Propane protonation

Aromatization of short alkanes can be carried out on a purely acidic HZSM-5 zeolite. In this case, activation of the alkane is thought to occur by protonation on the zeolite acid sites, with formation of a penta-coordinated carbonium ion (1). In the case of propane, protonation at a C-H bond will lead to the formation of H2 and a sec-propyl cation (dehydrogenation), while protonation at a C-C bond will produce methane and a primary ethyl cation (cracking) ... 

We have demonstrated that monofunctional carbonium ion initiation is predominant for propane activation on H-MH as the olefin concentration approaches zero. The monofunctional reaction step involves propane protonation by a Brpnsted acid site to yield H-proponium, C-proponium and ethanemethonium energetically close pentacoordinated ions. 

The decreased shielding caused by electronegative substituents is primarily an inductive effect and like other inductive effects falls off rapidly as the number of bonds between the substituent and the proton increases Compare the chemical shifts of the pro tons m propane and 1 mtropropane... 

Protons are equivalent to one another and have the same chemical shift when they are m equivalent environments Often it is an easy matter to decide simply by mspec tion when protons are equivalent or not In more difficult cases mentally replacing a proton m a molecule by a test group can help We 11 illustrate the procedure for a sim pie case—the protons of propane To see if they have the same chemical shift replace one of the methyl protons at C 1 by chlorine then do the same thing for a proton at C 3 Both replacements give the same molecule 1 chloropropane Therefore the methyl protons at C 1 are equivalent to those at C 3... 

Replacement of either one of the methylene protons of propane generates 2 chloro propane Both methylene protons are equivalent Neither of them is equivalent to any of the methyl protons... 

The H NMR spectrum of propane contains two signals one for the six equiva lent methyl protons the other for the pair of equivalent methylene protons... 

Vicinal CH coupling constants Hqh resemble vicinal HH coupling constants in the way that they depend on the cosine of the dihedral angle 9 between the CC bond to the coupled C atom and the C//bond to the coupled proton (cf Fig. 2.16), as illustrated by the Newman projections of the conformers 20a-c of a propane fragment. 

Enby 6 is an example of a stereospecific elimination reaction of an alkyl halide in which the transition state requires die proton and bromide ion that are lost to be in an anti orientation with respect to each odier. The diastereomeric threo- and e/ytAra-l-bromo-1,2-diphenyl-propanes undergo )3-elimination to produce stereoisomeric products. Enby 7 is an example of a pyrolytic elimination requiring a syn orientation of die proton that is removed and the nitrogen atom of the amine oxide group. The elimination proceeds through a cyclic transition state in which the proton is transferred to die oxygen of die amine oxide group. 

An isopropyl carbocation cannot experience a beta fission (no C-C bond beta to the carbon with the positive charge).It may either abstract a hydride ion from another hydrocarbon, yielding propane, or revert back to propene by eliminating a proton. This could explain the relatively higher yield of propene from catalytic cracking units than from thermal cracking units. 

Pentadienyl radical, 240 Perturbation theory, 11, 46 Propane, 16, 165 n-Propyi anion conformation, 34 n-Propyl cation, 48, 163 rotational barrier, 34 Propylene, 16, 139 Protonated methane, 72 Pyrazine, 266 orbital ordering, 30 through-bond interactions, 27 Pyridine, 263 Pyrrole, 231... 

Aquilante and Volpi indicate (2) that propanium ions formed by proton transfer from H3 + are not collisionally stabilized at propane pressures as great as 0.3 mm. and that they decompose by elimination of hydrogen or a smaller saturated hydrocarbon to form an alkyl carbonium ion. Others (16, 19) have proposed one or the other of these fates for unstabilized propanium ions. Our observations can be rationalized within this framework by the following mechanisms ... 

Note also that (1) d° Ta alkyhdene complexes are alkane metathesis catalyst precursors (2) the cross-metathesis products in the metathesis of propane on Ta are similar to those obtained in the metathesis of propene on Re they differ only by 2 protons and (3) their ratio is similar to that observed for the initiation products in the metathesis of propane on [(=SiO)Ta(= CHfBu)(CH2fBu)2]. Therefore, the key step in alkane metathesis could probably involve the same key step as in olefin metathesis (Scheme 27) [ 101 ]. 

Data related to the effect of P-halogen on the proton and carbon chemical shifts of CF2H or CF2 groups are scarce (Scheme 4.13), although there is a recent review of 13C spectra of chlorofluorocyclo-propanes.3... 

Thus reaction of the 1-propyl cation (13) with water (reaction type a) will yield propan-l-ol (14), elimination of a proton from (13) will yield propene (15, reaction type b), while rearrangement of (13, reaction type d)—in this case migration of He—will yield the 2-propyl cation... 

Reactions of this type are important in some gases and certainly in many gaseous mixtures. The basic proton donors are H3+, CH5+, and ions of the types C H+2n + L and C H,, + in addition to certain others. In some cases, such as in cyclo-propane, the product of the proton transfer reaction is a stable ion ... 

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