Chapter 9: Substitution and Elimination Flashcards
Which reactions have a carbocation intermediate?
E1 and SN1
H2O can be the base in the E1 mechanism. This is because the E1 mechanism creates a carbocation, which can allow beta H’s to be deprotonated even with ___ bases.
weak
The E2 Energy Diagram has only one hill (and no intermediates), because the base comes in at the ___ ___ as the LG leaves.
same time
Rank the following nucleophiles from best to worst if the solvent is H2O or ROH:
I-, Br-, Cl-, F-
I- > Br- > Cl- > F-
___ Mechanism:
primary LG needs a large base, i.e. KOtBu or LDA
secondary LG needs a strong base, i.e. OH- or OR-
tertiary LG needs a strong base, i.e. OH- or OR-
E2
Anions going DOWN the periodic table are better nucleophiles if the solvent is ___ or ___.
H2O or ROH
Rank the following nucleophiles from best to worst if the solvent is polar (No OH):
I-, Br-, Cl-, F-
F- > Cl- > Br- > I-
SN2 needs a ___ nucleophile and a ___ base.
good nucleophile, poor base
___ ___ is the opposite of Zaitsev’s Rule.
Hoffman’s Rule
In substitution reactions, the LG leaves for a ___ to take its place.
nucleophile
E2 Elimination needs the LG and H to be antiperiplanar. This means on the same ___ but opposite ___ of each other.
plane, sides
Anions going UP the periodic table are better nucleophiles if the solvent is ___.
polar (No OH)
poor base = Its conjugate acid has a pKa < ___ - ___.
pKa < 10 - 12
SN1/E1 need a ___ nucleophile and a ___ base.
bad nucleophile, poor base
E alkenes are favored over Z alkenes. Less steric ___.
clash
The more ___ (lower pKa) the LG’s conjugate acid is, the better the LG.
acidic
E2 needs a ___ nucleophile and a ___ base.
good nucleophile, good base
Examples of a poor base:
RS-, RCO2-, X-, CN-, N3-
E2 is a bimolecular reaction. rate = ___ ___ ___.
rate = k[Base][Elec]
Perform ___ at lower temperatures with weak nucleophiles around.
SN1
Polar aprotic solvents are good at stabilizing ___, not anions. This means the nucleophile will NOT H-bond, and will be free to react.
cations
___ works with hindered, uncharged, N bases.
Example: DBu.
E2
OH- is a ___ LG.
horrible
___ ___: The more substituted alkene is favored as a product of elimination. This is because the transition states leading up to the product are lower in energy (hill is lower).
Zaitsev’s Rule
The more ___ the LG, the better the LG.
stable
___ ___: The less substituted alkene of elimination is favored. This is due to the steric clash between the bulky base and the more substituted C.
Hoffman’s Rule
___ are better nucleophiles than neutral species.
i.e. HS- > H2S
Anions
In ___ reactions, the LG leaves for a nucleophile to take its place.
substitution
SN1 and E1 usually produce a ___ of products.
mixture of products
The E2 Energy Diagram has ___ hill (transition state) and ___ intermediates.
one, no
The ___ the base, the more favorited the less substituted alkene is.
larger (bulkier)
Resonance stabilized primary LG’s tend to be ___, as there are rarely beta H’s present for ___.
SN1, E1
___ has the same LG preferences as SN1.
E1
___ - Loss of H and LG.
elimination
___ works for primary and secondary LG’s, but never for tertiary LG’s.
SN2
Examples of bad nucleophiles which will NOT do SN2. Will probably do SN1 or E1 instead (need a carbocation to react):
H2O, ROH, Carboxylic Acid
SN1 is an unimolecular reaction. rate = ___ ___.
rate = k[Elec]
E1 is an unimolecular reaction. rate = ___ ___.
rate = k[Elec]
Examples of good nucleophiles (which are also good bases which will prefer to do E2 instead):
OH-, OR-, H2N-
SN1 loses ___ from SM -> product.
stereochemistry
SN2/E2 are faster in polar aprotic solvents, because the nucleophile does not ___, so it can ___. The transtition states of ___ SN2 and E2 are stabilized.
H-bond, react, only
In elimination, both of the Cs next to the H and LG must be ___ hybridized.
sp3
E1 has the same LG preferences as ___.
SN1
SN2 doesn’t work for tertiary LG’s because it is too hard for the nucleophile to attack ___ of the LG. It is blocked by steric ___.
backside, clash
___/___ are faster in polar protic solvents.
SN1/E1
A ___ hydrogen is a hydrogen on a C next to the LG.
beta hydrogen
H2O, ROH, Carboxylic Acid are bad nucleophiles which will not do SN2. Will probably do SN1 or E1 instead (need a ___ to react):
carbocation
___ needs a good nucleophile and a poor base.
SN2
SN2/E2 are faster in ___ ___ solvents.
polar aprotic solvents (polar but without OH)
The E2 Energy Diagram has one ___ (transition state), and no ___.
hill, intermediates
