What is the meaning of the bracket and cross symbol in the mechanism of organic reactions?

Medium parentheses. It means that it cannot be separated, and the double cross is called double dagger to indicate the transition state. Isomerism is prevalent in organic chemistry, so in order to synthesize a target molecule with a specific structure, it is necessary to introduce the concepts of regioselectivity, diastereoselectivity, and enantioselectivity.

The Woodward-Hoffman rule can be used to determine whether the pericyclic reaction can occur, while the Marhalanobis rule and the Chaitseff rule can be used as electrophilic addition reactions and elimination reactions, respectively.

of reference rules. The factors that determine the reactivity of organic reactions are similar to those of other chemical reactions, with special features involving: stability of reactants and products – factors including conjugation, superconjugation, and aromaticity; Generation and stability of active intermediates – e.g. free radicals.

Carboanion and carbocation. <>

In organic chemistry, the bracketed substances indicate intermediate states, and the unequal sign is added to indicate intermediate states that have not been proven to exist. Organic reaction is a chemical reaction involving organic compounds and is the basis of organic synthesis. The basic types of reactions are:

Substitution, addition, elimination, pericyclic reaction, rearrangement reaction, and redox reaction. Early organic reactions, including the combustion of organic fuels, and the saponification reaction used to make soap. Today's organic reactions have become more and more complex, and several of the Nobel Prize-winning reactions in chemistry are:

The Grignard reaction in 1912, the Dils-Alder reaction in 1950, the Vitig reaction in 1979, and the olefin metathesis reaction in 2005. <>

This is a very interesting question, the chemical industry and literature now have many expressions that are not very standardized, which causes us difficulties in understanding and reading, which is a historical reason, because the predecessors started to study from a poor two hundred, so they all follow their own habits to spread falsehoods. Standardization is a systematic project, which is inseparable from the individual, and needs to be paid attention to and corrected by our generation now. The middle brackets circle the synthetic route or the intermediate expounded by the mechanism (which may be stable or unstable, or it may just be an imaginary thing), that is, it is not the product of our goal, but only to show the reader the process of the middle, that is, it does not matter if we can completely omit this intermediate.

The above standard can be dispensable, positive or negative. But there are strict regulations for the International International International Dual Cross, which are limited to the transitional state, that is, one species of the transitional state theory. Experimentally, it is completely inseparable, and only exists in theory.

To put it simply, it must be represented by a dotted line, because its chemical bond is not clear. The positive and negative signs are easy to understand, which indicates the overall charge of the intermediate, whether it is a cation or an anion. Sometimes the parentheses are marked inside and outside.

As for the plus sign, I don't think it makes sense, because the product charge itself in the bracket is not matched, and it stands to reason that there is a negative charge above a to match the precursor neutral. In other words, there should still be no charge in the whole, and it doesn't make sense that the plus sign means a positive charge. I think organic is acid and alkali, nuclear and electricity two pairs of cp to mess around, the charm lies in its mechanism, to see a reaction in the mind of the electron cloud is a beautiful thing.