Separations, Purification and Spectroscopy

Q1A compound dissolved in water is extracted with dichloromethane (density 1.33 g/mL). In the separatory funnel, where is the organic layer?

C. Layer position depends on density. Dichloromethane is denser than water, so the organic layer is on the bottom. The top would be right for ether. The solvents are immiscible, so layers form, and the solute does not change which solvent is denser.

Q2Where should the thermometer bulb be placed during a distillation?

C. The bulb must sit in the vapor that is about to enter the condenser so it reads the boiling temperature of the distilling vapor. In the liquid it reads the pot temperature, and the condenser top or mantle would read neither the vapor temperature nor the boiling point.

Q3Pentane, hexane, and heptane are injected into a GC with a nonpolar column. Which elutes first?

A. In GC, order follows volatility, so the lowest-boiling alkane exits first. Larger molecules with higher boiling points are retained longer, and the three have different boiling points so they separate.

Q4In IEF, a protein with pI 6.0 is placed in a region of the gel at pH 9.0. In which direction does it first move?

D. At pH 9 (above its pI) the protein is net negative, so it migrates toward the anode, where the acidic end lies, until it reaches pH 6. It moves because it is charged, and it would only head toward the cathode if it were net positive.

Q52-Bromopropane gives which 1H NMR pattern?

D. The two equivalent CH3 groups each have one neighboring H, so a 6H doublet results, and the CH has six neighbors, so a septet results. A triplet and quartet describes ethyl, and a 9H singlet is tert-butyl.