Carbocations substituents

Free radicals like carbocations have an unfilled 2p orbital and are stabilized by substituents such as alkyl groups that release electrons Consequently the order of free radical stability parallels that of carbocations... 

Carbocations are stabilized by alkyl substituents attached directly to the positively charged carbon Alkyl groups are electron releasing sub stituents Stability increases in the order... 

In general alkyl substituents increase the reactivity of a double bond toward elec trophilic addition Alkyl groups are electron releasing and the more electron rich a dou ble bond the better it can share its tt electrons with an electrophile Along with the observed regioselectivity of addition this supports the idea that carbocation formation rather than carbocation capture is rate determining... 

We have seen this situation before m the reaction of alcohols with hydrogen halides (8ection 4 11) m the acid catalyzed dehydration of alcohols (8ection 5 12) and m the conversion of alkyl halides to alkenes by the El mechanism (8ection 5 17) As m these other reactions an electronic effect specifically the stabilization of the carbocation intermediate by alkyl substituents is the decisive factor The more stable the carbo cation the faster it is formed... 

Not all the properties of alkenes are revealed by focusing exclusively on the func tional group behavior of the double bond A double bond can affect the proper ties of a second functional unit to which it is directly attached It can be a sub stituent for example on a positively charged carbon in an allylic carbocation, or on a carbon that bears an unpaired electron in an allylic free radical, or it can be a substituent on a second double bond in a conjugated diene... 

A rule of thumb is that a C=C substituent stabilizes a carbocation about as well as two methyl groups Al though allyl cation (H2C=CHCH2 ) is a primary carbocation it is about as stable as a typical secondary carbocation such as isopropyl cation (CH3)2CH-"... 

Both compounds react by an S l mechanism and their relative rates reflect their acti vation energies for carbocation formation Because the allylic chloride is more reactive we reason that it ionizes more rapidly because it forms a more stable carbocation Struc turally the two carbocations differ m that the allylic carbocation has a vinyl substituent on Its positively charged carbon m place of one of the methyl groups of tert butyl cation... 

This chapter focused on the effect of a carbon-carbon double bond as a stabilizing substituent on a positively charged carbon m an allylic carbocation, on a carbon bearing... 

Additional phenyl substituents stabilize carbocations even more Triphenylmethyl The triphenylmethyl group is 1, , t 11 1. . 1. .. 

Section 11 16 Addition reactions to alkenylbenzenes occur at the double bond of the alkenyl substituent and the regioselectivity of electrophilic addition is governed by carbocation formation at the benzylic carbon See Table 11 2... 

All alkyl groups not just methyl are activating substituents and ortho para direc tors This IS because any alkyl group be it methyl ethyl isopropyl tert butyl or any other stabilizes a carbocation site to which it is directly attached When R = alkyl... 

The lone pair on oxygen cannot be directly involved m carbocation stabilization when attack is meta to the substituent... 

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... 

Deactivating and meta directing These substituents are strongly electron withdrawing and destabilize carbocations They include... 

Alkyl substituents stabilize a carbonyl group m much the same way that they sta bilize carbon-carbon double bonds and carbocations—by releasing electrons to sp hybridized carbon Thus as then heats of combustion reveal the ketone 2 butanone is more stable than its aldehyde isomer butanal... 

Other, removable cation-stabilizing auxiliaries have been investigated for polyene cyclizations. For example, a sdyl-assisted carbocation cyclization has been used in an efficient total synthesis of lanosterol. The key step, treatment of (257) with methyl aluminum chloride in methylene chloride at —78° C, followed by acylation and chromatographic separation, affords (258) in 55% yield (two steps). When this cyclization was attempted on similar compounds that did not contain the C7P-silicon substituent, no tetracycHc products were observed. Steroid (258) is converted to lanosterol (77) in three additional chemical steps (225). 

TT-Conjugating groups tend to favor attack at C, but the ratio of Ca. C attack depends strongly on a balance of steric and electronic factors arising from both substituent and nucleophile (Table 4). The results can be rationalized, to a first approximation, by assuming that with good vr-donors stabilization of the incipient carbocation in (50) offsets steric hindrance. 

Because carbocations are key intermediates in many nucleophilic substitution reactions, it is important to develop a grasp of their structural properties and the effect substituents have on stability. The critical step in the ionization mechanism of nucleophilic substitution is the generation of the tricoordinate carbocation intermediate. For this mechanism to operate, it is essential that this species not be prohibitively high in energy. Carbocations are inherently high-energy species. The ionization of r-butyl chloride is endothermic by 153kcal/mol in the gas phase. ... 

The triarylmethyl cations are particularly stable because of the conjugation with the aryl groups, which delocalizes the positive charge. Because of their stability and ease of generation, the triarylmethyl cations have been the subject of studies aimed at determining the effect of substituents on carbocation stability. Many of these studies used the characteristic UV absorption spectra of the cations to determine their concentration. In acidic solution, equilibrium is established between triarylearbinols and the corresponding carbocations. 

The pATr+ values allow for a comparison of the stability of carbocations. The carbocations that can be studied in this way are all relatively stable carbocations. The data in Table 5.1 reveal that electron-releasing substituents on the aryl rings stabilize the carbocation (more positive pA r+) whereas electron-withdrawing groups such as nitro are destabilizing. This is what would be expected from the electron-deficient nature of the carbocation. 

Aldiough diese structures have a positive charge on a more electronegative atom, diey benefit from an additional bond which satisfies file octet requirement of the tricoordinate carbon. These carbocations are well represented by file doubly bonded resonance structures. One indication of file participation of adjacent oxygen substituents is file existence of a barrier to rotation about the C—O bonds in this type of carbocation. 

Substitution reactions by the ionization mechanism proceed very slowly on a-halo derivatives of ketones, aldehydes, acids, esters, nitriles, and related compounds. As discussed on p. 284, such substituents destabilize a carbocation intermediate. Substitution by the direct displacement mechanism, however, proceed especially readily in these systems. Table S.IS indicates some representative relative rate accelerations. Steric effects be responsible for part of the observed acceleration, since an sfp- caibon, such as in a carbonyl group, will provide less steric resistance to tiie incoming nucleophile than an alkyl group. The major effect is believed to be electronic. The adjacent n-LUMO of the carbonyl group can interact with the electnai density that is built up at the pentacoordinate carbon. This can be described in resonance terminology as a contribution flom an enolate-like stmeture to tiie transition state. In MO terminology,.the low-lying LUMO has a... 

The C-NMR chemical shift of the trivalent carbon is a sensitive indicator of carbocation structure. Given below are the data for three carbocations with varying aryl substituents. Generally, the larger the chemical shift, the lower is the electron density at the carbon atom. 

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. 

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