Naming of Bicyclic Alkanes

IUPAC nomenclature of bicyclic alkanes in organic chemistry. They saturated cyclic hydrocarbon compounds. 

Key words: Bicyclic alkanes, Fused bicyclic, Bridged bicyclic, Spirocyclic, Nomenclature. 

As the name of the broad category indicates “bicyclic” means there are two rings which are joined together. In this category of alkanes three types of structures are found.                                                     

      Fused bicyclic

2.    Bridged bicyclic

3.    Spirocyclic

Let’s start with fused bicyclic system. In this type molecules to rings are adjacent and they share one carbon-carbon single bond. For example,

See these molecules A, B and C they consist of two rings. The rings share two adjacent carbons and a single bond between them.

How to assign names for these molecules?

The answer is simple; these types of fused bicyclic molecules are named based on the names of the alkanes having same number of carbons. Since these are bicyclic molecules so the names must include the prefix “bicyclo”. Also the names consist of bracket which shows the number of carbons in each ring.

Let’s consider molecule A, it consist of six carbons so the molecule is named as bicyclo-hexane. The molecule has TWO bridges of two carbons and ONE bridge of zero carbon. Therefore the molecule is named as “bicyclo[2.2.0]hexane”. Here larger number is written first in the bracket.

Similarly molecule B is has eight carbons so it is named as “bicyclo-octane”. Also it has TWO carbon bridges which consist of three carbons and ONE zero-carbon bridge. Therefore correct name of the molecule is “bicyclo[3.3.0]octane”

The molecule C has 10 carbons so it is “bicyclo-decane”. It has TWO carbon bridges which consist of four carbons and ONE zero-carbon bridge. Therefore correct name of the molecule is “bicyclo[4.4.0]octane”

Now let’s see bridged bicyclic molecules, they have fused rings but they share two carbons which are non-adjacent. They do not consist of zero carbon bridge. For example see the molecule D. It has six carbons so it is named as “bicyclo-hexane”. It consists of ONE carbon-bridge of 2 carbons and TWO carbon-bridges with one carbon. Therefore correct name of the molecule D is “bicyclo[2.1.1]hexane”

Similarly the molecule E has seven carbons along with TWO carbon bridges of two carbons and ONE carbon-bridge of one carbon. Therefore the it is named as “bicyclo[2.2.1]heptane”.

The molecule F has eight carbons along with THREE carbon bridges of two carbons. Therefore the it is named as “bicyclo[2.2.2]octane”

Now let’s consider spirocyclic system they have two adjacent rings and share one carbon atom. They are named as alkanes which is having same carbons. Also the names must include the prefix “spiro” .

For example see the molecule G which has two five member ring which share only one carbon atom.  

The molecule G has nine carbons and TWO carbon bridges with four carbons. Therefore correct name of the molecule G is “spiro[4.4]nonane”.

Let’s see the molecule H; it has 11 carbons and TWO carbon bridges with five carbons. Therefore correct name of the molecule H is “spiro[5.5]undecane”.

See the molecule I; it has 10 carbons and ONE carbon-bridge with five carbons and other ONE carbon bridge with four carbons. Therefore correct name of the molecule I is “spiro[4.5]decane”. Here in case of spiro compounds; in bracket lower number is written first followed by higher number.

To summarize this topic

•  Bicyclic alkanes are consists of three types Fused bicyclic, Bridged bicyclic and Spirocyclic.
• They are named based on the names of alkanes which are having same carbons.
• In case of fused bicyclic and bridged bicyclic system the prefix “bicyclo” is used before the name of alkane.
• In case of Spirocyclic system; the prefix “spiro” is used before the name of alkane.
• The name of the bicyclic system also consists of a bracket which includes number of carbons in different carbon-bridges 

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