Question

1. A student performed two m.p. determinations on a crystalline product whose reported m.p. is 143⁰C. Data for the two determinations are as follows:

Sample #     m.p. range, ⁰C        sample height in capillary tube

      1              141-142                                2 mm

      2              141-145                                5 mm

Explain the broader range observed in Sample 2.

2. A student performed a m.p. determination on the same product in Question 1. The reported m.p. range was 136-138 ⁰C, and the rate of heating using the Melt-Temp apparatus was 6 ⁰C/min. Explain the lower m.p.

3. The R_f value of compound A is 0.34 when a TLC plate was developed in pentane and 0.44 when the plate was developed in diethyl ether. Compond B has an R_f value of o.42 in pentane and 0.60 in diethyl ether. Which solvent would be better for separating a mixture of A and B by TLC? Explain.   

Answer 1

1. In sample 2, sample height in capillary tube is 5 mm which is broad. The temperature of sample is recorded at particular point. Hence, when sample is heated, it may happen that at the point where temperature is recorded 141 deg C, the temperature at some other point is still 135 deg C or so where melting will not occur. Hence, to melt that part of sample, more heat is required which increases the temperature to 145 deg C at a point where temperature is measured. At other point where temperature was 135 deg C, it increases to 141 deg C which melts the sample. Due to this, the temperature range for sample 2 is broad (141-145 deg C).