4. Absolute Configuration and R/S Sequence Rules. In 3 you constructed models with two stereocenters that represented 4 stereoisomers. 2,3-Dibromobutanoic acid exemplifies such a compound, Make models for the isomers of this compound. a. Draw solid/dashed-wedge structures for the 4 possible stereoisomers and label the enantiomers and diastereomers. b. Specify the sequence priority for the substituents on carbon-2 and carbon-3. Label each structure with the proper R/S notation in the drawings above, for example (2R, 35).

d. Draw Newman projections for all 4 possible stereoisomers. TY e. Specify the sequence of priority for the substituents on C2 and C3 for tartaric acid (see 3p). Write your answer for 3p. 5. Conformational Enantiomers and Cyclic Compounds a. Construct a model of methanol (CH3OH). Is methanol chiral? Why or why not? Orient your model so that the plane of symmetry (indicated by the dotted rectangle) contains the H-C-O-H bonds that are in the plane of the paper, as shown below:

b. Orient your model so that the plane of symmetry (indicated by the dotted rectangle) contains the H-C-O-H bonds that are in the plane of the paper, as shown below: Now rotate the C-O bond, so that the plane no longer contains the O-H bond. All of the bonds in methanol freely rotate at room temperature, so you are just showing a different conformation of methanol. Does this conformation of methanol have a plane of symmetry? Yes No Even though one conformation of methanol has a plane of symmetry, and one doesn’t, methanol is still considered to be achiral. It is only necessary to find one conformation with a plane of symmetry for a molecule to be achiral. Now consider cyclohexanol, shown below. Looking at the flat, hexagonal representation of cyclohexanol, does cyclohexanol contain a plane of symmetry? Yes No OH