Enols and Enolates

Question 1

what is the definition of a tautomer?

Answer 1

structural isomers that differ only in the position of a moveable proton and a π-system.

Question 2

why is a ketol form of a tautomer more stable than an enol form?

Answer 2

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.

Question 3

why can’t the enol form resonance?

Answer 3

it has a double bond between the C+O, but not in a conjugated system.

= not the same delocalisation of π electrons.

Question 4

what is the functional group of an enol?

Answer 4

contains both an alkene and an alcohol group bonded to each other.

Question 5

how does an enol form?

Answer 5

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.

Question 6

how does stabilisation of the enol occur?

Answer 6

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.

Question 7

what happens when enolisation occurs when catalysed by an acid?

Answer 7

acid donates a proton to alpha-carbon, now more acidic.

= easier to deprotonate.

deprotonation is facilitated by conjugate base.

Question 8

what happens when enolisation occurs when catalysed by a base?

Answer 8

base abstracts a proton from alpha-carbon, = enolate ion.

facilitated by presence of conjugate acid.

Question 9

why do carboxylic acids use acid catalysed enolisation?

Answer 9

pKa of alpha-hydrogen is high, not very acidic.

requires a strong acid to facilitate deprotonation.

Question 10

what is the functional group of an enolate?

Answer 10

has - charge C atom attached to C=O.

Question 11

why is an enolate more reactive than an enol?

Answer 11

the - charge on an enolate = strong Nu.

readily attacks E, leading to stabilisation of - charge.

Question 12

how does the formation of enolates depend on the strength of the base being used?

Answer 12

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).

Question 13

how do you go from an enolate to keto form?

Answer 13

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.

Question 14

how can the use of strong bases completely deprotonate?

Answer 14

can overcome the acidity of alpha-hydrogen.

Question 15

does the keto form react with Nu or E?

Answer 15

Nu.

Question 16

does the enol form react with Nu or E?

Answer 16

E.

Question 17

does the enolate form react with Nu or E?

Answer 17

E (is more electron-rich).

Question 18

what is noteworthy about silyl enol ethers?

Answer 18

steric hindrance = more stable and less reactive.

has EWG, reduces electron density on oxygen, less nucleophilic.

Question 19

what happens if E+ is more reactive than enolate? (major/minor product).

Answer 19

major product will be formed by attac of enolate on E+.

Question 20

what happens if E+ is less reactive than enolate?

Answer 20

the major product will be formed by the attack of E+ on enolate.

Question 21

how do you control alkylation position?

Answer 21

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.

Question 22

what is a hard electrophile and a hard site?

Answer 22

hard E = small with high charge density, very reactive.

hard site = high electron density and small.

Question 23

what is a soft electrophile and a soft site?

Answer 23

soft E = large with low charge density, highly reactive.

soft site = low electron density, large size.

Question 24

what type of bond is formed by a hard-hard interaction?

Answer 24

strong covalent.

Question 25

what type of bond is formed from soft-soft interactions?

Answer 25

weak electrostatic attractions.

Question 26

how does the increase in number of resonance forms explain high acidity?

Answer 26

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.

Question 27

why does the alkylation of 1,3-dicarbonyl compounds only require weak bases?

Answer 27

due to presence of acidic alpha-hydrogens which can easily deprotonate.

resonance stabilises enolate ions.

using a stronger base leads to overalkylation.

Question 28

tell me about the aldol reaction:

Answer 28

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.

Question 29

why is the intramolecular aldol reaction faster than the intermolecular?

Answer 29

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.

Question 30

why are aldehydes more electrophilic than ketones and how is this significant when observing the relative rates of carbonyl compounds?

Answer 30

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.

Question 31

why does a shorter wavelength = a weaker C=O bond when looking at conjugate addition?

Answer 31

higher energy photons can break the bonds.

wavelength decreasing means energy required to break the bonds is less.

Question 32

why does conjugation make a C=O bond weak?

Answer 32

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.

Question 33

what is the difference between 1,2-addition versus 1,4-addition?

Answer 33

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.