Nucleophilic aromatic substitution via additionelimination. Like s n2 reactions, nucleophilic aromatic substitution reactions involve nucleophiles and leaving groups, and they also. All activating group donate electrons through inductive effects andor resonance. There are 6 nucleophilic substitution mechanisms encountered with aromatic systems. Synthesis of aromatic heterocycles examples of commonly used strategies for heterocycle synthesis carbonheteroatom bond formation and choice of oxid ation state pyridines general properties, electronic structure electrophilic substitution of pyridines nucleophilic substitution of pyridines metallation of pyridines. Pdf specific nucleophileelectrophile interactions in nucleophilic. When a reaction proceeds this way, it is electrophilic aromatic substitution. Electrophilic aromatic substitution reactions of benzene. All electrophilic aromatic substitution reactions occur by similar mechanisms. The arene system contains an electron rich cc system which react with electrophiles via a substitution pathway to preserve aromaticity via what is known as electrophilic aromatic substitution ears. Nitric and sulfuric acid react to form the nitronium ion electrophile. In nucleophilic aromatic substitution, the formation of the addition intermediate is usually the ratedetermining step so the ease of cx bond breaking does not affect the rate. What is the increasing rate of electrophilic substitution.
A functional group is a substituent that brings with it certain chemical reactions that the aromatic compound itself doesnt display. Certainly, thinking of this as an electrophilic aromatic substitution, youd be right in thinking that the answer to what happens here. Electrophilic aromatic substitution reactions are one of the more fun topics that you will study in organic chemistry. This removal of electron density makes the ring a worse nucleophile, therefore the. Some of the most important electrophilic aromatic substitutions are aromatic nitration, aromatic halogenation, aromatic sulfonation, and alkylation and alkylating friedelcrafts reaction.
A nucleophilic aromatic substitution is a substitution reaction in organic chemistry in which the nucleophile displaces a good leaving group, such as a halide, on an aromatic ring. An electrophilic aromatic substitution consists of three main fundamental components. Electrophilic aromatic substitution is a typical reaction for bhs. A nucleophilic aromatic substitution reaction is a reaction in which one of the substituents in an aromatic ring is replaced by a nucleophile a meisenheimer complex is a negatively charged intermediate formed by the attack of a nucleophile upon one of the aromaticring carbons during the course of a nucleophilic aromatic substitution reaction. A nucleophilic aromatic substitution reaction is a reaction in which one of the substituents in an aromatic ring is replaced by a nucleophile a meisenheimer complex is a negatively charged intermediate formed by the attack of a nucleophile upon one of the aromatic ring carbons during the course of a nucleophilic aromatic substitution reaction. Y by an electrophile e such that e becomes bonded to y by the electron pair of the xy bond. We can picture this in a general way as a heterolytic bond breaking of compound x.
Bromine itself is not electrophilic enough to react with benzene. Partial rate factors relative rate of electrophilic aromatic substitution compared to benzene electron rich aromatic rings are more nucleophlic. How does resonance influence the rate of this reaction. Birch reduction reaction conditions involve sodium metal, in an alcoholic solution of ammonia. A substituent x is said to be deactivating if the rate of electrophilic aromatic substitution of the substituted benzene c 6h. Electrophilic aromatic substitution eas ucla chemistry. However, one disadvantage of this reaction is that the product is more nucleophilic than the reactant. The rate determining step in an electrophilic aromatic substitution is the formation of a resonance stabilized radical. An illustration describing the electrophilic substitution of a hydrogen atom belonging to a benzene molecule with a chlorine atom is provided below. Attack of the electrophile on the aromatic ring, creating a resonancestabilized. Evidence is provided to support a mechanism for cu1 catalysed aromatic nucleophilic substitution via innersphere electrontransfer and a cu111 intermediate, and to show the synthetic. This reaction is known as electrophilic substitution. The mechanism of this nucleophilic substitution is interesting since it cannot proceed by the sn2. General mechanism of electrophilic aromatic substitution, halogenation.
