Nucleophilic aromatic substitution snar as an approach to. Some of the most important electrophilic aromatic substitutions are aromatic nitration, aromatic halogenation, aromatic sulfonation, and alkylation and alkylating friedelcrafts reaction. Nucleophilic substitution and elimination walden inversion ooh oh ho o s malic acid ad 2. All activating group donate electrons through inductive effects andor resonance.
The two primary types of electrophilic substitution reactions undergone by organic compounds are electrophilic aromatic substitution reactions and electrophilic aliphatic substitution reactions. A substituent x is said to be deactivating if the rate of electrophilic aromatic substitution of the substituted benzene c 6h. Electrophilic aromatic substitution study guide cheat sheet. Electrophilic aromatic substitution mechanism and reaction. However, one disadvantage of this reaction is that the product is more nucleophilic than the reactant. Lecture handouts organic chemistry i chemistry mit. Lets examine some of the characteristics of this mechanism. The mechanism of electrophilic aromatic substitution follows two elementary steps. 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. General mechanism of electrophilic aromatic substitution, halogenation.
Meta substitution means a 1,3 arrangement on a benzene ring. General mechanism of electrophilic aromatic substitution. Apr 20, 2015 ericminikel cambridge, ma chem20 these are my notes from lecture 29 of harvards chemistry 20. Electrophilic aromatic substitution is a typical reaction for bhs. The aromatic comes in because you are going to reform an aromatic ring in your mechanism. Analysis of many variants of nucleophilic aromatic substitution of hydrogen. Nucleophilic aromatic substitution chemistry libretexts.
Examples of intermediates at the extreme of this scale that can be. If a catalyst and heat is used, electrophilic aromatic substitution occurs. This organic reaction is typical of aromatic compounds and a very u seful method for adding substit uents. Electrophilic aromatic substitution reactions are one of the more fun topics that you will study in organic chemistry. When a reaction proceeds this way, it is electrophilic aromatic substitution. Pdf on may 11, 2018, dr sumanta mondal and others published unitii. Finally, benzene has electrons above and below the ring making it nucleophilic and subject to electrophilic attack. 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. Electrophilic aromatic substitution reactions an organic reaction in which an electrophile substitutes a hydrogen atom in an aromatic compound. Understanding the holleman rule in the electrophilic. So thats where that thats where the electrophilic part comes in this. A functional group is a substituent that brings with it certain chemical reactions that the aromatic compound itself doesnt display. Snar mechanism simple aryl halides, are relatively unreactive toward nucleophilic substitution under conditions that would give rapid nucleophilic substitution with alkyl halides.
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. 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. Electrophilic aromatic substitution mechanisms and reactions. Electrophilic aromatic substitution mechanism video. But, as you may suspect, this isnt an electrophilic aromatic substitution. Electrophilic aromatic substitution ucla chemistry. These reactions are examples of nucleophilic aromatic substitution. Electrophilic and nucleophilic aromatic substitution cont.
There are two resonance structures which retain fully conjugated aromatic ring in the case of substitution at position 1. There are 6 nucleophilic substitution mechanisms encountered with aromatic systems. Electrophilic substitution the general equation for this reaction is. Initially, the approaching electrophile interacts with the nucleophilic, aromatic.
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. Nucleophilic aromatic substitution snar as an approach. Second, removal of a proton from that cation restores aromaticity. How does resonance influence the rate of this reaction. But perhaps you find it tricky due to the sheer volume of information that makes up the big picture.
Attack of the electrophile on the aromatic ring, creating a resonancestabilized. This removal of electron density makes the ring a worse nucleophile, therefore the. 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. In this respect electrophilic and nucleophilic aromatic substitution can be. Reactivity in the nucleophilic aromatic substitution. Some of the most important electrophilic aromatic substitutions are aromatic nitration, aromatic halogenation, aromatic sulfonation, and acylation and alkylating friedelcrafts reaction. This reaction is known as electrophilic substitution. Concerted nucleophilic aromatic substitution reactions rohrbach. In nucleophilic aromatic substitution nas, all the trends you learned in electrophilic aromatic substitution operate, but in reverse.
The facile incorporation of medicinally important cf. Birch reduction reaction conditions involve sodium metal, in an alcoholic solution of ammonia. The structure and properties of aromatic systems were discussed in chapter 11. Most of the electrophilic substitution reactions occur in the benzene ring in the presence of an electrophile a positive ion. Nucleophilic aromatic substitution dramatically different conditions when compared with the electrophilic aromatic substitution eas. Notably, double orthosubstitution can be tolerated on the aromatic partner, and this allowed e. Common mechanisms in the 118 series that use this terminology. 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. Electrophilic aromatic substitution chemistry libretexts. 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.
