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Experiment 2: aldol condensation reaction


The purpose of this laboratory exercise is to introduce the concept of the Carbon-Carbon bond forming reaction and the basic mechanism of the aldol condensation reaction.

Background information

A useful carbon-carbon bond-forming reaction is known as the Aldol Reaction or Aldol Condensation. It is an example of electrophilic substitution at the alpha carbon in enols or enolate anions. Traditionally, it is the acid- or base-catalyzed condensation of one carbonyl compounds with the enolate/enol of another, which may or may not be the same, to generate aβ-hydroxy carbonyl compound—an aldol. It is reversible in nature. The reaction may occur between two molecules of aldehyde, two molecules of ketones or one molecule of aldehyde and a molecule of ketone. When two different carbonyl compounds react, it is known as mixed aldol/ Crossed aldol condensation.

Reactions in which a larger molecule is formed from smaller components with the elimination of a very small by-product such as water are termed Condensations. Hence the following examples are properly referred to as aldol condensations. The dehydration step of an aldol condensation is also reversible in the presence of acid and base catalysts. Consequently, on heating with aqueous solutions of strong acids or bases, manyα,β-unsaturated carbonyl compounds fragment into smaller aldehyde or ketones, a process known as the retro-aldol reaction. Below, a few examples of aldol condensation reaction are:

The product has both a carbonyl and an alcohol group; since the chemistry was initially developed with aldehydes the name "Ald-ol" (aldehyde-alcohol) was coined and has been applied to the reaction that forms it. For aldol condensation, the presence ofα-H (acidic hydrogen atom) atom is necessary. In the absence ofα-H atom, it can undergo the Cannizaro reaction. Aldol condensation involves the following steps in the mechanism:

1. Addition phase:

(a) Formation of an enol or enolate anion.

(b) Nucleophilic addition

(c) Proton transfer

2. Dehydration phase (If strong base/acid or some heat is applied)

Using acetone as an example:

Step 1: An acid-base reaction occurs first. Ethoxide functions as a base and removes the acidic-hydrogen giving the reactive enolate.

Step 2: The nucleophilic enolate attacks another molecule of acetone at the electrophilic carbonyl C in a nucleophilic addition type process giving an intermediate alkoxide.

Step 3: An acid-base reaction. The alkoxide deprotonates a solvent molecule (here ethanol) creating hydroxide and the β size 12{β} {} hydroxyaldehydes or aldol product is formed.

There is little difference mechanistically between an acid or base catalyzed mechanism but examples of both have been given for completeness.

In next figure, the actual carbon-carbon bond formation has shown in different color for simplicity.

Acid catalyzed reaction mechanism:

Base catalyzed reaction mechanism:

From the mechanism it is clear that presence of electron withdrawing group in acceptor molecule facilitates the reaction and vice versa.

Questions & Answers

what is Nano technology ?
Bob Reply
write examples of Nano molecule?
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Introduction about quantum dots in nanotechnology
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s. Reply
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Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
is Bucky paper clear?
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Do you know which machine is used to that process?
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s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
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