HOMO-LUMO interactions carbonyl group with

Reactions of nucleophiles with protons and with carbonyl groups are heavily influenced by electrostatic attraction (as well as by HOMO-LUMO interactions). The proton is, of course, positively charged. The carbonyl group too has a substantial positive charge on the carbon atom, which comes from the uneven distribution of electrons in the C=0 k bond (Chapter 4). 

You have met a similar sequence before in Chapter 10, and it would be useful to review the terms we used then. Nucleophiles like R3P and RS-, the ones that react well with saturated carbon, are referred to as soft nucleophiles and those that are more basic and react well with carbonyl groups referred to as hard nucleophiles. These are useful and evocative terms because the soft nucleophiles are rather large and flabby with diffuse high-energy electrons while the hard nucleophiles are small with closely held electrons and high charge density. When we say hard (nucleophile or electrophile) we refer to species whose reactions are dominated by electrostatic attraction and when we say soft (nucleophile or electrophile) we refer to species whose reactions are dominated by HOMO-LUMO interactions. 

Ammonia, a soft nucleophile for being neutral, reacts with methyl acrylate 100 in methanol in conjugate manner to give the primary amine 101. The reaction continues in the same sense and the secondary amine 102 and the tertiary amine 103 are formed successively [39]. It is to be noted that ammonia, and other primary and secondary amines, do not react with simple esters to form amides. Combine this with the known observation that attack at the carbonyl group is irreversible and also rate determining [40], the above conjugate addition must necessarily be a product of kinetic control, supported by HOMO-LUMO interaction. 

Notice how the trigonal, planar sp hybridized carbon atom of the carbonyl group changes to a tetrahedral, sp hybridized state in the product. For each class of nucleophile you meet in this chapter, we will show you the HOMO-LUMO interaction involved in the addition reaction. These interactions also show you how the orbitals of the starting materials change into the orbitals of the product as they combine. Most importantly here, the lone pair of the nucleophile combines with the re of the carbonyl group to form a new a bond in the product. 

What does matter is the strength of the HOMO-LUMO interaction. In a nucleophilic attack on the carbonyl group, the nucleophile adds in to the low-energy n orbital. In a nucleophilic attack on a saturated carbon atom, the nucleophile must donate its electrons to the cr orbital of the C-X bond, as illustrated in the margin for an alkyl bromide reacting with the nonbonding lone pair of a nucleophile. 

Alkyl substituents accelerate electrophilic addition reactions of alkenes and retard nucleophilic additions to carbonyl compounds. The bonding orbital of the alkyl groups interacts with the n bonding orbital, i.e., the HOMO of alkenes and raises the energy (Scheme 22). The reactivity increases toward electron acceptors. The orbital interacts with jt (LUMO) of carbonyl compounds and raises the energy (Scheme 23). The reactivity decreases toward electron donors. 

Lewis acid catalysis enormously enriches the scope of Diels-Alder reactions, but it is limited to reagents containing Lewis basic sites, i.e. functional groups with lone pairs such as carbonyl, amino, ether or nitro close to the reaction centre. As we have seen in the discussion about the FMO aspects of Lewis acids, the major reason for catalysis is the reduction of the HOMO-LUMO gap. In case of Diels-Alder reactions with normal electron demand, it follows that the coordination of the Lewis acid lowers the LUMO energy of the dienophile. Such interactions are only possible if there is a spatial proximity or an electronic conjugation between the coordinated Lewis basic site and the reaction centre. Fortunately, in nearly every Diels-Alder reaction one of the reagents, mostly the dienophile, meets this requirement. 

As we have already seen, delocalization of electrons by conjugation decreases the energy difference between the HOMO and LUMO energy levels, and this leads to a red shift. Alkyl substitution on a conjugated system also leads to a (smaller) red shift, due to the small interaction between the cr-bonded electrons of the alkyl group with the K-bond system. These effects are additive, and the empirical Woodward-Fieser rules were developed to predict the 2max values for dienes (and trienes). Similar sets of rules can be used to predict the A ax values for a,P-unsaturated aldehydes and ketones (enones) and the Amax values for aromatic carbonyl compounds. These rules are summarized in Table 2.4. 

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