Carboanions with groups that can stabilize negative charges have high stability. These groups can be phenyl, heteroatoms with strong electronegativity, such as O and N, and groups such as-,-C(=O)-, -CN, etc.
Or terminal alkynes (which can also be regarded as electronegative), such as triphenylmethane, cyanomethane, nitromethane and 1, 3- dicarbonyl compounds have strong acidity.
2. Look at the aromaticity of the molecular structure.
Aromatic substances have a closed conjugated system in structure; The atoms participating in the yoke are on the same plane; ; The number of p electrons participating in the * * * yoke conforms to the law of 4n+2. Similarly, the cyclic * * * yoke carbanion with the above three conditions is aromatic and stable.
3. See the * * * yoke effect of molecular structure?
When negative carbon ions are directly connected to carbon-carbon double bonds or benzene rings, negative charges can be dispersed due to the yoke effect. Therefore, the more double bonds (or benzene rings) connected to the carboanion, the more stable the carboanion is.
Extended data
Ordinary carboanion
The negative carbon ion is negatively charged, the central carbon atom is trivalent, and the valence electron shell is full of eight electrons, with a pair of unused electrons. There are two possible configurations of the central carbon atom: one is a mixed plane configuration, and the other is a mixed pyramid configuration. Different carbanions have different configurations because of different groups attached to the central carbon atom, but generally simple hydrocarbyl anions are all hybrid pyramid configurations, and there are no electron pairs in the hybrid orbit.
This is mainly because the hybrid orbital contains more S orbital components than the P orbital, and the increase of orbital components means that the orbital is closer to the nucleus and the orbital energy decreases. The unused electron pair of carbanion is closer to the carbon atom nucleus in hybrid orbit than in P orbit, so the system energy is lower and more stable.
At the same time, in the carboanion system, there is repulsion between the unused electron pair and the other three pairs of bonding electrons. When the unused electron pair is in the hybrid orbit, it is similar to the orbits of the other three pairs of bonding electrons, while when it is in the P orbit, it is perpendicular to the three hybrid orbits. Therefore, in the hybrid pyramid configuration, the repulsion of electron pairs is smaller, which is more favorable.
Therefore, unlike carbocation, the simple hydrocarbon-based carbocation is in a pyramid configuration in the hybrid state, and it is in one of four hybrid orbits without electron pairs, which is the usual reasonable structure of carbocation. ?
Special carboanion
Although cyclopropyl positive ions are unstable because ring tension is not conducive to plane configuration, cyclopropyl negative ions do exist because pyramid configuration is relatively beneficial to carbon negative ions. Among the bridged ring compounds, the bridgehead carbocation is very unstable. Due to the limitation of ring geometry, it is not conducive to the existence of plane configuration, so there are few bridgehead carbocations generated.
However, the pyramid structure is relatively beneficial to the carbon anion at the bridgehead, so the carbon anion at the bridgehead is stable and can exist. Because of this, bridgehead organolithium compounds are easy to generate, for example, the following reaction caused by bridgehead carboanion is very smooth. This also provides further evidence for the pyramid configuration of carbon negative ions.
However, when the negatively charged central carbon atom is connected to a bond or an aromatic ring, the unused electron pair can generate a * * * yoke delocalization with the bond and be stable. At this time, the carbon anion will adopt hybrid plane configuration to achieve maximum orbital overlap and better delocalization, so that the system energy is the lowest and most stable.
References:
Baidu encyclopedia-carbon negative ion