Enols and Enolates

Question 1

what is an enol?

Answer 1

what you get when an alpha proton on a ketone or aldehyde moves up to the carbonyl’s oxygen

Question 2

what/where is the alpha carbon?

Answer 2

the carbon(s) directly attached to a carbonyl

Question 3

what/where is the alpha hydrogen?

Answer 3

attached to the alpha carbon

Question 4

are alpha hydrogens acidic or basic?

Answer 4

acidic

Question 5

what are the 2 functional groups that an enol can be made from?

Answer 5

ketone with alpha hydrogen or aldehyde with alpha hydrogen

Question 6

what are the two types of catalyst that can make a ketone or an aldehyde go into equilibrium with an enol?

Answer 6

acid catalyst and basic catalyst

Question 7

what do we call the relationship between the aldehyde/ketone and their enol?

Answer 7

they are tautonomers of each other

Question 8

what process/transition does tautonermization describe?

Answer 8

transition of aldehyde or ketone to enol (and vice versa since it’s an equilibrium rxn)

Question 9

what are the 2 steps involved in tautonemerization?

Answer 9

the structure receives a proton (on the oxygen) and losing an alpha proton

the order of these steps depend on whether the conditions are acidic or basic.

Question 10

what comes first in an ACID catalyzed tautonemerization?

Answer 10

protonation; oxygen gains a proton

Question 11

what comes first in an BASE catalyzed tautonemerization?

Answer 11

deprotonation; structure loses an alpha hydrogen

Question 12

where/how does a aldehyde/ketone lose its alpha hydrogen during a BASE catalyzed tautonemerization?

Answer 12

it gets taken by a base

Question 13

where/how does a aldehyde/ketone GAIN a proton during an ACID catalyzed tautonemerization?

Answer 13

the carbonyl’s oxygen takes the acid’s hydrogen

Question 14

what do you call the intermediate structure/s in a tautomerization rxn?

Answer 14

enolate

Question 15

what is the charge on an enolate intermediate that was formed under acidic conditions?

Answer 15

+

Question 16

what is the charge on an enolate intermediate that was formed under BASIC conditions?

Answer 16

-

Question 17

why are enols and enolates considered to be intermediates?

Answer 17

because we use them in reactions with other stuff

Question 18

what is the technical definition of a tautomer?

Answer 18

Tautomers are structural isomers of chemical compounds that readily interconvert.

This reaction commonly results in the relocation of a proton.

Question 19

why is the carbonyl the predominant resonance structure compared to the enol?

Answer 19

it is more stable

Question 20

what reagents can you use for the acid and base in a BASE catalyzed tautomerization?

Answer 20

hydroxide (for the H loss at the beginning) and hydronium (for the H gain at the end)

Question 21

fron where do are enolates do their nucleophilic attack?

Answer 21

They can attack with their O or their C but we will be learning about their C attacks

Question 22

why are enolates more reactive than enols?

Answer 22

enolates have a full negative charge (rather than just a pi bond)

Question 23

what is an alkoxide?

Answer 23

the conjugate of an alcohol

Question 24

Which bases give the most enolate: weaker bases or stronger bases?

Answer 24

Stronger bases

Question 25

Why isn’t the LDA base also a nucleophile?

Answer 25

It’s too big/too sterically hindered

Question 26

When it comes to how much enolate a base can produce, what is more important: size or pka?

Answer 26

Pka

Question 27

In a base catalyzed tautomerization, how can you tell if your reaction will to go completion rather than going to an equilibrium?

Answer 27

If the carbonyl’s pka is much higher than the pka of the base’s c.acid, the rxn will go to completion; all products

If the carbonyl and the base’s c.acid have very similar pkas, the rxn will go to equilibrium and you will have racemic mix of starting material and products

Question 28

In which situation can you use a weaker base like NaOEt to take your tautomerization rxn all the way to completion?

Answer 28

If your starting structure has 2 carbonyls, you can use something like NaOEt rather than the much strong LDA

LDA is more expensive.

Question 29

Which enolate is more stable: more substituted or less substituted?

Answer 29

More substituted

Question 30

Which product do you get from the kinetic tautonermization pathway: least substituted enolate or most substituted enolate?

Answer 30

Less substituted

Question 31

Which product do you get from the THERMO tautonermization pathway: least substituted enolate or most substituted enolate?

Answer 31

Most substituted

Question 32

why is base selection important with tautomerizing a carbonyl under basic conditions?

Answer 32

because of varying pka values, some bases will take the reaction to completion and some will instead establish and equilibrium

Question 33

what is the pka range for aldehydes and ketones?

Answer 33

16-20

Question 34

when/how is a base able to carry a tautomerization all the way to completion?

Answer 34

the base’s conjugate acid has a lower/more acidic pka than the carbonyl being tautomerized

Question 35

what 2 base characteristics impact the enolates you can produce?

Answer 35

1. base strength/pka (weaker bases give racemic products)

2. base size (large/sterically hindered bases give less substituted products)

Question 36

what is a beta hydroxy carbonyl?

Answer 36

A molecule containing a hydroxyl group bonded to the beta carbon of an aldehyde.

Question 37

how do you know when the product of an aldol rxn will undergo a condensation?

Answer 37

when you have heat

Question 38

does an aldol condensation work like a substitution or like an elimination?

Answer 38

elimination

Question 39

what’s the difference between a directed aldol and a regular cross aldol?

Answer 39

in a regular cross aldol, you mixed everything in the same beaker (and choose two reactants where only one of them has alpha carbons).

in directed aldol, you use LDA is create the enolate in a beaker by itself, THEN add the other molecule/electrophile to it

Question 40

when should you use LDA for directed cross aldol reactions?

Answer 40

When both starting materials have alpha hydrogens.

You want to make sure only one of the starting materials is made into an enolate so you use the LDA in a directed aldol so the enolate can be in it’s own container before you add the other starting material.