MCAT Organic Chemistry

Phosphorus Compounds

MCAT Organic Chemistry > Phosphorus Compounds

Background

Phosphoric Acid (H3PO4) is a widely used inorganic acid reagent in organic chemistry. Phosphoric acid can be used to create many different types of chemical compounds.

MCAT Phosphorus Compounds - Phosphoric Acid ModelMCAT Phosphorus Compounds - Phosphoric Acid

You should also know the simplest phosphoric acid anhydride: Pyrophosphate

MCAT Phosphorus Compounds - Pyrophosphate

Phosphodiester Bonds

See Phosphodiester Bonds

Wittig Reaction

MCAT Phosphorus Compounds - Wittig Reaction

In a Wittig reaction, a carbonyl group is essentially replaced with an alkene bond by reacting it with a phosphorous ylide. A more technical description of this reaction indicates that the Wittig reaction forms carbon-carbon double bonds by converting aldehydes and ketones to alkenes. This reaction usually does not involve stereochemistry.

Carbonyl + Phosphorus Yilde → Alkene

Steps of a Wittig reaction

Step 1: Formation of a phosphonium salt from the SN2 reaction of an alkyl halide with triphenylphosphine, a strong nucleophile

Step 2: The phosphonium salt is the deprotonated with a strong base, which causes the loss of the hydrogen α to the phosphorous, and yielding an ylide (neutral) or phosphorane, which will alternate at equilibrium.

The ylide form is a zwitterion (molecule with both positive and negative charges), and the phosphorane form has a double bond between phosphorous and carbon.

A common strategy to help solve this problem is to realize that the C1=O reacts with Ph3P=C2 to create a double between between C1=C2

MCAT Organic Chemistry

Organic Chemistry Topics