Water is less acidic than hydrogen peroxide because hydrogen is less electronegative than oxygen, and the covalent bond joining these atoms is polarized in the manner shown. The acidic hydrogen is colored red in all examples. The following diagram illustrates this factor for several simple inorganic and organic compounds (row #1), and shows how inductive electron withdrawal may also increase the acidity of carboxylic acids (rows #2 & 3). For example, alcohols have pK a's of 16 or greater but their acidity is increased by electron withdrawing substituents on the alkyl group. However, inductive effects also play a role. The resonance effect described here is undoubtedly the major contributor to the exceptional acidity of carboxylic acids. This stabilization leads to a markedly increased acidity, as illustrated by the energy diagram displayed by clicking the " Toggle Display" button. In the carboxylate anion the two contributing structures have equal weight in the hybrid, and the C–O bonds are of equal length (between a double and a single bond). Both the carboxyl group and the carboxylate anion are stabilized by resonance, but the stabilization of the anion is much greater than that of the neutral function, as shown in the following diagram. Water is the standard base used for pK a measurements consequently, anything that stabilizes the conjugate base (A: (–)) of an acid will necessarily make that acid (H–A) stronger and shift the equilibrium to the right. In an acid base equilibrium the equilibrium always favors the weaker acid and base (these are the more stable components). We know that an equilibrium favors the thermodynamically more stable side, and that the magnitude of the equilibrium constant reflects the energy difference between the components of each side. These relationships were described in an previous section of this text. Why should the presence of a carbonyl group adjacent to a hydroxyl group have such a profound effect on the acidity of the hydroxyl proton? To answer this question we must return to the nature of acid-base equilibria and the definition of pK a, illustrated by the general equations given below. When we compare these values with those of comparable alcohols, such as ethanol (pK a = 16) and 2-methyl-2-propanol (pK a = 19), it is clear that carboxylic acids are stronger acids by over ten powers of ten! Furthermore, electronegative substituents near the carboxyl group act to increase the acidity. The pK a 's of some typical carboxylic acids are listed in the following table. The high boiling points of the amides and nitriles are due in large part to strong dipole attractions, supplemented in some cases by hydrogen bonding. When the mouse pointer passes over the drawing, an electron cloud diagram will appear. A structural formula for the dimer of acetic acid is shown here. Carboxylic acids have exceptionally high boiling points, due in large part to dimeric associations involving two hydrogen bonds. 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. Physical Properties of Some Organic Compounds The following table lists a few examples of these properties for some similar sized polar compounds (the non-polar hydrocarbon hexane is provided for comparison). Hydrogen bonding is also a major factor in the water solubility of covalent compounds To refresh your understanding of these principles Click Here. In general, dipolar attractive forces between molecules act to increase the boiling point of a given compound, with hydrogen bonds being an extreme example. The factors that influence the relative boiling points and water solubilities of various types of compounds were discussed earlier. Again, changes in crystal packing and intermolecular forces are responsible. Thus, palmitoleic acid melts over 60º lower than palmitic acid, and similar decreases occur for the C 18 and C 20 compounds. In the table of fatty acids we see that the presence of a cis-double bond significantly lowers the melting point of a compound. This reflects differences in intermolecular attractive forces in the crystalline state. Unbranched acids made up of an even number of carbon atoms have melting points higher than the odd numbered homologs having one more or one less carbon. The boiling points increased with size in a regular manner, but the melting points did not. 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. Physical Properties of Some Carboxylic Acids
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