Alicyclic Hydrocarbons

 Chapter -9 

Alicyclic Hydrocarbons 

Open-cofilin and cyclic compounds

• In the compounds that we have studied in previous chapters, the carbon atoms are attached to one another to form chains; these are called open-chain compounds, In many compounds, however, the carbon atoms are arranged to form rings; these are called cyclic compounds.

• In this chapter we shall take up the alicyclic hydrocarbons (aliphatic cyclic hydrocarbons). Much of the chemistry of cycloalkanes and cycloalkenes we already know, since it is essentially the chemistry of open-chain alkanes and alkenes. But the cyclic nature of some of these compounds confers very special properties on them. It is because of these special properties that, during the past fifteen years, alicyclic chemistry has become what Professor Lloyd Ferguson, of the California State College at Los Angeles, has called "the playground for organic chemists." It is on some of these special properties that we shall focus our attention.

 Nomenclature

• Cyclic aliphatic hydrocarbons are named by prefixing cycle- to the name of the corresponding open-chain hydrocarbon having the same number of carbon atoms as the ring. For example:

• Polycyclic compounds contain two or more rings that share two or more carbon atoms. We can illustrate the naming system with norbornane, whose systematic name is bicycle[2.2.1] heptane: (a) heptane, since it contains a total of seven carbon atoms; (b) bodycon, since it contains two rings, that is, breaking two carbon.

• carbon bonds converts it into an open-chain compound; (c) [2.2.1], since the number of carbons between bridgeheads (shared carbons) is two (C-2 and C-3), two (C-5 and C-6), and one .

• Polycyclic compounds in a variety of strange and wonderful shapes have been made, and their properties have revealed unexpected facets of organic chemistry. Underlying much of this research there has always been the challenge; can such a compound be Madel.

•  The ultimate polycycle; aliphatic system is diamond which is, of course, not a hydrocarbon at all, but one of the allotropic forms of elemental carbon. In diamond each.

Industrial source

• We have already mentioned (Sec. 3.13) that petroleum from certain areas, (in particular California) is rich in cycloalkanes, known to the petroleum industry as naphthene. Among these are cyclohexane, methylcyclohexane, methyl cyclopentane, and 1,2-dimethylcyclopentane. These cycloalkanes are converted by catalytic reforming into aromatic hydrocarbons, and thus provide one of the major sources of these important compounds (Sec. 12.4). For example: 

Preparation

• Preparation of alicyclic hydrocarbons from other aliphatic compounds generally involves two stages: (a) conversion of some open-chain compound or compounds into a compound that contains a ring, a process called cyclization; (b) conversion of the cyclic compound thus obtained into the kind of compound that we want: for example, conversion of a cyclic alcohol into a cyclic alkene* or of a cyclic alkene into a cyclic alkane^ Very often, cyclic compounds are made by the adapting of a standard method of preparation to the job of closing a ring. For example, we have seen (Sec. 3.17) that the alkyl groups of two alkyl halides can be coupled together through conversion of one halide into an organometallic compound (a lithium dialkylcopper) : CH3CH2-C1 - CH3CH2-M - , CH3CH2 CH3CH2-C1 - ' CH3CH2 Ethyl chloride w-Butane. 

Reactions

With certain very important and interesting exceptions, alicyclic hydrocarbons undergo the same reactions as their open-chain analogs.

Reactions of small-ring compounds. Cyclopropane and cyclobutene

• Besides the free-radical substitution reactions that are characteristic of cycloalkanes and of alkanes in general, cyclopropane and cyclobutene und addition reactions. These addition reactions destroy the cycldprofttrie and ctyplp* butane Hahg systems and yield open-chain products. For example:

• Cyclobutene does not undergo most of the ring-opening reactions of cyclopropane; it is hydrogenated, but only under mote vigorous conditions than those required for cyclopropane, thus cyclobutene undergoes addition less readily than cyclopropane and, with some exceptions, cyclopropane less readily than an alkene. The remarkable thing is tata these cycloalkanes undergo addition at all.

Baeyer strain theory

• In 1885 Adolf von Baeyer (of the University of Munich) proposed a theory to account for certain aspects of the chemistry of cyclic compounds. The part of his theory dealing with the ring-opening tendencies of cyclopropane and cyclobutene is generally accepted today, although it is dressed in more modern language. Other parts of his theory have been shown to be based on false assumptions and have been discarded. Baeyer's argument was essentially the following. In general, when carbon is bonded to four other atoms, the angle between any pair of bonds is the tetrahedral angle 109.5. But the ring of cyclopropane is a triangle with three angles of 60, and the ring of cyclobutene is a square with four angles of 90. In cyclopropane or cyclobutene, therefore, one pair of bonds to each carbon cannot assume the tetrahedral angle but must be compressed to 60 or 90 to fit the geometry of the ring.

