Answer to Question #95290 in Organic Chemistry for Stephen

Question

1. You and your lab partner take melting point of the same sample. You observe a melting point of 101-107°C, while your partner observe a value of 110-112°C. Explain how you can get two different values with exactly the same sample.

2. An unidentified compound is observed to melt sharply at 111°C with the vigorous evolution of a gas. The sample then solidifies and does not melt until the temperature reaches 155°C, at which time it again melts sharply. Briefly explain these observations .

Answer 1

1. If the sample is really the same (taken from one vial), then problem can be in a procedure of measurement (e.g. different equipment calibration), contamination of a measured sample while transferring, or simply different point of observation (one person can detect start of the melting at the very beginning of the process while another a bit later).

If the are two samples of one compound (prepared independently or stored in different vials) then there can be some different impurities in the samples which can slightly affect melting point numbers.

2. Decarboxylation can proceed during heating which explains the evolution of the gas (CO2). Then newly formed compound then melts at higher melting point value.