Organic Chemistry Laboratory I - Chem 341

Quiz 1b

1. Draw the Newman projection of the highest energy conformation (rotomer) for 1,2-dibromoethane. (2 pts)

The highest energy rotomer has the bromines eclipsed with each other and the lowest energy conformation is in the staggered when the bromine atoms are on opposite sides from each other.

2. What is the IUPAC name for the following compound? (2 pts)

3-Methylpentane, 1pt for methyl or pentane, 1pt for 2-ethylbutane

3. Define “flash point” as it pertains to organic solvents? (1 pt)

The flash point is the temperature at which an organic compound can spontaneously ignite. For many solvents this is very low, for instance the f.p. for diethyl ether is –40C.

4. Consider the following compound and answer the questions below.

(a) Draw the highest energy chair conformation. (1 pt)

A and B are equal in energy since there is always one chlorine atom in the axial and equatorial position for the trans compound.

(b) How many axial chlorine atoms are in the lowest energy conformation? (1 pt)

One chlorine is always in the axial position for this trans compound

5. Explain the process of “seeding” as it pertains to a recrystallization? (1 pt)

The process of seeding is used to induce crystal growth when crystallization does not occur upon cooling to room temperature. To seed a solution, you take a pure crystal and add it to your solution. This provides a surface for additional crystals to grow from. Other techniques used to induce crystallization include cooling, scratching the flask with a glass rod, and evaporating more solvent to make the solution more concentrated.

6. How do we assess the purity of our compounds after recrystallization? (2 pts)

The purity of our compounds in this experiment were assessed by melting point. A pure compound will have a well-defined melting point range, which is higher and narrower than that of an impure compound. The m.p. should be as close as possible to the literature value.