong BULKY BASE!!!)
what hydrogen do you abstract in an E2 rxn?
beta hydrogen
