Question

Make3 a model of cyclopentane. Does this molecule seem to have any ring strain in the sense that the three and four-membred rings do? The molecule is not completely strin-free because there is some eclipsing. Identify the eclipsed atoms or groups when the molecule is planar. In fact the molecule is not planar, but is puckered in either of two ways; (i) One C-atom is abover or below the plane defined by the other four coplanar C-atoms defining an envelope conformation, (ii) Two adjacent C-atoms are arranged so that one is above and the other is below the plane defined by the other three C-atoms giving a pseudo-chair conformation. Try to make these conformations and note that eclipsing is diminished compared to the planar molecule. These conformations interconvert rapidly. (c) make both cis- and trans-1, 2-dimethylcyclopentane. Classify each as being chiral or achiral. How can you be sure of your assignments? Identify any symmetry elements. (a) Prepare a model of cyclohexane and make a boat conformation (see the illustration earlier). This is a high-energy conformation. Why? (Look for eclipsing and note the opposed 1, 4-C-H bonds.) Now twist the model a little bit to give a twist-boat conformation. Note that eclipsing is somewhat relieved. (b) make a chair conformation. Is there any eclipsing? Look at any four-carbon sequence and note that it has the same shape as a gauche conformation of butane. Next put white spheres on all six of the axial positions and leave the equatorial positions empty. Now do a chair-chair interconversion or "flip." To do so, you must twist the ring through a chair, half-chair, twist-boat, half-chair, chair sequence. The half-chair conformations represent energy maxima in the interconversion. What happens in a chair-chair flip? Repeat this intercoversion a few times.

Answer 1

(b) drawn beow is cyclopentane. Due to the structure of the molecule, the ring is not completely strain free. It has an angle strain due to angle becomes 120 oC. Also it has torsional strain due to bonds coming close to each other. The carbon-carbon bonds are eclipsed (2 of these) and two more carbon-carbon bonds are partially eclipsed in form.

Drawn below are the structures.

(c) Cis-1,2-dimethylcyclopenaten as shown below has a plane of symmetry whereas, the trans isomer is chiral.

8. (a) Drawn below is the boat conformation of cyclohexane. The two carbons 1 and 4 are forced towards each other (flag pole hydrogens) and have decreased van-der waals distance and give rise of additional strain.A twist boat has lesser straing than boat but have hgher strain than the chair conformation.

(b) drawn below are chair conformation for cyclohexane. The C-1 bonds are eclipsing with the C-3 and C-5 bonds in the structure as shown below.