Organic Mechanisms. Xiaoping Sun

Organic Mechanisms - Xiaoping Sun


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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.

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

equation

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

      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.

      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


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