Carbocations 2-propyl

One example of substituent replacement involves cleavage of a highly branched alkyl group. The alkyl group is expelled as a carbocation, and for this reason, substitution is most common for branched alkyl groups. The nitration of l,4-bis(i-propyl)benzene provides an example ... 

One possible explanation is that adamantyl cation, an intermediate in the reaction, is particularly unstable because it cannot accomodate a planar carbocation center (see Chapter 1, Problem 9). Examine the geometry of adamantyl cation. Does it incorporate a planar carbocation center Compare electrostatic potential maps of adamantyl cation and 2-methyl-2-propyl cation. Which cation better delocalizes the positive charge Assuming that the more delocalized cation is also the more stable cation, would you expect adamantyl tosylate to react slower or faster than tcrf-butyl tosylate Calculate the energy of the reaction. 

Compare electrostatic potential maps for ethyl, 2-propyl, 2-methyl-2-propyl and 2-butyl cations. Does the extent to which positive charge is localized at the carbocation center parallel proton affinity Explain. 

For some tertiary substrates, the rate of SnI reactions is greatly increased by the relief of B strain in the formation of the carbocation (see p. 366). Except where B strain is involved, P branching has little effect on the SnI mechanism, except that carbocations with P branching undergo rearrangements readily. Of course, isobutyl and neopentyl are primary substrates, and for this reason they react very slowly by the SnI mechanism, but not more slowly than the corresponding ethyl or propyl compounds. 

Owing to the involvement of carbocations, Friedel-Crafts alkylations can be accompanied by rearrangement of the alkylating group. For example, isopropyl groups are often introduced when n-propyl reactants are used.33... 

We now consider an example of a non-planar carbocation, i. e., the tetrahedral 1-propyl cation. The two conformers to be considered are the syn and anti conformers shown below ... 

Ethylation, which involves an unstable ethylcarbenium ion as intermediate, is much slower (1500 times over AICI3) than isopropylation. It is also the case in benzene alkylation with propene for the undesired formation of n-propylbenzene, which involves a primary n-propyl carbocation. Furthermore, as alkyl substituents activate the aromatic ring, consecutive alkylation of the primary product occurs with a greater rate than the first alkylation step ( 2 > i)-... 

Tertiary electrophiles alkylate bydroxylamines through the SatI mechanism. These reactions (e.g. equation 10) are practically feasible only for compounds forming highly stabilized carbocations such as trityl , or 2-(p-alkoxyphenyl)propyl. All these reactions proceed exclusively on the nitrogen atom and have been used for A-protection of the amino groups in bydroxylamines. 

A second limitation is the penchant for the alkylating reagent to give rearrangement products. As an example, the alkylation of benzene with 1-chloropropane leads to a mixture of propylbenzene and isopropylbenzene. We may write the reaction as first involving formation of a propyl cation, which is a primary carbocation ... 

With propanamine, loss of nitrogen from the diazonium ion gives the very poorly stabilized propyl cation, which then undergoes a variety of reactions that are consistent with the carbocation reactions discussed previously (see Sections 8-9B and 15-5E) ... 

The modern view of HX addition is that H+ is transferred from HX to the alkene to give a carbocation. The major product is the one derived from the more stable carbocation. Compare the energies of 1-propyl and 2-propyl cations (protonated propene), 2-methyl-1-propyl and 2-methyl-2-propyl cations (protonated 2-methylpropene), and 2-methyl-2-butyl and 3-methyl-2-butyl cations (protonated 2-methyl-2-butene). Identify the more stable cation in each pair. Is the product derived from this cation the same product predicted by Markovnikov s rule Is the more stable carbocation also the one for which the positive charge is more delocalized Compare atomic charges and electrostatic potential maps for one or more pairs of carbocations. 

The photoreactions of saturated five-membered heterocycles are generally characterized by initial carbon-heteroatom bond homolysis. Tetrahydro-furans150 and 1,3-dioxolans151 behave in this way, and the major photoproducts of 2,2-dimethyl-1,3-dioxolan, for example, are acetone, propyl acetate, ethylene, acetaldehyde, methyl acetate, and oxiran. The vinyltetrahy-drofuran (180) is converted on irradiation in methanol to the ketal (181) and the ketone (182) by way of a Wagner-Meerwein shift in the carbocation... 

The solvolysis mechanisms of 2,2-dimethyl-3-pentyl- and l-(l-adamantyl)-propyl sulfonates appeal- to involve partial reversible ionization to the ultimate ion pair followed by competing elimination and solvent separation, substitution products being formed from the separated ions.27 The lifetimes of simple tertiary carbocations may be some 100 times shorter than previously thought several 3-(4-methoxyphenyl)-l,l-dimethylpropyl species hydrolyse in 50% aqueous TFE with rate constants estimated at some 3.5 x 1012 s-1.28 Much elimination was also observed.28 Two studies concerning proposed carbocation intermediates in enzymatic processes are reported.29,30... 

Further calculations39 on secondary and tertiary systems at the MP2/6-31G(d) level showed that for the 2-propyl cation, a /1-silyl group in the bisected orientation provides 22.1 kcalmol-1 of stabilization compared to a /1-hydrogen, as opposed to 6.6 kcalmol-1 provided by a /)-methyl group. For the secondary system, the bridged carbocation was found to have a stabilization of 18.4 kcalmol-1, i.e. less than the open form. 

Halogen as Heteroatom. In 1966 Olah, Cupas, and Comisarow511 reported the first a-fluoromethyl cation. Since then, a large variety of fluorine-substituted carbocations have been prepared. a-Fluorine has a particular ability to stabilize carbocations via back-donation of its unshared electron pairs into the vacant p orbital of the carbocationic carbon atom. 19F NMR spectroscopy is a particularly efficient tool for the structural investigations of these ions.512,513 The 2-fluoro-2-propyl cation 247 (NMR spectra, Figure 3.16) and 1-phenylfluoroethyl cation 248 are representative examples of the many reported similar ions.514... 

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