Page 11 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

organometallic reagents, enolates, enols, and enamines. The mechanism of the aldol xi
addition is discussed. The acidity of hydrocarbons and functionalized molecules is
Introduction
considered. Chapter 7 discusses the fundamental reactions of carbonyl groups. The
reactions considered include hydration, acetal formation, condensation with nitrogen
nucleophiles, and the range of substitution reactions that interconvert carboxylic acid
derivatives. The relative stability and reactivity of the carboxylic acid derivatives is
summarized and illustrated. The relationships described in Chapters 6 and 7 provide the
broad reactivity pattern of carbonyl compounds, which has been extensively developed
and is the basis of a rich synthetic methodology.
Chapter 8 discusses the concept of aromaticity and explores the range of its appli-
cability, including annulenes, cyclic cations and anions, polycyclic hydrocarbons, and
heterocyclic aromatic compounds. The criteria of aromaticity and some of the methods
for its evaluation are illustrated. We also consider the antiaromaticity of cyclobutadiene
and related molecules. Chapter 9 explores the mechanisms of aromatic substitution
with an emphasis on electrophilic aromatic substitution. The general mechanism is
reviewed and the details of some of the more common reactions such as nitration,
halogenation, Friedel-Crafts alkylation, and acylation are explored. Patterns of position
and reactant selectivity are examined. Recent experimental and computational studies
that elucidate the role of aromatic radical cations generated by electron transfer in
electrophilic aromatic substitution are included, and the mechanisms for nucleophilic
aromatic substitution are summarized. Chapter 10 deals with concerted pericyclic
reactions, including cycloaddition, electrocyclic reactions, and sigmatropic rearrange-
ments. This chapter looks at how orbital symmetry influences reactivity and introduces
the idea of aromaticity in transition structures. These reactions provide interesting
examples of how stereochemistry and reactivity are determined by the structure of the
transition state. The role of Lewis acids in accelerating Diels-Alder reactions and the
use of chiral auxiliaries and catalysts to achieve enantioselectivity are explored.
Chapter 11 deals with free radicals and their reactions. Fundamental structural
concepts such as substituent effects on bond dissociation enthalpies (BDE) and radical
stability are key to understanding the mechanisms of radical reactions. The patterns of
stability and reactivity are illustrated by discussion of some of the absolute rate data
that are available for free radical reactions. The reaction types that are discussed include
halogenation and oxygenation, as well as addition reactions of hydrogen halides, carbon
radicals, and thiols. Group transfer reactions, rearrangements, and fragmentations are
also discussed.
Chapter 12 ventures into the realm of photochemistry, where structural concepts
are applied to following the path from initial excitation to the final reaction product.
Although this discussion involves comparison with some familiar intermediates,
especially radicals, and offers mechanisms to account for the reactions, photochemistry
introduces some new concepts of reaction dynamics. The excited states in photo-
chemical reactions traverse energy surfaces that have small barriers relative to most
thermal reactions. Because several excited states can be involved, the mechanism
of conversion between excited states is an important topic. The nature of conical
intersections, the transition points between excited state energy surfaces is examined.
Fundamental concepts of structure and its relationship to reactivity within the
context of organic chemistry are introduced in the first three chapters, and thereafter
the student should try to relate the structure and reactivity of the intermediates and
transition structures to these concepts. Critical consideration of bonding, stereochem-
istry, and substituent effects should come into play in examining each of the basic