Enols and Enolates Flashcards
what is the definition of a tautomer?
structural isomers that differ only in the position of a moveable proton and a π-system.
why is a ketol form of a tautomer more stable than an enol form?
more stable carbonyl group and C=O bond.
the C is sp² hybridised, leading to greater ‘s’ character and being more electronegative than sp³.
the C-O bond is also more polarised.
keto form has resonance that stabilises the carbonyl by delocalising π-electrons.
why can’t the enol form resonance?
it has a double bond between the C+O, but not in a conjugated system.
= not the same delocalisation of π electrons.
what is the functional group of an enol?
contains both an alkene and an alcohol group bonded to each other.
how does an enol form?
if C=O has hydrogen atoms on alpha-carbon.
alpha-carbon must be acidic enough to be deprotonated by a conjugate base = enol intermediate.
can then tautomerise to form enol.
proton from alpha-carbon to oxygen on carbonyl forms a db.
how does stabilisation of the enol occur?
conjugation to another π-system, where π-electrons delocalise along the entire system.
resonance structures, where π-electrons delocalise into Oxygen on the carbonyl.
hydroxyl group can form H-bonds and steric effects.
what happens when enolisation occurs when catalysed by an acid?
acid donates a proton to alpha-carbon, now more acidic.
= easier to deprotonate.
deprotonation is facilitated by conjugate base.
what happens when enolisation occurs when catalysed by a base?
base abstracts a proton from alpha-carbon, = enolate ion.
facilitated by presence of conjugate acid.
why do carboxylic acids use acid catalysed enolisation?
pKa of alpha-hydrogen is high, not very acidic.
requires a strong acid to facilitate deprotonation.
what is the functional group of an enolate?
has - charge C atom attached to C=O.
why is an enolate more reactive than an enol?
the - charge on an enolate = strong Nu.
readily attacks E, leading to stabilisation of - charge.
how does the formation of enolates depend on the strength of the base being used?
stronger base = more likely to deprotonate.
= higher concentration of enolate ions.
= shift in eq. towards enolate form.
(a weaker base shifts eq. closer to the ketol form).
how do you go from an enolate to keto form?
loss of proton from carbon, enolate no longer stabilised by resonance.
C lost = charge no longer delocalised onto the carbonyl group.
ketol form is stabilised by H-bonds, which lower molecule energy.
how can the use of strong bases completely deprotonate?
can overcome the acidity of alpha-hydrogen.
does the keto form react with Nu or E?
Nu.
does the enol form react with Nu or E?
E.
does the enolate form react with Nu or E?
E (is more electron-rich).
what is noteworthy about silyl enol ethers?
steric hindrance = more stable and less reactive.
has EWG, reduces electron density on oxygen, less nucleophilic.
what happens if E+ is more reactive than enolate? (major/minor product).
major product will be formed by attac of enolate on E+.
what happens if E+ is less reactive than enolate?
the major product will be formed by the attack of E+ on enolate.
how do you control alkylation position?
if alkyl halide is EWG, it will react with the more nucleophilic C of enolate.
steric hindrance may cause alkyl halide to react with less hindered C.
what is a hard electrophile and a hard site?
hard E = small with high charge density, very reactive.
hard site = high electron density and small.
what is a soft electrophile and a soft site?
soft E = large with low charge density, highly reactive.
soft site = low electron density, large size.
what type of bond is formed by a hard-hard interaction?
strong covalent.
what type of bond is formed from soft-soft interactions?
weak electrostatic attractions.
how does the increase in number of resonance forms explain high acidity?
delocalisation of charge = greater stability of conjugate base = more acidic.
- charge can be delocalised over different atoms through resonance.
as resonance increases, electron density decreases.
why does the alkylation of 1,3-dicarbonyl compounds only require weak bases?
due to presence of acidic alpha-hydrogens which can easily deprotonate.
resonance stabilises enolate ions.
using a stronger base leads to overalkylation.
tell me about the aldol reaction:
enolate ion formed by deprotonation of alpha-C.
Nu enolate attacks carbonyl C of another carbonyl compound = beta-hydroxy.
aldol undergoes dehydration by heating/acid catalyst to eliminate water.
= alpha,beta-unsaturated carboyl compound.
why is the intramolecular aldol reaction faster than the intermolecular?
Intra:
reactants are in close proximity, effective concentration of reacting species is higher.
leads to higher frequency of collisions and faster reaction rate.
Inter:
reactants are seperated, effective concentration of the reactive species is lower.
= fewer collisions and slower rate of reaction.
why are aldehydes more electrophilic than ketones and how is this significant when observing the relative rates of carbonyl compounds?
ketones have steric hindrance, preventing Nu from approaching.
carbonyl C has H attached, so no e- donating, C is more electron deficient.
ketones have e- donating inductive effects, reduces electrophilic character of the carbonyl compound.
why does a shorter wavelength = a weaker C=O bond when looking at conjugate addition?
higher energy photons can break the bonds.
wavelength decreasing means energy required to break the bonds is less.
why does conjugation make a C=O bond weak?
delocalisation of electrons means stabilisation of π-system.
overlapping p-orbitals allow resonance structures to be made, reduces electron density.
C=O bond is now weaker, reduced electron density makes it more susceptible to Nu attack and easier to break.
what is the difference between 1,2-addition versus 1,4-addition?
1,2:
Nu adds directly to C adjacent to alpha-C and beta-C.
new bond is formed between alpha-C and Nu.
π-bonds between alpha/beta C are broken, Nu replaces a substituent on alpha-C.
1,4:
Nu adds to beta-C.
π-bond between alpha-C and beta-C remains in tact.
new bond formed between Nu and beta-C.
