Organic Mechanisms. Xiaoping Sun

the stability of allylic cation (CH₃=CHCH₂⁺) and benzylic cation (PhCH₂⁺) is even higher than a regular tertiary carbocation such as (CH₃)₃C⁺. The stabilization is due to large conjugation effects of the unsaturated groups. In each of the allylic and benzylic cations, the positively charged empty p orbital overlaps in sideways with the π bond of the unsaturated group (conjugation effect), which delocalizes the positive charge to the vinyl or phenyl group and lowers energy of the cation.

Usually, a carbocation is produced by dissociation of a tertiary or secondary haloalkane, which in turn is attacked by a nucleophile resulting in an S_N1 reaction (Reaction 1.63) [1, 6].

(1.63)

      Dissociation of the haloalkane to a carbocation can also be facilitated by a Friedel–Crafts catalyst, such as AlCl₃ [1, 6]:

Another common route for generation of a carbocation is the electrophilic addition of a hydrogen halide to an alkene (Reaction 1.64) [1, 6].

(1.64)

      Once formed, the intermediate carbocation reacts fast with the halide (a nucleophile) to give a haloalkane addition product.

The concentrations of the carbocations in Reactions 1.63 and 1.64 are extremely low, and their existence cannot be detected by common spectroscopic methods. However, in certain conditions, a secondary or a tertiary carbocation can be stabilized and identified experimentally. For example, 2‐chloromethane dissociates in the medium of antimony pentafluoride (SbF₅, a very strong Lewis acid) to give isopropyl cation (CH₃)₂CH⁺ (Reaction 1.65) [1, 7].

(1.65)

      Due to the extremely weak basicity (nucleophilicity) of the (SbClF₅)⁻ anion, the concentration of (CH₃)₂CH⁺ can be enhanced to a sufficient level in SbF₅ so that the carbocation has been identified by ¹H NMR spectroscopy. The NMR spectrum shows a one‐proton (the CH proton) signal at δ = 13.5 ppm, split into septets by six methyl protons. The –CH₃ proton appears at δ = 5.1 ppm. The very low‐field signal of the CH proton (13.5 ppm) is consistent with the strong deshielding effect of the positive charge on the secondary CH carbon [1, 7]. Another NMR‐characterized carbocation is the secondary diphenylmethyl cation Ph₂CH⁺, which is greatly stabilized by two aromatic groups, and this makes the experimental identification of the carbocation practically possible. In contrast to the secondary and tertiary carbocations, primary carbocations (e.g., the ethyl CH₃CH₂⁺ ion) are usually not observable experimentally due to their high instability. This is comparable to the above-mentioned situation for the methyl CH₃⁺ cation.

The neutral haloalkane molecules RX (X = Cl, Br, or I) function as another common type of electrophiles owing to polarity of the carbon–halogen bond. The slightly positively charged carbon in the carbon–halogen bond of a primary or secondary haloalkane can be attacked by a strong nucleophile (usually, an anion) effecting an S_N2 reaction (Reaction 1.66).

(1.66)

A carbonyl group (C=O) functions as an electrophile, with the reactive (electrophilic) center being the slightly positively charged carbon atom as described earlier in Section 1.8.3 (Fig. 1.13). As a result, a ketone (or an aldehyde) can undergo a nucleophilic addition with a strong nucleophile as shown in Reaction 1.67:

(1.67)

A Bronsted (protic) acid (a proton donor) can always accept a pair of electrons from nucleophiles via the proton attached to the acid molecule. Therefore, the Bronsted acid is also a Lewis acid or an electrophile. For example, a hydrogen halide (H–X) molecule functions as an electrophile in Reaction 1.64, with the reactive center being the electrophilic proton. In addition, a protic acid can protonate the carbonyl oxygen, which is nucleophilic because of the lone pairs of electrons (Reaction 1.68):

(1.67)

      Some strong Lewis acids such as AlCl₃, BF₃, and BH₃ (generated by dissociation of B₂H₆) are also strong electrophiles commonly used in many organic reactions. The electrophilicity of these species lies in the active empty p orbitals in the central aluminum and boron atoms. The functions of the compounds will be discussed extensively in Chapters 3 and 5.

      

      1.9.2 Common Nucleophiles

      Many anions that contain a lone pair of electrons (e.g., OH⁻, OR⁻, RCO₂⁻, HS⁻, Br⁻, and CN⁻) are good (strong) nucleophiles. On the other hand, many electrically neutral molecules that contain a lone pair of electrons (e.g., H₂O, ROH, and RCO₂H) act as poor (weak) nucleophiles.

      When we determine the relative strength of nucleophiles, consideration is focused on the reactivity for the nucleophiles toward electrophilic carbon atoms of organic substrates. The general guideline is The greater the electron density does the species have on its reactive center and the larger is the size of the reactive center, the more nucleophilic is the species. On the basis of this guideline, we have the following general rules:

      1 An anion is stronger in nucleophilicity than a neutral molecule given that the reactive (nucleophilic) centers for both the anion and neutral molecule are the atoms of the same element. This is because in general an anion, due to its negative charge, has greater electron density than does a neutral molecule. It can be readily seen by comparison of hydroxide (OH−, a strong nucleophile) and water (H2O, a weak nucleophile) and by comparison of an alkoxide (RO−, a strong nucleophile) and an alcohol (ROH, a weak nucleophile).For both pairs of species, the reactive centers are in oxygen atoms. The above