Organic Chemistry Lab 318

Spring, 2012

DUE DATES• Today

– Banana Oil Report at beginning of lab– “Quiz” notebook copy of today’s experiment

– At end of lab -- copy of the remainder of the laboratory notebook pages for today's experiment

• Next Week-Dibenzalacetone Report at beginning of lab

Synthesis of Dibenzalacetone

• This reaction is an example of an aldol condensation

• The reaction is also a mixed aldol, between and aldehyde and a ketone

• The product is quite stable because of conjugation (resonance stabilization), and so product formation is highly favored

CH

O

CH3 C

O

CH3NaOH

CH=CH C

O

CH=CH+2

Synthesis of Dibenzalacetone

• Only the ketone has an alpha-H that can be removed by OH- base to form the enolate:

• The carbonyl group in the aldehyde is more reactive than the carbonyl of the ketone:

HO H2O CH2 C CH3

O

CH2 C CH3

O

CH2 C CH3

O

H

C HPh

O

CPh

H

CH2 C CH3

OO

CH2 C CH3

O

Synthesis of Dibenzalacetone

• The tetrahedral intermediate from addition is protonated:

• Then dehydration occurs to give a stable conjugated product (-unsaturated ketone):

HOCPh

H

CH2 C CH3

OO

CPh

H

CH2 C CH3

OOH

H2O

H2OHO CPh

H

CH

O

CH3C

CPh

H

O

CH

O

CH3C

H

H

HO

Synthesis of Dibenzalacetone

• The reaction is repeated by removing the -H from the ketone and the enolate reaction with the aldehyde, followed by dehydration:

• The product has two trans double bonds (more stable).

CPh

H

CH

O

CH3C

HO

C

O

C CH

HCC

H

H

The Experiment

• The experiment is very simple – follow the instructions on p. 103 of the Manual.

• You will need to calculate the quantity of acetone to use based on the amount of benzaldehyde that is in your vial.

• Weigh the vial + contents; pour contents into smallest Erlenmeyer flask; weigh empty vial.

• Add the calculated volume of acetone to the flask

The Experiment

• Use another smallest Erlenmeyer flask to prepare a mixture of 95% ethanol and 10% NaOH

• Add half of the PhCHO/acetone mixture to the ethanol/base mixture

• Swirl for 15 min.• Add the remainder of the PhCHO/acetone

mixture to the reaction flask, and swirl for 15 min.

The Experiment

• Isolate the crude product by vacuum filtration and wash with 3 portions of distilled water (the dibenzalacetone is very insoluble in water).

• Recrystallize the crude product from 95% ethanol using about 10 ml of ethanol for every 4 g of crude product.

• Use cold ethanol to wash the filtered product.• Put product in pre-weighed vial. Weigh vial +

product (“wet” product – no m.pt. this week).

Experiment Notes

• Empty your vial containing the aldehyde/ketone unknown into the Waste container in the hood.

• Wash the vial.

• Clean-up: Tiffany Nguyen