• Thus, Baeyer considered that tings smaller or larger than cyclopentane or cyclohexane were unstable; it was because of this instability that the three- and Fo unmembered rings underwent ring-opening reactions; it was because of this instability that great difficulty had been encountered in the synthesis of the larger How does Baeyer's strain theory agree with the facets.

Heals of combustion and relative stabilities of the killalaites

• We recall that the heat of combustion is the quantity of heat evolved when one mole of a compound is burned to carbon dioxide and water. Lik$ heats of hydrogenation (Sees. 6.4 and 8.16), heats of combustion can often furnish valuable information about the relative stabilities of organic compounds. Let us see if the heats of combustion of the various cycloalkanes support Baeyer's proposal that rings smaller or larger than cyclopentane and cyclohexane are unstable. Examination of the data for a great many compounds has shown that the heat of combustion of an aliphatic hydrocarbon agrees rather closely with that calculated by assuming a certain characteristic contribution from each structural unit. For open-chain alkanes each methylene group, CH2 , contributes very close to 157.4 kcal/mole to the heat of combustion. Table 9.2 lists the heats of combustion Tjia have been measured for some of the cycloalkane.

• the methods that are used successfully to make large rings take this fact into consideration. Reactions are carried out in highly dilute solutions where collisions between two different chains are unlikely; under these conditions the ring-closing reaction, although slow, is the principal one. Five- and six-membered rings are the kind most commonly encountered in organic chemistry because they are large enough to be free of angle strain, And small enough that ring closure is likely.

Orbital picture of angle strain

• What is the meaning of Baeyer's angle strain in terms of the modern picture of Tha codling bond? We have seen (Sec. 1.8) that, for a bond to form* two atoms must be located so that an orbital of one overlap an orbital of the other. For a given pair of atoms, the greater the overlap of atomic orbitals, the stronger the bond. When carbon is bonded to four other atoms, its bonding orbitals (spa* orbitals) are directed to the corners of a tetrahedron; the angle between any pair of orbitals is thus 109.5. Formation of a bond with another carbon atom involves overlap of one of these sp* orbitals with a similar spa* orbital of the other car.

• Both calculations and experimental measurements show that the final result is a compromise, and that few molecules have the idealized conformations that we assign them and, for convenience, usually work with. For example, probably no tetravalent carbon compound except one with four identical substituents has exactly tetrahedral bond angles: a molecule accepts a certain amount of angle strain to relieve van der Waals strain or dipole-dipole interaction. In the gauche conformer of w-butane (Sec. 3.5), the dihedral angle between the methyl groups is not 60, but almost certainly larger: the molecule accepts some torsional strain to ease van der Waals strain between the methyl group.

Conformations of cycloalkanes

• Let us look more closely at the matter of puckered rings, starting with cyclohexane, the most important of the cycloalkanes. Let us make a model of the mocap. clue, and examine the conformations that are free of angle strain. 

• Between the chair form and the Twi*t*boat forth life's the highest barrier of all; a transition state conformation (the half-chair) which, with angle strain and tor* siopao strain, lies about 1 1 kcal above the chair form. The overall relationships are summarized In Fig. 9.5. Equilibrium exists between the chair and twist-boat forms, with the more stable chair form being favored 10,000 to 1 at room temperature.

Equatorial and axial bonds in cyclohexane

Let us return to the model of the chair conformation of cyclohexane (see Fig. 9.8). Although the cyclohexane ring is not flat, we can consider that the carbon atoms lie roughly in a plane. If we look at the molecule in this way, we see that the hydrogen atoms occupy two kinds of position: six hydrogens lie in the plan

As a simple example of the importance of 1,3-diaxial interactions, let us consider methylcyclohexane. In estimating relative stabilities of various conformations of this compound, we must focus our attention on methyl, since it is the largest substituent on the ring and hence the one most subject to crowding. There are

We notice that 0.9 kcal is nearly the same value that we earlier assigned to a gauche interaction in /z-butane; examination of models shows that this is not just accidental. Let us make a model of the conformation of methylcyclohexane with axial methyl. If we hold it so that we can sight along the Q 2 bond, we see something like this, represented by a Newman projection:

Stereoisomerism of cyclic compounds: cis and train isomers

• Let us turn for the moment from conformational analysis and look at configurational isomerism in cyclic compounds. We shall begin with the glycol of cyclopentene, 1,2-cyclopentanediol. Using models, we find that we can Ananke the atoms of this molecule as in I, in which both hydroxyls lie below (or above) the plane of the ring, and as in II, in which one hydroxyl lies above and the other lies below the plane of the .

