Propyl carbocation

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)-... 

The isopropyl benzene results from a rearrangement of the initially formed propyl carbocation to the more stable isopropyl carbocation. 

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

Reaction of 1-chloropropane with the Lewis acid AICI3 forms a carbocation. The less stable propyl carbocation undergoes a hydride shift to produce the more stable isopropyl carbocation, which reacts with benzene to give isopropylbenzene. 

Thus, the order of the stability of the benzyl, allyl and n-propyl carbocations is as follows ... 

A carbocation may lose a proton to form an alkene. For example, 1-propyl carbocation generated from diazonium salt may lose a proton (H+) to form an alkene (propene). Alternatively, 1-propyl carbocation may rearrange to more stable secondary carbocation, which may also lose a proton to give propene (Scheme 2.1). 

Transient Three-membered Ring Compounds.—The existence of stable protonated alkylcyclopropanes as reactive intermediates has been discussed in detail. The collisional activation spectra of the gaseous ions formed upon protonation of pro-pene and cyclopropane are identical and the low activation energy for the conversion of protonated cyclopropane into the 2-propyl carbocation thus indicated is inconsistent with MINDO/3 calculations. Protonated cyclopropanes have been implicated in the reduction of (225) in the presence of strong acids and in the decomposition of cyclopropane on Zeolite HY. The involvement of the 3-methyl function in the deamination of 3-methyl-2-aminobutane has been evident for some time. However, studies with optically active and labelled substrates have now confirmed... 

Butyl carbocation Methyl 2-Propyl carbocation Ethyl... 

The reason for this is simple. Since we are forming a complex with carbocationic character, it is possible for a carbocation rearrangement to occur. It is not possible for a methyl carbocation to rearrange. Similarly, an ethyl carbocation cannot rearrange to become any more stable. But a propyl carbocation CAN rearrange (via a hydride shift) ... 

And since we are forming a propyl carbocation, we can expect that sometimes it will rearrange before reacting with benzene (while other times, it will not get a chance to rearrange before it reacts with benzene). And that is why we observe a mixture of products. So you need to be careful when using a Friedel-Crafts alkylation to look out for unwanted carbocation rearrangements. 

The reaction of propene with HBr is shown below. In this case, two different carbocations are possible If the hydrogen atom had bonded to C-2, a primary, -propyl carbocation would have formed if the hydrogen bonds to C-3 of propene, a secondary, isopropyl carbocation forms. Since a secondary carbocation is more stable than a primary carbocation, only 2-bromoproane forms. 

The isopropyl carbocation has the positive charge on a secondary carbon atom. The -propyl carbocation has the positive charge on a primary carbon atom. The isopropyl carbocation forms, rather than the -propyl cation, because the larger number of alkyl groups attached to a positively charged carbon atom help to stabilize the charge. 

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 ... 

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) ... 

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