Cyclopentane half-chair

FIGURE 3 12 The (a) planar (b) envelope and (c) half chair conformations of cyclopentane... 

Cyclopentane has two nonplanar conformations that are of similar sta bility the envelope and the half chair... 

Half chair (Section 3 6) One of the two most stable conforma tions of cyclopentane Three consecutive carbons in the half chair conformation are coplanar The fourth and fifth carbon he respectively above and below the plane... 

Cyclopentane is nonplanar, and the two minimum-energy geometries are the envelope and half-chair. In the envelope conformation, one carbon atom is displaced from the plane of the other four. In the half-chair conformation, three carbons are coplanar, vdth one of the remaining two being above the plane and the other below. The energy differences between the conformers are very small, and interconversion is rapid. All of the carbon atoms r idly move through planar and nonplanar positions. The process is called pseudorotation. 

Identify the lowest-energy conformer from among those provided cyclopropane, planar and puckered cyclobutane, planar and puckered cyclopentane and chair, half-chair, boat and twist-boat cyclohexane. (If... 

Two non-planar conformations are believed to exist for cyclopentane which is the envelope form 10.9(Z>), and half chair form 10.9(c). 

In the envelope conformation four of the carbon atoms are coplanar. The fifth carbon is out of the plane of the other four. There are three coplanar carbons in the half-chair conformation, with one carbon atom displaced above that plane and another below it. In both the envelope and the half-chair conformations, in-plane and out-of-plane carbons exchange positions rapidly. Equilibration between conformations of cyclopentane is very fast and occurs at rates similar to that of rotation about the carbon-carbon bond of ethane. 

In cyclopentane, as opposed to cyclobutane and cyclopropane, the bond angles have values close to the optimum. Therefore, the strain in the molecule arises essentially from bond opposition and is partly relieved by puckered conformations. Two flexible forms of cyclopentane exist, namely the so-called envelope (LXXVlIIa) and half-chair (LXXVIIIb) forms. The former has four carbons in the same plane, and... 

The two conformers LXXVlIIa and LXXVIIIb are extremes of symmetry in what is known as the pseudorotational circuit of cyclopentane. If in structure LXXVlIIa, the out-of-plane carbon (arbitrarily designated C-l here) is pushed down together with C-2, a half-chair is obtained (C-l above, C-2 below the plane). If the motion is continued, another envelope is reached (C-2 below the plane). The process then repeats with C-2 and C-3, and so on 10 envelope forms and 10 half-chair forms interconvert by this process, which is not unlike a wave on a water surface ([100], and refs, therein). 

Cyclobutane derivatives are usually, but not always, nonplanar. Cy-clobutane itself exists as two butterflylike conformers that are easily interconvertible by inversion. Cyclopentane is a cyclic five-membered ring structure that can exist as a set of half-chair (twist-boat) forms (C2) and a set of envelope (C5) conformations. These are low-energy conformations and are readily interconverted by twists about bonds without any bond angle changes, only changes in torsion angles. These interconversions are called pseudorotations. 

Half chair conformer One of the puckered conformations of cyclopentane, the other being the envelope. 

The cyclobutane ring exists in a non-planar, open-book conformation, which may flex further open or shut. The cyclopentane ring exists in two puckered conformations, the half chair and the envelope, both of which are non-planar. The position of the puckering may migrate around the ring, and this is called pseudorotation. 

As has been pointed out by Hart <84CHEC-I(7)185>, thiiranes fused to cyclopentanes adopt a boat conformation with C(3) and the sulfur atom on the same side of the C(1)C(2)C(4)C(5) plane. The angle of deviation from this plane is about 30° for C(3) and 60-70° for the S atom. Cyclohexane rings having fused thiiranes prefer a twisted half-chair conformation. Thiiranes fused to larger cycloalkanes can adopt a wide variety of conformations. 

---