In organic synthesis, various reactions are often accompanied by complex reaction processes, and different products are generated through different reaction mechanisms. Studying the various steps and possible intermediates involved in the conversion of reactants into products can not only explain the reaction mechanism, but also predict the reaction products and possible by-products, and design new synthetic routes.
Carbenium ion is an unstable organic compound with a positive charge. Like free radicals, it is an active intermediate with a positive charge and an outermost layer of 6 electrons.
The classic carbocation is a planar structure. A positively charged carbon atom is in a sp2 hybridized state, where three sp2 hybridized orbitals form a sigma bond with the orbitals of the other three atoms, forming a plane with a bond angle close to 120 °. The remaining p orbitals of the carbon atom are perpendicular to this plane, and there are no electrons in the p orbitals. Analyzing this substance is crucial for discovering the ability to manufacture dozens of modern essential chemical products at a low cost.
According to the position of positively charged carbon atoms, they can be divided into primary carbocations, secondary carbocations, and tertiary carbocations. The structure and stability of carbocations are directly influenced by the functional groups connected to them. The general rule of their stability is as follows:
(2) The other carbocations are: 3 °>2 °>1 °;
However, there is still controversy over the stability comparison between vinyl and benzyl carbocations and secondary carbocations. The stability of different carbocations can be explained by hyperconjugation. There is also controversy over the stability of cyclopropylmethyl carbocation and benzyl carbocation.
Carbon cations can be divided into classical carbon cations and non classical carbon cations based on their structural characteristics.
Carbon negative ions are negatively charged particles with even valence electrons, whose negative charge (unshared electron pairs) is localized on a carbon atom. The methyl anion can be regarded as the parent of all carbon anions, and each carbon anion can be named after an alkyl anion. Carbon negative ions stabilized by electron withdrawing conjugation (- R effect) have the property of being called carbon negative ions due to the fact that negative charges are mainly distributed on oxygen atoms in the actual resonant structure. Solitary pair electrons occupy a hybrid orbital. Compare the acidity of the corresponding acids to determine the stability of negative ions.
Carbon negative ions carry a negative charge, with a trivalent central carbon atom and a valence electron layer filled with eight electrons, possessing a pair of unshared electrons. There are two possible configurations for the central carbon atom: a hybrid planar configuration and a hybrid pyramidal configuration.
Different carbon negative ions have different configurations due to the different functional groups connected to the central carbon atom. However, simple alkyl negative ions generally have a hybrid pyramid configuration and do not share electron pairs in hybrid orbitals. This is mainly because hybrid orbitals contain more S orbital components compared to P orbitals, and an increase in S components in orbitals means that the orbitals are closer to the atomic nucleus, resulting in a decrease in orbital energy. When the unshared electron pairs of carbon negative ions are in hybrid orbitals, compared to being in P orbitals, the unshared electron pairs are closer to the carbon nucleus, resulting in lower system energy and greater stability. Meanwhile, in the carbon negative ion system, there is also a repulsive force between the unshared electron pairs and the other three pairs of bonding electrons. When the unshared electron pairs are in hybrid orbitals, the orbitals they are in are similar to those of the other three pairs of bonding electrons, while when they are in P orbitals, they are perpendicular to the three hybrid orbitals. Therefore, the pyramid configuration in a hybrid state has a smaller repulsive effect on electron pairs and is more advantageous. So unlike carbocations, simple alkyl carbocations are generally in a pyramid configuration in a hybrid state, with unshared electron pairs in one of the four hybrid orbitals, which is the typical rational structure of carbocations.
Free radicals are paramagnetic substances with unpaired electrons that can generate electron spin resonance spectra. Therefore, electron spin resonance spectra (ESR) are generally used to detect free radicals, while spin traps, nuclear magnetic resonance spectra, and free radical inhibitors can also be used to detect free radicals.
The mechanism of free radical reaction includes three steps: initiation, transmission, and termination. The rate of free radical reaction depends on the concentration of free radicals, which mainly depends on the concentration of initiators and is a first-order semi reaction in kinetics. Most organic compounds can undergo homolysis at high temperatures to generate free radicals, and the difficulty of free radical generation depends on their dissociation energy, which is related to the molecular structure:
2. The more branches there are, the more stable the free radicals are and the easier they are to generate. The stability of alkyl radicals is tertiary carbon>secondary carbon>primary carbon.
3. The longer the bond length, the smaller the difference in electronegativity between atoms, and the easier it is to generate free radicals. Single bonds between heteroatoms such as N, O, S, P, etc. generally have lower bond energies.
In summary, based on the structure of the molecule, especially its conjugation and charge dispersion characteristics, the stability and ease of generation of active intermediates can be determined. Similarly, the chemical reactivity of different mechanisms can be determined based on the charge distribution state of active intermediates.
Carbene, also known as carbene or carbene. Generally represented by R2C:, it refers to a highly active intermediate with only two valence bonds attached to the carbon atom and two unbound electrons remaining. It is usually formed by the elimination of a neutral molecule from a molecule containing an easily leaving group. Like carbon radicals, it belongs to neutral and reactive intermediates without positive or negative charges. Carbene is a general term for H2C: and its substituted derivatives. Carbene contains an electrically neutral divalent carbon atom with two unbound electrons on it. It is a highly active intermediate containing two unbound electrons (see active intermediate). The lifespan of carbenes is much less than 1 second and can only be captured at low temperatures (below 77K), separated and observed in the lattice. Its existence has been proven by numerous experiments.
Nitrogen carbene, also known as nitrogen carbene, nitrogen ene, or nitrate, is an electron deficient monovalent nitrogen active intermediate similar to carbon ene. Multiple reactions can occur and are widely present in organic synthesis.
The N atom of nitrogene has 6 valence electrons and only one bond is connected to other atoms or groups. It also has two structures: singlet and triplet. The energy of singlet nitrogen atoms is 154.9 KJ/mol higher than that of triplet nitrogen atoms.
