Most of what we know about the structure of atoms and molecules comes from studying their interaction with light (electromagnetic radiation). Different regions of the electromagnetic spectrum provide different kinds of information as a result of such interactions. Electronic Spectroscopy: Theory Electronic Spectroscopy: Application Electronic Spectroscopy: Interpretation Rotational Spectroscopy of Diatomic Molecules Photoelectron Spectroscopy: Application Photoelectron Spectroscopy: Theory
If E is a strong electrophile, as in the first equation, it will attack the nucleophilic oxygen of the carboxylic acid directly, giving a positively charged intermediate which then loses a proton. If E is a weak electrophile, such as an alkyl halide, it is necessary to convert the carboxylic acid to the more nucleophilic carboxylate anion to facilitate the substitution. The reaction is easily followed by the evolution of nitrogen gas and the disappearance of the reagent's color.
Although nitriles do not have a carbonyl group, they are included here because the functional carbon atoms all have the same oxidation state. The top row (yellow shaded) shows the general formula for each class, and the bottom row (light blue) gives a specific example of each. As in the case of amines, amides are classified as 1º, 2º or 3º, depending on the number of alkyl groups bonded to the nitrogen. The Greek letter locates the nitrogen relative to the carbonyl group of the amide.
Using the combustion of propane as an example, we see from the following equation that every covalent bond in the reactants has been broken and an entirely new set of covalent bonds have formed in the products. Since all the covalent bonds in the reactant molecules are broken, the quantity of heat evolved in this reaction is related to the strength of these bonds (and, of course, the strength of the bonds formed in the products).
Recall that the molecular formula of a hydrocarbon (C n H m ) provides information about the number of rings and/or double bonds that must be present in its structural formula. These are illustrated by the examples in the table above, taken from the previous list of naturally occurring amines. • The presence of oxygen does not alter the relationship. • All halogens present in the molecular formula must be replaced by hydrogen. • Each nitrogen in the formula must be replaced by a CH moiety.
The stereoselectivity of Brønsted acid addition is sensitive to experimental conditions such as temperature and reagent concentration. The selectivity is often anti, but reports of syn selectivity and non-selectivity are not uncommon. Of all the reagents discussed here, these strong acid additions (E = H in the following equation) come closest to proceeding by the proposed two-step mechanism in which a discrete carbocation intermediate is generated in the first step.
We can account both for the high stereoselectivity and the lack of rearrangement in these reactions by proposing a stabilizing interaction between the developing carbocation center and the electron rich halogen atom on the adjacent carbon. The positive charge is delocalized over all the atoms of the ring, but should be concentrated at the more substituted carbon (carbocation stability), and this is the site to which the nucleophile will bond.
Since boron is electron deficient (it does not have a valence shell electron octet) the reagent itself is a Lewis acid and can bond to the pi-electrons of a double bond by displacement of the ether moiety from the solvated monomer. Indeed, this hydride shift is believed to occur concurrently with the initial bonding to boron, as shown by the transition state drawn below the equation, so the discrete intermediate shown in the equation is not actually formed.
The resulting amine substituent strongly activates an aromatic ring and directs electrophilic substitution to ortho & para locations. (iii) The activating character of an amine substituent may be attenuated by formation of an amide derivative (reversible), or even changed to deactivating and meta-directing by formation of a quaternary-ammonium salt (irreversible). (iv) Conversion of an aryl amine to a diazonium ion intermediate allows it to be replaced by a variety of different groups (includin…