Lab H: Aldol Reaction Flashcards
aldol reaction definition
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
where does the name “aldol” come from?
because many aldol products contain an ALDehyde and an alcohOL
how are enolates formed?
enolates are formed when a strong base removes a proton on the alpha-C (aka the alpha-H)
alpha-Carbon definition
the carbon adjacent to a carbonyl group/ Carbon
why is the C-H bond on the alpha-C more acidic?
because the resulting enolate (when an electron pair is on the alpha-C) is RESONANCE STABILIZED
compare the pKa values and acidity of an alpha-H in an ald/ketone, alkanes, alcohols, and acids
pKa of alpha-H:
(least acidic)
alkanes: 50
ald/ket: 20
alcohols: 17
acids: 4
(most acidic)
pKa value decrease, acidity increase
what are the 2 types of aldol reactions?
- symmetrical aldol reactions
- crossed/mixed aldol reactions
symmetrical aldol reaction definition
an aldol reaction that occurs between 2 IDENTICAL ALD or KET as the starting components
crossed/mixed aldol reaction definition
aldol reactions that can occur between 2 DIFFERENT carbonyl compounds
what can be concluded for a crossed aldol reaction where 2 different carbonyl compounds BOTH have alpha-Hs
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)
when are crossed aldol reactions most successful?
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)
what are 4 examples of aldehydes with no alpha-Hs?
formaldehyde
benzaldehyde
furfural
2,2-dimethylpropanal
what is our aldol reaction for this lab?
2 benzaldehyde + acetone —> (KOH, 95% aq. EtOH) dibenzalacetone + 2 H2O
what is the order of incorporating chemicals in our aldol reaction?
- solvent 95% ethanol goes into reaction flask
- benzaldehyde
- aq. KOH
- acetone last
why is acetone added last into the reaction flask?
acetone is added last to prevent it from reacting with itself (aka self-aldol reaction)
what allows dibenzalacetone to absorption sunlight?
CONJUGATION
- it is also a component of sunscreens
what are the health hazards of KOH and acetone?
KOH - corrosive (strong base)
acetone - flammable
why is a crossed-aldol condensation reaction hard to control?
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)
which 2 methods can allow you to control a crossed aldol condensation reaction (aka get only 1 product instead of a mixture)
- careful choice of the base used for enolate formation
- control the molar proportions of the 2 carbonyl compounds
in this lab, what type of reaction is it?
a crossed aldol condensation reaction
how do we control the product we get for this lab’s specific reaction?
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
what are our 3 starting reagents for this lab?
1mL benzaldehyde
6mL 95% ethanol
4mL 2M KOH
–> all of them are liquids
what is the order in which we add the reagents into the reaction flask?
- ethanol
- benzaldehyde
- KOH
- acetone (LAST) - after the other 3 are stirred for 2 min
why is a hot plate used for this lab?
for the STIRRING function ONLY (no heat)
what happens to the solution after adding acetone and stirring?
the solution turns from CLEAR –> YELLOW
what is the overall procedure of this lab?
- reaction occurs - stirring
- vacuum filtration to separate the product produced
what do you rinse the reaction flask with for vacuum filtration?
ICE COLD ethanol
what does the final aldol product look like?
yellow powder
waste disposal procedure?
there are specially labeled containers for this lab
KOH health hazard?
extremely CORROSIVE
the general aldol condensation reaction is ___ and consists of which few steps (3)
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)
what does it mean for a reaction to be crossed-aldol condensation vs general aldol condensation?
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)
