Carbonyl groups terminology

Substitution reactions by the ionization mechanism proceed very slowly on a-halo derivatives of ketones, aldehydes, acids, esters, nitriles, and related compounds. As discussed on p. 284, such substituents destabilize a carbocation intermediate. Substitution by the direct displacement mechanism, however, proceed especially readily in these systems. Table S.IS indicates some representative relative rate accelerations. Steric effects be responsible for part of the observed acceleration, since an sfp- caibon, such as in a carbonyl group, will provide less steric resistance to tiie incoming nucleophile than an alkyl group. The major effect is believed to be electronic. The adjacent n-LUMO of the carbonyl group can interact with the electnai density that is built up at the pentacoordinate carbon. This can be described in resonance terminology as a contribution flom an enolate-like stmeture to tiie transition state. In MO terminology,.the low-lying LUMO has a... 

But before we can get started, we need to know some basics about C=0 bonds. Let s start with a bit of terminology that we will use throughout the entire chapter. Instead of constantly using the expression C=0 double bond, we will call it a carbonyl group. This term is NOT used for nomenclature. You will never see the term carbonyl appearing in the lUPAC name of a compound. Rather, it is just a term that we use when we are talking about mechanisms, so that we can quickly refer to the C=0 bond without having to say C=0 double bond all of the time. 

This terminology can also be applied to the faces of trigonal planar groups such as carbonyl. Thus the two faces of acetophenone are enantiotopic as seen by the fact that reaction of hydride ion at one as opposed to the other gives enantiomeric products (14) and (15) (section 1.3). The faces of the carbonyl group in (55) are diastereotopic because reduction on one or other gives diastereomers (56) and (57) (section 1.11). 

Fig. 3.1. Glucose and its two anomeric forms, which result from the formation of a ring on opening up of the carbonyl (—C=0) group. Note the several ways of drawing molecular structure. Maltose consists of two alpha-glucoses linked by a 1,4 bond, Amylose consists of a chain of such linkages. Glycogen, in addition, contains 1,6 links, which result in a branched structure. See Appendix 1, page 73, for further review of isomer terminology.

The carbonyl oxygen lone pairs, NBOs 11 and 12, are again seen to be of distinctly inequivalent form. The on-axis no NBO 11, is relatively inert, usually only weakly involved in carbonyl intra- and intermolecular interactions. In contrast, the off-axis NBO 12 (in-plane p-Tt-type, labeled Py in Zimmerman s terminology), is the primary active site of co-ordinative H-bonding (Chapter 9) and photochemical n—>n excitation (Chapter 11). Thus, a rabbit ears depiction of carbonyl lone pairs is seriously erroneous and misleading with respect to important chemical properties of amide groups. 

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