In Chapter 8 (p. 175) we talked about how hybridization affects acidity. Alkynes, with their C–H bonds formed from sp orbitals, are the most acidic of hydrocarbons, with pKas of about 25.

They can be deprotonated by more basic organometallics such as butyllithium or ethyl magnesium bromide. Alkynes are sufficiently acidic to be deprotonated even by nitrogen bases and you saw on p. 171 that a common way of deprotonating alkynes is to use NaNH₂ (sodium amide), obtained by reacting sodium with liquid ammonia. An example of each is shown here. Propyne and acetylene are gases, and can be bubbled through a solution of the base.

The metal derivatives of alkynes can be added to carbonyl electrophiles, as in the following examples. The fi rst (we have reminded you of the mechanism for this) is the initial step of an important synthesis of the antibiotic erythronolide A, and the second is the penultimate step of a synthesis of the widespread natural product farnesol.

Ethynylo estradiol The ovulation-inhibiting component of almost all oral contraceptive pills is a compound known as ethynyloestradiol, and this compound too is made by an alkynyl lithium addition to the female sex hormone oestrone. A range of similar synthetic analogues of hormones containing an ethynyl unit are used in contraceptives and in treatments for disorders of the hormonal system.

Triple bonds: stability and acidity

You have now met all the more important compounds with triple bonds. They all have electrons in low-energy sp hybrid orbitals (shown in green on the diagrams below), a feature which gives them stability or even unreactivity. Remember, an sp orbital has 50% s character, so electrons in this orbital are on average closer to the nucleus, and therefore more stable, than electrons in an sp² or sp³ orbital. Nitrogen, N₂, has sp orbitals at both ends and is almost inert. It is neither basic nor nucleophilic and a major achieve ment of life is the ‘fixing’ (trapping in reductive chemical reactions) of nitrogen by bacteria such as those in the roots of leguminous plants (peas and beans). HCN has an sp orbital on nitrogen and a C–H σ bond at the other end. The nitrogen’s sp lone pair is not at all basic, but HCN is quite acidic with a pKa of 10 because the negative charge in the conjugate base (CN−) is in an sp orbital. Nitriles have similar bonds and they are non-nucleophilic and non-basic. Finally, we have just met alkynes, which are among the most acidic of hydrocarbons, again because of the stability of an anion with its charge in an sp orbital.

مواضيع ذات صلة

The product of nucleophilic addition to a carbonyl group is not always a stable compound

Oxidation of alcohols

?Secondary and tertiary alcohols: which organometallic, which aldehyde, which ketone

Making primary alcohols from organometallics and formaldehyde

Making carboxylic acids from organometallics and carbon dioxide

Using organometallics to make organic molecules

Halogen–metal exchange

Organometallics as bases

How to make organolithium reagents

Making organometallics, How to make Grignard reagents

Organometallic compounds contain a carbon–metal bond

Lewis acids and bases