The table at the beginning of this page gave the melting and boiling points for a homologous group of carboxylic acids having from one to ten carbon atoms. In the table of fatty acids we see that the presence of a cis-double bond significantly lowers the melting point of a compound. The first five entries all have oxygen functional groups, and the relatively high boiling points of the first two is clearly due to hydrogen bonding.
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
The following equations, in which the hydroxyl oxygen atom of the carboxylic acid is colored red and that of the alcohol is colored blue, illustrate this distinction (note that the starting compounds are in the center). Since this oxygen is found in the ester product and not the water, the hydroxyl group of the acid must have been replaced in the substitution.
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).
The following table lists several general examples of this strategy in the rough order of decreasing nucleophilicity of the nitrogen reagent. Acid chlorides react with ammonia to give amides, also by an addition-elimination path, and these are reduced to amines by LiAlH 4 . The 6th example is a specialized procedure for bonding an amino group to a 3º-alkyl group (none of the previous methods accomplishes this).
One third of the hydride is lost as hydrogen gas, and the initial product consists of metal salts which must be hydrolyzed to generate the alcohol. In the first, bromine replaces the carboxyl group, so both the carboxyl carbon atom and the remaining organic moiety are oxidized. Finally, the third example illustrates the general decarboxylation of β-keto acids, which leaves the organic residue in a reduced state (note that the CO 2 carbon has increased its oxidation state.).
35 kcal/mole), but in this case is polarized so that the acyloxy group is negative and the hydroxyl group is positive (recall that the acidity of water is about ten powers of ten weaker than that of a carboxylic acid). It is important to remember that whenever an atom or group is reduced, some other atom or group is oxidized, and a balanced equation must balance the electron gain in the reduced species with the electron loss in the oxidized moiety, as well as numbers and kinds of atoms.