Introduction

    Molecules with several chiral carbon atoms are usually displayed using Fischer projection formulas or using wedge formulas. Other useful representations are Newman projections, particularly for looking at eclipsed and staggered conformations. To follow this discussion, it would be very convenient that you have a molecular model that you can manipulate with your hands (balls and sticks); if you have one available (*), prepare a model for erythrose.
    The model for D-erythrose is shown in the conformation required for the Fischer projection. Both C1 and C4 lie on the back, while C2 and C3 are closer to you. OH groups in both C2 and C3 are to the right. Move the model around until you are confident with this arrangement. Position the model such that it matches the Newman perspective in the right-hand drawing, with the C2-C3 bond in the middle.
    To compare a model with a Fischer projection, you must focus on one chiral carbon at a time, and orientate the model so that the -OH and -H groups are horizontal and point towards you.

eclipsed conformation
Fischer projection D-erythrose Newman projection
 

Exercise 2.6

If you have the means available (*), prepare a physical model of D-threose to help you.
Draw a Newman projection for D-threose in the eclipsed conformation, similar to the one displayed above for D-erythrose.
D-threose
* You can improvise a physical ball and stick model using toothpicks and plasticine, or even toothpicks and olives, which make a model that has other enticing uses later!