Alkanes stereochemistry

Other sulfonate derivatives are obtained by the use of trifluoromethanesulfonyl hypochlorite and hypobromite (CF3SO2OQ and CF3S020Br) in reactions with petfluoroalkyl halides and their derivatives [30. These reactions lead to the corresponding trifluoromethanesulfonate derivatives of alkanes (equation 28) (Table 11). The reaction proceeds with complete retention of stereochemistry at the carbon center [30]. 

CHAPTER 3 Organic Compounds Alkanes and Their Stereochemistry... 

A second hydrogen is transferred from the metal to the second carbon, giving the alkane product and regenerating the catalyst. Because both hydrogens are transferred to the same face of the alkene, the reduction has syn stereochemistry. 

Complete reduction to the alkane occurs when palladium on carbon (Pd/C) is used as catalyst, but hydrogenation can be stopped at the alkene if the less active Lindlar catalyst is used. The Lindlar catalyst is a finely divided palladium metal that has been precipitated onto a calcium carbonate support and then deactivated by treatment with lead acetate and quinoline, an aromatic amine. The hydrogenation occurs with syn stereochemistry (Section 7.5), giving a cis alkene product. 

The Lead-Off Reaction Addition of HBr to Alkenes Students usually attach great-importance to a text s lead-off reaction because it is the first reaction they see and is discussed in such detail. 1 use the addition of HBr to an alkene as the lead-off to illustrate general principles of organic chemistry for several reasons the reaction is relatively straightforward it involves a common but important functional group no prior knowledge of stereochemistry or kinetics in needed to understand it and, most important, it is a polar reaction. As such, 1 believe that electrophilic addition reactions represent a much more useful and realistic introduction to functional-group chemistry than a lead-off such as radical alkane chlorination. 

The structure of alkanes can be difficult to establish as their KMR spectra are difficult to interpret. The stereochemistry as well as the structure of (34) was in doubt, but synthesis by three different routes established both. 

Apparently the geometry of the transition state for adsorption is approximately that of a ir-complexed olefin in that its structure seems to be only slightly distorted from that of the isolated alkene. However, this does not necessarily mean that the adsorbed state which is formed in the elementary reaction to which the stereochemistry refers is a tt complex, because the same geometry also represents a stage in the progression of olefin to the eclipsed 1,2-diadsorbed alkane. Hopefully other experi-... 

Although the transition state for the exchange reaction may be described as the critical complex for the conversion of the half-hydrogenated state to either a jr-complexed olefin or an eclipsed vicinal diadsorbed alkane, the stereochemistry of hydrogenation of cycloalkenes on platinum at low pressures can be understood if the transition state has a virtually saturated structure. 

Concerning the stereochemistry of the olefins in the semireduction reaction, different results were obtained depending on the structure of the starting alkanes, so the corresponding mechanism is not clear. 

Elimination of HX from an alkane or cycloalkane typically follows an E2 mechanism and occurs with anti stereochemistry, e.g., elimination of HCl from chlorocyclohexane. ... 

As with alkanes, the boiling points and melting points of alkenes decrease with increasing molecular weight, but show some variations that depend on the shape of the molecule. Alkenes with the same molecular formula are isomers of one another if the position and the stereochemistry of the double bond differ. For example, there are four different acyclic structures that can be drawn for butene (C4H8). They have different b.p. and m.p. as follows. 

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