James Richard Fromm
Saturated hydrocarbons occur in three forms: straight-chain forms (alkanes), branched chain forms (alkanes), and cyclic forms (cycloalkanes). The cycloalkanes contain only single bonds, and have the general formula CnH2n. Cyclomethane and cycloethane obviously cannot exist, but cyclopropane can; it is a triangular stable structure, though somewhat reactive because the bond angles are somewhat strained to form the triangular structure. The bond angles in cyclopropane are those of an equilateral triangle, 60 degrees, as compared to the tetrahedral bond angle of 109.5 degrees.
Cyclobutane is a square structure; it is less reactive than cyclopropane because the bond angle strain is less, 90 degrees compared to the tetrahedral bond angle of 109.5 degrees. Cyclopentane and larger cycloalkanes are, like the normal alkanes, quite unreactive; there is no significant bond angle strain in these molecules.
|Cyclobutane (C4H8 )|
Because these diagrams are somewhat unwieldy, organic chemists use stylized drawings to represent cyclic compounds. In the cyclic compound symbols, a carbon atom is understood to be at the intersection of each pair of straight lines. Each carbon atom is understood to be bonded to a sufficient number of hydrogen atoms to produce a total number of four bonds (the number required for carbon). The standard symbols used to represent the first four cycloalkanes are:
Cycloalkanes with a single ring are named analogously to their normal alkane counterpart of the same carbon count: cyclopropane, cyclobutane, cyclopentane, cyclohexane, etc. The larger cycloalkanes, with greater than 20 carbon atoms are typically called cycloparaffins.
The naming of polycyclic alkanes such as bicyclic alkanes and spiro alkanes is more complex, with the base name indicating the number of carbons in the ring system, a prefix indicating the number of rings (eg, "bicyclo"), and a numeric prefix before that indicating the number of carbons in each part of each ring, exclusive of vertices. For instance, a bicyclooctane which consists of a six-member ring and a four member ring, which share two adjacent carbon atoms which form a shared edge, is [4.2.0]-bicyclooctane. That part of the six-member ring, exclusive of the shared edge has 4 carbons. That part of the four-member ring, exclusive of the shared edge, has 2 carbons. The edge itself, exclusive of the two vertices that define it, has 0 carbons.
Fused ring systems that share more than two atoms are called bicyclic molecules.
To name bicyclic alkanes, you follow these three steps:
Example: Name the following molecule by the IUPAC system of nomenclature.
1. Count the total number of carbons. In this case there are 7 carbons making the parent name heptane.
2. count the total number of carbons bewtween bridgeheads. In this case there are 2, 2 and 1 carbons between the bridgehead. Placed in descending order between brackets it is [2, 2,1].
3. Place the word bicyclo in front of the name.
The name therefore is Bicyclo[2,2,1]heptane.
We have seen that if two (or more) carbon rings in a molecule are completely separate from each other (i.e. they have no carbons common to the two rings), then the naming is straight forward. However it is possible for the two rings to be sharing one or two carbons, and special ways have been formulated to name these situations.
Bicyclic alkanes are named differently depending whether the two rings are connected at one carbon or at two. When naming rings sharing only one carbon the following rules will apply. Examples
|Example I||Example II||Example III|
Note: The two rings are not in the same plane, in fact they are at right angles to each other at the common carbon. This is often not shown when the structures are put into two dimensions.
Instead of naming these compounds as bicycloalkanes as might be supposed, the whole two-ring structure is given the prefix spiro after the way the two rings connect at the single carbon (in a sort of spiral).
To name these compounds, the root name is taken from the total number of carbons in both rings.
Thus for example I above, there are 9 (nine) carbons in both rings together leading to the name spirononane. However, the size of the two rings may vary, for if you look closely at Examples II and III above, you will count 9 (nine) carbon atoms in both rings for each of these compounds as well. So all are spirononanes, but quite obviously they are constitutional isomers (differing in atom connectivity) of each other.
To complete the naming of these compounds, the size of the two rings is used to differentiate them: a count is made of the number of non- common carbons in each ring, and the two numbers obtained in this count are used in the middle of the name, in square brackets [..].
Example I above has counts 4 and 4 in the two rings and so is spiro[4.4]nonane.
Example II is spiro[3.5]nonane.
Example III is spiro[2.6]nonane.
Hint: the sum of the two numbers plus 1 equals the number of carbons in the two rings giving the root name: 4 + 4 + 1 = 9; 3 + 5 + 1 = 9; 2 + 6 + 1 = 9.
|Note the Rules of Syntax|
Numbering for positioning ring substituents.
If substituents are present on the ring, numbering of the rings starts in the small ring next to the ring junction, passes around this ring and through the ring junction into the larger ring. The numbering goes around the two rings in the direction which produces the lowest numbers in each ring. Examples:
Previous Topic: Introduction to the Alkanes
Next Topic: Introduction to the Alkenes
Return To Course Outline