Electrophilic addition reaction(EA), referred to as electrophilic addition, isElectrophilic reagent(a positively charged group) an addition reaction caused by attacking an unsaturated bond.In the reaction, the unsaturated bond (double bond or triple bond) is opened andsubstrateTwo new σ bonds are formed.The most common in electrophilic additionUnsaturated compoundThey are alkenes and alkynes.
As for the concept in organic chemistry, if the attack reagent has a strong tendency to obtain electrons, such as H in halogen and hydrogen chloride+In addition reaction with alkene and alkyne, the electrophilic part (H+、X+）It attacks the heavy bonds of multi electron alkenes and alkynes, called electrophilic addition.Unsaturated hydrocarbon is subject toElectrophilic reagentAfter the attack, the π bond breaks, and two parts of the reagent are added to the carbon atoms at both ends of the heavy bond.The decisive step of the reaction is the addition reaction caused by the attack of electrophilic reagent, so it is calledElectrophilic addition reaction, the most common in electrophilic additionUnsaturated compoundThey are alkenes and alkynes.
When the electrophile attacks the reaction center, the positive part of the reagent is more active, and it is always added to the atoms with large electron cloud density in the reaction center, that is, the double bond carbon with large electron cloud density.Common electrophilic reagents include halogen (Cltwo、Brtwo）, inorganic acid (HtwoSOfour、HCl、HBr、Hl、HOCl、HOBr），Organic acid (FthreeC—COOH、CIthreeC-COOH), etc.
chemical reaction
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The electrophilic addition reaction is an addition reaction caused by the attack of unsaturated bonds by electrophilic reagents (positively charged groups).In the reaction, the unsaturated bond (double bond or triple bond) opens and forms two new σ bonds with another substrate.The reaction is carried out in two steps:
2、NucleophileIt is combined with the carbocationic intermediate to form an addition product.
reaction mechanism
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There are many mechanisms of electrophilic addition, including carbocation mechanism, ion pair mechanism, cyclic onium ion mechanism and three center transition state mechanism.
Which mechanism is adopted for the reactionUnsaturated compoundThe nature of bromine, the polarity of solvent and the stability of transition state are all closely related. Generally speaking, in halogen addition reaction, the addition reaction of bromine and olefin is mainly carried out according to the mechanism of cyclic onium ion intermediate, while the addition reaction of chlorine and olefin is mainly carried out according to the first two mechanisms (sometimes through cyclic chlorine atoms).This is mainly because the electronegativity and atomic radius of the two halogen atoms are different, and the lone electron pair of bromine is easy to be compared with the carbon positive ionpOrbits overlap, causing electron cloud delocalization, while chlorine does not.
Electrophilic addition of ethylene and bromine (cyclic onium ion mechanism)
Different mechanisms can also produce products with different stereoselectivity.The mixture of cis addition and trans addition products is obtained by the carbocation mechanism, the cis addition products are obtained by the ion pair mechanism, and the trans addition products are obtained by the cyclic onium ion mechanism.
For asymmetricElectrophilic addition reactionIn general, the reaction conforms to the Markov rule, and the product is regioselective.However, the double bond carbon is attached with an electron withdrawing group orOrganoboron compounddoElectrophilic reagentThe product is anti Mahalanobis rule, for example, the reaction of olefin and ethane to form alkyl boron.
The main types of electrophilic addition reactions are halogen addition reaction, hydrogenation reaction, hydration reaction, hydrogenation reactionHydroxymercury reaction, borohydride oxidation reactionPrins reaction And addition reaction with sulfuric acid, hypohalogen acid, organic acid, alcohol and phenol;For alkynes, there are mainly halogen addition reaction, hydrogen halide reaction and hydration reaction.becausespElectronegativity ratio of carbon atomsptwoCarbon atoms have strong electronegativity and are more closely bound to electrons, so the electrophilic addition reaction of alkynes is generally slower than that of alkenes.
The π bond electron cloud in the olefin molecule is distributed above and below the double bond, and the electron cloud is exposed outside. It belongs to the electron rich system, has nucleophilic property, and is vulnerable to the attack of electron deficient reagents (that is, electrophilic reagent).The addition reaction triggered by the attack of electrophilic reagent is called electrophilic addition reaction.
The π bond with nucleophilic effect is different fromElectrophilic reagentThe reaction between.Common can occur with olefinsElectrophilic addition reactionThe electrophilic reagents of are halogen, hydrogen halide, water, sulfuric acid, etc.
Alkenes are prone to addition reaction with halogens to generate corresponding ortho dihalides.Olefin and brominatedcarbon tetrachlorideAfter the solution is mixed, reaction occurs, and the reddish brown color of bromine can be faded, which is obvious.The laboratory can use this reaction to identify alkenes.
Electrophilic addition reaction of ethylene with bromine
In addition reaction with halogen, the addition reaction of fluorine with olefin is too intense, which will lead to the fracture of carbon chain.Iodine is difficult to react with alkenes, so the reaction of alkenes with halogens usually refers to the addition of chlorine or bromine.The addition activity sequence of halogen and olefin is Ftwo＞Cltwo＞Brtwo＞Itwo。When ethylene reacts with bromine in sodium chloride aqueous solution, in addition to the main product 1,2-dibromoethane, 1-chloro-2-bromoethane and2-Bromoethanol。
Electrophilic addition reaction of ethylene and bromine in sodium chloride aqueous solution
Since sodium chloride or water does not add alkenes under this experimental condition, the results show that the addition of alkenes and bromine is carried out step by step, and the positive ion intermediate is generated first in the reaction process.When the bromine molecule is close to the olefin molecule, the π electron in the olefin induces the polarization of the electron cloud of the bromine molecule, and one of the bromine atoms is partially positively charged (δ+）, another bromine atom is partially negatively charged (δ-）。Bromine molecule after polarization is Brδ+Attack alkenes and combine with carbon carbon double bonds to form cyclic bromonium ion and bromine anion.In this process, the breaking of π bond of olefin molecule and the breaking of σ bond of bromine molecule require energy, and the rate is slow, which is the rate determining step of the whole reaction.[2]
Addition to water
Generally, water cannot directly react with olefins, but olefins can react with water to form alcohols under the catalysis of acids (such as phosphoric acid).
The first step is also to attack the double bond carbon with protons to generate carbonium ions, then combine with water to generate salts, and finally remove the protons to generate alcohols. The products followMarkov rule。The whole reaction process is equivalent to adding a molecule of water to the double bond, which is also called electrophilic hydration reaction.
Acid catalyzed electrophilic addition reaction of olefins with water
In industry, it is called the direct hydration of olefins, which is one of the most important methods to prepare ethanol and isopropanol in industry.
For example, ethylene and water are hydrated to produce ethanol at 300 ℃ and 7MPa under the catalysis of phosphoric acid:
Electrophilic addition reaction of ethylene with water catalyzed by phosphoric acid
Due to the development of petroleum industry, the sources of ethylene, propylene, etc. are abundant, and this method is relatively simple. Ethanol and isopropanol can be produced in large scale by this method.
