Specific rotation changes

In special cases of optically active photochromes the change in molecular form is accompanied by a change in specific rotation. Singh (170) noted that as (l-naphthylamino)camphor (0.726 g. in 100 cc. chloroform) turned from colorless to green with a 6-min. exposure to sunlight, the specific rotation changed from 126° to 186.6°. Reversal of the system required days. 

PROBLEM 16.13 When crystalline maltose is dissolved in water, the initial specific rotation changes and gradually reaches an equilibrium value. Explain. 

There is a curious phenomenon, however, called mutarotation. When pure a-d-mannopyranose is allowed to strand in aqueous solution, the specific rotation changes and eventually stabilizes at +14.5°. Similarly, when a sample of pure P-d-mannopyranose is allowed to strand in solution, the specific rotation of the final mixture is also measured to be +14.5°. This observation indicates two things. First, the sign of specific rotation is positive, which means that there is more of the a-d-mannopyranose than the P-d-mannopyranose, indicating that there is more of the axial anomer than the equatorial anomer. Remember from Chapter 8 (Section 8.6.1) that a substituent in the equatorial position has less A -strain than when that substituent is in the axial position, so it is expected to be lower in energy. Because a-d-mannopyranose is the major conformation, a different effect must dominate the equilibrium. 

When the pure a anomer is dissolved in water, it begins to equilibrate with the (3 anomer, resulting in a mixture that ultimately achieves the expected equilibrium concentrations. At first, the pure anomer exhibits a specific rotation of + 112°, but as it equilibrates with the (3 anomer, the specific rotation changes until the equilibrium is established, and the specific rotation is measured to be +52.6°. A similar result is achieved when the pure (3 anomer is dissolved in water. At first, the pure anomer has a specific rotation of +18.7°, but as the mixture equilibrates, the specific rotation changes and ultimately is measured to be +52.6°. This phenomenon is called mutarotation, a term commonly used to describe the fact that a and P anomers can equilibrate via the open-chain form. Mutarotation occurs more rapidly in the presence of catalytic acid or base. 

Indole is oxidized in the animal body to indoxyl or /3-hydroxyindole. Indoxyl occurs normally in the urine as an ethereal sulfate but when dogs were fed indole (308) the glucuronoside of indoxyl was isolated from the urine as a barium double salt of indoxyl glucuronoside and indoxyl sulfuric acid (LXIII). The salt was levorotatory and its specific rotation changed in 24 hrs. from —64.9° to —34°. 

D-Glucose can be crystallized in two different forms, a-glucose and p-glucose . The forms have different melting points, and different specific rotations when dissolved in water. These rotations change upon standing to give a solution whose specific rotation is +52.7°. 

Both anomers of o-glucopyranose can be crystallized and purified. Pure a-n-glucopyranose has a melting point of 146 °C and a specific rotation, lo-Jn, of +112.2 pure /3-D-glucopyranose has a melting point of 148 to 155 °C and a specific rotation of +18.7. When a sample of either pure anomer is dissolved in water, however, the optical rotation slowly changes and ultimately reaches a constant value of +52.6. That is, the specific rotation of the a-anomer solution decreases from +112.2 to +52.6, and the specific rotation of the /3-anomer solution increases from +18.7 to +52.6. Called mutarotation, this change in optical rotation is due to the slow conversion of the pure anomers into a 37 63 equilibrium mixture. 

By measuring the optical rotation as it changes with time, after a gelatin solution is rapidly cooled to the temperature of interest, and extrapolating back to zero time, one can determine the initial specific rotation. It is approximately constant with the concentration, but varies with temperature. This initial specific rotation probably represents that of the sol molecule at that temperature before it is converted into the gel form. 

Marvel, Dec, and Cooke [J. Am. Chem. Soc., 62 (3499), 1940] have used optical rotation measurements to study the kinetics of the polymerization of certain optically active vinyl esters. The change in rotation during the polymerization may be used to determine the reaction order and reaction rate constant. The specific rotation angle in dioxane solution is a linear combination of the contributions of the monomer and of the polymerized mer units. The optical rotation due to each mer unit in the polymer chain is independent of the chain length. The following values of the optical rotation were recorded as a function of time for the polymerization of d-s-butyl a-chloroacrylate... 

Neomycins B and C have been shown to differ in their specific rotation values, neomycin B having a specific rotation of + 80° and neomycin C a specific rotation of + 120° 1 7. Brooks et al -2 made this fact the basis for a number of methods to determine the B C content of commercial neomycin. The specific rotation of the test solution is determined at 25°C and total neomycin determined either titrimetrically or spectro-photometrically. By substitution of these values in a suitable equation the concentration of neomycins B and C are calculated. In a second method the same authors determined the specific rotation of the test-solution at temperatures of 25° and 75°C. The change in the value of specific rotation on increasing the temperature from 25°to 75°C can then be used to calculate the amounts of neomycin B and C in the sample. 

When a sample of pseudo-ir-bromoni trocamphor is dissolved in chloroform a spontaneous change in specific rotation, a, occurs and an equilibrium is established in which the pseudo and normal forms exist. The tabulated data refer to the mutarotation at 14 C (Lowry, J Chem Soc, p 211, 1899). Find the sum of the forward and specific rates, k1+k2. 

The specific rotations of the plane of polarized light of solutions of the so-called laevo-rotary malic acid and its sodium salt change with increasing dilation from being dextro-rotary to being laevo-rotary, as may be seen from the data of Woringer1 and of Thomsen2 in table 1. 

The reaction is pseudo-first order and rate is proportional to [Sucrose], The progress of the reaction can be studied by measuring the change in specific rotation of a plane of polarised light by sucrose. Let r0, r, and r are the rotation at initially (when t = 0), at any time t and final rotation, respectively. The initial concentration a is proportional to (r0 - r, ) and concentration at any time t, (a - x) is proportional to (r0 - rt). Thus, the rate constant may be obtained as... 

Fig. 2.—Partial hydrolysis of fructofnranoside sirups with invertase. Plot I, change in specific rotation of the entire methyl fructoside sirup plot II, same for benzyl fructoside sirup plot III, change in calculated specific rotation of fructose from methyl fructoside plot IV, same for fructose liberated from benzyl fructoside sirup.

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