Does the nomenclature of spiro compounds follow the same pattern as that for bicyclo compounds? That is, in absence of functional groups do we start form the largest chain to the smallest chain. What would be the case if there were functional groups?
No, according to the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), numbering of spiro compounds starts in the smaller ring.
P-24.2.1 Monospiro alicyclic ring systems
Mono spiro parent hydrides consisting of two saturated cycloalkane rings are named by placing the nondetachable prefix ‘spiro’ before the name of the unbranched acyclic hydrocarbon with the same total number of skeletal atoms. The number of skeletal atoms linked to the spiro atom in each ring is indicated by arabic numbers separated by a full stop, cited in ascending order and enclosed in square brackets; this descriptor (called in these recommendations the ‘von Baeyer spiro descriptor’) is placed between the spiro prefix and the name of the acyclic alkane. Numbering starts in the smaller ring, if one is smaller, at a ring atom next to the spiro atom and proceeds first around that ring, then through the spiro atom and around the second ring.
The fixed numbering of the spiro hydrocarbon ring is not modified by the introduction of heteroatoms, double bonds, or functional groups. However, low locants must be attributed if there is a choice.
In Spiro compounds its actually the opposite,we start from the smaller chain to the larger chain.
General Name structure: Secondary Prefix Spiro [X,Y] Root word Primary Suffix Secondary Suffix
X,Y are the number of carbons present on both sides of bridge carbon.
X,Y should be written in ascending order.
Numbering should be started from carbon which is adjacent to fused/bridge carbon in the smaller ring and it should continue into bigger ring through the fused/bridge carbon.
Even if functional groups we will follow the same rules above (even if functional group is there in the bigger ring).