Carbocations protonation

The notion that carbocation formation is rate determining follows from our previous experience and by observing how the reaction rate is affected by the shucture of the aUcene Table 6 2 gives some data showing that alkenes that yield relatively stable carbocations react faster than those that yield less stable carbocations Protonation of ethylene the least reactive aUcene m the table yields a primary carbocation protonation of 2 methylpropene the most reactive m the table yields a tertiary carbocation As we have seen on other occa sions the more stable the carbocation the faster is its rate of formation... 

C4 is nucleophilic (enol ether), and CIO is electrophilic. The Lewis acid makes CIO more electrophilic by coordinating to 013. After conjugate addition, 08 traps the C3 carbocation. Proton-Li+ exchange gives the product. 

Protonation at end of diene unit gives a carbocation Protonation at C-2 gives a carbocation that is both secondary and allylic most stable that is secondary but not allylic less... 

The best approach is to work through this reaction mechanistically. Addition of hydrogen halides always proceeds by protonation of one of the terminal carbons of the diene system. Protonation of C-l gives an allylic cation for which the most stable resonance form is a tertiary carbocation. Protonation of C-4 would give a less stable allylic carbocation for which the most stable resonance form is a secondary carbocation. 

Fig. 6. Plot of proton affinities (PA) of carboxylic acids and esters versus the a value of the substituent R. These values are 1.000 (hydrogen), 0.938 (methyl), 0.915 (ethyl), 0.895 (i-propyl), and 0.883(ferf-butyl), as explained in the chapter on carbocations. Proton affinity data have been taken from Ref. [15]...

When the source is neutral, protonation can occur to produce the heteroatom-protonated species or the delocalized heteroatom-substituted carbocation. Protonation on the more basic heteroatom lone pair is the lower-energy process, but the heteroatom-protonated species usually just returns to reactants. 

Radicals can be generated by cathodic reduction from carbocations, protonated C=X bonds, and the reduction of halides or onium salts. The reduction potentials of carbocations range from 1.87 V (vs nhe) for NCCH(4-CN-C6H4)+, 0.97 V for the benzylcation, 0.33 V for the tert-butyl cation, and 0.0 V for the methoxymethyl-cation to -0.88 V for Et2N=CHCH3+ [143],... 

The double bonds of 1,4-pentadiene are equivalent and it will not matter which one becomes protonated. What is important is which carbon of the double bond becomes protonated. Protonation at carbon 1 gives a secondary carbocation whereas protonation of carbon 2 gives a primary carbocation. Neither species can undergo electron delocalization. Since secondary carbocations are more stable than primary carbocations, protonation will occur at carbon 1. 

As with alkenes, the regioselectivity of electrophilic addition to conjugated dienes is governed by the stability of the resulting carbocation. Protonation of a conjugated diene always occurs at the end of the diene unit because an allylic carbocation results. 

Proton transfer to carbon 3 of this alkene gives a tertiary (3°) carbocation. Proton transfer to carbon 2 gives a secondary (2 ) carbocation. 

Bock CW, George P, Glusker JP (1993) Ab initio molecular orbital studies on CjHjO and C H jPO+i oxonium ion, carbocation, protonated aldehyde, and related transition-state structures. J Org Chem 58(21) 5816-5825... 

Protonation of ethylene, the least reactive alkene of the three, would give a primary carbocation protonation of 2-methylpropene, the most reactive, gives a tertiary carbocation. The more stable the carbocation, the faster its rate of formation and the faster the overall reaction rate. 

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