Exercise 8.5 Group Charges

Compute the group charges for the CH and CH2 groups with each method for allyl cation using each of the methods from the previous exercise. 

On account of the relatively low water regain of cellulose acetate, the molal concentration of ionic groups in the swollen material exceeds Smmolal. This is comparable to the concentration of 300 ppm sodium chloride, a typical reverse osmosis product solution. Our homogeneous membranes are believed to be very similar to the active layer of an asymmetric membrane as developed by Loeb and Sourirajan. It is evident therefore that the concentration of fixed charges in the membrane is sufficient to exercise a significant Donnan exclusion of co-ions on the downstream side of the membranes in a reverse osmosis plant. 

A likely explanation is that the anionic oxy-Cope transition state is polarized and can be modeled by an acrolein (free or as an ate complex) interacting with an allylic anion. The isopropenyl group stabilizes the transition state by delocalizing the negative charge. If this argument is valid, then an attractor substituent put at position 4 or 6 will accelerate the reaction and a donor substituent at the same positions will impede it (see Exercise 12). 

The presence of both amino groups and carboxyl groups in amino acids makes it possible for amino acids to exist in several ionic forms, including the form of a zwitterion. Objective 2 (Section 9.2), Exercise 9.12 The ZwitteriOU iS a dipolar form in which the net charge on the ion is zero. 

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