Saturated hydrocarbons, or alkanes, can form rings as well as straight
and branched chains (right). Open-chain alkanes, straight or branched,
have the general formula CnH2n
+ 2 , in which n is the number of carbon atoms.
In ring alkanes the chain bends back to connect with itself again,
and the overall formula is CnH2n.
The smallest of these, cyclopropane (C3H6),
is strained because the C-C-C angle is 600
which is far less than the optimum of 109.50
for tetrahedral bonding.
The overlap of sp3 orbitals on
adjacent carbon atoms is poor, hence the bonds are weak. Cyclobutane
and cyclopentane (C5H10)
are less strained, and the cyclohexane ring (C6H12)
has no strain at all.
Cyclohexane can adopt either the "chair" or the "boat"
configurations shown above, with the "chair" being favored
because it keeps the ends of the molecule farther apart.
The chair form of cyclohexane is the prototype of the six-membered
ring found in glucose and similar sugar molecules, which we shall
see in Chapter 21.