Transition states carbonium ion

Carbon atom, 4. See also Atomic orbitals Carbon dioxide hydration, 197-199. See also Carbonic anhydrase Carbonic anhydrase, 197-199,200 Carbonium ion transition state, 154, 159 Carboxypeptidase A, 204-205 Catalysis, general acid, 153,164,169 in carboxypeptidase A, 204-205 free energy surfaces for, 160, 161 in lysozyme, 154... 

These differences were explained by solvation effects in the liquid phase. The carbenium ions are more efficiently stabilized by solvation than carbonium ions, because the former have unsaturated trivalent carbon atoms. In this way, the energy barrier between the (solvated) carbenium ion and the carbonium ion transition state increases. 

The catalytic homogeneous isomerization of olefins by protonic acids as well as by Lewis acids is well-known and there is little doubt that such isomerizations proceed through carbonium ion transition states or intermediates. Thus, strong acids isomerize 1-hexene to all possible hexene isomers, including cis and trans isomers where these are possible. The isomerization to 2-hexene may be written as a proton addition-elimination ... 

From these experiments, some general conclusions can be drawn concerning the behavior of small alkanes in the strongest HF-SbF5 system. (1) The reversible protonation of the alkanes (i) is very fast in comparison with the ionization step (ii) it takes place on all cr-bonds independently of the subsequent reactivity of the alkane (iii) it involves carbonium ions (transition states), which do not undergo molecular rearrangements. (2) Protonation of an alkane is atypical acid-base reaction and carbon monoxide has no effect on this step. 

For this reason when Magic Acid (HS03F-SbF5,1 1 molar ratio) is used under the same experimental conditions as above at room temperature, isobutane undergoes very slow ionization and the formation of the ferf-butyl ion can be monitored. However, recovered isobutane shows no exchange because the reversible protonation via carbonium ion transition state does not take place and because the ferf-butyl ion, stable in this solution at room temperature, does not deprotonate. 

More recently, it has been proposed (78, 79) that on zeolite catalysts the reaction can also start by protonation of a C-C bond by the acid site of the zeolite forming a pentacoordinated carbonium ion transition state. This can then eliminate H2 or a short alkane molecule leaving an adsorbed carbenium ion on the zeolite (protolytic cracking), as shown below ... 

In such reactions, any steric effect of a neighboring hydroxyl group is weak, compared to its electron-withdrawing effect which, presumably, destabilizes one of the two carbonium ion transition states more than the other, as pointed out by Lemieux. ... 

One observation raises the question of whether enough is known about the carbonium-ion lifetime in enzymic sites to be certain that it is always as short as estimated for reactions in water. The crystal structure of soybean beta-amylase complexed either with /8-maltose or maltal shows features strongly suggesting that the reaction center is shielded from solvent.101 A mechanism involving a carbonium-ion transition state with C-l unsubstituted and subject to attack by a structurally positioned water molecule, to form a product of /3-configurat.ion, is likely for the inverting reactions... 

For reactions that can proceed through different reaction channels, each channel implies the formation of a different transition state with different shape, energy and entropy. In the CH cleavage reaction (Fig. 4.8a), the protonated carbonium ion transition state (with five-fold coordination on the protonated carbon) has the approximate geometry... 

Zheng A, Deng F, Liu S-B (2011) Regioselectivity of carbonium ion transition states in zeolites. Catal Today 164(l) 40-45... 

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