3-methyl-2-butyl cation

These results indicate an energy profile for the 3-methyl-2-butyl cation to 2-methyl-2-butyl cation rearrangement in which the open secondary cations are transition states, rather than intermediates, with the secondary cations represented as methyl-bridged species (comer-protonated cyclopropanes) (Fig. 5.10). 

Fig. 5.10. Energy surface for the reanangement of the 3-methyl-2-butyl cation to the 2-methyl-2-butyl cation. [Adapted from D. Farcasiu and S. H. Norton, J. Org. Chem. 62 5374 (1997).]...

Electrostatic potential map for 3-methyl-2-butyl cation shows most positively-charged regions (m blue) and less positively-charged regions (m red). 

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 r-amyl cation (39) exchanges the two types of methyl groups (Ea=13.2 kcal / mol) and, at higher temperatures, the two methylene protons with the nine methyl protons (Ea=18.8 kcal/mol)44 69 70. Most of the former activation energy may be attributed to the formation of the secondary 3-methyl-2-butyl cation (40). The more... 

The same direction of the equilibrium isotope effect was observed in the nondegenerate 1,2-hydride shift equilibrium of 2-(4 -trifluoromethyl-phenyl)-3-methyl-2-butyl cation [142] with one trideuteriomethyl group at C-2 or C-3 respectively (Forsyth and Pan, 1985). The isotope shifts in the spectrum are much smaller (1.3 ppm-1.45 ppm) than in degenerate cations like [141] because Ky is very much in favour of the benzylic cation structure for [142]. 

The properties of tert butyl cation can be understood by focusing on its structure which IS shown m Figure 4 9 With only six valence electrons which are distributed among three coplanar ct bonds the positively charged carbon is sp hybridized The unhybridized 2p orbital that remains on the positively charged carbon contains no elec Irons Its axis is perpendicular to the plane of the bonds connecting that carbon to the three methyl groups... 

FIGURE 4 9 tert Butyl cation (a) The positively charged carbon is sp hybridized Each methyl group IS attached to the positively charged carbon by a cr bond and these three bonds he in the same plane (b) The sp hybridized car bon has an empty 2p orbital the axis of which is perpen dicular to the plane of the carbon atoms... 

Methyl cation Ethyl cation (primary) Isopropyl cation (secondary) tert Butyl cation (tertiary)... 

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... 

TIricooRlinate caibocations are fiequendy called carbonium ions. The terms methyl cation, butyl cation, etc., are used to describe the c >rTesixiiulir.ji tricoordinate cations. Chemical Abstracts uses as specific names methylium, ethyUum, propylium. We will use carbocation as a generic term for trivalent carbon cations. 

The 2-methyl-2-butyl cation provides the opportunity to explore the effect of C—C hyperconjugation. At the 6-31G /MP4 level of calculation, little energy difference is found between structures C and D which differ in alignment of CH3 or H with the empty p orbital. ... 

Draw a Lewis structure (or series of Lewis structures) foi 2-norbornyl cation which adequately describes its geometry, charge distribution and bond density surface, Relate this structure to your description of 3-methyl-1-butyl cation. 

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. 

Step through the sequence of structures depicting rotation about the C i - C+ bond in 2-methyl-2-butyl cation. Plot energy (vertical axis) vs. CCCC dihedral angle (horizontal axis). What is the preferred conformation, with the ethyl group in plane or perpendicular to the plane ... 

In view of the chemical nature of alkylaluminums and methyl halides, complexation is most likely to be rapid and complete, i. e. K is large. Indeed Me3 Al and a variety of Lewis bases were found to complex rapidly2. Initiation, i.e., f-butyl cation attack on monomer, is also rapid since it is an ion molecule reaction which requires very little activation energy. Thus, it appears that Rj t. and hence initiator reactivity are determined by the rate of displacement Ri and ionization R2. 

Neither methyl nor ethyl fluoride gave the corresponding cations when treated with SbFs. At low temperatures, methyl fluoride gave chiefly the methylated sulfur dioxide salt, (CH3OSO) ShF while ethyl fluoride rapidly formed the rert-butyl and ferf-hexyl cations by addition of the initially formed ethyl cation to ethylene molecules also formed ° At room temperature, methyl fluoride also gave the tert-butyl cation. In accord with the stability order, hydride ion is abstracted from alkanes by super acid most readily from tertiary and least readily from primary positions. 

Figure 7.2. X-ray structure of the 2-methyl-2-butyl cation and (CHBnMe5Br5) from reference 19.)...

Benzyl, methyl, and t-butyl esters are rapidly cleaved, but secondary esters react more slowly. In the case of t-butyl esters, the initial silylation is followed by a rapid ionization to the t-butyl cation. 

The tert-butyl cation structure (7) with Cs symmetry is better suited for hyperconjugation than the C3h form and is thus energetically slightly favored.29 The energy surface for methyl-group rotation is however very flat. 

The 2-butyl cation is the smallest secondary cation that can be stabilized either by C-C or C-H hyperconjugation. Experimental results give evidence for two equilibrating isomers.33 MP2/6-311G(d,p) calculations show that the symmetrically hydrido-bridged structure 11 is marginally more stable than the partially methyl-bridged structure 10.34 35... 

The 2-methyl-2-butyl cation (12) is the smallest tertiary carbocation structurally suitable for stabilization through C-C hyperconjugation. 

When -butenes are used, the initiation produces a secondary carbenium ion/butoxide. This species may isomerize via a methyl shift (Reaction (2)) or accept a hydride from isobutane to form the tertiary butyl cation (Reaction (3)). Isobutylene forms the tertiary cation directly. 

This assignment was queried [16-18] and detailed investigations [19a] showed that the principal originator of this spectrum is the 3-(l-methyl, 3-phenylindanyl) cation (I), and that, according to the conditions of the experiment, other ions of the diphenylmethyl type may contribute. The spectrum of the 1-phenylethyl ion has still not been identified but that of the homologous 1,3-diphenyl-n-butyl cation (II) was shown to have only a single important absorption, at 315 pm [19a]. 

This absorption is in fact due to the ions derived from l-methyl-3-phenylindane (the cyclic dimer of styrene) and its higher homologues (oligostyrenes with indanyl end groups). There can be no doubt that the ions formed at the end of the polymerisation of styrene belong to the same families of compounds (indanyl and various phenyl alkyl carbonium ions [7]). Our evidence showed that the 1-phenylethyl cation is absent from the ions formed from styrene by excess of acid its dimeric homologue, the l,3-diphenyl- -butyl cation, is a minor component of the ion mixture. We refer to this mixture of ions formed rapidly from styrene by excess acid, or at the end of a styrene polymerisation, as SD (styrene-derived) ions. 

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