Chemistry · Ch 15 — Hydrocarbons
Conformations in alkanes
Conformations in alkanes
Alkanes contain only single covalent (sigma) bonds, and a sigma bond -- formed by coaxial (head-on, cylindrically symmetric) overlap of orbitals along the bond axis -- keeps the same extent of orbital overlap no matter how the two bonded atoms are rotated relative to each other. Because of this cylindrical symmetry, a carbon-carbon single bond can rotate freely, and as it rotates, the atoms attached to one carbon continuously change their position in space relative to the atoms attached to the other carbon. Each distinct spatial arrangement reached this way is called a conformation, and because rotation through 360 degrees passes through infinitely many such arrangements, an alkane technically has innumerable conformations. This phenomenon -- interconverting spatial arrangements around a single bond, without ever breaking a bond -- is a form of stereoisomerism, distinct from the structural (constitutional) isomerism of section 15.1.1 and from the configurational isomerism of Chapter 14, precisely because conformations freely interconvert at room temperature rather than being fixed, separable compounds. Ethane, the simplest case, has two extreme conformations out of its infinite possibilities: the eclipsed conformation, in which the three C-H bonds on the front carbon line up directly behind (eclipse) the three C-H bonds on the back carbon, and the staggered conformation, in which the back set is rotated 60 degrees away so it falls exactly between the front set. These can be drawn either as a Sawhorse formula (a perspective, ball-and-stick-like view of both carbons and all six hydrogens) or as a Newman projection (the molecule viewed end-on along the C-C axis, with the front ca …
What this figure shows. Shows the two extreme conformations reached on rotating ethane's C-C single bond through 360 degrees, drawn two ways. In the Sawhorse formula, the front carbon's three C-H bonds and the back carbon's three C-H bonds are drawn as two tripods joined by the central C-C bond; in the eclipsed form the front and back C-H bonds point in exactly the same directions when viewed along the C-C axis, while in the staggered form the back set is rotated 60 degrees so each back C-H bond bisects the angle between two front C-H bonds. In the Newman projection (the molecule viewed end-on along the C-C bond, front carbon as a dot with three bonds radiating out, back carbon as a circle with three bonds emerging from behind it), the eclipsed conformation shows the front and back C-H lines directly overlapping, and the staggered conformation shows the back C-H lines …