Electrophilic aromatic substitution furan, thiophene, and pyrrole, like benzene and naphthalene, undergo electrophilic aromatic. Electrophilic aromatic substitution chemistry libretexts. Its application goes way beyond electrophilic aromatic substitution reactions. Aromatic nitro compounds are used as explosives, antibiotics, and synthetic intermediates in the production of dyes, foams, analgesics, antidegradants for rubber, and synthetic fibers. The reactivity and regioselectivity of this category of reactions is significantly impacted by the group that is already attached to the aromatic ring. How does resonance influence the rate of this reaction. Electrophilic substitution is the typical reaction type for aromatic rings.
Take a benzene with a group like och3 attached to it. Kinetic products are always more stable than thermodynamic. Electrondonating and withdrawing groups on polycyclic structures. In electrophilic aromatic substitution, the attacking species electrophile necessarily is a lewis acid the electrophilic bromination or chlorination of benzene requires, in addition to the halogen. Electrophilic aromatic substitution directing groups master. Electrophillic substitution of benzene linkedin slideshare. The second difference is that the br in the electrophilic aromatic substitution reaction replaces the hydrogen while both hydrogens are still there when they are on the alkene. The first three examples have two similar directing groups in a metarelationship to each other. Sn1 leads to phenyl cation which is less stable than a primary carbocation two types of mechanisms that operate in nucleophilic substitutions are, 1. Electrophilic aromatic substitution electrophilic aromatic. In addition to exerting an effect on the speed of reaction, substituents on the benzene ring also influence the regiochemistry of the reaction. Electrophilic aromatic substitution is a multistep process. The halogens are in a class by themselves, being deactivating but ortho.
This organic reaction is typical of aromatic compounds and a very u seful method for adding substit uents. Acidity of pyrrole and indole pyrrole and indole are weak acids. Aromatic compounds are more stable than antiaromatic compounds. The nature of orthopara and meta group directing in electrophilic aromatic substitu tion article pdf available in the journal of chemical physics 14119. In the first step, the addition of an electrophile yields a highenergy carbocation in which the aromatic.
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. Herein lies the difference between aromatic substitution and alkene addition. This is the rate determining step as it destroys the aromaticity of the arene. Partial rate factors relative rate of electrophilic aromatic substitution compared to benzene electron rich aromatic rings are more nucleophlic. Many functional groups can be added to aromatic compounds via electrophilic aromatic substitution reactions. The mechanism of electrophilic aromatic substitution. Fill in the starting materials and reagents needed to obtain the major product shown via electrophilic aromatic substitution. There are five general types of electrophilic aromatic substitution reactions. An electrophilic aromatic substitution consists of three main fundamental components.
Nitric and sulfuric acid react to form the nitronium ion electrophile. Recall that the basis for orthopara and meta directing effects is whether the. We study several common electrophiles how each is generated. 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. That is, they control where the new substituent appears in the product.
The major products of electrophilic substitution, as shown, are the sum of the individual group effects. 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. Oxford university press online resource centre multiple. Deactivating groups decrease the rate of electrophilic aromatic substitution, relative to hydrogen. A functional group is a substituent that brings with it certain chemical reactions that the aromatic compound itself doesnt display the bromination of benzene. This is a part of a detailed tutorial video series. It deactivates benzene ring towards electrophilic substitution reaction. An electrophilic aromatic substitution is a process where one atom or group on an aromatic ring is replaced by an incoming electrophile. While the mechanism undergoes a broken pi bond and addition to the former sp2 carbon atom, the eas reaction is very very different from the alkene addition reactions youve studied back.
