Carbon-nitrogen bonds nucleophilic properties

Chapter 22 continues the study of carbonyl compounds with a detailed look at nucleophilic acyl substitution, a key reaction of carboxylic acids and their derivatives. Substitution at sp hybridized carbon atoms was introduced in Chapter 20 with reactions involving carbon and hydrogen nucleophiles. In Chapter 22, we learn that nucleophilic acyl substitution is a general reaction that occurs with a variety of heteroatomic nucleophiles. This reaction allows the conversion of one carboxylic acid derivative into another. Every reaction in Chapter 22 that begins with a carbonyl compound involves nucleophilic substitution. Chapter 22 also discusses the properties and chemical reactions of nitriles, compounds that contain a carbon-nitrogen triple bond. Nitriles are in the same carbon oxidation state as carboxylic acids, and they undergo reactions that form related products. 

Chemical Properties The formation of salts with acids is the most characteristic reaction of amines. Since the amines are soluble in organic solvents and the salts are usually not soluble, acidic products can be conveniendy separated by the reaction with an amine, the unshared electron pair on the amine nitrogen acting as proton acceptor. Amines are good nucleophiles reactions of amines at the nitrogen atom have as a first step the formation of a bond with the unshared electron pair of nitrogen, eg, reactions with acid anhydrides, haUdes, and esters, with carbon dioxide or carbon disulfide, and with isocyanic or isothiocyanic acid derivatives. 

The principal feature of the chemical reactivity of QBA is the addition of a nucleophile to the iminium bond C=N (Scheme 1). The carbon atom C-6 displays the lowest 7C-electron density [97,98]. This process is associated with a number of significant alterations in the constitution, physical appearance, solubility, spectral properties, etc. The quaternary cation is a brightly coloured, polar, water-soluble species. The tertiary-nitrogen adduct has lost the colour and is non-polar and water insoluble. In the case of aminoacetal and aminal derivatives (Scheme 1, Nu = OR, NHR), the reaction is essentially reversible, i.e. the action of acid immediately converts the adduct back to the quaternary salt. Viewed from another perspective the emergence of colour is a sensitive indicator of the presence of some acid and ipso facto of deconq)osition of the adduct. 

Owing to the high polarizability of the carbon-selenium double bond, selenoesters can easily be attacked by nucleophiles. This property makes a selenoester a useful reagent for the synthesis of hetero-cycles. Condensation of selenoesters with bifiinctional aromatic or aliphatic amines has yielded a variety of nitrogen-containing heterocyclic compounds (Scheme 19). 

Thermal Stability According to a literature search, it is known that ionic liquids are thermally stable (300°C to 400°C). The imidazohum salts are reported to be thermally stable up to 300°C. Above 300°C, there is a cleavage of the C-N bond of the imidazolium nitrogen and the carbon of the alkyl chain (57, 66-68). It was found that different anions also affect the thermal properties of the ionic hquids (66-68). Ionic liquids with less nucleophilic or coordinating anions show a higher thermal stability (66-69). 

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