Organic Chemistry Laboratory I - Chem 341

Quiz 1c

 

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)

 

 

 

2-bromobutane,     1pt for bromo or butane

 

 

3. What is meant by term Òoiling outÓ as it pertains to a recrystallization?  (1 pt)

 

 

Sometimes a solid will melt into an oil that phase-separates from the solvent.  Therefore, it does not dissolve at first.  Usually the oil dissolves after a few minutes, but sometimes you need to add more solvent until it completely dissolves.  The compound will not be purified if you cool the oil into a solid, since impurities will remain mixed with the oil.  Only when the compound crystallizes slowly can we ensure that impurities stay in solution.

 

 

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

 

 

(a) Draw the lowest 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 cis compound.

 

 

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

 

 

one

 

 

5. Why is flameless heating used in a recrystallization when hexane or diethyl ether is used as the solvent?  (1 pt)

 

 

Flameless heating is used because most organic solvents are flammable, especially diethyl ether, which has a very low flash point (-40C).

 

 

6. When is it appropriate to use a Òmixed solventÓ recrystallization?  (2 pts)

 

 

A mixed solvent recrystallization is used when the compound is too soluble in most common solvents and we cannot find a single solvent where it is only soluble at elevated temperature.  In this case we use a solvent where it is very soluble (e.g. methanol) and then add a second solvent that it is not soluble in (e.g. water).  Thus, the mixture of the two solvents makes the compound soluble at elevated temperatures, but not at room temperature.