Add time:08/04/2019         Source:sciencedirect.com

Rotation of the XCH2, X2CH, or X3C group about the carbonyl group for haloacetic acids causes erratic ν(CO) and νop(CO)2 frequency behavior that appears to offset the inductive effect. The inductive effect causes both ν(CO) and νop(CO)2 to either increase or decrease in frequency, depending upon whether sigma electrons of the group joined to the carbonyl group withdraw or donate sigma electrons to the carbonyl group, respectively. The X atoms of halogenated acetic acid in certain rotational configurations shield the carbonyl group from the reaction field of the solvent, and this factor is what we believe the one that offsets the inductive effect. Shielding of the carbonyl group by the halogen atoms in haloacetic acids from the reaction field of the solvent decreases in the order of I, Br, Cl and F. This is in the order of the decreasing atomic radius of the halogen atoms. The absorbance ratio A[ν(CO)]A[νop(CO)2] increases as the mole% CHCl3/CCl4 is increased. It also increases in the monohaloacetic acid series, I through F, in the dihaloacetic acid series, Br2 through F2, and in the trihaloacetic acid series, Br3 through F3. Thus, as the mole% CHCl3/CCl4 is increased, the concentration of acetic acid dimer decreases with an increase in the concentration of “monomeric” acetic acid. The ratio of “monomer” to “dimer” concentration with change in the mole% CHCl3/CCl4 also increases as the pKa of the haloacetic acid is increased. This can be explained on the basis that as the acid proton becomes more acidic it forms a stronger intermolecular complex with chloroform. For example, which is in equilibrium with the intermolecularly hydrogen bonded carboxylic acid dimer. Support for this argument is that the acid ν(OH) frequency occurs at lower frequency in CHCl3 solution than in CCl4 solution even though the Cl atoms in CHCl3 are less basic than the Cl atoms in CCl4.

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