Nucleophilic aromatic substitution chemistry libretexts. Choose from 500 different sets of electrophilic aromatic substitution flashcards on quizlet. In examples 4 through 6, oppositely directing groups have an ortho or pararelationship. All activating group donate electrons through inductive effects andor resonance. All electrophilic aromatic substitution reactions share a common mechanism. In an electrophilic aromatic substitution reaction, existing substituent groups on the aromatic ring influence the overall reaction rate or have a directing effect on positional isomer of the products that are formed. In cases of opposing effects prediction is more difficult and mixtures may result. Electrophilic aromatic substitution video khan academy. Such groups are called electron withdrawing groups and are meta directing groups in electrophilic aromatic substitution. For example, the complete mechanism for substitution into the 4 position is. Electron withdrawing groups deactivate the benzene ring to electrophilic aromatic substitution. The origin of this selectivity will be discussed in the next section. A free powerpoint ppt presentation displayed as a flash slide show on id. Directing group effects in electrophilic aromatic substitution aromatic rings containing very strongly activating groups such as oh phenol and nh 2.
Electrophilic substitution involves the attack of a electrophile which is deficit of electrons. Electrophilic aromatic substitution reactions, course notes archive, 1. Learn electrophilic aromatic substitution with free interactive flashcards. The nitronium ion no2 is the reactive electrophile in this reaction. Electrophilic aromatic substitution reactions ucla chemistry. Water functions as a base to remove the proton from the sp 3 c bearing the nitro group and reforms the cc and the aromatic system. Electrophilic aromatic substitution objective the objective of this exercise is to explain the observed effects of orthopara and meta directing substituents of electrophilic aromatic substitution in terms of the partial charge on each aromatic carbon atom.
In this friedelcrafts acylation, we use the acylium ion for electrophilic attack on the aromatic ring. Green iodination of vanillin chem 22l unc charlotte. Abstracs the electrophilic substitution reactions nitration and bromination in benzene and its some monoand dialkylsubstituted derivatives with alkyl groups c1c4 are explored. 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. Electrophilic aromatic substitution eas an aromatic proton is replaced by an.
Sets of substituents are all orthopara directing in electrophilic aromatic substitution reactions. The chloro and bromo groups are orthopara directors. However, even though they contain unshared electron pairs, they are deactivating toward electrophilic aromatic substitution because of the electronegative effect of the halogens. The first difference of benzene being less reactive brings the need for using a lewis acid febr 3 which turns the br 2 into a stronger electrophile and makes the reaction possible. Start by comparing eas to alkene reactions then understand how this differs. An electrophilic aromatic halogenation is a type of electrophilic aromatic substitution. Electrophilic aromatic substitution in polycyclic structures.
Position of electrophilic attack for disubstituted benzenes 1 where two groups reinforce each other, the outcome is obvious. In an electrophilic aromatic substitution reaction, a hydrogen atom is replaced. 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. Halogenation of benzene with br2, cl2 or i2 occurs through the same mechanism.
Followed by detailed tutorial videos on the individual aromatic reactions. The topic of eas or electrophilic aromatic substitution reactions is one that covers a key reaction pathways studied in the average organic chemistry course. Ppt electrophilic aromatic substitution powerpoint. Groups promoting substitution at the orthopara and meta position are called orthopara and meta directing. Substitution reactions of benzene and its derivatives benzene is aromatic. Sets of substituents are all deactivating groups in electrophilic aromatic substitution reactions. Electrophilic aromatic substitution directing groups. The greater acidity of pyrroles and indoles is a consequence of. Substituents already attached to benzene exert an influence on additional atoms or groups attempting to bond to the benzene ring via electrophilic aromatic substitution reactions. Resonance effect of activating and deactivating groups it is also important to note that when an electrophilic aromatic substitution reaction is performed on a monosubstituted benzene ring containing an activating group, the new electrophile will. There are 6 nucleophilic substitution mechanisms encountered with aromatic systems. Substituent directing effect the electrophile attacks the nucleophilic pibond on the benzene ring, but the.
If you substitute a nitro group, no 2, into the benzene ring in methylbenzene, you could possibly get any of the following products the carbon with the methyl group attached is thought of as the number 1 carbon, and the ring is then numbered around from 1 to 6. Electrophilic aromatic substitution is a typical reaction for bhs. Attack of the electrophile on the aromatic ring, creating a resonancestabilized. Thus, the nitro group is a meta directing group because all electrophilic substitution reactions of nitrobenzene occur at the meta position. If the conditions of the reaction are not too acidic, aniline and its derivatives undergo rapid ring substitution. Aromatic amines can undergo electrophilic aromatic substitution reactions on the ring sec. In electrophilic aromatic substitution reactions the hydroxyl group is an o,pdirector because.