Just because a base b has extra electrons, that doesnt necessarily make it a good nucleophile. Mechanism of electrophilic and nucleophilic substitution. Most of the electrophilic substitution reactions occur in the benzene ring in the presence of an electrophile a positive ion. Jun 15, 2016 what is the difference between electrophilic and nucleophilic substitution.
Substrates of nucleophiles are commonly alkyl halides, while aromatic compounds are. Leaving group is necessary electron deficient aromatic rings react fastest deactivated toward eas strong base is used as the nucleophile this can be thought of as an additionelimination reaction no2. King chapter 18 electrophilic aromatic substitution i. Nucleophilic aromatic substitution snar as an approach. Concerted nucleophilic aromatic substitution reactions rohrbach. The electrophilic aromatic substitution reaction mechanism involves an initial loss of aromaticity. The aromatic comes in because you are going to reform an aromatic ring in your mechanism. A functional group is a substituent that brings with it certain chemical reactions that the aromatic compound itself doesnt display the bromination of benzene.
Understanding the holleman rule in the electrophilic. Electrondonating groups stabilize the carbocation intermediate of electrophilic aromatic substitution. Electrophilic and nucleophilic aromatic substitution. Using the drawing window on the left below, draw the major product from each of the electrophilic aromatic substitution reactions shown below. General mechanism of electrophilic aromatic substitution. Electrophilic substitution reaction mechanism, types. But, as you may suspect, this isnt an electrophilic aromatic substitution. Many functional groups can be added to aromatic compounds via electrophilic aromatic substitution reactions. The facile incorporation of medicinally important cf. Nucleophilic aromatic substitution dramatically different conditions when compared with the electrophilic aromatic substitution eas. Electrophilic aromatic substitution mechanisms and reactions. Electrophilic aromatic substitution reactions an organic reaction in which an electrophile substitutes a hydrogen atom in an aromatic compound. Additionelimination s nar groups which favor substitution no 2, cn, co. What is nucleophilic aromatic substitution and how does it differ from electrophilic aromatic substitution.
Video 1 introduction to electrophilic aromatic substitution. For product ratios, the two easiest peaks to use are at 4. It also discusses the nucleophilic aromatic substitution reactions. What is the difference between electrophilic and nucleophilic substitution. Analysis of many variants of nucleophilic aromatic substitution of hydrogen proceeding according to an additionelimination pattern reveals that this is the major reaction pathway, whereas nucleophilic replacement of halogen or another nucleofugal group is the secondary process, i. All electrophilic aromatic substitution reactions share a common mechanism. Nucleophilic aromatic substitution snar as an approach to.
The facile incorporation of medicinally important cf 3 groups is a potentially useful aspect of the current. Electrophilic aromatic substitution mechanism video. Br i, is often found in studies of rates of s n ar reactions of activated aryl halides. Aromaticity and electrophilic aromatic substitution. If a catalyst and heat is used, electrophilic aromatic substitution occurs. A substituent x is said to be activating if the rate of electrophilic aromatic substitution of the substituted benzene c 6h 5x is faster than benzene. The two primary types of electrophilic substitution reactions undergone by organic compounds are electrophilic aromatic substitution reactions and electrophilic aliphatic substitution reactions. May 12, 2018 it discusses the electrophilic aromatic substitution reactions of 5 membered heterocycles such as pyrrole, furan, and thiophene. Nucleophilic aromatic substitutions have been studied at least since the 1870s. Snar mechanism simple aryl halides, are relatively unreactive toward nucleophilic substitution under conditions that would give rapid nucleophilic substitution with alkyl halides. Some of the most important electrophilic aromatic substitutions are aromatic nitration, aromatic halogenation, aromatic sulfonation, and acylation and alkylating friedelcrafts reaction. Nucleophilic aromatic substitution chemistry libretexts. These reactions are examples of nucleophilic aromatic substitution.
