Addition of Bisulfite to Aldehydes

Demonstration Experiment on Video

Addition of Bisulfite to Aldehydes

Objective: Nucleophilic Addition to the Carbonyl Function

Peter Keusch

German version

Chemicals:
benzaldehyde
sodium disulfite

Apparatus and glass wares:
3 graduated cylinder with stopper 500 mL
beaker 200 mL
Erlenmeyer flask with stopper 250 mL
temperature probe>
temperature measuring device

Hazards and safety precautions:

Benzaldehyde may cause a skin allergy. It can affect you when breathed.
Sodium disulfite may cause irritation and may be harmful if swallowed. May cause allergic reaction, skin and eye irritant.

Safety goggles and protective must be worn. The experiment should be carried out in a well ventilated room.

Experimental procedure:

Using an Erlenmeyer flask with stopper, 108 g of sodium disulfite are dissolved in 200 mL of dist. water. Sodium disulfite dissolves in water to form bisulfite ions.

The solution of disulfite is poured into a graduated cylinder containing 116 mL of benzaldehyde. The stoppered cylinder is shaken vigorously. After removing the stopper a temperature probe connected to a temperature measuring device is inserted into the reaction mixture.

Results:

With an exothermic reaction the content of the graduated cylinder solidifies.

Video clip (Download RealPlayer .rm file)

Discussion:

The characteristic reaction of aldehydes and ketones is addition across the carbon-oxygen double bond. Because of polarization of the C=O bond, the carbon atom of the carbonyl group becomes electron-deficient, acquiring a partial positive charge. This makes it susceptible to nucleophilic attack by an electron-rich chemical species. In the present case, bisulfite ion is added to the electrophilic center. Since the sulfur atom of bisulfite has an unshared pair of electrons it can act as a nucleophile and form a bond to carbonyl carbon (1).

In general, aldehydes are more reactive than ketones. There is a combination of steric hindrance and inductive effects that makes ketones to react slower than aldehydes (2).

· Bulky alkyl groups sterically hinder the approach of nucleophile.

· The electronic effects of alkyl substituents are weakly electron donating. So they make the C atom in carbonyl less electrophilic.

The addition of bisulfite is usually employed to purify aldehydes. Aldehydes are isolated from reaction mixtures through its bisulfite derivatives. The addition compound can be split easily to regenerate the aldehyde by treating it with either dilute mineral acid or dilute alkali.

References:
Demonstration Experiment on Video Addition of 2,4-Dinitrophenyl hydrazine to Aldehydes and Ketones
Demonstration Experiment on Video Schiff's Reagent
Microscale Projection Experiments Test for Aldehydes - Schiff's Reagent

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