Addition via Carbocation

Recall that a carbocation is a cation with a +1 formal charge on a carbon (the carbocation carbon). The carbocation carbon lacks an octet. This is unfavorable and drives up its potential energy. 

An adjacent electron-donating group will help complete the octet of a carbocation carbon by donating some electron density to its electron-deficient neighbor. 

Memorization Task 8.1 Aikyi groups (e.g., CHs) are more eiectron-donating than H 

In a later activity we -will discuss an argument for why alkyl groups are more electron-donating than hydrogens. 

Is your answer to the previous CTQ consistent with the fact that highest PE/least stable carbocation in Fig. 8.1 is at the top  

---

The second mechanism is the one followed when addition occurs opposite to Markovmkov s rule Unlike electrophilic addition via a carbocation intermediate this alternative mechanism is a chain reaction involving free radical intermediates It is pre sented m Figure 6 7... 

Trimers may be formed through the addition of a tert-butyl carbocation to 2 or via addition of carbocation 1 to isobutylene. The trimer fraction mainly consists of 4-methylene-2,2,6,6-tetramethylheptane and 2,2,4,6,6-pentamethyl-3-heptene. 

According to quantum chemical calculations, additions of carbocations to alkenes are expected to proceed via 7r-complexes [84, 85]. The high antistereoselectivity of carbocationic cyclizations had been explained by this hypothesis [86-88]. 

HBr addition to propene without added light, heat, or peroxides gives 2-bromopropane the H atom is added to the less substituted carbon. This reaction occurs via carbocation intermediates (Section 10.10). 

Hydrocarbons undergo formal addition to vinylcyclopropanes via carbocation intermediates and short-lived organometallic species the addition is facilitated according to the nature of the double bond. Although a carbocation will attack a nucleophilic alkene, the reactive substrates have, almost without exception, contained a 3-cyclopropylprop-2-enone moiety, which undergoes 1,4-addition on treatment with a variety of reagents, including anions... 

The ( )-olefin GPP (3) can then be easily isomerized to the (Z)-configured neryl diphosphate (NPP 4). This isomerization most reasonably proceeds via carbocation formation and diphosphate addition to the tertiary carbon to give the conformationally flexible linalyl diphosphate (LPP, 5) (Scheme 6.2). In addition there are also enzymes known that... 

Olefin hydration catalyzed by acids proceeds via the mechanism of electrophilic addition. Here carbocations play an important role as intermediates (see Section 8.2.4). 

Alkenes add electrophiles readily. Most reactions proceed via carbocations, and initial addition of the electrophile gives rise to the most stable carbocation (Markovnikov rule). The addition of hydrogen halides, and water, proceeds by this mechanism. These reactions are almost invariably regiospecific, but not stereospecific. 

Self-Assessment Exercises 79a. Nucleophilic substitution corresponds to a substitution (either Sxjl or S[.j2) for aliphatic compounds. Electrophilic aromatic substitution is typical for aromatic compounds (an atom is replaced by an electrophile) 79b. An addition reaction is the opposite of an elimination reaction. In an addition reaction, two or more atoms (molecules) combine to form a larger one. 79c. S[,jl reaction involves the formation of carbocation. Sf.j2 reaction, on the other hand, is a one-step process in which bond breaking and bond making occur simultaneously at a carbon atom with a suitable leaving group. 79d. El reactions are unimolecular elimination reactions that proceed via carbocation intermediates. E2 reactions are bimolecular, one-step reactions that require an antiperiplanar conformation at the time of rr-bond formation and /3-bond breaking and do not involve carbocations. 

In this group of intramolecular additions the reaction apparently proceeds via an intermediate carbocation whose presence was established by NMR data. 

It is not difficult to incorporate this result into the general mechanism for hydrogen halide additions. These products are formed as the result of solvent competing with halide ion as the nucleophilic component in the addition. Solvent addition can occur via a concerted mechanism or by capture of a carbocation intermediate. Addition of a halide salt increases the likelihood of capture of a carbocation intermediate by halide ion. The effect of added halide salt can be detected kinetically. For example, the presence of tetramethylammonium... 

Carbocations initially formed upon addition of an electrophile to an alkene may be able to undergo skeletal rearrangement depending on whether or not a more stable cation exists and, if it does exist, whether or not it can be reached via a low-energy pathway. Consider addition of HBr to 3-methyl-1-butene, the product of which is 2-methyl-2-butyl bromide. 

As we saw in the previous section, Markovnikov s rule tells us to place the H on the less substituted carbon, and to place the X on the more substituted carbon. The rule is named after Vladimir Markovnikov, a Russian chemist, who first showed the regiochemical preference of HBr additions to alkenes. When Markovnikov recognized this pattern in the late 19th century, he stated the rule in terms of the placement of the proton (specifically, that the proton will end up on the less substituted carbon atom). Now that we understand the reason for the regiochemical preference (carbocation stability), we can state Markovnikov s rule in a way that more accurately reflects the underlying principle The regiochemistry will be determined by the preference for the reaction to proceed via the more stable carbocation intermediate. 

In order to understand the regiochemistry of HX additions to alkenes, we focused our attention on the intermediate carbocation. We argued that the reaction would proceed via the more stable carbocation. This all-important principle will also help explain why some reactions will involve a rearrangement. For example, consider the following reaction ... 

---