Part 1

Carbonyl compounds

edited by Roger Hill and Peter Taylor

based on Carbonyl compounds, by Jim Iley

INTRODUCTION: CARBONYL COMPOUNDS IN CONTEXT 1

Organic compounds are conveniently subdivided into classes, based on their functional group; each member within a class reacts in the same way with a particular reagent. This Part of the Book explains the chemistry of the carbonyl group (C=O); you can see in Table 1.1 that the carbonyl group turns up in a number of functional groups. You have met many of these before, and, no doubt, have encountered them in everyday life, probably without realizing it (see Box 1.1). Carbonyl compounds are so widespread that some understanding of their behaviour is an essential requirement for every student of organic chemistry.

We'll start our examination of the topic with a close look at the structure of the carbonyl group.

THE STRUCTURE OF THE CARBONYL GROUP 2

X-ray crystallography shows that the carbonyl group is planar. The carbon and oxygen atoms of the carbonyl group, together with the two carbon atoms attached to the carbonyl carbon, all lie in the same plane, with the three bond angles all close to 120° (2.1). This implies a particular arrangement of molecular orbitals.

* What hybridization of s and p orbitals is implied at the carbon atom?

* Since the central carbon atom is bound to three other atoms, it must form three σ bonds and so it will be sp2-hybridized.

* What about the oxygen atom?

* It forms one σ bond to carbon, but has two non-bonding electron pairs, so is also sp2-hybridized.

* So both the oxygen atom and the central carbon atom contribute one electron to the σ bond between them, which leaves one over for each. How are these deployed?

* They are each contained in a p orbital perpendicular to the plane and overlapping sideways with each other, forming a π bond.

The bond between carbon and oxygen is thus a double bond, with two electrons in a σ bond and two electrons in a π bond (Figure 2.1).

The carbonyl bond is polar; that is, the four shared electrons are closer to one atom than to the other, leaving one atom electron-deficient or slightly positive (δ+), and the other with a surplus or slightly negative (δ-).

* Which atom bears the δ+ and why?

* Oxygen is more electronegative than carbon, and claims more than an equal share of the four electrons, leaving the carbon atom electron deficient. This is shown in Figure 2.2.

This polarization of C=O (2.2) is extremely useful in understanding the chemical behaviour of carbonyl compounds.

* Which of the two atoms would be prone to attack by nucleophiles?

* Nucleophiles have an electron-rich centre (they are 'nucleus-liking') and therefore seek to associate with the electron-deficient (positively charged) carbon atom.

* What is the simplest electrophile, and how would it seek to associate with a carbonyl compound?

* The hydrogen ion (proton) is the simplest electrophile, and would associate with the oxygen atom; that is, the carbonyl oxygen atom can be protonated.

You will see shortly how a large part of the chemistry of carbonyl compounds can be explained by the answers to the last two questions. But the electronic structure of a group also determines its spectroscopic properties, so this is a good place for a brief look in that direction.

* Why should you want to know about spectroscopic properties?

* The spectra (e.g. infrared and nuclear magnetic resonance) of a class of organic compounds have features common to that class. Conversely, some features of the spectra are diagnostic for that class of compound, so organic chemists use spectroscopy to tell whether or not a compound belongs to that class — that is, whether it contains the relevant group within its molecular structure.

Each of our exemplar compounds in Table 1.1 fall within the range of IR and 13C NMR values usually quoted for carbonyl groups, namely 1 650–1 850 cm-1 and 160–220 p.p.m., respectively. Details for each carbonyl functional group are given in Table 23.1 of the Data Book, which is available from the CD-ROM. We quote a range because, although the spectroscopic values differ from one functional group to another, there is a specific range for each functional group depending on the nature of the R group(s). When we use these ranges to determine the structure of an unknown compound, there is some overlap; in such cases we need to bring together all the information we have about a compound before we can be sure of the nature of the functional group.

2.1 Summary of Sections 1 and 2

1 The carbonyl group is found in the following classes of compound: aldehydes, ketones, carboxylic acids and their salts, esters, amides, acid halides and acid anhydrides.

2 The carbonyl group is planar, and the three bond angles around the central carbon atom are all close to 120°.

3 The bonding in the carbonyl group involves σ bond formation by overlap of an sp2 hybrid orbital on carbon with an sp2 hybrid orbital on oxygen, and π bond formation by overlap of a p orbital on carbon with a p orbital on oxygen.

4 The carbon–oxygen double bond is...