Pdf on may 11, 2018, dr sumanta mondal and others published. In this respect electrophilic and nucleophilic aromatic substitution can be. Aromatic nitro compounds are used as explosives, antibiotics, and synthetic intermediates in the production of dyes, foams, analgesics, antidegradants for rubber, and synthetic fibers. Electrophilic aromatic substitution the most characteristic reaction of aromatic compounds is electrophilic aromatic substitution, in which one of the ring hydrogens is substituted by a halogen, nitro group, sulfonic acid group, alkyl or acyl group. The mechanism of electrophilic aromatic substitution follows two elementary steps. Initially, the approaching electrophile interacts with the nucleophilic, aromatic. But perhaps you find it tricky due to the sheer volume of information that makes up the big picture. Reagents that acquire an electron pair in chemical reactions are said to be electrophilic electronloving. Electrophilic aromatic substitution mechanism and reaction. The reactivities of aryl halides, such as the halobenzenes, are exceedingly low toward nucleophilic reagents that normally effect displacements with alkyl halides and activated aryl halides. Apr 20, 2015 ericminikel cambridge, ma chem20 these are my notes from lecture 29 of harvards chemistry 20. Lecture handouts organic chemistry i chemistry mit. Finally, benzene has electrons above and below the ring making it nucleophilic and subject to electrophilic attack.
The structure and properties of aromatic systems were discussed in chapter 11. And so, in addition to my eas tutorial video series ive put together a thorough eas cheat sheet. The eas intro video below gives you a detailed overview of the eas reaction, along with a comparison to alkene addition reactions and the need for a superelectrophile. Herein lies the difference between aromatic substitution and alkene addition. Electrophilic and nucleophilic aromatic substitution cont. This video provides an overview of both nucleophilic and electrophilic aromatic substitution reactions. Meta substitution means a 1,3 arrangement on a benzene ring.
Second, removal of a proton from that cation restores aromaticity. So thats where that thats where the electrophilic part comes in this. Bimolecular nucleophilic substitution s n2 of haloalkanes unimolecular nucleophilic substitution s n1 of haloalkanes electrophilic addition to alkenes or alkynes. It discusses the electrophilic aromatic substitution reactions of 5 membered heterocycles such as pyrrole, furan, and thiophene. Electrophilic aromatic substitution is an organic reaction in which an atom that is attached to an aromatic system usually hydrogen is replaced by an electrophile. Concerted nucleophilic aromatic substitution with 19f and 18f. Examples of intermediates at the extreme of this scale that can be. Difference between electrophilic and nucleophilic substitution. An electrophilic aromatic halogenation is a type of electrophilic aromatic substitution.
Electrophilic aromatic substitution chemistry libretexts. Electrophilic substitution involves the attack of a electrophile which is deficit of electrons. Note that in this reaction and in others that follow, only one of the six benzene hydrogens is shown explicitly to emphasize that one hydrogen is lost in the reaction. In the mo treatment, some indices such as free valence 40, localization energy 41, and other quantities 42,43 have been introduced to predict the orientation of electrophilic aromatic substitution. The first trend to understand is that electron withdrawing groups ewgs dramatically increase the rate of reaction, not decrease it. The nucleophilic aromatic substitution s n ar reactions of activated substrates follow a twostep, additionelimination mechanism and have been well studied. Reactivity in the nucleophilic aromatic substitution. This organic reaction is typical of aromatic compounds and a very u seful method for adding substit uents. Lecture handouts section contains handouts for topics covered in the course. There are two resonance structures which retain fully conjugated aromatic ring in the case of substitution at position 1. Pdf on may 11, 2018, dr sumanta mondal and others published unitii. In nucleophilic aromatic substitution nas, all the trends you learned in electrophilic aromatic substitution operate, but in reverse.
Lets examine some of the characteristics of this mechanism. Aromaticity nucleophilic aromatic substitution, benzyne. Electrophilic aromatic substitution ucla chemistry. Electrophilic substitution the general equation for this reaction is. Pdf we herein report results obtained from an integrated experimental and theoretical study on aromatic nucleophilic substitution snar. Notably, double orthosubstitution can be tolerated on the aromatic partner, and this allowed e. Common mechanisms in the 118 series that use this terminology. Electrophilic aromatic substitution study guide cheat sheet. Nucleophilic aromatic substitution for hydrogen reduction in chemical waste generation elimination of 74% of organic waste 99% of inorganic waste eliminates use of chlorine reduction in waste water more than 97% savings eliminates use of xylene a sara chemical improves process safety lower reaction temperatures.
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