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Cycloalkane or Alicyclic Compounds

The cyclic counterpart of aliphatic compounds is called as ‘Alicyclic Compounds’ (ali = aliphatic and cyclic = ring structure). They resemble aliphatic compounds in most of their properties but due to their cyclic structures, they also differ from aliphatic compounds in some respects. Aliphatic compounds corresponding to alkanes (paraffins) are known as ‘cycloalkane’, whereas those corresponding to alkenes and alkynes are called ‘cycloalkenes’ and ‘cycloalkyne’ respectively. In this chapter, we shall discuss nomenclature, general methods of preparation, chemical properties, and stereochemistry of some small ring cycloalkanes.

Nomenclature:

The general formula of cycloalkane is CnH2n where n = 3, 4, 5, 6, etc. Alkenes also possess the same formula. Therefore, cycloalkanes are isomeric with alkenes. Initially, these compounds were known as *polymethylenes’ since they contain a number of methylene (CH,) units joined together to form a ring. In the I.U.P.A.C.system, saturated alicyclic hydrocarbons are called ‘cycloalkanes’. The name of an individual cycloalkane is obtained by prefixing the word cyclo to the name of the corresponding alkane.

General Methods of Preparation :

The methods commonly used to prepare alicyclic compounds are outlined below.

1. Wislicenus Method (1893) or From salts of dicarboxylic acids :

When the calcium, barium, or thorium salts of certain dicarboxylic acids are distilled, cyclic ketones are formed. These cyclic ketones can be converted into alicyclic compounds by Clemmensen reduction as shown below.

This method is useful for the preparation of only five and six-membered rings.

Limitation: Cyclo propane and its derivatives can not be prepared by this method.

  1. Freund’s Method (1882):

This method is actually an extension of the Wurtz reaction which may be regarded as an internal Wurtz reaction. It is an intermolecular cyclization where two halogens present almost at two terminal positions of the chain with the presence of Na-Zn in aq. alcohol.

In general; This method has been used for the preparation of three, four, five, and six-membered rings but gives good yields only for cyclopropane i.e. when n = 1 and its derivatives. For example,

  1. Dieckmann’s Cyclisation Reaction (1901):

Esters of 0,6-dicarboxylic acids undergo intramolecular Claisen condensation called ‘Dieckmann’s cyclization’ in the presence of sodium ethoxide to form cyclic B -ketoesters. Hydrolysis of these B – ketoesters followed by decarboxylation yield cycloalkanone which upon Clemmensen reduction form cycloalkanes. For example, condensation of the esters of adipic, pimelic or to form five. six and seven-membered cyclic ketoesters: Which on hydrolysis followed by decarboxylation and Clemmensen reduction vibe 5, 6, and 7-membered cycloalkanes respectively.

Mechanism: It involves the following steps:

 

  1. Hydrogenation of Aromatic Hydrocarbons :

Cyclohexane and its derivatives are easily prepared by the catalytic hydrogenation of aromatic compounds at high temperatures and pressure. For example :

  1. From Active Methylene Compounds :

When active methylene compounds like malonic ester and acetoacetic ester condense with certain a, w – dihalides to form cyclic compounds.

 

Note :

i). Direct decarboxylation breaks the cyclopropane ring; therefore it is not employed.

This method is used to prepare 3 to 6-membered ring compounds.

  1. By Cycloaddition Reactions :

These are reactions in which two unsaturated compounds (molecules ) add each other to form a cyclic product. During this reaction, n-electrons form two new 6-bonds. Cycloaddition reactions generally do not involve any sort of intermediates like ions or free radicals. Reactants are directly transformed into products via cyclic transition states. Therefore, such concerted reactions come under the category of ‘pericyclic reactions’ (pericyclic means at the periphery of the cycle.)

Another significant theory about these reactions is that they do not require any catalyst and are initiated either by heat or light. Cycloaddition reactions are of different types depending upon the number of t-electrons involved from each compound as discussed below.

  • [2 + 2] Cycloaddition Reactions: These cycloaddition reactions involve two -electrons each from two compounds e.g. alkenes.
  • [4 + 2 ] Cycloaddition Reactions (Diels-Alder Reactions) :

The cycloaddition reaction between an alkene or an alkyne and a conjugated diene, resulting in the formation of a six-membered ring is called Diels – Alder reaction’. In these reactions, the 21-electron system is called ‘dienophile’ and conjugated. diene or 411 -electrons system is called ‘diene’. For example, when a mixture of butadiene and ethylene is. : heated at 475K cyclohexene is formed.

