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
Zaitsev’s Rule: The ___ substituted alkene is favored as a product of elimination. This is because the ___ ___ leading up to the product are ___ in energy (hill is lower).
more, transition states, lower
The more acidic (___ pKa) the LG’s conjugate acid is, the better the LG.
lower
___ works for primary, secondary, and tertiary LG’s.
E2
___ needs a good nucleophile and a poor base.
SN2
___ needs a bad nucleophile and a poor base.
SN1/E1
Some examples of a leaving group:
A Halide (I, Br, Cl), An Electrophile
A(n) ___ hydrogen is a hydrogen on the same C as the leaving group.
alpha hydrogen
SN2 needs a ___ nucleophile and a ___ base.
good, poor
H2O can be the base in the ___ mechanism.
E1
Elimination has 2 mechanisms:
E2, E1
Perform E1 at ___ temperatures with ___ around. Also ___ nucleophiles around.
higher, H2SO4 around, NO nucleophiles around
Perform SN1 usually at ___ temperatures with ___ nucleophiles around.
lower, weak nucleophiles around
Anions going DOWN the periodic table are ___ nucleophiles if the solvent is H2O or ROH.
better
H2O, ROH, Carboxylic Acid are bad nucleophiles which will NOT do SN2. Will probably do ___ or ___ instead (need a carbocation to react):
SN1, E1
Syn elimination CAN happen in E2 elimination.
Anti elimination is just much ___.
faster
H2O, ROH, Carboxylic Acid are bad nucleophiles which will NOT do ___. Will probably do SN1 or E1 instead (need a carbocation to react):
SN2
___ alkenes are favored over ___ alkenes. Less steric clash.
E, Z
___ ___ = Its conjugate acid has a pKa < 10 - 12.
poor base
SN1/E1 are faster in polar protic solvents, because the ___ are stabilized.
carbocations
Substitution has two mechanisms:
SN2, SN1
SN1 ___ stereochemistry from SM -> product.
loses
___ works for tertiary and secondary LG’s, but not for primary LG’s, unless they are resonance stabilized.
E1
E2 works for ___, ___, and ___ LG’s.
primary, secondary, and tertiary
___ ___ - Reaction rate is dependent on the concentration of 2 molecules.
bimolecular reaction
You can make OH- a better LG by either adding ___ or ___.
H+, tosylate
When drawing Newman Projections to illustrate E2 Eliminations, ___ the LG and opposite H, and draw a ___ ___ down their plane to visually split up the resulting alkene.
erase, dotted line
SN1 works for tertiary and secondary LG’s, but not for primary LG’s, unless they are ___ ___.
resonance stabilized
When the nucleophile is adding during the SN2 mechanism, ___ occurs to the nucleophile being added.
inversion
Hoffman’s Rule: The ___ substituted alkene of elimination is favored. This is due to the steric ___ between the bulky base and the more substituted C.
less, clash
E1 works for ___ and ___ LG’s, but not for ___ LG’s, unless they are resonance stabilized.
tertiary, secondary, primary
E2 Mechanism:
___ LG needs a large base, i.e. KOtBu or LDA
___ LG needs a strong base, i.e. OH- or OR-
___ LG needs a strong base, i.e. OH- or OR-
primary, secondary, tertiary
*IMPORTANT: The most stable conformation may not always be the most ___ conformation. This is true for both Newman Projections and Chair Conformations.
reactive
Which reactions produce an alkene?
E2 and E1
For E2 Elimination to happen to cyclohexane, the H and LG need to be ___.
anti-axial (one up, one down)
LG = ___ ___.
Leaving Group
SN1/E1 need bad nucleophiles and bad bases.
Examples:
H2O, ROH, Carboxylic Acid
Examples of good nucleophiles/good bases:
OH-, OR-, tert-butyl O-
E2 Mechanism:
primary LG needs a ___ base, i.e. KOtBu or LDA
secondary LG needs a ___ base, i.e. OH- or OR-
tertiary LG needs a ___ base, i.e. OH- or OR-
large, strong, strong
Anions going UP the periodic table are ___ nucleophiles if the solvent is polar (No OH).
better
Polar protic solvent means the solvent is ___ AND has a ___ group.
polar (delta negative somewhere), OH
___ is a good LG (as a result of changing OH-). It has a pKa of 0.6.
tosylate
Rank the following nucleophiles from best to worst:
NH3, H2O, HF
NH3 > H2O > HF
The E2 Energy Diagram is similar to the ___ Energy Diagram.
SN2
___ can be the base in the E1 mechanism.
H2O
___ needs a strong base, ___ does NOT.
E2, E1
In elimination reactions, the LG leaves for a ___ to steal a ___, and for an ___ to form.
base, proton, alkene
Rank the following LG’s from best to worst:
I-, Br-, Cl-, F-
I- > Br- > Cl- > F-
E2 also works with ___, ___, ___ bases.
Example:
hindered, uncharged N bases
Example: DBu
Examples of good nucleophiles (which are actually good bases as well, so they will instead prefer to do E2).
OH-, OR-, H2N-
Leaving Group = ___.
LG
___ ___ - Functional group/atom that makes off with the electrons from its old sigma bond.
Leaving Group
E2 needs a strong ___, E1 does NOT.
base
SN1 works for ___ and ___ LG’s, but not for ___ LG’s, unless they are resonance stabilized.
tertiary, secondary, primary
E alkenes are ___ over Z alkenes. Less steric clash.
favored
Perform ___ at higher temperatures with H2SO4 around. Also NO nucleophiles around.
E1
___/___ are faster in polar aprotic solvents.
SN2/E2
___ isn’t usually a LG (compared to other halides).
F-
Examples of a “bulky” base:
KOtBu (KO-tert-butyl), LDA (Li-N-2-iso-propyl)
Across the periodic table, the ___ of a nucleophile the molecule is.
worse
SN2 works for ___ and ___ LG’s, but never for ___ LG’s.
primary, secondary, tertiary
Examples of good nucleophile/good base are anions with pka > ___ - ___.
pka > 13 - 15
SN2 is a bimolecular reaction. rate = ___ ___ ___.
rate = k[Nuc][Elec]
E2 Elimination needs the LG and H to be antiperiplanar. This means on the ___ plane but ___ sides of each other.
same, opposite
In ___ reactions, the LG leaves for a base to steal a proton, and for an alkene to form.
elimination
Polar aprotic solvent means the solvent is ___ but DOES NOT have a ___ group.
polar (delta negative somewhere), OH
___ ___ solvents are good at stabilizing cations, not anions. This means the nucleophile will NOT H-bond, and will be free to react.
polar aprotic
___ works for tertiary and secondary LG’s, but not for primary LG’s, unless they are resonance stabilized.
SN1
___ is a horrible LG, because its conjugate acid of H2O has a high pKa of 15.
OH-
In substitution reactions, the C next to the LG has to be ___ hybridized.
sp3
E2 Elimination needs the LG and H to be ___. This means on the same plane but opposite sides of each other.
antiperiplanar
SN1/E1 are faster in ___ ___ solvents.
polar protic solvents (polar solvents with OH)
OH-, OR-, H2N are good nucleophiles and good bases too which will prefer ___.
E2
___ ___ - Reaction rate is dependent on the concentration of a single molecule.
unimolecular reaction
___ needs a good nucleophile and a good base.
E2
Anions are ___ nucleophiles than neutral species.
i.e. HS- > H2S
better
E1 works for tertiary and secondary LG’s, but not for primary LG’s, unless they are ___ ___.
resonance stabilized
