Organic Chemistry Laboratory I - Chem 341

Quiz 1e

 

1. Draw the Newman projection of the lowest energy 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)

 

 

 

1-chloropropane,     1pt for chloro or propane

 

 

3. 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. 

 

 

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

 

 

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

 

 

 

 

B is the highest energy conformation because both chlorine atoms are in the axial position for the cis compound.

 

 

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

 

 

Zero because both chlorines are equatorial in A.

 

 

5. Explain the process of ÒseedingÓ 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. Does a pure compound have a higher or lower melting point than an impure compound? (1 pt)

 

 

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.