Carbocations tert-butyl carbocation

Ethyl carbocation iso-Propyl carbocation tert-Butyl carbocation... 

The transition state is closer m energy to the carbocation (tert butyl cation) so Its structure more closely resembles the carbocation than it resembles tert butyloxonium ion The transition state has considerable carbocation character meaning that a significant degree of positive charge has developed at carbon... 

As we have just seen the rate determining intermediate m the reaction of tert butyl alco hoi with hydrogen chloride is the carbocation (CH3)3C Convincing evidence from a variety of sources tells us that carbocations can exist but are relatively unstable When carbocations are involved m chemical reactions it is as reactive intermediates formed slowly m one step and consumed rapidly m the next one... 

Carbocations are classified according to their degree of substitution at the positively charged carbon The positive charge is on a primary carbon m CH3CH2" a secondary car bon m (CH3)2CH" and a tertiary carbon m (CH3)3C Ethyl cahon is a primary carbocation isopropyl cation a secondary carbocation and tert butyl cation a tertiary carbocation... 

The major difference between the two mechanisms is the second step The second step m the reaction of tert butyl alcohol with hydrogen chloride is the ummolecular dis sociation of tert butyloxonium ion to tert butyl cation and water Heptyloxonium ion however instead of dissociating to an unstable primary carbocation reacts differently It IS attacked by bromide ion which acts as a nucleophile We can represent the transition state for this step as... 

These common features suggest that carbocations are key intermediates m alcohol dehydra tions just as they are m the reaction of alcohols with hydrogen halides Figure 5 6 portrays a three step mechanism for the acid catalyzed dehydration of tert butyl alcohol Steps 1 and 2 describe the generation of tert butyl cation by a process similar to that which led to its for matron as an intermediate m the reaction of tert butyl alcohol with hydrogen chloride... 

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... 

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... 

Benzylic halides resemble allylic halides m the readiness with which they form carbocations On comparing the rate of S l hydrolysis m aqueous acetone of the fol lowing two tertiary chlorides we find that the benzylic chloride reacts over 600 times faster than does tert butyl chloride... 

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 tert butyl group is cleaved as the corresponding carbocation Loss of a proton from tert butyl cation converts it to 2 methylpropene Because of the ease with which a tert butyl group is cleaved as a carbocation other acidic reagents such as trifluoroacetic acid may also be used... 

Carbinolamines are formed by nucleophilic addition of an amine to a carbonyl group and are intermediates in the for mation of imines and enamines Carbocation (Section 4 8) Positive ion in which the charge re sides on carbon An example is tert butyl cation (CH3)3C Carbocations are unstable species that though they cannot normally be isolated are believed to be intermediates in certain reactions... 

There is direct evidence, from IR and NMR spectra, that the re/T-butyl cation is quantitatively formed when tert-butyl chloride reacts with AICI3 in anhydrous liquid HCl. In the case of alkenes, Markovnikov s rule (p. 984) is followed. Carbocation formation is particularly easy from some reagents, because of the stability of the cations. Triphenyhnethyl chloride and 1-chloroadamantane alkylate activated... 

Replacing an a-alkyl snbstituent by an a-aryl group is expected to stabilize the cationic center by the p-Jt resonance that characterizes the benzyl carbocations. In order to analyze such interaction in detail, the cumyl cation was crystallized with hexafluoroantimonate by Laube et al. (Fig. 13) A simple analysis of cumyl cation suggests the potential contributions of aromatic delocalization (Scheme 7.3), which should be manifested in the X-ray structure in terms of a shortened cationic carbon—aromatic carbon bond distance (C Cat). Similarly, one should also consider the potential role of o-CH hyperconjugation, primarily observable in terms of shortened CH3 distances. Notably, it was found experimentally that the Cai distance is indeed shortened to a value of 1.41 A, which is between those of typical sp -sp single bonds (1.51 A) and sp -sp double bonds (1.32 A). In the meantime, a C -CH3 distance of 1.49 A is longer than that observed in the tert-butyl cation 1 (1.44 A), and very close to the normal value for an sp -sp single bond. 