All electrophilic aromatic substitution reactions occur by similar mechanisms. Jun 15, 2016 what is the difference between electrophilic and nucleophilic substitution. What is nucleophilic aromatic substitution and how does it differ from electrophilic aromatic substitution. All electrophilic aromatic substitution reactions share a common mechanism. 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.
Nucleophilic aromatic substitutions have been studied at least since the 1870s. What is the difference between electrophilic and nucleophilic substitution. Pdf on may 11, 2018, dr sumanta mondal and others published. King chapter 18 electrophilic aromatic substitution i. An illustration describing the electrophilic substitution of a hydrogen atom belonging to a benzene molecule with a chlorine atom is provided below. Nitric and sulfuric acid react to form the nitronium ion electrophile. The mechanism of this nucleophilic substitution is interesting since it cannot proceed by the sn2. 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. 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. Many functional groups can be added to aromatic compounds via electrophilic aromatic substitution reactions. Br i, is often found in studies of rates of s n ar reactions of activated aryl halides. Nucleophilic aromatic substitution via additionelimination. Y by an electrophile e such that e becomes bonded to y by the electron pair of the xy bond.
Pdf specific nucleophileelectrophile interactions in nucleophilic. Using the drawing window on the left below, draw the major product from each of the electrophilic aromatic substitution reactions shown below. The facile incorporation of medicinally important cf 3 groups is a potentially useful aspect of the current. An electrophilic aromatic halogenation is a type of electrophilic aromatic substitution. We can picture this in a general way as a heterolytic bond breaking of compound x. Pdf we herein report results obtained from an integrated experimental and theoretical study on aromatic nucleophilic substitution snar.
Like s n2 reactions, nucleophilic aromatic substitution reactions involve nucleophiles and leaving groups, and they also. 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. Reagents that acquire an electron pair in chemical reactions are said to be electrophilic electronloving. Electrophilic and nucleophilic aromatic substitution. Electrophilic substitution reaction mechanism, types. Difference between electrophilic and nucleophilic substitution. Additionelimination s nar groups which favor substitution no 2, cn, co. Electrophilic aromatic substitution video khan academy. Lecture handouts section contains handouts for topics covered in the course. 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. This video provides an overview of both nucleophilic and electrophilic aromatic substitution reactions. A functional group is a substituent that brings with it certain chemical reactions that the aromatic compound itself doesnt display the bromination of benzene.
Certainly, thinking of this as an electrophilic aromatic substitution, youd be right in thinking that the answer to what happens here. Aromaticity and electrophilic aromatic substitution. Bimolecular nucleophilic substitution s n2 of haloalkanes unimolecular nucleophilic substitution s n1 of haloalkanes electrophilic addition to alkenes or alkynes. For product ratios, the two easiest peaks to use are at 4. An electrophilic aromatic substitution consists of three main fundamental components. Video 1 introduction to electrophilic aromatic substitution. Aromaticity nucleophilic aromatic substitution, benzyne. 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. May 12, 2018 it discusses the electrophilic aromatic substitution reactions of 5 membered heterocycles such as pyrrole, furan, and thiophene. 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 eas ucla chemistry. It discusses the electrophilic aromatic substitution reactions of 5 membered heterocycles such as pyrrole, furan, and thiophene. Substrates of nucleophiles are commonly alkyl halides, while aromatic compounds are. 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. Partial rate factors relative rate of electrophilic aromatic substitution compared to benzene electron rich aromatic rings are more nucleophlic.
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. Herein lies the difference between aromatic substitution and alkene addition. Just because a base b has extra electrons, that doesnt necessarily make it a good nucleophile. And so, in addition to my eas tutorial video series ive put together a thorough eas cheat sheet. Mechanism of electrophilic and nucleophilic substitution. Electrophilic substitution involves the attack of a electrophile which is deficit of electrons. What is the increasing rate of electrophilic 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. The rate determining step in an electrophilic aromatic substitution is the formation of a resonance stabilized radical. It also discusses the nucleophilic aromatic substitution reactions.
917 605 873 630 200 524 1033 149 674 314 974 61 1041 183 1063 755 571 169 1040 1368 264 784 391 200 772 217 1491 293 638 269 872 371 957 1521 1524 572 1409 1089 576 1424 1465 1202 122 285 132 822 569 594