Nuclear Magnetic Resonance of Alkenes

Ethenyllithium (vinyllithium) is not generally prepared by direct deprotonation of ethene but rather from chloroethene (vinyl chloride) by metallation (Section 8-7). 

Upon treatment of ethenyllithium with acetone followed by aqueous work-up, a colorless liquid is obtained in 74% yield. Propose a structure. 

In Summary The presence of the double bond does not greatly affect the boiling points of alkenes, compared with alkanes, but cis-disubstituted alkenes usually have lower melting points than their trans isomers, because the cis compounds pack less well in the solid state. Alkenyl hydrogens are more acidic than those in alkanes because of the electron-withdrawing character of the ip -hybridized alkenyl carbon. 

The double bond exerts characteristic effects on the H and chemical shifts of alkenes (see Tables 10-2 and 10-6). Let s see how to make use of this information in structural assignments. 

The pi electrons exert a deshielding effect on alkenyl hydrogens 

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Monomer (Section 6 21) The simplest stable molecule from which a particular polymer may be prepared Monosaccharide (Section 25 1) A carbohydrate that cannot be hydrolyzed further to yield a simpler carbohydrate Monosubstituted alkene (Section 5 6) An alkene of the type RCH=CH2 in which there is only one carbon directly bonded to the carbons of the double bond Multiplicity (Section 13 7) The number of peaks into which a signal IS split in nuclear magnetic resonance spectroscopy Signals are described as singlets doublets triplets and so on according to the number of peaks into which they are split... 

Frei and co-workers also extended this reaction to other zeolites showing that almost identical behavior was observed in BaY, BaX, and in the K+ and Ba " forms of zeolite L [45,46]. Xiang et al. [47] have also studied the photooxidations of a series of 1-alkenes in the more acidic BaZSM-5 [48] and Ba- 3. The extensive polymerization of propylene in these zeolites demonstrates the detrimental effect of Bronsted acid sites on the reaction selectivity. These workers also used ex situ nuclear magnetic resonance (NMR) allowing more detailed... 

Infrared (IR) spectroscopy was the first modern spectroscopic method which became available to chemists for use in the identification of the structure of organic compounds. Not only is IR spectroscopy useful in determining which functional groups are present in a molecule, but also with more careful analysis of the spectrum, additional structural details can be obtained. For example, it is possible to determine whether an alkene is cis or trans. With the advent of nuclear magnetic resonance (NMR) spectroscopy, IR spectroscopy became used to a lesser extent in structural identification. This is because NMR spectra typically are more easily interpreted than are IR spectra. However, there was a renewed interest in IR spectroscopy in the late 1970s for the identification of highly unstable molecules. Concurrent with this renewed interest were advances in computational chemistry which allowed, for the first time, the actual computation of IR spectra of a molecular system with reasonable accuracy. This chapter describes how the confluence of a new experimental technique with that of improved computational methods led to a major advance in the structural identification of highly unstable molecules and reactive intermediates. 

Fullerenes can be derivatized by various means. For example, reaction with fluorine gas proceeds stepwise to the formation of colorless CeoFeo, which, according to the 19F nuclear magnetic resonance (NMR) spectrum, contains just one type of F site and so evidently retains a high degree of symmetry.9 In view of the low adhesion typical of fluorocarbons, this spherical molecule is expected to have extraordinary lubricant properties. Curiously, bromination of Ceo is reversible on heating otherwise, the reactions of fullerenes resemble those of alkenes or arenes (aromatic hydrocarbons). 

Having introduced the correlation of Fig. 1, we may return to the stabilities of alkyl cations. Rate constants for the hydration of secondary and primary alkenes have been measured in concentrated solutions of aqueous sulfuric acid by Lucchini and Modena97 and by Tidwell and Kresge42 using proton nuclear magnetic resonance (NMR) or UV to monitor progress of the reactions. It is conceivable that the reactions involve a concerted addition of a proton and water molecule to the alkenyl double bonds. However, the very weak basicity of water under the conditions of reaction makes this unlikely, and the steep acidity dependences of the reactions (e.g., m =-1.65) is... 

Hydrocarbons may be differentiated by their solubility in sulphuric acid since unsaturated hydrocarbons are soluble in concentrated sulphuric acid as are those arenes which are readily sulphonated, whereas the saturated alkanes and lesser reactive arenes are insoluble in this reagent. The presence of an alkene, alkyne or arene is usually readily apparent from an inspection of the infrared and nuclear magnetic resonance spectra, the characteristic features of which are fully discussed in Sections 5.2., 5.3 and 6.1 respectively and in Chapter 3. 

Analysis with chiral nuclear magnetic resonance shift reagents revealed that the isotactic poly (1,4-ketone) products were formed with an average or overall degree of enantioselectivity that was >90%. Using the same catalyst, Jiang and Sen also described the first example of alternating co-polymerization between an internal alkene (2-butene) and carbon monoxide to form an isotactic, optically active poly(l,5-ketone). 

A silver-silyl intermediate was detected by Si nuclear magnetic resonance (NMR) at 5 = 97 ppm and inverse saturation behavior was detected with mono-substituted alkenes. Based on their studies, a silver-mediated reversible releasing of silylene followed by an irreversible electrophilic addition of the silver-silyl intermediate to the alkene was suggested (Fig. 32). 

Stereochemical and kinetic analyses of the Brpnsted acid-catalysed intramolecular hydroamination/deuterioamination of the electronically non-activated cyclic alkene (13) with a neighbouring sulfonamide nucleophile have been found to proceed as an anh-addition (>90%) across the C=C bond to produce (15). No loss of the label was observed by and NMR (nuclear magnetic resonance) spectroscopies and mass spectrometry (MS). The reaction follows the second-order kinetic law rate = 2 [TfOH] [13] with the activation parameters being = 9.1 0.5 kcal moP and = -35 5 cal moP An inverse a-secondary kinetic isotope effect of d/ h = (1-15 0.03), observed for (13) deuteration at C(2), indicates a partial CN bond formation in the transition state (14). The results are consistent with a mechanism involving concerted, intermolecular proton transfer from an N-protonated sulfonamide to the alkenyl C(3) position coupled with an intramolecular anti-addition by the sulfonamide group. ... 

Isomerization of an Alkene Nuclear Magnetic Resonance Analysis... 

Nuclear magnetic resonance spectroscopic measurements have provided powerful evidence that these and other selected alkenes form stable bridged bromonium ion salts. A solid bromonium ion tribromide salt of adamantyUdene... 

Cis- and trans-[(Ph3P)2PdMe(E- and Z-CH=CHPh)] eliminate to give E- and Z-MeCH=CHPh. Product analysis from decomposition of the cis-E isomer in the presence of deuterated resonance material suggests an intramolecular mechanism. Nuclear magnetic resonance points to the presence of alkene species, e.g., (Tj -PhCH=CHMe)PdL, while the effect on rate of free phosphine and of the alkene product suggest other intermediates. 

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