Enols and Enolates

Question 1

instead of the chemistry happening on the carbonyl, where is the chemistry with enols?

Answer 1

in alpha carbons

Question 2

what is an alpha carbon

Answer 2

a carbon adjacent to the carbon of the carbonyl

Question 3

what are alpha protons

Answer 3

hydrogens attached to alpha carbons

Question 4

are alpha protons mildly acidic or basic? what is the effect of this

Answer 4

mildly acidic, so they may be deprotonated with a strong base

Question 5

after an alpha proton is deprotonated, what is left behind

Answer 5

a conjugate base that has resonance

Question 6

the conjugate base of a deprotonated molecule has resonance. Where is the negative charge?

Answer 6

on the O

Question 7

when a ketone has an alpha proton and is in the presence of an acid or a base, what happens

Answer 7

the ketone exists in equilibrium with an enol

Question 8

what is an enol

Answer 8

alkane alcohol

Question 9

is the ketone/enol equilibrium centered?

Answer 9

no, it lies heavily towards the ketone

Question 10

what word is used to describe the relationship between ketones and enols

Answer 10

tautomers

Question 11

define tautomer

Answer 11

the two molecules differ by an alpha proton

the ketone has it on a carbon, the enol has it on the oxygen

Question 12

how might the ketone/enol equilibrium be shifted to the right

Answer 12

acid or base catalysis

Question 13

what is it called when the enol is missing a proton

Answer 13

enolate

Question 14

describe acid catalyzed tautomerism

Answer 14

ketone attacks H+ to make the enol. Both ketone and enol have resonance. Water can attack the enol resonance contributor

Question 15

describe base catalyzed tautomerism

Answer 15

OH- attacks the ketone to make the enol. Both form resonance contributors, and the one from the enol can attack water

Question 16

how might we add an electrophile onto the carbon of an enol

Answer 16

must do acid/base catalysis first to make the enol, and then the enol can attack the electrophile from the pi bond, electrons move around, giving us a ketone with the E attached to an alpha carbon

Question 17

what is it called when we add an electrophile onto a ketone

Answer 17

alpha addition

Question 18

what happens in the Hell-Volhard-Zelinsky mechanism

Answer 18

it’s alpha addition onto a carboxylic acid (not a ketone this time)

Question 19

describe the mechanism of the HVZ

Answer 19

convert carb acid into an acyl halide, then tautomerize, then use Br2 (which is the alpha addition step), then convert back into an acid via hydrolysis

Question 20

how do we make enolates

Answer 20

deprotonate an enol (gives the alpha position a negative charge)

Question 21

T or F: you can draw the enolate with one structure

Answer 21

false; you need to show both resonance contributors

Question 22

in making enolates, what side of the equilibrium is favored when OH- is used

Answer 22

the ketone

Question 23

in making enolates, what side of the equilibrium is favored when LDA is used

Answer 23

the enolate, because LDA is stronger than OH-

Question 24

describe the strength and size of LDA

Answer 24

strong and bulky

Question 25

which will be more selective and why: OH- or LDA

Answer 25

LDA is more selective because it’s bigger and needs to consider sterics

Question 26

what type of enolate is OH- best used for making

Answer 26

double enolates or the haloform reaction

Question 27

what is the haloform reaction

Answer 27

you start with a methyl ketone, use OH- and Br2 to make a carb acid. It’s used as a test to see if we started with a methyl ketone

Question 28

what two bases are used to make enolates (not OH-)

Answer 28

LDA and NaH

Question 29

describe the strength and size of NaH

Answer 29

super small and strong

Question 30

compare the size and strength of NaH and LDA

Answer 30

NaH: small and strong
LDA: big and strong

Question 31

describe what a kinetic product is

Answer 31

a less stable product, to make it there isn’t interference

Question 32

describe what a thermodynamic product is

Answer 32

a more stable product, to make it there is more sterics

Question 33

which base is used to make a kinetic product

Answer 33

LDA (bulky)

Question 34

which base is used to make the thermodynamic product

Answer 34

NaH (small)

Question 35

how do we grow the carbon chain of an enolate

Answer 35

first we make the enolate, then react it with RX and R will attach to the alpha carbon

Question 36

what are enolates analogous to

Answer 36

enamines

Question 37

describe why enamines are analogous to enolates

Answer 37

enamines can also do alpha addition. If we push the lone pair from the N into the ring, we can move electrons from the double bond to form an enolate (neg charge on the ring, pos charge on N)

Question 38

describe how to do alpha addition on an enamine

Answer 38

enamine attacks RX, adding R onto the ring, then we do hydrolysis to turn the =N part into a carbonyl