Organic Mechanisms. Xiaoping Sun

photochemical reactions may become possible via the singly occupied molecular orbitals (SOMOs) (Chapter 4).

1.9 ELECTROPHILES/NUCLEOPHILES VERSUS ACIDS/BASES

      In organic chemistry, any chemical species that function as acceptors of an electron pair (2e) from another species are termed electrophiles. In contrast, any species that function as donors of an electron pair (2e) to an electrophile are termed nucleophiles. In inorganic chemistry, the electron‐pair acceptors (electrophiles) are called Lewis acids. The electron‐pair donors (nucleophiles) are called Lewis bases. Therefore, by the nature, electrophiles and Lewis acids are equivalent terms, and they are used to describe the same type of chemical species. Nucleophiles and Lewis bases are another set of equivalent terms, and they are used to describe another same type of chemical species.

Electrophiles are electron‐deficient species so that in chemical reactions they can accept a pair of electrons from a nucleophile to form a covalent bond. In contrast, nucleophiles are electron‐rich in order to be able to donate a pair of electrons to an electrophile to form a covalent bond (Eq. 1.62) [1, 6].

(1.62)

An electrophile can be a cation (positively charged) or a neutral molecule. In general, in order for a neutral molecule to be an electrophile, the molecule usually should contain a polar covalent bond, with the reactive center (the atom that accepts an electron pair) being partially positively charged. Alternatively, if an atom in a neutral molecule contains an empty orbital (typically, a p orbital), the molecule can also be an electrophile, with the reactive center being on the atom that contains the empty orbital. A nucleophile can be an anion (negatively charged) or a neutral molecule. In general, a nucleophile contains at least one lone‐pair of electrons which is relatively active and able to be donated to an electrophile forming a covalent bond (Eq. 1.62). In some types of molecules, a bonding electron‐pair including σ‐bond and π‐bond electrons can also be donated to an electrophile forming a covalent bond.

      Now let us go over briefly some common types of electrophiles and nucleophiles. In Chapters 6 and 7 on individual types of reactions, we will present more intensive discussions on electrophiles and nucleophiles.

      

      1.9.1 Common Electrophiles

The first type of common electrophiles we will study is carbocations. A carbocation is a species that contains a positively charged trivalent carbon atom possessing a trigonal plannar structure. The simplest carbocation is, in principle, the methyl (CH₃⁺) cation (Fig. 1.16). CH₃⁺ has not been identified experimentally presumably due to its extremely high instability. It is, however, employed as a prototype from which all the carbocations are derived by replacing one or more hydrogen atoms with different groups. The central carbon atom in CH₃⁺ is sp² hybridized, with an empty p orbital holding a positive charge and perpendicular to the trigonal plane defined by the three sp² orbitals. Each C─H bond is formed by the 1s‐sp² overlap. When one hydrogen atom in CH₃⁺ is replaced by an alkyl (R) group, a primary (1°) carbocation RCH₂⁺ is formed. Replacement of the second and third hydrogen atoms with alkyl groups results in a secondary (2°) carbocation R₂CH⁺ and a tertiary (3°) carbocation R₃C⁺, respectively (Fig. 1.16). In general, all types of carbocations are energetic. Thus, they are usually very unstable and highly reactive. Most of them act as strong electrophiles primarily owing to the active empty p orbital in the central carbon atom which has highly strong tendency to accept a pair of electrons from a nucleophile (almost any types of nucleophiles).

The relative stability for different types of carbocations increases in the order of methyl cation CH₃⁺, 1° carbocation RCH₂⁺, 2° carbocation R₂CH⁺, and a 3° carbocation R₃C⁺. This is mainly attributed to the hyperconjugation effect of the C─H bonds in the alkyl groups, which can be well explained using the ethyl CH₃CH₂⁺ cation (a primary carbocation) (Fig. 1.17a). In CH₃CH₂⁺ one of the C─H bonds in the methyl group can overlap in sideway with one lobe of the unhybridized p orbital (hyperconjugation effect) in the primary CH₂ carbon. It results in delocalization of the positive charge into the C─H bond domain. In addition, the positively charged CH₂ carbon (sp² hybridized) attracts electrons from the CH₃ carbon (sp³ hybridized) through the C─C σ‐bond (inductive effect) also leading to delocalization of the positive charge. The charge delocalization lowers energy of the carbocation. The hyperconjugation effect is further demonstrated in Figure 1.17b using a MO model. The interactions (linear combinations) of a low‐energy C─H bond and a high‐energy p orbital lead to the formation of a bonding MO (with the energy level lower than that of the C─H bonding orbital) and an antibonding MO (with the energy level essentially the same as that of the p orbital). The difference in energy between the C─H bond and the bonding MO represents stabilization of the carbocation (decrease in energy) by a methyl group.

FIGURE 1.16 Structure of different types of carbocations.

FIGURE 1.17 (a) Overlap of a C─H bond of the methyl group in the ethyl cation (CH₃CH₂⁺) with one lobe of the empty p orbital in the carbocation (hyperconjugation) and (b) linear combination of the C─H bonding orbital with the empty p orbital giving rise to formation of bonding and antibonding molecular orbitals.

      In (CH₃)₂CH⁺ (a secondary carbocation), two C─H bonds (each from one methyl group) can overlap simultaneously with one lobe of the unhybridized p orbital in the secondary CH carbon. In (CH₃)₃C⁺ (a tertiary carbocation), three C─H bonds (each from one methyl group) can overlap simultaneously with one lobe of the unhybridized p orbital in the tertiary carbon. As a result, the increase in number of the C─H bonds overlapping with the unhybridized p orbital (hyperconjugation effects) makes the positive charge delocalize to greater domains and further lowers the energies of the carbocations. In addition, the inductive effects through the methyl–C⁺ σ bonds are getting more appreciable as the number of methyl groups on the positive carbon increases. This also makes the positive charge delocalize to greater domains and further lowers the energies of the carbocations.

      When unsaturated groups such as vinyl and phenyl are attached to a positively charged carbon, the carbocations