Lab H: Aldol Reaction

Question 1

aldol reaction definition

Answer 1

2 molecules of an aldehyde or ketone react with each other in the presence of a base to form a beta-hydroxy carbonyl compound
- a new C-C bond at the alpha-C forms as a result

Question 2

where does the name “aldol” come from?

Answer 2

because many aldol products contain an ALDehyde and an alcohOL

Question 3

how are enolates formed?

Answer 3

enolates are formed when a strong base removes a proton on the alpha-C (aka the alpha-H)

Question 4

alpha-Carbon definition

Answer 4

the carbon adjacent to a carbonyl group/ Carbon

Question 5

why is the C-H bond on the alpha-C more acidic?

Answer 5

because the resulting enolate (when an electron pair is on the alpha-C) is RESONANCE STABILIZED

Question 6

compare the pKa values and acidity of an alpha-H in an ald/ketone, alkanes, alcohols, and acids

Answer 6

pKa of alpha-H:
(least acidic)
alkanes: 50
ald/ket: 20
alcohols: 17
acids: 4
(most acidic)
pKa value decrease, acidity increase

Question 7

what are the 2 types of aldol reactions?

Answer 7

1. symmetrical aldol reactions
2. crossed/mixed aldol reactions

Question 8

symmetrical aldol reaction definition

Answer 8

an aldol reaction that occurs between 2 IDENTICAL ALD or KET as the starting components

Question 9

crossed/mixed aldol reaction definition

Answer 9

aldol reactions that can occur between 2 DIFFERENT carbonyl compounds

Question 10

what can be concluded for a crossed aldol reaction where 2 different carbonyl compounds BOTH have alpha-Hs

Answer 10

when 2 different compounds BOTH have alpha-Hs, a crossed aldol reaction is NOT SYNTHETICALLY USEFUL since 4 possible aldol products can form (aka a misture)

Question 11

when are crossed aldol reactions most successful?

Answer 11

when only ONE carbonyl compound has alpha-Hs –> thus, only one enolate anion can be formed from the 2 reagents –> only ONE product is formed

(aka 1 carbonyl compound has no alpha-Hs and thus cannot form an enolate anion)

Question 12

what are 4 examples of aldehydes with no alpha-Hs?

Answer 12

formaldehyde
benzaldehyde
furfural
2,2-dimethylpropanal

Question 13

what is our aldol reaction for this lab?

Answer 13

2 benzaldehyde + acetone —> (KOH, 95% aq. EtOH) dibenzalacetone + 2 H2O

Question 14

what is the order of incorporating chemicals in our aldol reaction?

Answer 14

1. solvent 95% ethanol goes into reaction flask
2. benzaldehyde
3. aq. KOH
4. acetone last

Question 15

why is acetone added last into the reaction flask?

Answer 15

acetone is added last to prevent it from reacting with itself (aka self-aldol reaction)

Question 16

what allows dibenzalacetone to absorption sunlight?

Answer 16

CONJUGATION
- it is also a component of sunscreens

Question 17

what are the health hazards of KOH and acetone?

Answer 17

KOH - corrosive (strong base)
acetone - flammable

Question 18

why is a crossed-aldol condensation reaction hard to control?

Answer 18

because both carbonyl compounds can act as the enolate and as the electrophile –> result in a mixture of 4 different products (aka a mixture of products)

Question 19

which 2 methods can allow you to control a crossed aldol condensation reaction (aka get only 1 product instead of a mixture)

Answer 19

1. careful choice of the base used for enolate formation
2. control the molar proportions of the 2 carbonyl compounds

Question 20

in this lab, what type of reaction is it?

Answer 20

a crossed aldol condensation reaction

Question 21

how do we control the product we get for this lab’s specific reaction?

Answer 21

the crossed aldol condensation will be controlled by using 1 carbonyl compounds WITHOUT any alpha-Hs (aka benzaldehyde)
without alpha-Hs, benzaldehyde cannot form an enolate and thus it can only act as the electrophile in the reaction

Question 22

what are our 3 starting reagents for this lab?

Answer 22

1mL benzaldehyde
6mL 95% ethanol
4mL 2M KOH
–> all of them are liquids

Question 23

what is the order in which we add the reagents into the reaction flask?

Answer 23

1. ethanol
2. benzaldehyde
3. KOH
4. acetone (LAST) - after the other 3 are stirred for 2 min

Question 24

why is a hot plate used for this lab?

Answer 24

for the STIRRING function ONLY (no heat)

Question 25

what happens to the solution after adding acetone and stirring?

Answer 25

the solution turns from CLEAR --> YELLOW

Question 26

what is the overall procedure of this lab?

Answer 26

1. reaction occurs - stirring 2. vacuum filtration to separate the product produced

Question 27

what do you rinse the reaction flask with for vacuum filtration?

Answer 27

ICE COLD ethanol

Question 28

what does the final aldol product look like?

Answer 28

yellow powder

Question 29

waste disposal procedure?

Answer 29

there are specially labeled containers for this lab

Question 30

KOH health hazard?

Answer 30

extremely CORROSIVE

Question 31

the general aldol condensation reaction is ___ and consists of which few steps (3)

Answer 31

the general aldol condensation reaction is a MULTI-STAGE reaction consists of ... 1. enolate formation 2. nucleophilic attack of the enolate on an ELECTROPHILIC carbonyl compound (aldol reaction) 3. elimination of H2O to form an alpha-beta-unsaturated carbonyl compound (loss of water forms a double bond in conjugation with the C=O double bond)

Question 32

what does it mean for a reaction to be crossed-aldol condensation vs general aldol condensation?

Answer 32

crossed-aldol condensation means that the enolate is derived from a DIFFERENT carbonyl compounds than the ELECTROPHILE - aka the enolate and electrophile are DIFFERENT COMPOUNDS the general aldol reaction is 2 equivalents of the SAME carbonyl compound (aka the enolate and electrophile are from the SAME carbonyl compound)