ALevel化学 Organic chemistry中比较难理解的就是反应机理（Mechanism）。

　　有机化学中常见的反应像取代反应（substitution），消去反应（elimination），加成反应（addition），酯化反应（esterification），水解反应（hydrolysis），氧化反应（oxidation），还原反应（reduction），缩合反应（condensation）等等，既多且难区分。

　　要想理解反应机理，为大家总结17个必备定义哦！

　　1

　　ImageFunctional group

　　a group of atoms responsible for the characteristic reactions of an organic compound.

　　2

　　ImageHomolytic fission

　　the splitting of a covalent bond where each atom retains one electron from the bonding pair.

　　3

　　ImageHeterolytic fission

　　the splitting of a covalent bond where one atom retains both electrons from the bonding pair.

　　4

　　ImageFree radical

　　an uncharged molecule with an unpaired electron.

　　5

　　ImageInitiation

　　an initial chemical reaction which triggers further reactions.

　　6

　　ImagePropagation

　　a secondary reaction where there is no net gain or loss of free radicals.

　　07

　　ImageTermination

　　the final step in a chain reaction where a reactive intermediate is rendered inactive.

　　8

　　ImageNucleophile

　　a molecule or substance that donates electrons.

　　9

　　ImageElectrophile

　　a molecule or substance that acts as an electron pair acceptor.

　　10

　　ImageAddition

　　a reaction where two or more molecules react to form a larger molecule.

　　11

　　ImageSubstitution

　　a reaction where an atom or group is replaced by another atom or group.

　　12

　　ImageElimination

　　a reaction in which two substituents are removed from a molecule in a mechanism with one or two steps.

　　13

　　ImageHydrolysis

　　the splitting up of a compound or molecule using water.

　　14

　　ImageCondensation

　　the formation of a compound with the release of water.

　　15

　　ImageOxidation

　　the formation of a compound with the release of water.

　　16

      ImageReduction

　　gain of electrons.

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