Multiple bonds and aromaticity

Unsaturated fluorocarbons are much more reactive toward nucleophiles than then hydrocarbon counterparts owing to fluorme s ability to both stabihze carban ions and mductively increase the electrophihcity of multiple bonds and aromatic nngs Nucleophihc attack dominates the chemistry of unsaturated fluorocarbons, and the role of fluonde ion in fluorocarbon chemistry is analogous to that of the proton in hydrocarbon chemistry [129] Like the related electrophilic reactions for hydrocarbons, there are fluonde-promoted isomenzations and dimenzations (equation 9), oligomenzations (equation 10), additions (equation 11), and amomc Fnedel-Crafts alkylations (equation 12) that all proceed via carbamomc intermediates [729 7 7]... 

This chapter begins with an introduction to the basic principles that are required to apply radical reactions in synthesis, with references to more detailed treatments. After a discussion of the effect of substituents on the rates of radical addition reactions, a new method to notate radical reactions in retrosynthetic analysis will be introduced. A summary of synthetically useful radical addition reactions will then follow. Emphasis will be placed on how the selection of an available method, either chain or non-chain, may affect the outcome of an addition reaction. The addition reactions of carbon radicals to multiple bonds and aromatic rings will be the major focus of the presentation, with a shorter section on the addition reactions of heteroatom-centered radicals. Intramolecular addition reactions, that is radical cyclizations, will be covered in the following chapter with a similar organizational pattern. This second chapter will also cover the use of sequential radical reactions. Reactions of diradicals (and related reactive intermediates) will not be discussed in either chapter. Photochemical [2 + 2] cycloadditions are covered in Volume 5, Chapter 3.1 and diyl cycloadditions are covered in Volume 5, Chapter 3.1. Related functional group transformations of radicals (that do not involve ir-bond additions) are treated in Volume 8, Chapter 4.2. 

One of the mildest general techniques to extend a carbon chain entails the addition of a carbon-centered radical to an alkene or alkyne. The method for conducting these addition reactions often determines the types of precursors and acceptors that can be used and the types of products that are formed. In the following section, synthetically useful radical additions are grouped into chain and non-chain reactions and then further subdivided by the method of reaction. Short, independent sections that follow treat the addition of carbon-centered radicals to other multiple bonds and aromatic rings and the additions of hete-roatom-centered radicals. 

ADDITIONS OF CARBON-CENTERED RADICALS TO OTHER MULTIPLE BONDS AND AROMATIC RINGS... 

This chapter gives a nearly exhaustive overview of the performance of Rh and Ru nanoparticles as soluble catalysts. Rh and Ru nanoparticles are mainly used in traditional hydrogenation reactions, in particular in the hydrogenation of carbon-carbon multiple bonds and aromatic compounds, but a few examples of other substrates containing carbonyl groups or other reducible functions such as the nitro group are also reported. Finally, in the main fields of homogeneous... 

The bond of lowest energy is the first bond to be cleaved. Conversely, polymers with multiple bonds and aromatic structures are less prone to thermal degradation. 

Dye molecules are of commercial importance because they are very intensely colored. Most dye molecules possess many multiple bonds and are often aromatic. Why is this important to the properties of the dye molecule ... 

There are a number of structural features in R—H that promote the removal of H by bases through making it more acidic, and also features that serve to stabilise the resultant carbanion, Re in some cases both effects are promoted by the same feature. The main features that serve to stabilise carbanions are (cf. factors that serve to stabilise carboca-tions, p. 104) (a) increase in s character at the carbanion carbon, (b) electron-withdrawing inductive effects, (c) conjugation of the carbanion lone pair with a polarised multiple bond, and (d) aromat-isation. 

Examples of other cases of prediction of complex organic reactions have been given elsewhere ( ). Functions applicable to the reactivity of multiple bonds and of aromatic systems have been developed in an analogous manner. 

The carboxylic group in carboxylic acids can be reduced to an aldehyde group, to an alcoholic group and even to a methyl group. Unsaturated acids and aromatic acids can be reduced at the multiple bonds or aromatic rings. 

Reagents where a carbon-carbon or a carbon-hydrogen bond is broken are classified as neutral (0) and most frequently include carbon-carbon multiple bonds or aromatic C-H bonds. 

All functional groups do not have polar bonds, e.g. alkenes, alkynes, and aromatic compounds have covalent multiple bonds and since space between the multiple bonded carbons is rich in electrons and is therefore nucleophilic. Thus, the nucleophilic centre in these molecules is not a specific atom, but the multiple bond ... 

Dehydrogenations, which involve the elimination of hydrogen Ifom organic molecules, lead to compounds containing double bonds, multiple bonds, or aromatic rings. For practical reasons, only the formation of carbon-carbon double bonds, of carbon-nitrogen double bonds in cyclic amines, and of aromatic rings (both carbocyclic and heterocyclic) will be discussed in this chapter. The conversion of alcohols into aldehydes and ketones and of amines into imines and nitriles will be discussed in the chapter Oxidations (Chapter 3). 

The simplest descriptors of the degree of unsaturation of a molecule are -> count descriptors based on the presence of double bonds, triple bonds, and aromatic bonds they are the double-bond count, the triple-bond count n= and the aromatic-bond count The MCB index was proposed as the number of multiple C-C bonds in the molecule accounting for double, triple, and aromatic bonds [Bakken and Jurs, 1999b]. Another simple global multiple bond count is defined as ... 

Electronic transition absorption losses of organic materials in the visible wavelength region are caused by multiple bonds (such as double bonds, triple bonds, and aromatic rings) whose electronic transitions involve the electrons in the rc-bonds. The accurate estimation of oteiec therefore requires very detailed quantum mechanical calculations which can use up large amounts of computer time. Instead, a simple empirical correlation is used for CKeiec-... 

ALKENES, ALKYNES, AND AROMATIC HYDROCARBONS (SECTION 24.3) The names of alkenes and alkynes are based on the longest continuous chain of carbon atoms that contains the multiple bond, and the location of the multiple bond is specified by a numerical prefix. Alkenes exhibit not only structural isomerism but geometric (cts-trans) isomerism as well. In geometric isomeis, the bonds are the same, but the molecules have different geometries. Geometric isomerism is possible in alkenes because rotation about the C=C double bond is restricted. 

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