Alkene protonated

The couplings of vicinal protons in 1,2-disubstituted alkenes lie in the range 6-12 Hz for cis protons (dihedral angle 0°) and 12-17 Hz for trans protons (dihedral angle 180°), thus also following the Karplus-Conroy equation. Typical examples are the alkene proton AB systems of coumarin (16a, cis) and tra 5-cinnamic acid (16b), and of the cis-trans isomers 17a and b of ethyl isopente-nyl ether, in addition to those in problems 3, 4, 8, 11, 13 and 38. 

Since the olefmic CC double bond is trisubstituted, the relative configuration cannot be determined on the basis of the cis and trans couplings of vicinal alkene protons in the H NMR spectrum. What is the relative configuration given the C NMR spectra 19 ... 

The alternative tran5-p-phenylcinnamic aldehyde F would display an additional coupling between the aldehyde proton and the vicinal alkene proton of the double bond whieh is not observed in speetmm 8 (but in speetmm 4 for eomparison). 

An examination of the eross signals of the HH COSY diagram leads to the proton eonneetivities shown in A starting from the alkene proton at 5// =5.67. 

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. 

Fragments E and F which include all 16 carbon atoms detected by C NMR can be attached to each other in two ways G or H the structure G is realised, as follows from the NOE difference spectra, which show a significant NOE between the methyl protons at Sh = 1-28 and the alkene proton at Sh = 6.28 and vice versa (Table 44.3). 

An NOE between the the alkene proton at 3h = 6.22 and the methyl protons at Sh = 1.17 establishes the relative eonfiguration (exo) of the respeetive methyl group. The exo attaehment of the six-membered ring in the stereostrueture I follows, in partieular, from the NOE between the methyl protons at 5h = 1.26 and the bridgehead proton at Sh = 3.22 as well as the absenee of effeets between the alkenyl proton pair with Sh = 5.44/6.22 and the bridgehead proton pair with Sh = 2.85/3.22. 

In a similar way, the linking of the earboxy function with a CC double bond follows from the correlation of the earboxy resonance (5c = 170.4) with the alkene protons (d/y = J.Ji and 6.] 8)-, the latter give correlation signals with the C atom at 5c = 38.5, as do the protons at Sff = ].33 and 1.53, so that taking into account the molecular unit B which is already known, an additional substructure D is established. 

A significant modification in the stereochemistry is observed when the double bond is conjugated with a group that can stabilize a carbocation intermediate. Most of the specific cases involve an aryl substituent. Examples of alkenes that give primarily syn addition are Z- and -l-phenylpropene, Z- and - -<-butylstyrene, l-phenyl-4-/-butylcyclohex-ene, and indene. The mechanism proposed for these additions features an ion pair as the key intermediate. Because of the greater stability of the carbocations in these molecules, concerted attack by halide ion is not required for complete carbon-hydrogen bond formation. If the ion pair formed by alkene protonation collapses to product faster than reorientation takes place, the result will be syn addition, since the proton and halide ion are initially on the same side of the molecule. 

When a hydrogen halide adds to an alkene, protonation of the double bond occurs in the direction that gives the more stable car-bocation. 

Electrophilic addition of HX to an alkene involves a two-step mechanism, the overall rate being given by the rate of the initial protonation step. Differences in protonation energies are usually explained by considering differences in carbocation stability, but the relief or buildup of strain can also be a factor. One of the following alkenes protonates much more easily than the other. 

How does the Hammond postulate apply to electrophilic addition reactions The formation of a catbocation by protonation of an alkene is an endergonic step. Thus, the transition state for alkene protonation structurally resembles the... 

We can imagine the transition state for alkene protonation to be a structure in which one of the alkene carbon atoms has almost completely rehybridized from sp2 to sp- and in which the remaining alkene carbon bears much of the positive charge (Figure 6.16). This transition state is stabilized by hyperconjuga-lion and inductive effects in the same way as the product carbocation. The more alkyl groups that are present, the greater the extent of stabilization and the faster the transition state forms. 

Alkene protonation at pore mouths can exclusively lead to secondary carbenium ions. In addition, the alkene standard protonation enthalpies increase with the number of carbon atoms inside the micropore because charge dispersive effects are supposed to be more effective on carbon atoms inside the micropores. 

Table 5.6 Estimation of chemical shifts for alkene protons.

It should always be remembered of course, that the NMR spectrum reflects a compound s behaviour in solution. It is quite possible for a compound and a weak acid to crystallise out as a stoichiometric salt and yet in solution, for the compound to give the appearance of a free base. For this reason, care should be taken in attempting to use NMR as a guide to the extent of protonation. If the acid has other protons that can be integrated reliably, e.g., the alkene protons in fumaric or maleic acid, then there should be no problem but if this is not the case, e.g., oxalic acid, then we would council caution Do not be tempted to give an estimate of acid content based on chemical shift. With weak acids, protonation may not occur in a pro rata fashion though it is likely to in the case of strong acids. 

