Before we leave substitution reactions of carbonyl groups, there is one more reaction that we must introduce. It is an important one and we will come back to it again later in this book, particularly in Chapter 27. It also has a rather different mechanism from most you have met in recent chapters, but we talk about it here because the overall consequence of the Wittig reaction is the substitution of a C=C bond for a C=O bond. We don’t normally tell you the name of a reaction before even mentioning how to do it, but here we make an exception because the reagents are rather unusual and need explaining in detail. The Wittig reaction is a reaction between a carbonyl compound (aldehyde or ketone only) and a species known as a phosphonium ylid. An ylid (or ylide) is a species with positive and negative charges on adjacent atoms, and phosphonium ylids are made from phospho nium salts by deprotonating them with a strong base. You have already met phosphonium salts in Chapter 5, where you saw the reaction of a phosphine (triphenylphosphine) with an alkyl halide (methyl iodide) to give the tetrahedral phosphonium salt.

So here is a typical Wittig reaction: it starts with a phosphonium salt, which is treated with a strong base such as BuLi or sodium hydride, and then with a carbonyl compound; the alkene forms in 85% yield.

What about the mechanism? We warned you that the mechanism is rather different from all the others you have met in this chapter, but nonetheless it begins with attack on the

carbonyl group by a nucleophile; the nucleophile is the carbanion part of the phosphonium ylid. This reaction generates a negatively charged oxygen that attacks the positively charged phosphorus and gives a four-membered ring called an oxaphosphetane.

Now, this four-membered ring (like many others) is unstable, and it can collapse in a way that forms two double bonds. Here are the curly arrows: the mechanism is cyclic and gives the alkene, which is the product of the reaction along with a phosphine oxide.

The chemistry of some elements is dominated by one particular property, and a theme running right through the chemistry of phosphorus is its exceptional affinity for oxygen. The P=O bond, with its bond energy of 575 kJ mol−1, is one of the strongest double bonds in chemistry, and the Wittig reaction is irreversible and is driven forward by the formation of this P=O bond. No need here for the careful control of an equilibrium necessary when making acetals or imines.

مواضيع ذات صلة

Energy, enthalpy, and entropy: ΔG, ΔH, and ΔS

Typical method for making an ester

How to make the equilibrium favour the product you want The direct formation of esters

A small change in ΔG makes a big difference in K

The sign of ΔG tells us whether products or reactants are favoured at equilibrium

How the equilibrium constant varies with the difference in energy between reactants and products

Stability and energy levels

Cyanide will attack iminium ions: the Strecker synthesis of amino acids

Lithium aluminium hydride reduces amides to amines

Iminium ions can react as electrophilic intermediates

Secondary amines react with carbonyl compounds to form enamines

Iminium ions and oxonium ions