Organic Chemistry Laboratory I - Chem 341

Quiz 1d

1. Draw the Newman projection of the eclipsed conformation (rotomer) for n-butane looking down the C2-C3 bonds. (2 pts)

The highest energy rotomer has the methyl groups eclipsed with each other and the lowest energy conformation is in the staggered when the methyls are on opposite sides from each other.

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

2-methylpentane, 1pt for methyl or pentane

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 lowest energy chair conformation. (1 pt)

A is the lowest energy conformation because both chlorine atoms are in the equatorial position for the trans compound.

(b) How many equatorial chlorine atoms are in the highest energy conformation? (1 pt)

zero since both chlorines are axial in B

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 was 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.