Chemistry Made Simple For You: Spiro Hydrocarbons

Spiro Hydrocarbons

A "spiro union" is one formed by a single atom which is the only common member of two rings. A "free spiro union" is one constituting the only union direct or indirect between two rings senior

. The common atom is designated as the "spiro atom". According to the number of spiro atoms present, the compounds are distinguished as monospiro-, dispiro-, trispirocompounds, etc. The following rules apply to the naming of compounds containing free spiro unions.

Rule A-41. Compounds: Method 1

41.1 - Monospiro compounds consisting of only two alicyclic rings as components are named by placing "spiro" before the name of the normal acyclic hydrocarbon of the same total number of carbon atoms. The number of carbon atoms linked to the spiro atom in each ring is indicated in ascending order in brackets placed between the spiro prefix and the hydrocarbon name.

Examples to Rule A-41.1

41.2 - The carbon atoms in monospiro hydrocarbons are numbered consecutively starting with a ring atom next to the spiro atom, first through the smaller ring (if such be present) and then through the spiro atom and around the second ring.

Example A-41.2

41.3 - When unsaturation is present, the same enumeration pattern is maintained, but in such a direction around the rings that the double and triple bonds receive numbers as low as possible in accordance with Rule A-11.

Example A-41.3

41.4 - If one or both components of the monospiro compound are fused polycyclic systems, "spiro" is placed before the names of the components arranged in alphabetical order and enclosed in brackets. Established numbering of the individual components is retained. The lowest possible number is given to the spiro atom, and the numbers of the second component are marked with primes. The position of the spiro atom is indicated by placing the appropriate numbers between the names of the two components.

Example A-41.4

41.5 - Monospiro compounds containing two similar polycyclic components are named by placing the prefix "spirobi" before the name of the component ring system. Established enumeration of the polycyclic system is maintained and the numbers of one component are distinguished by primes. The position of the spiro atom is indicated in the name of the spiro compound by placing the appropriate locants before the name.

Example A-41.5

41.6 - Polyspiro compounds consisting of a linear assembly of three or more alicyclic systems are named by placing "dispiro-", "trispiro-", "tetraspiro-", etc., before the name of the unbranched-chain acyclic hydrocarbon of the same total number of carbon atoms. The numbers of carbon atoms linked to the spiro atoms in each ring are indicated in brackets in the same order as the numbering proceeds about the ring. Numbering starts with a ring atom next to a terminal spiro atom and proceeds in such a way as to give the spiro atoms as low numbers as possible after numbering all the carbon atoms of the first ring linked to the terminal spiro atom.

Example A-41.6

41.7 - Polycyclic compounds containing more than one spiro atom and at least one fused polycyclic component are named in accordance with Part .4 of this rule by replacing "spiro" with "dispiro", "trispiro", etc., and choosing the end components by alphabetical order.

Example A-41.7

See Recommendations'93 R-2.4.3

Rule A-42. Compounds: Method 2

42.1 - (Alternate to Rules A-41.1 and A-41.2) - When two dissimilar cyclic components are united by a spiro union, the name of the larger component is followed by the affix "spiro" which, in turn, is followed by the name of the smaller component. Between the affix "spiro" and the name of each component system is inserted the number denoting the spiro position in the appropriate ring system, these numbers being as low as permitted by any fixed enumeration of the component. The components retain their respective enumerations but numerals for the component mentioned second are primed. Numerals I may be omitted when a free choice is available for a component.

Examples A-42.1

42.2 - (Alternate to A-41.3) - Rule A-41.3 applies also with appropriate different enumeration, where nomenclature is according to Rule A-42.1, but the spiro junction has priority for lowest numbers over unsaturation.

Example A-42.2

42.3 - (Alternate to A-41.5) - The nomenclature of Rule A-41.5 is applied also to monocyclic components with identical saturation, the spiro union being numbered 1.

Examples A-42.3

but

42.4 - (Alternate to A-41.6 and A-41.7) - Polycyclic compounds containing more than one spiro atom are named in accordance with Rule A-42.1) starting from the senior

end-component irrespective of whether the components are simple or fused rings.