Midterm #1 Flashcards
describe a 1,2- addition
ch. 22 slide 6:
a nucleophile attacks the carbonyl carbon and then the double bond goes to the oxygen.
products formed:
FS: enolate and a nucleophile attached to former carbonyl carbon (treat with acid to form…)
SS: hydroxide and a nucleophile (alcohol formed)
describe a 1,4- addition
ch. 22 slide 6:
a nucleophile attacks one of the carbons of the double bond. The double bond pushes up to form a double bond with the carbonyl carbon. the double bond of the carbonyl goes up to the oxygen.
products formed:
FS: an enolate is formed and the Nu is attached to where it attacked (treated with acid to form…)
SS: carbonyl regained and Nu attached. (no alcohol formed)
what addition is kinetically controlled
1,2- addition and it is generally irreversible
what addition is thermodynamically controlled
1,4- addition and it is typically reversible. it is also more stable because carbonyls are more stable
hard vs. soft nucleophiles and examples of each
hard sites: have their lone pair and charge very localized and are not polarizable. ex. grignard / organolithium. hard acids prefer hard bases.
soft sites: have their lone pair either quite delocalized, or in a large orbital. ex. organocuperates. soft acids prefer soft bases.
describe orbital reactivity
- orbitals closer in energy react forwardly
- orbitals further apart in energy don’t
how can the alpha carbon be deprotonated
by a reasonably strong base
describe alpha carbon pka values
Ch. 22 slide 12
- due to resonance stability of the anion (charge and space), the pka values are lower
would a carbonyl with an alkene/ketone have a higher/lower pka, compared to a carbonyl with an ester
Ch. 22 slide 12
a carbonyl with an alkene/ketone would have a pka around 16-20. This has a lower pka because groups can share electron density.
a carbonyl with an ester would have a pka around 25 because the oxygen is an EDG which destabilizes
how would having two carbonyls on a compound affect pH?
ch.22 slide 13
- the pka would be even lower
there would be three alpha carbons that have hydrogens. see resonance forms
why would a compound with just two carbonyls have a lower pka than a compound with two carbonyls and an extra oxygen attached?
ch.22 slide 13
- the oxygen is an EDG via resonance, which is destabilizing because it is donating into a negative charge.
keto enol tautomerization in basic conditions
ch.22 slide 14
- the base attacks a hydrogen on the alpha carbon.
- forms an enolate which shifts to form a double bond between carbons
- acid workup forms enol form
- since there are two alpha carbons, the compound could be protonated from either side. follow regioselectivity rules and draw the most stable form as the product. This mechanism is similar to E2, which is direct and with no rearrangements.
keto enol tautomerization in acidic conditions
ch.22 slide 14
- carbonyl oxygen attacks the acid
- unstable carbonyl formed which shifts to form a carbocation via resonance
- an enol form was produced
- under acidic conditions, you don’t get an enolate, you go right to an enol. enolates are helpful because they are easier to control. this is similar to an E1 mechanism because it can rearrange (less certainty).
what reactants push keto / enol tautomerization under basic conditions
LDA because it is a strong and big base. small bases don’t work because SN2 would be able to happen.
what reactants push keto / enol tautomerization under acidic conditions
an acid. this process is reversible
does the equilibrium favor keto or enol form
ch. 22 slide 16
The preference for the keto or enol tautomeric form depends on…
- Resonance stabilization –> keto form is more stable when there is resonance stabilization of the carbonyl group. Ex. conjugated systems or when the carbonyl group is adjacent to an EWG.
- Steric effects –> Steric hindrance affects the stability of the enol form. If there are bulky groups near the potential enol site, the enol form may be disfavored due to steric repulsion.
describe the halogenation of the alpha carbon (acidic)
ch. 22 slide 18
halogens are electrophilic - the enol / enolates are nucleophilic
- extra acidic hydrogens can continue to react to form halogenated products, however, these new EWGs make it difficult to keep halogenating. its especially bad to have EWGs and a carbocation right next to each other
explain how alpha halogenation to alpha beta unsaturated ketones occurs
ch.22 slide 19
- a monosubstituted halogenation can easily produced an alpha-beta unsaturated ketone
- the halogen is a good LG for E2
explain the halogenation of the alpha carbon under basic conditions
Ch.22 slide 21
reagents: X2, basic
FS: the enolate is in equilibrium with enol
- the X is an EWG, which is good because you’re distributing electron density. this makes it more reactive, which is why it can proceed further than in acidic conditions.
Produces: a haloform
SS: the haloform (C-X3) is a great LG, so OH comes in to form a tetrahedral intermediate. this leaves a carboxylic acid derivative.
describe LDA use in reactions via an enolate
ch.22 slide 22
- 1 equiv of LDA and aprotic THF are used to produce products
- with LDA, you have a lot of control. you can stop it.