Additionally, the aldehyde group adds steric hindrance to its surrounding environment. Electrondonating groups stabilize the carbocation intermediate of electrophilic aromatic substitution. Substituents and electrophilic aromatic substitution eas reaction rates. King chapter 18 electrophilic aromatic substitution i. 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. Substituents stabilize positive charge on substituted carbon. In electrophilic aromatic substitution reactions the nitro group is. What is the increasing rate of electrophilic substitution. Eas electrophilic aromatic substitution reaction mechanism. In an electrophilic aromatic substitution reaction, existing substituent groups on the aromatic ring influence the overall reaction rate or have a directing effect on. Electrophilic aromatic substitution electrophilic aromatic substitution.
Pdf electrophilic aromatic substitution as one of the most fundamental chemical. Although precise reactivity ratios depend on the particular reaction, the relative rates of bromination are typical. The electrophilic nitronium ion reacts with the nucleophilic cc of the arene. Substitution into the 3 position the first step methyl groups direct new groups into the 2 and 4 positions, but a nitro group, no 2, already on the ring directs incoming groups into the 3 position. Back side attack as in sn2 and inversion is precluded by the geometry of the ring 1. Which of the following statements regarding electrophilic aromatic substitution is wrong. Activating groups increase the rate of electrophilic aromatic substitution, relative to hydrogen.
All electronwithdrawing groups are metadirectors, except the halogens. A substituent group is either an ortho, paradirecting group or a metadirecting group in all electrophilic aromatic substitution reactions. A monosubstituted benzene, when treated with an electrophile, could undergo three electrophilic aromatic substitution reactions. Its one thing to learn about electrophilic aromatic substitution reactions of benzene itself. Electrophilic substitution the general equation for this reaction is. Every one of these groups, therefore, makes the ring more electron rich than benzene and thereby makes the ring more susceptible to attack by electrophiles i. Since sigma complex formation is the rate determining step of eas reactions, benzene derivatives are divided into two groups based on how the substituent stabilizes or destabilizes the positively charged sigma complex.
Electrophilic aromatic substitution eas is a substitution reaction usually involving the benzene ring. If the relative yield of the ortho product and that of the. Why is nitro group a meta directing in electrophilic aromatic substitution reaction ans in nitrobenzene, nitro group no2 is attached to the benzene ring. A functional group is a substituent that brings with it certain chemical reactions that the aromatic compound itself doesnt display. Today well describe the two main patterns by which substituents direct electrophilic aromatic substitution. However, the aldehyde functional group is a meta directing deactivating group that will promote electrophilic attraction at the only meta positionthe bottom vertexand restrict substitution at the carbon atoms adjacent to it. Regardless of what electrophile is used, the electrophilic aromatic substitution mechanism can be divided into two main steps. Some of the most important electrophilic aromatic substitutions are aromatic nitration, aromatic halogenation, aromatic sulfonation, and alkylation and alkylating friedelcrafts reaction. Mar 14, 2014 eas tutorial video series breaking down electrophilic aromatic substitution from a logical perspective. Electrophilic aromatic substitution reactions an organic reaction in which an electrophile substitutes a hydrogen atom in an aromatic compound. Chlorine is a meta directing, activating substituent for electrophilic aromatic substitution. Experiment 16 electrophilic aromatic substitution page 4 of 8 figure 6. An atom or group already attached to a benzene ring may direct an incoming electrophile to either the ortho.
1420 1537 132 753 1527 937 762 999 700 1182 383 634 55 1312 498 1157 679 1519 672 322 1028 293 519 542 722 637 178 1274 461 1248 280 1474 1190 892 768 990 632 774 849 1194