Bridged hydrocarbons

In organic chemistry, the term bridged hydrocarbons is used to summarize saturated and unsaturated hydrocarbons with several linked ring structures. They can be derived from cycloalkanes in which two non-adjacent carbon atoms are connected directly or with one or more methylene groups. This gives bicyclic hydrocarbons ( bicycloalkanes ).

A bicyclo [m, n, r] alkane is constructed from a cycloalkane with m + n + 2 carbon atoms by inserting a chain of r carbon atoms

The connecting carbon atoms are called bridges , the two connection points ( tertiary carbon atoms) are known as bridgeheads or bridgehead atoms .

For the characterization of the bridges can be of the alkanes derived designations methano (-CH ₂ -), ethano (-CH ₂ CH ₂ -), propano (-CH ₂ CH ₂ CH ₂ -), etc. to use. If the bridgehead atoms are directly bound to each other, this connection is called a “zero bridge” and there is a connection with condensed rings.

nomenclature

The naming of the bridged hydrocarbons is based on the Von Baeyer nomenclature , which was originally developed for bicyclic compounds and later expanded to include polycyclic compounds. This nomenclature is part of the IUPAC rules. In particular for the bicyclic hydrocarbons, common names are often used, which in many cases are derived from natural substances.

Saturated bicyclic hydrocarbons

According to the Von Baeyer nomenclature , the saturated bicyclic hydrocarbons are referred to as bicycloalkanes (bicyclobutane, bicyclopentane, bicyclohexane, etc.). The number of carbon atoms in the three bridges is given in square brackets in descending order after the word part “Bicyclo”; the numbers are separated by periods.

Examples:

Bicyclo [2.2.2] octane, bicyclo [3.3.1] nonane and bicyclo [3.3.3] undecane

Condensed bicyclic compounds: bicyclo [4.4.0] decane, bicyclo [4.3.0] nonane, bicyclo [3.2.0] heptane and bicyclo [2.1.0] pentane

The IUPAC rule A-31.2 applies to the numbering of the ring positions (locants):

“ The numbering of a bridged hydrocarbon begins at a bridgehead; you number on the longest possible path to the other bridgehead, from this atom on the longer of the two remaining paths back to the first bridgehead and then on the shortest path to the end. "

Example: bicyclo [3.2.1] octane

This molecule has the spatial structure shown on the left (stereoskeletal formula ). The planar formula with numbering is shown on the right. It arises when the molecule is projected “from above” onto the plane of the drawing.

Unsaturated bicyclic hydrocarbons

Bicyclic hydrocarbons often contain one or more C =C double bonds. As with the cycloalkenes, they are characterized by the ending "-en" . As usual, only the lowest position of the carbon atoms involved in double bonds is given. “ If there are multiple options for numbering, the multiple bonds are given the lowest numbers. "

Example: unsaturated bicyclo [3.2.1] octanes:

Bicyclo [3.2.1] oct-6-ene and bicyclo [3.2.1] octa-2,6-diene. The crossed-out formula is intended to explain incorrect figures

Polycyclic hydrocarbons

If further bridges are introduced into bicyclic hydrocarbons, polycyclic compounds are obtained. With a bridge, tricyclic systems are created; double bridging leads to tetracyclic hydrocarbons and so on.

As examples of tricyclic hydrocarbons, adamantane and its synthesis precursors dicyclopentadiene and tetrahydrodicyclopentadiene should be considered:

Synthesis of adamantane (right) by hydrogenation of dicyclopentadiene (left) and isomerization of the tetrahydro compound (middle).

To determine the systematic name of adamantane, it is first determined how many rings the molecule contains.

IUPAC rule A-32.12:

" The number of rings in a polycyclic system is the number of cuts that are necessary to convert the system into an open-chain connection "

With adamantane, three cuts are necessary to achieve an open-chain connection. Since the molecule consists of ten carbon atoms, it is therefore a tricyclodecane .

The next step is to determine the so-called main ring , the branches of the main ring, and the "secondary bridge".

After the first cut you can see that the main ring contains eight carbon atoms; its longest branches consist of three carbon atoms each.

The main bridge is C ₁ , so it is a bicyclo [3.3.1 ] system.

The secondary bridge is also C ₁ , so there is a tricyclo [3.3.1.1] system.

The side bridge connects C-3 with C-7.

Thus the systematic name is: Tricyclo [3.3.1.1 ³^{, 7} ] decane .

Tetrahydrodicyclopentadiene (see above) contains a "zero bridge". Of the two possible stereoisomers, the endo stereoisomer is shown in the figure (left) . However, as defined, the Von Baeyer nomenclature does not distinguish between stereoisomers.

Three cuts (red) through the bonds are necessary to create an open-chain alkane (right). If one cuts the zero bridge, one obtains the bicyclo [5.2.1] system. Linking the carbon atoms C-2 and C-6 gives the systematic name: Tricyclo [5.2.1.0 ²^{, 6} ] decane .

Dicyclopentadiene also contains two double bonds that begin at C-3 and C-8: tricyclo [5.2.1.0 ²^{, 6} ] deca-3,8-diene .

Individual evidence

↑ ^a ^b ^c ^d International Union for Pure and Applied Chemistry (IUPAC): Rules for the nomenclature of organic chemistry , German edition (Ed. H. Grünewald), Vol. 1, Section A - Hydrocarbons, Verlag Chemie, Weinheim, 1975.

literature

International Union for Pure and Applied Chemistry (IUPAC): Rules for the nomenclature of organic chemistry. Volume 1, Section A - Hydrocarbons, Verlag Chemie, Weinheim, 1975.
Dieter Hellwinkel: The systematic nomenclature of organic chemistry: instructions for use, 5th edition, Springer, Berlin, Heidelberg u. a. O., 2006, ISBN 3-540-26411-6 .