Lab 11: Aldol Condensation

is this acidic or base catalyzed

basic

what is involved in this reaction

it is a base catalyzed coupling of two carbonyl compounds (either can happen between identical carbonyl compounds or dissimilar carbonyl compounds)

how does our experiment this week differ from the usual method

1. only one of the species has alpha hydrogens to form the enolate intermediate, so there will not be a mixed reaction/crossed aldol reaction but only one product

2. there are two sets of symmetrical alpha carbons, so two reaction enolate intermediates will form (in a stepwise fashion). this is why we have two equivalents of the aldehyde because it will add on both sides to form a disubstituted product

what is an enone

a combination of alkene and carbonyl group (ketone)

techniques used

suction filtration, recrystallization, melting point determination, IR spec

reagents

Acetone (structure, MW, BP, density)
p-Tolualdehyde (structure, MW, BP, density)
o-Tolualdehyde (structure, MW, BP, density)
4-Methoxybenzaldehyde (structure, MW, BP, density)
Benzaldehyde (structure, MW, BP, density)
Ethanol (structure, MW, BP, density)
Sodium hydroxide (structure, MW, MP)
(1E,4E)-1,5-diphenylpenta-1,4-dien-3-one (structure, MW, MP = 110-111 oC)
(1E,4E)-1,5-di-o-tolylpenta-1,4-dien-3-one (structure, MW, MP = 98-100 oC)
(1E,4E)-1,5-di-p-tolylpenta-1,4-dien-3-one (structure, MW, MP = 174-177 oC)
(1E,4E)-1,5-bis(4-methoxyphenyl)penta-1,4-dien-3-one (structure, MW, MP = 128-132 oC

broad description of the protocol

1. add 95% ethanol to your unknown aldehyde and acetone in a conical vial and mix for like 15 mins

2. once formed, chill vial in water

3. then collect crude product by suction filtration

4. wash with water about three times

5. after fourth wash, test the pH of the solution (you want it to be neutral and not basic)

6. recrystallize in warm ethanol

describe alpha hydrogens acidity in nature

weakly acidic (resonance stablization of enolate anion)

how to Bronsted-Lowry bases catalyze keto-enol tautomerization

by abstracting an alpha hydrogen and forming a reacting enolate intermediate (which will then act as a nucleophile and react with the electrophil to yield the addition products)

why are enolates more reactive than their neutral version

because of the higher overall electron density

what is the electrophile in this?

the carbonyl group

what is the product of the first elemination

b-hydroxy aldehyde, aka an aldol (basically an aldehyde and an alcohol)

for the dehydration of an aldol product, are we in basic or acidic conditions

basic conditions

what is the dehydration mechanism known as

E1cB

which one is more reactive: an aldehyde or a ketone

aldehyde because it is more electrophilic

what is the molar ratio of the electrophilic starting material to the final product

2:1

what are the four possible reactants today

benzaldehyde, p-tolualdehyde, o-tolualdehyde, 4-methoxybenzaldehyde

draw the enolate intermediate for this reaction that will act as nucleophile

enolate of acetone

what is used to catalyze this reaction

sodium hydroxide (hence the basic conditions)

draw the four possible products

balanced equation

2 aldehyde

1 acetone

1 product

draw the mechanism

first part: form the enolate or when the H gets eliminated to form that enolate

nucleophilic attack

• nucleophile will attack benzaldehyde to form the enol. then the alpha H in the middle will get eliminated to form a double bond near the once carbonyl. then rearrangement will happen to reform the carbonyl and kick off the OH

compare and contrast IR of the starting

First, benzaldehyde has two weak peaks around 2720 and 2820 cm-1 because of the aldehyde C-H bonds. Also the C=O stretch in benzaldehyde is around 1700-1720 cm-1. These two aspects change in the product IR. First the C=O stretch in the product is a lower wavenumber—around 1650-1680 cm-1. This is because of the conjugation with the adjacent C=C bonds to the C=O bond in the product. Next, there is a strong peak around 1600 cm-1 because of the C=C bonds in the alkene chain. Lastly, the aldehyde C-H bonds in the benzaldehyde IR should be gone because these bonds are not present in our product.

p-chlorobenzaldehyde is incapable of undergoing an aldol reaction with itself, explain why. Be specific in your answer:

it lacks alpha hydrogens so it cannot form the enolate

what reagent was in excess and why?

aldehyde because it was added twice to the nucleophile since there were symmetrical alpha carbon sets on both sides

draw the product if only one aldehyde is present

there should only be benzyl group present in this product

Students were instructed to mix the benzaldehyde and acetone starting materials in their conical vials before adding the ethanolic sodium hydroxide solution. Why was this essential to the success of the reaction?

this ensure cross-aldol will occur instead of acetone reacting with itself

what would it look like if acetone reacted with itself (balanced equation)

Provide skeletal structures of the aldol products that form when the following compounds or mixtures of compounds are reacted under basic (NaOH) conditions: