Chapter 4: Organic Compounds: Cycloalkanes and Their Stereochemistry
Naming cycloalkanes
Cycloalkanes are saturated hydrocarbons whose carbons form a ring. Closing a ring removes two hydrogens relative to an open chain, so the general formula is CₙH₂ₙ. The name attaches the prefix cyclo- to the parent alkane name for the ring size: cyclopropane, cyclobutane, cyclopentane, cyclohexane. Substituents follow the alkane rules. A single substituent requires no locant, as in methylcyclohexane. Two or more substituents are numbered around the ring to give the lowest set of locants, with alphabetical order breaking ties, as in 1-ethyl-2-methylcyclopentane. One decision is specific to…
Two faces of a ring: cis and trans isomers
An open chain can rotate about each of its carbon–carbon bonds, but the bonds of a ring are locked into the cycle, so a ring presents two distinguishable faces. When two carbons of the ring each bear a substituent, the substituents either project from the same face — the cis isomer — or from opposite faces — the trans isomer. These are configurational stereoisomers, not conformations. Converting cis-1,2-dimethylcyclopentane into its trans isomer requires breaking a carbon–carbon or carbon–hydrogen bond; no amount of bond rotation or ring puckering accomplishes it. The two isomers are…
Angle and torsional strain in small rings
Heats of combustion show that not all rings are equally stable, and the differences are explained by two kinds of strain. Angle strain arises when a ring's geometry forces the C–C–C angle away from the tetrahedral value of 109.5°; an equilateral triangle demands 60°, so cyclopropane's bonding orbitals overlap poorly, at an angle. Torsional strain arises when ring geometry eclipses the bonds on adjacent carbons, the same effect met in eclipsed ethane. Cyclopropane suffers both effects maximally — severe angle compression and six fully eclipsed C–H bonds — giving a total strain near 115 kJ/mol.…
The strain-free chair
A planar cyclohexane would combine 120° internal angles with twelve eclipsed C–H bonds; the molecule avoids both penalties by folding. In the chair conformation, four carbons define a plane while the remaining two pucker to opposite sides. In this geometry every bond angle returns to nearly 109.5°, and a Newman projection along any C–C bond — the end-on view introduced with alkane conformations, in which the front carbon's three bonds are compared with the back carbon's — shows perfect staggering, with no two bonds lined up. The chair is therefore strain-free, which is why the cyclohexane…
Axial and equatorial positions interchange in the ring flip
The chair's geometry sorts the twelve C–H bonds of cyclohexane into two families. Six axial bonds stand parallel to the ring axis, alternating up and down on successive carbons. Six equatorial bonds extend outward from the ring's equator, angled only slightly up or down. Each carbon carries exactly one of each. Cyclohexane is not static: at room temperature it undergoes a rapid ring flip in which one chair passes through the half-chair and twist-boat geometries into the alternative chair, roughly 100,000 times per second. The flip's defining consequence is positional exchange — every bond…
Substituents prefer equatorial positions
A substituent on a chair occupies either an axial or an equatorial position, and the two chairs related by the ring flip place it differently. In methylcyclohexane the equatorial-methyl chair predominates (about 95:5 at room temperature) because the axial methyl approaches the axial hydrogens on carbons 3 and 5 across the ring — two carbons away on each side, which is what the label 1,3-diaxial records. This is steric strain: the plain cost of pushing two groups closer than their electron clouds will comfortably allow, as distinct from the angle and torsional strain of the previous section.…
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