Cycloalkanes three-bond

Conformational analysis is far simpler m cyclopropane than m any other cycloalkane Cyclopropane s three carbon atoms are of geometric necessity coplanar and rotation about Its carbon-carbon bonds is impossible You saw m Section 3 4 how angle strain m cyclopropane leads to an abnormally large heat of combustion Let s now look at cyclopropane m more detail to see how our orbital hybridization bonding model may be adapted to molecules of unusual geometry... 

Stereochemistry refers to chemistry in three dimensions Its foundations were laid by Jacobus van t Hoff and Joseph Achille Le Bel m 1874 Van t Hoff and Le Bel mde pendently proposed that the four bonds to carbon were directed toward the corners of a tetrahedron One consequence of a tetrahedral arrangement of bonds to carbon is that two compounds may be different because the arrangement of their atoms m space IS different Isomers that have the same constitution but differ m the spatial arrangement of their atoms are called stereoisomers We have already had considerable experience with certain types of stereoisomers—those involving cis and trans substitution patterns m alkenes and m cycloalkanes... 

C-C bond Fluonnation increases the bond strengths in cycloalkanes, including cyclobutanes [75, 94], but by contrast, it decreases C-C bond strengths and increases nng strain in cyclopropanes and other three-membered nng compounds [75 94. 9S]... 

There is some increase in selectivity with functionally substituted carbenes, but it is still not high enough to prevent formation of mixtures. Phenylchlorocarbene gives a relative reactivity ratio of 2.1 1 0.09 in insertion reactions with i-propylbenzene, ethylbenzene, and toluene.212 For cycloalkanes, tertiary positions are about 15 times more reactive than secondary positions toward phenylchlorocarbene.213 Carbethoxycarbene inserts at tertiary C—H bonds about three times as fast as at primary C—H bonds in simple alkanes.214 Owing to low selectivity, intermolecular insertion reactions are seldom useful in syntheses. Intramolecular insertion reactions are of considerably more value. Intramolecular insertion reactions usually occur at the C—H bond that is closest to the carbene and good yields can frequently be achieved. Intramolecular insertion reactions can provide routes to highly strained structures that would be difficult to obtain in other ways. 

Conformational isomerism in cyclopropane Cyclopropane is the first member of the cycloalkane series, and composed of three carbons and six hydrogen atoms (CsHe). The rotation about C-C bonds is quite restricted in cycloalkanes, especially in smaller rings, e.g. cyclopropane. 

For replacement names the parent carbocycles, apart from benzene, are all named as cycloalkanes (modified to -ene, -diene, etc., as necessary) with the heteroatom replacing /CH2, CH, or C— as appropriate to its valency. Replacement names derived from benzene are not used unless the three formal double bonds are retained otherwise names of the type oxacyclohexa-2,4-diene are appropriate [see 1-3 and 57-60 replacement names for 7-11 are thiacyclopenta-2,4-diene (7), 1,3-diazabenzene (8), l-oxa-2,5-diazacyclopenta-2,4-diene (9), l-oxa-4-azacyclohexane (10), and l,2-diazacyclopent-3-ene (11)]. 

Results of protoly tic reactions of hydrocarbons in superacid media were interpreted by Olah as indication for the general electrophilic reactivity of covalent C H and C—C single bonds of alkanes and cycloalkanes. The reactivity is due to the tr-donor ability of a shared electron pair (of cr-bond) via two-electron, three-center bond formation. Consequently, the transition state of the reaction, is of three-center bound pentacoordinate carbonium ion nature [Eq. (5.5)]. 

Baeyer fi theory was wrong- for a very simple reason Ho assumed that rings are flat. In fact, though, most cycloalkanes are not flat they adopt puckered Ihree-dimensioival conformations that allow bond angles tv be nearly tetrahedral. Only for three- and four-membered rings in his concept of angle strain important. 

A cycloalkane is a saturated cyclic hydrocarbon with the general tomiula C H2 . In contrast to open-chain alkanes, where nearly free rotation occurs around C- C bonds, rotation is greatly reduced in cycloalkanes, ni ubsiiluted cycU)alkancs can therefore exist as cis-trans isomers. The cis isomer has both substituents on the same face of the ring the trans isomer has substituents on opposite faces. Cis-trans isomers are just one kind of stereoisomers—i.somcrs that have the same connections between atoms but different three-dimensional arrangements. 

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