Lecture 16 - Enols and Enolates

Question 1

Resonance=

Answer 1

electrons arranged differently

Question 2

Tautomerisation=

Answer 2

the transfer of one proton (hydrogen) and shift of a double bond

Question 3

Enols can be formed in keto-enol tautomerisation. What is a keto? What is an enol?

Answer 3

• Keto: carbonyl compounds with protons at the alpha position (bonded to alpha carbon- the carbon adjacent to C=O) that can enolize.
• Enol: enols have a C=C double bond (alkENe) and an OH group (alcohOL) directly bonded to it.

Question 4

Ketos and enols are NOT resonance forms. Instead they are…

Answer 4

structural/constitutional isomers - they have different physical and chemical properties. They are different compounds with different structures just with the same formula.

Question 5

The Keto/Enol structural isomers can interconvert and are in ……………..

Answer 5

equilibrium

Question 6

In a neutral solution keto/enol tautomerisation is a slow process. How can we speed it up?

Answer 6

acid/base catalyst.

Question 7

In the acid catalysed keto/enol tautomerisation reaction, what occurs?

Answer 7

• the oxygen in C=O is protonated first by the acid catalyst
• the inductive effect pulls electron density from the protons at the alpha position towards the positively charged oxygen atom
  a proton is then lost from the alpha carbon- C-H bond breaks. So, a C=C bond forms. So, the C=O bond breaks

Question 8

Draw the mechanism for the acid catalysed keto/enol tautomerisation reaction. Use a ketone as the example.

Answer 8

Question 9

In the base catalysed elonization reaction, what occurs?

Answer 9

• inductive effect pulls electron density from the alpha protons towards the partially positive carbon atom in the C=O bond of the keto. This makes the alpha protons more acidic
• an alpha proton is removed first by forming a bond with the base. A bond is broken between C-H
• So, a double bond forms - C=C
• But as a result, the C=O bond breaks
• An enolate forms

Question 10

What is an enolate?

Answer 10

negatively charged enol.

Question 11

Draw a mechanism for the base catalysed enolization of a keto

Answer 11

Question 12

What effects how likely it is for a keto to convert to an enol? For enolization to occur?

Answer 12

The acidity of the hydrogens at the alpha position

Question 13

Rank ketone, ester, amides and acyl chloride in order of most likely to enolize to least likely

Answer 13

acyl chloride
ketone
ester
amides

Question 14

pKa of acyl chloride

Answer 14

~15

Question 15

pKa of ketone

Answer 15

~20

Question 16

pKa of ester

Answer 16

~25

Question 17

pKa of amides

Answer 17

~30

Question 18

What makes the hydrogens at the alpha position more acidic in a keto and therefore makes the compound more likely to undergo tautomerisation to form an enol?

Answer 18

If the partially positive carbon in the C=O bond is more positive, there is stronger inductive effect, pulling electron density from the hydrogen atoms at the alpha position, making them more acidic

Question 19

The enolate ion- formed in the basic enolization reaction, has two resonance forms. Draw and name them. Give curly arrows to show how they interconvert.

Answer 19

Question 20

Why do we always draw enolate ions in their oxyanion resonance form?

Answer 20

• oxygen is more electronegative than carbon
• O- is more stabilised than C-
• C- is therefore more reactive than O-
  Less stable= more reactive.

The oxyanion contains the O- and is more stable than the carbanion, containing C-

This is why we draw enolate as the oxyanion

Question 21

Unlike Ketos which contain the electrophilic C=O, enols are…

Answer 21

nucleophilic

Question 22

Because enols are nucleophiles, they usually react through…

Answer 22

the alpha carbon

Question 23

Give the acidic halogenation reaction of enols - start off using the keto propanone, produce an enol through tautomerisation with an acid catalyst and then react enol with a halogen- Br2. Name the product.

Answer 23

Question 24

Give the basic halogenation reaction of enols - start off using keto propanone, produce an enolate with a base catalyst and then react enol with iodine. What is this reaction called?

Answer 24

Question 25

Why do aldol reactions occur?

Answer 25

If we add a small amount of base catalyst- like NaOH to a keto only some of it will turn into the enolate ione. The hydroxide is not basic enough to enolize the keto completely. Therefore, the enolate is surrounded by molecules that have not been enolized and still have their electrophilic C=O group. The nucleophilic enolates react with these electrophilic keto molecules

Question 26

importance of aldol reactions?

Answer 26

Carbon-to-carbon bonds form in the reaction

Question 27

Draw the aldol reaction and name all intermediates/products.

Question 28

What is an aldol?

Answer 28

aldehyde with a hydroxy group.