Cations 1-propyl

Thus reaction of the 1-propyl cation (13) with water (reaction type a) will yield propan-l-ol (14), elimination of a proton from (13) will yield propene (15, reaction type b), while rearrangement of (13, reaction type d)—in this case migration of He—will yield the 2-propyl cation... 

We have already seen one example of this type (p. 107), in which the 1-propyl cation rearranged to the 2-propyl cation by the migration of a hydrogen atom, with its electron pair (i.e. as He), from C2 to the carbocationic Ct, a 1,2-hydride shift ... 

The above analysis also applies to substituted 1-propyl cations which can exist in either of the geometries shown below. 

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 ... 

Some systems show NMR coalescence consistent with a rearrangement to an intermediate cation that is less stable. The NMR of the isopropyl cation 9, for example, shows a coalescence of the two proton signals between 0 and 40 °C. The activation barrier is 16 kcal/mol, close to the difference between a secondary and a primary cation. Calculations show that a comer protonated cyclopropane (11) is the only other minimum on the C3H energy surface. " The 1-propyl cation 10 is a transition state. 

The presence of other hexane isomers and a typical hexane isomer distribution of 26% 2,3-dimethylbutane, 28% 2-methylpentane, 14% 3-methylpentane, 32% n-hexane, far from equilibrium, indicate that the 1-propyl cation (although significantly delocalized with protonated cyclopropane nature) is also involved in alkylation. It yields n-hexane and 2-methylpentane through primary or secondary C—H bond insertion, respectively (Scheme 5.3). 

The major product is isopropylbenzene. Protonation of propene gives the 2-propyl cation, a secondary carbocation, rather than the less stable 1-propyl cation, a primary carbocation. 

Early calculations at the STO-3G level by Pople and coworkers indicated the 1-propyl cation (42) to be lower in energy than edge (47) and corner-protonated cyclopropanes 45 and 46. Higher level 4-3IG calculations reduced both the energy of corner-protonated cyclopropane (46) and 1-propyl cation in conformations 42 and 43. Calculations at the STO-3G, 4-3IG and at the 6-3IG levels favor a conformation of the 1-propyl cation (42) where the C(2)-C(3) cr-bond can hyperconjugate effectively with the vacant p orbital at C(l). This cation at the HF/6-31G level was suggested to convert without activation to corner-protonated cyclopropane (45, 46) on optimization. 

The energy for the 1-propyl cation (42) at the projected MP4 SDQ/6-31G level is 19.7 kcal mol less stable than the 2-propyl cation (49) which is higher than the earlier reported experimental value but an even higher experimental value of 23.5 kcal mol" ... 

The experimental value given for 1-propyl cation is actually that for the ethyl cation. The values should not be much different, for the open propyl cation will receive a small stabilization because of its greater size, but the experimentally studied ethyl cation has a small stabilization from bridging. 

The classical structures of propan-1-yhum ion (1-propyl cation) are not minima on the potential energy surface of the C3Hy manifold. They are some 20kcalmol above 4, as determined computationally. This result is in fair agreement with gas-phase photoionization experiments on the w-propyl radical by Schultz,and Dyke and co-workers.These structures are transition states involved in the hydrogen scrambling of 4 in superacidic media. ° ... 

A radical-based homodesmotic reaction gives a value of 30.4 kcal/mol, which compares with 29.1 kcal/mol for benzene by the same approach. " The gas phase heterolytic bond dissociation energy to form cyclopropenium ion from cyclopropene is 225 kcal/mol. This compares with 256 kcal/mol for formation of the allyl cation from propene and 268 kcal/mol for the 1-propyl cation from propane. It is clear that the cyclopropenyl cation is highly stabilized. 

There is no minimum structure for the 1-propyl cation but one other critical point, the bisected 1-propyl cation 15, is 1.2 kcal mol lower in energy than 13 and is the... 

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