  1. From Alkenes: When an alkene is treated with diazomethane, it forms cycloalkane. For example :

Physical Properties :

Cycloalkanes resemble open-chain analogs in their physical properties, although their boiling points, melting points, and densities are somewhat higher. Cycloalkanes because of their rigid ring structure overcome the oscillation of aliphatic compounds and hence comparatively are close packed. Cyclopropane(- 33°C) and cyclobutane (12°C) are gases. The higher members of the series are liquids. Like alkanes, cycloalkanes are also non-polar and hence are soluble in non-polar solvents like CCl4, ether, benzene, etc. but are insoluble in polar solvents like water, alcohol, etc.

Chemical Properties:

Cycloalkanes being saturated, resemble alkanes in their chemical properties. All carbon in cycloalkanes is sp3 hybridized forming only o-bonds. Higher cycloalkanes are stable and unreactive, they are not attacked by chemical reagents like strong acids, strong agents, and reducing agents, but undergo free radical substitution reactions. However, lower cycloalkanes like cyclopropane and cyclobutane show a higher degree of reactivity. Cyclopropane is more reactive than cyclobutane. This is due to the fact that these are highly strained molecules and have a tendency to undergo ring cleavage forming additional products. But they are much less reactive than alkenes. Some of the important chemical reactions of cycloalkanes are discussed below.

  1. Free radical Substitution Reactions :

Cycloalkanes undergo typical free radical substitution when treated with halogens at high temperatures in the dark or at room temperature in the presence of light. For example.

  1. Addition Reactions :

Small ring compounds like cyclopropane and cyclobutane undergo unexpected addition reactions when treated with halogen, halogen acids, or hydrogen in presence of a catalyst. In this reaction, small rings get opened up, and open chain saturated products are obtained, whereas the other members are not affected at all.

The action of heat: Lower cycloalkanes may be isomerized into the corresponding alkenes either thermally or catalytically. e.g.

The above properties show that the lower cycloalkanes viz. cyclopropane and cyclobutane are less stable and the rings are easily opened by the action of suitable reagents. Moreover, these reactions also indicate the stability of the ring increases as the ring becomes larger, i.e. the stability of the various cycloalkanes 50 6-membered can be represented as follows:

Cyclopropane < Cyclobutane < Cyclopentane < Cyclohexane

Causes of the stability of Cycloalkanes: The relative stability of these cycloalkanes was explained by Baeyer(1895) and Sachse Mohr(1918) in the form of the theories known as ‘Baeyer strain theory’ and ‘Sachse strainless’ theory respectively.

  1. Baeyer strain theory: This theory was based upon the postulation that when an open chain organic compound, having the normal carbon tetrahedral bond angle of 109°28′ is converted into a Dychic compound, a definite distortion(deviation) of this normal bond angle takes place leading to the development of a strain in the molecule. According to this theory, the greater the deviation from the normal bond angle (109°28′ ), the greater will be the strain in the molecule and hence greater instability of the 3 and 4- membered rings strain is greater so 3 and 4- membered ring compounds are unstable. By assuming that the rings are planar the amount of strain in the various cycloalkanes can be expressed in terms of the valence angle distortion(d) which can be calculated from the following expression :

Where a is the internal angle in cycloalkane. On the assumption that 3 to 8 membered rings are planar, the internal strain in various cycloalkanes is given below:

The positive and negative values of valence angle distortion(d) indicate whether the bond angle(α) is lower or greater than the normal tetrahedral bond angle(109°28′) i.e. whether the strain is inward or outward. It is concluded from the above data that the value of valence deviation is minimum in the case of cyclo cyclopentane and not too large with cyclohexane, hence these rings have minimum strain and maximum stability.

Advantages of Baeyer strain theory :

Baeyer strain theory explains beautifully the relative stability of the various cyclic compounds up to 5. membered rings but it failed beyond this.

Sachse strainless theory :

Mohr(1918) proposed that the rings larger than C5 exist without valence deviation i.e. without any strain(strainless) which is possible only when the rings are puckered but not planar. Thus, cyclohexane molecules may exist in two strainless or puckered conformations having the normal tetrahedral angle around 109°28’about each carbon atom. These two puckered conformations are the chair or Zform (l) and the boat, flexible or C-form (II). These two conformational isomers are rapidly interconvertible at ordinary temperature and have not yet been isolated individually

In these two conformational isomers, the chair form is considerably more stable than the boat form. The relative instability of the boat form to the chair form is due to the fact that in the chair conformation of cyclohexane the hydrogens are staggered, i.e. they are present far away from each other, and hence no steric repulsion occurs with the result of the conformation has lower energy and maximum stability, whereas in the boat from the hydrogens on four of the carbon atoms, namely 1, 2, 4 and 5 are eclipsed which bring them substantially close together and hence a definite steric repulsion dive ops between the hydrogens. Due to this steric repulsion between the hydrogen atoms in the boat form, it is less stable than the chair form having no such repulsion between the hydrogen atoms.

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