Bonding carbocations

Hydride-transfer reactions, which formally involve two electrons and three centres, show close analogy with proton transfer reactions which comprise four electrons and three centres. Stable hydrogen-bonded complexes can be formed with the proton symmetrically or unsymmetrically spaced between the hydrogen-bonding atoms. Such structures correspond to the stable hydride bonded carbocations [8] and [9] respectively discovered by Sorensen and coworkers (Kirchen and Sorensen, 1978 Kirchen et al., 1981a) ... 

Carbocations are usually formed either by loss of a leaving group or by addition of an electrophile to a pi bond. Carbocation formation is very rare in nonpolar solvents that cannot stabilize the cation. Polar solvent molecules can cluster their lone pairs around the cationic center, forming an intermolecular donation of electron density that stabilizes the carbocation. Some carbocations are stable enough to be on the pA a chart the more stable ones are the weaker acids. Recall that when the pH equals the pATa. the conjugate acid and base concentrations are equal. A protonated amide with a pATa of -0.5 requires very acidic water before it is half-cation and half-neutral. A protonated ester with a pATa of -6.5 is much less stable. However, since most carbocations are not on the pAfa chart, we need another method to rank carbocation stability. 

See Table 6.6. Hydrogen halide addition to alkenes proceeds by electrophilic attack of the reagent on the tt electrons of the double bond. Carbocations are intermediates. Addition to unsymmetrical alkenes is regioselective. 

Trivalent ( classical carbenium ions contain an sp -hybridized electron-deficient carbon atom, which tends to be planar in the absence of constraining skeletal rigidity or steric interference. The carbenium carbon contains six valence electrons thus it is highly electron deficient. The structure of trivalent carbocations can always be adequately described by using only two-electron two-center bonds (Lewis valence bond structures). CH3 is the parent for trivalent ions. 

The remarkable stability of the gold complexes is due to significant metal-metal bonding. However, their isolation and structural study are remarkable and greatly contributed to our knowledge of higher-coordinate carbocations. 

An alkyl radical is neutral and has one more electron than the corresponding carbocation Thus bonding m methyl radical may be approximated by simply adding an electron to the vacant 2p orbital of sp hybridized carbon m methyl cation (Figure 4 19a) Alternatively we could assume that carbon is sp hybridized and place the unpaired elec tron m an sp orbital (Figure 4 9b)... 

Water may remove a proton from either Cl or C 3 of this carbocation Loss of a proton from C 1 yields the minor product 2 3 dimethyl 1 butene (This alkene has a disubstituted double bond )... 

One possibility is the two step mechanism of Figure 5 12 m which the carbon-halogen bond breaks first to give a carbocation intermediate followed by depro tonation of the carbocation m a second step... 

The alkyl halide m this case 2 bromo 2 methylbutane ionizes to a carbocation and a halide anion by a heterolytic cleavage of the carbon-halogen bond Like the dissoci ation of an aUcyloxonmm ion to a carbocation this step is rate determining Because the rate determining step is ummolecular—it involves only the alkyl halide and not the base—It is a type of El mechanism... 

Primary alcohols do not dehydrate as readily as secondary or tertiary alcohols and their dehydration does not involve a primary carbocation A proton is lost from the (3 carbon m the same step m which carbon-oxygen bond cleavage occurs The mechanism is E2... 

Section 5 15 Dehydrohalogenation of alkyl halides by alkoxide bases is not compli cated by rearrangements because carbocations are not intermediates The mechanism is E2 It is a concerted process m which the base abstracts a proton from the p carbon while the bond between the halogen and the a carbon undergoes heterolytic cleavage... 

In the acid catalyzed dehydration of 2 methyl 1 propanol what carbocation would be formed if a hydride shift accompanied cleavage of the carbon-oxygen bond in the alkyloxonium lon" What ion would be formed as a result of a methyl shift" Which pathway do you think will predominate a hydnde shift or a methyl shift" ... 

We can gam a general understanding of the mechanism of hydrogen halide addi tion to alkenes by extending some of the principles of reaction mechanisms introduced earlier In Section 5 12 we pointed out that carbocations are the conjugate acids of alkenes Therefore strong acids such as HCI HBr and HI can protonate the double bond of an alkene to form a carbocation... 

Both steps m this general mechanism are based on precedent It is called elec trophilic addition because the reaction is triggered by the attack of an acid acting as an electrophile on the rr electrons of the double bond Using the two rr electrons to form a bond to an electrophile generates a carbocation as a reactive intermediate normally this IS the rate determining step... 

Figure 6 6 focuses on the orbitals involved and shows how the rr electrons of the double bond flow in the direction that generates the more stable of the two possible carbocations... 

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... 

Addition begins m the usual way by protonation of the double bond to give m this case a secondary carbocation This carbocation can be captured by chloride to give 2 chloro 3 methylbutane (40%) or it can rearrange by way of a hydride shift to give a tertiary carbocation The tertiary carbocation reacts with chloride ion to give 2 chloro 2 methylbutane (60%)... 

Step 1 Protonation of the carbon-carbon double bond in the direction that leads to the more stable carbocation... 

Carbocation intermediates are not involved m hydroboration-oxidation Hydration of double bonds takes place without rearrangement even m alkenes as highly branched as the following... 

Neither bromine nor ethylene is a polar molecule but both are polarizable and an induced dipole/mduced dipole force causes them to be mutually attracted to each other This induced dipole/mduced dipole attraction sets the stage for Br2 to act as an electrophile Electrons flow from the tt system of ethylene to Br2 causing the weak bromine-bromine bond to break By analogy to the customary mechanisms for electrophilic addition we might represent this as the formation of a carbocation m a bimolecular elementary step... 

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... 

The two dimers of (CH3)2C=CH2 are formed by the mechanism shown m Figure 6 16 In step 1 protonation of the double bond generates a small amount of tert butyl cation m equilibrium with the alkene The carbocation is an electrophile and attacks a second molecule of 2 methylpropene m step 2 forming a new carbon-carbon bond and generating a carbocation This new carbocation loses a proton m step 3 to form a mixture of 2 4 4 tnmethyl 1 pentene and 2 4 4 tnmethyl 2 pentene... 

Step 2 The carbocation acts as an electrophile toward the alkene A carbon-carbon bond is formed resulting in a new carbocation—one that has eight carbons... 

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