Carbocations substitution

Figure 7.1 shows the interaction diagrams relevant to a carbocation substituted by X , Z, and C substituents ... 

For similar reasons the stability of polyalkoxymethanes to acid-catalyzed hydrolysis is unexpectedly dependent on the number of carbon-bound alkoxy groups. Contrary to chemical intuition, orthocarbonates are more difficult to hydrolyze than orthoesters [51] (Scheme 3.13), although a carbocation substituted with three alkoxy groups is more stable than one with only two alkoxy groups. Similarly, the tendency of fluoride to add to fluorinated alkenes is much greater than that of other halides [53], despite its low nucleophilicity (see also Section 4.2.2). 

A secondary carbocation is formed first in this reaction. This carbocation then rearranges to form a more stable tertiary carbocation. Substitution and elimination products from both of these carbocations may also be formed. 

J. Zhang and C. A. Wilkie, A carbocation substituted clay and its styrene nanocomposite. Polymer Degradation and Stability, 83 (2004), 301-7. 

An example from Ref. [7] will be treated here with the simpler Arrhenius equation and will be used again later to compare this method to more sophisticated treatments. The data consists of measurement of the solvolysis of an alkyl halide (1-chloro, 1-methyl cycloheptane) in a solution of 80% ethanol for what is nominally an SNl replacement of the chloride ion. In a later chapter, we will examine this reaction in more detail as a model of a steric hindrance in a carbocation substitution reaction but here we are just interested in the measured reaction rates at various temperatures. 

The beta silicon effect , referring to the strong stabilization of carbocations -substituted with silicon, is another well-known manifestation of hyperconjugation. The C-Si empty orbital on a cationic center, very significant delocalization is observed. Finally, an important feature in the conformational analysis of alkenes is the tendency for allylic sp centers to prefer to eclipse the C=C double bond (Scheme I) (see Conformational Analysis I Conformational Analysis 2 and Conformational Analysis 3). This preference, typically on the order of 2 kcal moI , is ascribed to improved hypercon-jugative interactions of the non-eclipsing C-H bonds with the jr orbital of the alkene for this conformation. 

To be able to prepare and study these elusive species in stable form, acids billions of times stronger than concentrated sulfuric acid were needed (so called superacids). Some substituted carbocations, however, are remarkably stable and are even present in nature. You may be surprised to learn that the fine red wine we drank tonight contained carbocations which are responsible for the red color of this natural 12% or so alcoholic solution. I hope you enjoyed it as much as I did. 

Reactions of the 2-amino-4,5-substituted thiazole (52) in acetic acid with ethylene oxide has been reported to give the N-exocyclic disubstitution product (S3) (201) in a 40% yield (Scheme 38). The reactive species in this reaction is probably the carbocation generated in acetic acid by ethvlene oxide. 

This order compares well with that of the decreasing susceptibility to elearophilic substitution of the three positions of the ring and of benzene. This comparison is justified by the analogy of the transition states of both reactions. The observed order agrees also with that of the calculated v net charge on the site of fixation of the incipient carbocation (133) ... 

Carbocations are classified according to their degree of substitution at the positively charged carbon The positive charge is on a primary carbon m CH3CH2" a secondary car bon m (CH3)2CH" and a tertiary carbon m (CH3)3C Ethyl cahon is a primary carbocation isopropyl cation a secondary carbocation and tert butyl cation a tertiary carbocation... 

To summarize the most important factor to consider m assessing carbocation sta bility IS the degree of substitution at the positively charged carbon... 

This suggests that as water attacks the bromonium ion positive charge develops on the carbon from which the bromine departs The transition state has some of the character of a carbocation We know that more substituted carbocations are more stable than less substituted ones therefore when the bromonium ion ring opens it does so by breaking the bond between bromine and the more substituted carbon... 

Step 2 The carbocation formed m step 1 reacts rapidly with a water molecule Water IS a nucleophile This step completes the nucleophilic substitution stage of the mechanism and yields an alkyloxonium ion... 

Primary carbocations are so high m energy that their intermediacy m nucleophilic substitution reactions is unlikely When ethyl bromide undergoes hydrolysis m aqueous formic acid substitution probably takes place by an 8 2 like process m which water is the nucleophile... 

Formation of a racemic product by nucleophilic substitution via a carbocation... 

Additional evidence for carbocation intermediates in certain nucleophilic substitutions comes from observing rearrangements of the kind normally associated with such species For example hydrolysis of the secondary alkyl bromide 2 bromo 3 methylbutane yields the rearranged tertiary alcohol 2 methyl 2 butanol as the only substitution product... 

Illustrate the stereochemistry associated with unimolecular nucleophilic substitution by con structmg molecular models of cis 4 tert butylcyclohexyl bromide its derived carbocation and the alcohols formed from it by hydrolysis under S l conditions... 

The three resonance forms of the intermediate leading to meta substitution are all secondary carbocations... 

The major influence of the methyl group is electronic The most important factor IS relative carbocation stability To a small extent the methyl group sterically hinders the ortho positions making attack slightly more likely at the para carbon than at a single ortho carbon However para substitution is at a statistical disadvantage because there are two equivalent ortho positions but only one para position... 

Oxygen stabilized carbocations of this type are far more stable than tertiary carbocations They are best represented by structures m which the positive charge is on oxygen because all the atoms have octets of electrons m such a structure Their stability permits them to be formed rapidly resulting m rates of electrophilic aromatic substitution that are much faster than that of benzene... 

Sections How substituents control rate and regioselectivity m electrophilic aro 12 10-12 14 matic substitution results from their effect on carbocation stability An electron releasing substituent stabilizes the cyclohexadienyl cation inter mediates corresponding to ortho and para attack more than meta... 

The sp hybridized carbon of an acyl chloride is less sterically hindered than the sp hybridized carbon of an alkyl chloride making an acyl chloride more open toward nude ophilic attack Also unlike the 8 2 transition state or a carbocation intermediate m an Stvfl reaction the tetrahedral intermediate m nucleophilic acyl substitution has a stable arrangement of bonds and can be formed via a lower energy transition state... 

The carbocation is stabilized by delocalization of the tt electrons of the double bond and the positive charge is shared by the two CH2 groups Substituted analogs of allyl cation are called allylic carbocations Allyl group (Sections 5 1 10 1) The group... 

Arenium ion (Section 12 2) The carbocation intermediate formed by attack of an electrophile on an aromatic substrate in electrophilic aromatic substitution See cyclohexadienyl cation... 

Substitution nucleophilic unimolecular(SNl) mechanism (Sec tions 4 9 and 8 8) Mechanism for nucleophilic substitution charactenzed by a two step process The first step is rate determining and is the ionization of an alkyl halide to a carbocation and a halide ion... 

The reactivity sequence furan > tellurophene > selenophene > thiophene is thus the same for all three reactions and is in the reverse order of the aromaticities of the ring systems assessed by a number of different criteria. The relative rate for the trifluoroacetylation of pyrrole is 5.3 x lo . It is interesting to note that AT-methylpyrrole is approximately twice as reactive to trifluoroacetylation as pyrrole itself. The enhanced reactivity of pyrrole compared with the other monocyclic systems is also demonstrated by the relative rates of bromination of the 2-methoxycarbonyl derivatives, which gave the reactivity sequence pyrrole>furan > selenophene > thiophene, and by the rate data on the reaction of the iron tricarbonyl-complexed carbocation [C6H7Fe(CO)3] (35) with a further selection of heteroaromatic substrates (Scheme 5). The comparative rates of reaction from this substitution were 2-methylindole == AT-methylindole>indole > pyrrole > furan > thiophene (73CC540). 

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