Q9. At first glance, the proton spectrum for this compound looks excellent. The protons are, with the exception of two aromatic protons, well separated and this is always a bonus The alkene protons draw immediate attention as they sit on either side of the aromatic protons and the doublet at about 8.4 ppm is definitely the alkene closest to the aromatic ring. Its coupling partner, closest to... 

When a terminal alkene protonates by using its tt electrons to bond a proton at the terminal carbon, a carbocation forms at the second carbon of the chain (The carbocation could also form directly from the 1° alcohol by a hydride shift from its P-carbon to the terminal carbon as the protonated hydroxyl group departs). 

Fig. 2 Free energy reaction coordinate profiles for the stepwise acid-catalyzed hydration of an alkene through a carbocation intermediate (Scheme 5). (a) Reaction profile for the case where alkene protonation is rate determining (ks kp). This profile shows a change in rate-determining step as a result of Bronsted catalysis of protonation of the alkene. (b) Reaction profile for the case where addition of solvent to the carbocation is rate determining (ks fcp). This profile shows a change in rate-determining step as a result of trapping of the carbocation by an added nucleophilic reagent.

Different rate-determining steps are observed for the acid-catalyzed hydration of vinyl ethers (alkene protonation, ks kp) and hydration of enamines (addition of solvent to an iminium ion intermediate, ks increasing stabilization of a-CH substituted carbocations by 71-electron donation from an adjacent electronegative atom results in a larger decrease in ks for nucleophile addition of solvent than in kp for deprotonation of the carbocation by solvent. 

Spectra were recorded at 90 MHz with tetramethylsilane as the internal standard. 4 117 contains only one (alkenic) proton at C-3. c Signals overlap with others. d The signals for H-8,8 almost coalesce, but are still noticeably different e Not determined. 

The a-methylbenzyl cation (1) can be approached from the alcohol dehydration direction or the alkene protonation direction, as shown, and both of these processes have been the subject of ab initio molecular orbital calculations. It was found that the alcohol dehydration has a transition state about half way between the two stmctures shown, with the transition state and the carbocation having about the same amount of 7T-orbital overlap. However, the alkene protonation has an earlier transition state with less effective 7r-orbital overlap than that in the cation. This is held to explain the different Yukawa-Tsuno r+ values found for the two processes, 0.7-1.1 for alkene... 

As a simple example, note that the major products obtained as a result of addition of HBr to the alkenes shown below are not always those initially expected. For the first alkene, protonation produces a particularly favourable carbocation that is both tertiary and benzylic (see Section 6.2.1) this then accepts the bromide nucleophile. In the second alkene, protonation produces a secondary alkene, but hydride migration then leads to a more favourable benzylic carbocation. As a result, the nucleophile becomes attached to a carbon that was not part of the original double bond. Further examples of carbocation rearrangements will be met under electrophilic aromatic substitution (see Section 8.4.1). 

Benzvalene (18) is a tricyclic benzene isomer containing a bicyclobutane ring system bridged by an ethylene moiety its radical cation is accessible by PET or radiolysis. CIDNP indicated negative hfcs for the alkene protons (H ), strong positive hfcs for the non-allylic bridgehead protons (Hy), and negligible hfcs for the... 

Figure 4.43 500 MHz LC-NMR spectrum of approximately 3 gg of prednisolone 21-acetate in D2O/CH3CN/0.1 % HCO2H, with solvent suppression at 5 1.90 (CH3CN) and 4.16 (residual water). The peak at 5 8.05 is due to HCO2H. The aliphatic protons are masked by the acetonitrile peak, but the methylene and alkene protons (highlighted in bold) can be seen in the range S 4.5-7.5...

The direct Jc 3 H., coupling constants decrease regularly along the series 0>NH>S>Se>Te. The values for 7C 2 H-2> are appreciably larger than the 159 Hz observed for benzene and the 170 Hz for the alkenic protons of cyclopentadiene, while the 7C.3 H.3 coupling constants span this range. 

The H NMR spectrum of the 3-isomer (81) is relatively simple. The alkenic protons appear as a doublet at 5.43 p.p.m. (3/P H = 34 Hz). The CH2 protons are slightly nonequivalent and as such appear as a pair of doublets. Similar observations pertain when the OPh group in (81) is substituted by either Me or Ph. The H NMR spectrum of (82) is more complicated as it has a seven-spin system (disregarding the downfield aromatic... 

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