Molecular Recognition of Aromatic and Aliphatic Carboxylic Adds

Molecular Recognition of Aromatic and Aliphatic Carboxylic Adds 

Three aromatic carboxylic acids, benzoic add (Arl), phenylacetic acid (Ar2), and 4-methoxyphenylacetic acid were selected as guests to probe the depth of penetration in host 1, and, as well, two aliphatic carboxylic acid guests, cydo-hexylacetic acid (All), and l-adamantanecarboxyHc acid (A12), were also used to further study the importance of hydrophobic effects in the molecular recognition process with host 1. According to their plQ values, all of these carboxylic adds existed in the anionic form at pH 7 [4a, b]. The CICS values of the three guests, Arl, Ar2, and 4-methoxyphenylacetic acid, by host 1 are presented in a similar fashion as described above for the substituted aromatic carboxylic acids (Fig. 10.6). 

For aliphatic carboxylic acid guests. All and A12, the CICS values of the two guests with host 1 are shown in Fig. 10.7. 

In order to further determine the relative importance of K-n and hydrophobic effects in the molecular recognition process, a competition study between aromatic guest Ar2, and its closely related aliphatic guest. All, was undertaken. During this study, the concentrations of both competitors were kept constant, while that of 1 increased from 0 to 1 equivalent. Two control experiments, which held constant the concentration of one guest while varying the concentration of 1, were also performed. The results show similar plots for the control and the competitive experiments for Ar2 and/or All, which suggest that the K-Kand hydrophobic effects may he of similar importance in aqueous solution. 

Two other guest molecules, o-methoxybenzoic acid and o-nitrobenzoic acid, were selected to study the steric and electronic influences of the different functional groups at the 2 position of benzoic acid with host 1. Their CICS values, together with those of Ar3 and Arl, were shown in Fig. 10.9 as before. 

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