Figure 6.10 Electrostatic potential maps for (a) tert-butyl (3°), (b) isopropyl (2°), (c) ethyl (1°), and (d) methyl carbocations show the trend from greater to lesser delocalization (stabilization) of the positive charge. (The structures are mapped on the same scale of electrostatic potential to allow direct comparison.)...

Numerous studies suggest that alkyl-aluminumsilyl oxonium ions should be the real intermediates in hydrocarbon reactions over zeolite, whereas carbocations should be just transition states (J). Equilibrium between the alkyl-aluminumsilyl oxonium ion and the carbocation, although suggested in some cases, has never been experimentally or theoretically proven, but recent calculations indicated that the tert-butyl carbenium ion is an intermediate on some specific zeolite structures 6,7). 

ControUed-potential oxidations of a number of primary, secondary, and tertiary alkyl bromides at platinum electrodes in acetonitrile have been investigated [10]. For compounds such as 2-bromopropane, 2-bromobutane, tert-butyl bromide, and neopentyl bromide, a single Ai-alkylacetamide is produced. On the other hand, for 1-bromobutane, 1-bromopentane, 1-bromohexane, 1-bromo-3-methylbutane, and 3-bromohexane, a mixture of amides arises. It was proposed that one electron is removed from each molecule of starting material and that the resulting cation radical (RBr+ ) decomposes to yield a carbocation (R" "). Once formed, the carbocation can react (either directly or after rearrangement) with acetonitrile eventually to form an Al-alkylacetamide, as described above for alkyl iodides. In later work, Becker [11] studied the oxidation of 1-bromoalkanes ranging from methyl to heptyl bromide. He observed that, as the carbon-chain length is increased, the coulombic yield of amides decreases as the number of different amides increases. 

Carbocations are a class of reactive intermediates that have been studied for 100 years, since the colored solution formed when triphenylmethanol was dissolved in sulfuric acid was characterized as containing the triphenylmethyl cation. In the early literature, cations such as Ph3C and the tert-butyl cation were referred to as carbonium ions. Following suggestions of Olah, such cations where the positive carbon has a coordination number of 3 are now termed carbenium ions with carbonium ions reserved for cases such as nonclassical ions where the coordination number is 5 or greater. Carbocation is the generic name for an ion with a positive charge on carbon. 

In the early days of stable ion chemistry, the experimental measurements of parameters such as NMR chemical shifts and IR frequencies were mainly descriptive, with the structures of the carbocations being inferred from such measurements. While in cases such as the tert-butyl cation there could be no doubt of the namre of the intermediate, in many cases, such as the 2-butyl cation and the nonclassical ions, ambiguity existed. A major advance in reliably resolving such uncertainties... 

Table 1.3 provides rate constants for the decay of selected carbocations and oxocar-bocations in H2O, TFE, and HFIP. As a general comment, water, methanol, and ethanol are highly reactive solvents where many carbocations that are written as free cations in standard textbooks have very short lifetimes. The diphenylmethyl cation, with two conjugating phenyl groups, has a lifetime in water of only 1 ns. Cations such as the benzyl cation, simple tertiary alkyl cations such as tert-butyl, and oxocarbocations derived from aldehydes and simple glycosides, if they exist at all, have aqueous lifetimes in the picosecond range, and do not form and react in water as free ions. This topic is discussed in more detail in Chapter 2 in this volume. 

This linear correlation was then assumed to hold for the formation and reaction of aliphatic tertiary carbocations, and values of = 3.5 and 1.6 X 10 s for addition of 50 50 (v/v) water/trifluoroethanol to 5+ and the tert-butyl carbocation, respectively, were estimated from the values of fcobsd for reaction of the corresponding tertiary aliphatic chlorides using Eq. 2. ... 

Finally, two studies have reported on the reactions of carbocations with Mg atoms using mass spectrometry The types of products formed depend on the nature of the carbocation. The labeled methanium ion, CH4D+, reacts via proton transfer (equation 11), deuteron transfer (equation 12) and charge transfer (equation 13). The ethyl cation reacts via charge transfer (equation 14) while the tert-butyl cation reacts via proton transfer (equation 15). In all cases there was no evidence for formation of an organomagnesium species. 

Addition of HBr to 2-methylpropene gives mainly tert-butyl bromide, because the product with the more stable carbocation intermediate always predominates in this type of reaction. 

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. 

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