Configuration of alkenes

Since carbon atoms in alkenes are interconnected with double bonds, every C-atom has three neighbors. As we know from previous chapters, the molecule is more stable if groups surrounding certain central atom are positioned as far as possible from each other (VSEPR method). If the central atom is carbon as in alkenes, its neighbors must 

The largest electron density in planar molecules like alkenes is not located between the two carbon atoms but rather above and below the molecular plane. Such distribution of electron density is rationalized by the molecular orbital model in which the double bond includes jc-orbitals as shown in the following figure. 

Subsequently, A may experience rotation around the C=C bond and return back to form A or generate a new molecule B  

Transformation A = A = B includes the breaking and re-forming of a chemical bond (in this case a jc-bond). The process in which a chemical bond is broken or formed is known as chemical reaction. If A is transformed into B by a chemical reaction then A and B are isomers. However, these isomers differ only in the spatial arrangement of functional groups these isomers have different configurations. Isomers which differ from each other only in spatial configuration are called stereoisomers. 

This rule for naming stereoisomers is practical, but still equivocal because most alkene molecules have structures in which it is impossible to decide whether they belong to the cis, or to the trans configuration. For instance, let us examine the following structure  

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The LUMOs are it of alkenes and of ketenes. The LUMO-LUMO interaction occurs between the C=C bond of alkenes and the C=0 bond of ketenes, promoting the reaction across the C=0 bond of kentenes. The important pseudoex-cited configuration D A is the locally-excited nn configuration of alkenes. 

The configuration of alkenes can be determined from the frequency of their C—bending vibrations. Cfs-isomers absorb between 840 and 700cm while trans-isomers absorb between 1000 and 930 cm" The latter band can be used as the basis for an analytical method. 

Determine the configuration of each of the following alkenes as Z or f as appropriate... 

Isomeric alkenes may be either constitutional isomers or stereoisomers There is a sizable barrier to rotation about a carbon-carbon double bond which corresponds to the energy required to break the rr component of the double bond Stereoisomeric alkenes are configurationally stable under normal conditions The configurations of stereoisomeric alkenes are described according to two notational systems One system adds the prefix CIS to the name of the alkene when similar substituents are on the same side of the double bond and the prefix trans when they are on opposite sides The other ranks substituents according to a system of rules based on atomic number The prefix Z is used for alkenes that have higher ranked substituents on the same side of the double bond the prefix E is used when higher ranked substituents are on opposite sides... 

Section 7 13 Addition reactions of alkenes may generate one (Section 7 9) or two (Sec tion 7 13) chirality centers When two chirality centers are produced then-relative stereochemistry depends on the configuration (E or Z) of the alkene and whether the addition is syn or anti... 

Overall the stereospecificity of this method is the same as that observed m per oxy acid oxidation of alkenes Substituents that are cis to each other m the alkene remain CIS m the epoxide This is because formation of the bromohydrm involves anti addition and the ensuing intramolecular nucleophilic substitution reaction takes place with mver Sion of configuration at the carbon that bears the halide leaving group... 

Both ( )- and (Z)-l-halo-l-alkenes can be prepared by hydroboration of 1-alkynes or 1-halo-l-alkynes followed by halogenation of the intermediate boronic esters (244,245). Differences in the addition—elimination mechanisms operating in these reactions lead to the opposite configurations of iodides as compared to bromides and chlorides. 

Table 2.12. chemical shifts (relative configurations of cycloalkanes, pyranoses and alkenes (application of y-effects) The shifts which are printed in boldface reflect y-effects on C atoms in the corresponding... 

Hence the compound is nona-2,6-dienal. The relative configuration of both CC double bonds follows from the HH coupling constants of the alkene protons in the H NMR spectrum. The protons of the polarised 2,3-double bond are in trans positions Jhh 5.5 Hz) and those on the 6,7-double bond are in cis positions Jhh = 10.5 Hz). The structure is therefore nom.-2-tmns-6-cis-dienal, D. 

Fumaric acid is converted to L-malic acid by hydration in the presence of the enzyme fumamse. From the structure of the substrate and the configuration of the product, it is apparent that the hydroxyl group has been added to the si fiice of one of the carbon atoms of the double bond. Each of the trigonal carbon atoms of an alkene has its fiice specified separately. The molecule of fumaric acid shown below is viewed fixjm the re-re fiice. 

How many alkenes have the molecular formula C5H10 Write their structures and give their lUPAC names. Specify the configuration of stereoisomers as cis or trans as appropriate. 

A simple approach for the formation of 2-substituted 3,4-dihydro-2H-pyrans, which are useful precursors for natural products such as optically active carbohydrates, is the catalytic enantioselective cycloaddition reaction of a,/ -unsaturated carbonyl compounds with electron-rich alkenes. This is an inverse electron-demand cycloaddition reaction which is controlled by a dominant interaction between the LUMO of the 1-oxa-1,3-butadiene and the HOMO of the alkene (Scheme 4.2, right). This is usually a concerted non-synchronous reaction with retention of the configuration of the die-nophile and results in normally high regioselectivity, which in the presence of Lewis acids is improved and, furthermore, also increases the reaction rate. 

Chirality center, 292 detection of, 292-293 Eischer projections and, 975-978 R,S configuration of, 297-300 Chitin, structure of, 1002 Chloral hydrate, structure of, 707 Chloramphenicol, structure of, 304 Chlorine, reaction with alkanes, 91-92,335-338 reaction with alkenes, 215-218 reaction with alkynes, 262-263 reaction with aromatic compounds, 550 Chloro group, directing effect of, 567-568... 

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