What are naphthenes



The Cycloalkanes (Cyclanes, older name: Naphthenes, cycloparaffins) are a group of substances consisting of ring-shaped, saturated hydrocarbons. In the systematics of organic chemistry they are counted among the cycloaliphatic compounds, the cycloalkanes are a subgroup of the alkanes, the chain-like, saturated hydrocarbons; due to certain differences, they are usually treated separately from the alkanes. The cycloalkanes form a homologous series with the general empirical formula C.nH2n + 2 (1-x), where x is the number of rings.

Historical information

Naturally occurring cycloalkanes (cyclopentane, cyclohexane, cycloheptane) were first found by the chemist Vladimir Wassiljewitsch Markovnikov in the petroleum fraction, also called naphtha, of the Caucasian petroleum. Hence the name Naphthawhich is occasionally used for all cycloalkanes. Usually this imprecise name is only used for the derivatives of cyclopentane and cyclopentane. In the language of the petroleum industry, naphthene is still a common name for these cycloalkanes.

Classification

One criterion for classifying cycloalkanes is the size of the carbon ring. Cycloalkanes whose cyclic carbon chain contains three to four carbon atoms are called small, from five to seven as normal, from eight to eleven as medium and more than eleven as larger.

In addition to simple rings of the monocycles, there are cycloalkanes, which consist of several rings connected to one another, they are called polycyclic alkanes. In polycyclic alkanes there are, in turn, different ways of connecting the rings: A distinction is made here connected (condensed) of bridged or Spiro compounds of the polycyclic alkanes.

nomenclature

The names of the cycloalkanes are derived from those of the corresponding open-chain alkanes with the same number of carbon atoms with the prefix Cyclo educated.

Polycyclic alkanes are also named after the alkanes with the same number of carbons. The number of rings is indicated by the prefix Bicyclo, Tricyclo etc. specified. The number of carbon atoms between carbon atoms that connect rings (so-called "bridgehead atoms") is placed in front of the name in square brackets in descending order. Spiro compounds are named in the same way, only that instead of the prefix Bicyclo, Tricyclo etc. Spiro is used. If the molecule has substituents, their location is marked with a number at the beginning, followed by a separated by a - the name of the substituent and finally the name of the cycloalkane ring.

structure

In the structural formula drawings of the cycloalkanes, the rings are shortened by polygons. Cycloalkanes can be thought of as a form of alkane in which the two ends of the carbon chain are linked together. They are not isomers of the alkanes. Like alkanes, they are saturated compounds.

Isomerism

Since the rotation of a substituent around a ring carbon atom is impossible, a special form of isomerism occurs, the cis-trans isomerism. In the case of cis-trans isomers, the spatial arrangement of substituents is different. The substituents can be on the same side (cis) or on different sides (trans) of the ring bond.

Conformations

To avoid molecular tensions, the cycloalkanes are not planar, as is often shown. They are all in such conformations that the interior angle of the tetrahedral shape (109.45 °) is retained as far as possible. With cyclopropane, cyclobutane and cyclopentane this angle is not quite reached, so-called Baeyer stress occurs; because of this tension, these molecules are more reactive. In the case of the cyclohexane molecule, however, the voltage no longer has any influence.

properties

The ring-shaped structure of the cycloalkanes affects their reactivity and the melting and boiling points. The melting and boiling point of a monocyclic alkane is always higher than that of the corresponding n-alkane, since the London interaction is better with the cycloalkanes, since the cycloalkanes are relatively rigid.
Cyclopropane, cyclobutane and cyclopentane are more reactive than the higher cycloalkanes because the Baeyer stress occurs with them.

Monocyclic unsubstituted alkanes with 3 to 4 carbon atoms are gaseous under normal conditions, from five carbon atoms they are liquid. In the case of the cycloalkanes, in contrast to the alkanes, the cis-trans isomerism can occur.

Cycloalkanes are poorly or insoluble in water. Cycloalkanes are easily flammable, but relatively inert. They essentially undergo the same reactions as the alkanes.

Occurrence

In addition to the six-membered ring of cyclohexane, which occurs in many steroids and terpenes, there are many other derivatives of small rings and macrocycles in nature. The steroids are based on the cycloalkane Gonan.

Many cycloalkanes such as cyclohexane, methylcyclohexane and cyclopentane occur in petroleum. Small amounts of the crystalline solids of the diamondoids, the simplest representative of which is adamantane, have also been found in petroleum.

Cycloalkanes with a carbon content of 14 to 18 occur, for example, in fragrances such as musk, which is obtained from a gland of the musk deer (Moschidae).

use

Cyclohexane and cyclododecane are starting materials for the synthesis of caprolactam and laurolactam, respectively. Just like the dicarboxylic acids derived from them, these are required for the production of polyamides.

Cyclohexane, decalin, cyclopentane, methylcyclohexane are used as solvents.

see also:Methylcyclopentane

literature

  • Josef Houben, Theodor Weyl: Methods of Organic Chemistry, Ln; Methods of Organic Chemistry, Ln, Bd.5 / 1a, Alkane, Cycloalkane. Thieme, Stuttgart 1994, ISBN 3-13-202204-7

Categories: Cycloalkane | Alicyclic Compound | Substance group