- with NaOH, its more disorganized. LDA is preferred.
describe the reaction via an enolate
- enolates are formed through a base mediated reaction
- one of the best bases for this is LDA - strong and non-nucleophilic
what happens to the products if the pka of the products are the same
you get a mixture of products
how does the equivalents of LDA used affect the halogenation
1 equiv gives monohalogenated
2 equiv gives dihalogenated
this is because using LDA prevents the cleavage of the haloform
explain how enolates are great nucleophiles and what do you have to worry about for the reactant when using them
- enolates can be used to form carbon-carbon bonds (i.e., an alkylation)
- if the reactant is symmetrical, you dont have to worry about kinetic / thermodynamic control
where is the enolate nucleophilic
at the alpha carbon
what does dess martin (DMP) do
oxidizes –> turns -OH into double bond O
what does NaH do to an -OH
becomes ONa
What does CH3I do when added to an enolate
adds the CH3 to the O-
kinetic vs. thermodynamic
see Ch.22 slide 28
Kinetic control: A reaction in which the product ratio is determined by the rate at which the products are formed. “sterics” (irreversible)
Thermodynamic product: The more stable product formed in a chemical reaction.
Thermodynamic control: A reaction in which the product ratio is determined by the relative stability of the products. “electronics” (reversible)
what temp must LDA be at and why
Ch. 22 slide 29
- LDA must be at -78 degrees celsius. this is because kinetic control must be irreversible. even though LDA is very bulky and it should go to the less steric (kinetic position), due to the pka it goes to the thermodynamic position. so it must be cooled.
what product is favored when the base pka is close to the alpha C-H pka
the thermodynamic product is favored
explain how carbonyls can react with themselves
see Ch.23 slide 5
- carbonyls can act as both nucleophiles and electrophiles, so they can react with themselves (one attacks the equivalent of another)
what groups do keto and enol forms of molecules react as
keto –> reacts like a carbonyl group
enol –> reacts like an alkene
what is the keto form
H3C-carbonyl- H
what is the enol form
H2C=C-OH
|
H
draw the mechanism for the aldol reaction
draw
what is the best group to use for an aldol reaction
if E+ is an aldehyde, it works better because it is less sterically hindered. a ketone still works, though
describe an aldol reaction
it is a reaction between a nucleophile and an electrophile. a new bond is formed between the alpha carbon and the carbn that was formerly the carbonyl carbon. it forms conjugated double bonds between two carbons and a water is produced
where does the carbon double bond for an aldol condensation go?
the more stable (conjugated) location is favored (ex. C-CH3 is more favored than C-H)
what reactants are used for the aldol condensation
describe the E1cB process of the aldol condensation
see ch. 23 slide 9
- due to the conjugation (i.e., increased stability) condensation can occur under basic conditions, as well. However, the mechanism is neither E1 nor E2
draw mechanism
when can the aldol condensation occur without heat
ch. 23 slide 10
if the compound produced has increased conjugation, the condensation occurs spontaneously. this is due to resonance
draw resonance examples
where will the aldol condensation reaction attached to if reacted with LDA at -78 degrees C
the kinetic side because LDA
what does [H] symbolize
reduction (gaining atoms)
what does [O-]
oxidation (losing atoms)
how can you control a crossed aldol condensation reaction without LDA
two main routes:
1. use one carbonyl that lacks alpha-hydrogens
2. use kinetic control wit LDA
3. make your enolate first (slowly)
4. use knowledge of EWGs / EDGs to facilitate reactivity
how do you control a crossed aldol condensation by adding the enolate first
add one equivalent of LDA on only the reactant you want it to (to form the enolate), then add the other reagent
how can you control a crossed aldol condensation by using a carbonyl that lacks alpha-hydrogens
add a coupling partner that cannot form an enolate
how do you control a crossed aldol condensation using pka
ch.23 slide 17
ex. a compound with lower pka would deprotonate most easily and would be the donor, the other compound would be the acceptor
describe the claisen condensation
its similar to an aldol reaction, though with esters –> so a tetrahedral intermediate will be present.
- crossed claisens are controlled in the same manner as an aldol
draw the claisen condensation reaction
draw it
what is the difference between the claisen condensation and the aldol addition rxn
ch. 23 slide 22
claisen condensation:
- forms a pi bond by elimination of RO-
- produces a beta-keto ester
- forms a tetrahedral intermediate
aldol addition:
- the protonation of O-
- produces a beta-hydroxyaldehyde
how do unstable vs. stable intermediates impact claisen rxns
The stability of tetrahedral intermediate depends on the ability of the groups attached to the new tetrahedral carbon atom to leave with the negative charge.
how do enolates change if they are conjugated
see slide 24 ch. 23
- conjugate addition is usually soft/soft interaction, so a soft base can facilitate conjugate addition
- ” harder” enolate is one with just the enolate ion
- “softer” enolate is one with the enolate and a carbonyl right next to it
*both are considered soft Nus, but one is a bit harder
**both are considered 1,4- additions
describe a michael addition
- a 1,4- addition to a conjugated enone/enal –> one needs a weak Nu
what is an enone
C=C-carbonyl-C
what is an enal
C=C-carbonyl-H
draw the michael addition mechanism
draw it