• Stereoisomerism of this same sort should be possible for compounds other than glycols, and for rings other than cyclopentane. Some examples of isomers that have been isolated are:

• Now, what can we say about the possible chirality of the 1,2-dimethylcyclohexanes? Let us make a model of /raw5-l,2-dimethylcyclohexane in the more stable Di equatorial conformation, say and a model of its mirror image. We find

• To summarize, then, 1,2-dimethylcyclohexane exists as a pair of (configurational) diastereomers: the cis- and /ran/w-isomers. The c/. v-isomer exists as a pair of conformational enantiomers. The trans-homer exists as a pair of configurational enantiomers, each of which in turn exists as two conformational diastereomers (axial -axial and equatorial-equatorial). Because- of the ready interconvertibility of chair conformations, it is possible to use planar drawings to predict the configurational stereoisomerism of cyclol

Carbonics. Methylene

• The difference between successive members of a homologous series, we have seen, is the CH2 unit, or methylene. But methylene is more than just a building block for the mental construction of compounds; it is an actual molecule, and its chemistry and the chemistry of its derivatives, the carbenes, has become one of the most exciting and productive fields of organic research, Methylene is formed by the photolysis of either diazomethane, CH2 N2 , or ketene, CH2- OO. (Notice that the two starting materials and the two other Diazomethane Methylene uuravtolctllght CH2 + C products, nitrogen and carbon monoxide, are pairs of isoelectronic molecules, that is, molecules containing the same number of valence electrons.) Methylene as a highly reactive molecule was first proposed in the 1930s to account for the fact that something formed by the above reactions was capable of removing certain metal mirrors (compare Problem 16, p. 72). Its existence was definitely) established in 1959 by spectroscopic studies.

• In the gas phase, with low alkene concentration and in the presence of an inert gas, addition of methylene to the 2-butenes is, we have seen, nonstereospecific. If, however, there is present in this system a little oxygen, addition becomes completely stereospecific (syn). Account in detail for the effect of oxygen.

Substituted carbenes. a-Elimination

• A more generally useful way of making cyclopropanes is illustrated by the reaction of 2-butene with chloroform in the presence of potassium tert-butoxide (f-Bu = tar/-butyl):

• The dichlorocyclopropanes obtained can be reduced to hydrocarbons or hydrolyzed to ketones, the starting point for many syntheses (Chap. 19). Here, too, reaction involves a divalent carbon compound, a derivative of methylene: dichlorocarbene12. It is generated in two steps, initiated by attack on chloroform by the very strong base, /erf-butoxide ion, and then adds to the alkene.

• There are many ways of generating what appear to be carbenes. But in some cases at least, it seems clear that no free carbene is actually an intermediate; instead, a carbenoid (carbene-like) reagent transfers a carbene unit directly to a double bond. For example, in the extremely useful Simmons-Smith reaction

Analysis of alicyclic hydrocarbons

• A cyclopropane readily dissolves in concentrated sulfuric acid, and in this resembles an alkene or alkyne. It can be differentiated from these unsaturated hydrocarbons, however, by the fact that it is not oxidized by cold, dilute, neutral permanganate. Other alicyclic hydrocarbons have the same kind of properties as their open chain counterparts, and they are characterized in the same way: cycloalkanes by their general inertness, and cycloalkenes and cycloalkanes by their response to tests for unsaturation (bromine in carbon tetrachloride, and aqueous permanganate). That one is dealing with cyclic hydrocarbons is shown by molecular formulas and by degradation products. The properties of cyclohexane, for example, show clearly that it is an alkane. However, combustion analysis and molecular weight determination show its molecular formula to be C6H 12. Only a cyclic structure (although not necessarily a six-membered ring) is consistent with both sets of data. Similarly, the absorption of only one mole of hydrogen shows that cyclohexane contains only one carbon-carbon double bond; yet its molecular formula is C6Hi, which in an open-chain compound would correspond to two carboncarbon double bonds or one triple bond. Again, only a cyclic structure fits the fact.

• Cleavage products of cycloalkenes and Cyd alkynes also reveal the cyclic structure. Ozonolysis of cyclohexene, for example, does not break the molecule into two aldehydes of lower carbon number, but simply into a single six-carbon compound containing two aldehydes.

Both cyclohexene and 1,7-octadiene yield the di-aldehyde OHC(CH2)4CHO upon ozonolysis. What other facts would enable you to distinguish between the two compounds.