Electrophilic Aromatic Substitution Ucla Chemistry And-Books Pdf

Electrophilic Aromatic Substitution UCLA Chemistry and
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Try writing a mechanism for this reaction using the two steps outlined above Include all. resonance contributors of the arenium ion, This is a bromination reaction which means that we must first generate the electrophile. by adding a strong Lewis acid to the Br2, 1 Generation of the nucleophile. 2 Attack of the elctrophile on the aromatic ring, note There are arenium ion resonance contributors and aromaticity is lost in this step. 3 Deprotonation of the arenium ion by a weak base to regain aromaticity. Directing effects of Elecrophilic Aromatic Substitution. What happens if there is a substituent on the aromatic ring How do we determine which. isomer of the product is formed, Example 14D Thinkbook CFQ 3. 3 Factors that determine where the electrophile attacks in decreasing order of. importance, 1 Resonance, 2 Probability, 3 Steric hinderance.
First we will talk about resonance, There are two types of substituent directors. Note we want to choose the most likely attack which creates the most resonance. stabilized carbocation Furthermore full octets are highly favored over open octets. 1 Ortho Para directors groups that donate electron density to the ring These groups. are referred to as activating groups because they speed up the reaction. a Three types of ortho para directors, i Alkyl groups stabilize the aromatic ring by providing electron. density through inductive effects, ii Pi bonds stabilize the aromatic ring by providing electron density. through resonance, iii Lone pairs stabilize the aromatic ring by providing resonance. b Some examples of ortho para directors, i Alkyl group benzene ring OH OR NH2 NHR NR2 F CL BR I.
Ortho attack, Para attack, 2 Meta directors usually groups that withdraw electron density from the ring. These groups are often referred to as deactivating groups because they slow. down the reaction, a Some examples of meta directors. i NO2 CN NR3 C O CF3 SO3H, Meta attack, Try this problem From 14D Thinkbook PP 3. Determine which products dominate the ortho and para product or the meta product. Solution To determine which products dominate we need to look at the resonance. contributors of each of the arenium ion intermediates. Ortho attack four resonance contributors one contributor with a full octet on all atoms. Para attack four resonance contributors one contributor with a full octet on all atoms. Meta attack three resonance contributors no contributors have a full octet on all. Because the ortho para attack results in more resonance contributors with full octets. than the meta attack the ortho para attack is favored and ortho para positions are. favored for the substituent, Other factors that determine where the electrophile attacks. Next we will talk about probability, Note that there are two locations for ortho and meta attack but only one position for para.
attack Therefore in terms of probability ortho and meta attacks are favored over para. Next we will talk about steric hinderance, Usually steric hinderance doesn t affect where the electrophile attacks When the. electrophile is relatively small However if the electrophile is larger then para attack is. favored over ortho attack to reduce steric hinderance between the substituent and the. electrophilic group, Try this problem From 14D Thinkbook PP 9. Determine whether this reaction has ortho para or meta attack based on arenium ion. stability probability and steric effects, Based on arenium ion stability we see that ortho and para attack are favored because. the arenium ion has three resonance contributors two of the contributors have. secondary carbocations and the other has a tertiary carbocation However meta attack. results in three resonance contributors but all three contributors have secondary. carbocations, Based on probability we see that there are two ortho and meta positions but only one. para position so probability favors attack at the ortho and para positions. Based on steric effects we see that there is steric hinderance of the methyl group. however this is not significant steric hinderance so ortho attack is still favored over. para If the group had been large para attack would have been favored over ortho. Special Case Substituent effects of Halogens, Halogens are generally ortho para directors because they provide a small amount of.
resonance stabilization due to their high electronegativity and small overlap with carbon. p orbitals by elements not in the same period as carbon However halogens are. deactivators because they are electron withdrawing due to the inductive effect they are. highly electronegative, If there is more than one substituent director. Activators dominate deactivators while stronger activators dominate over weaker. activators and weaker deactivators dominate over stronger deactivators. Six Common Electrophilic Aromatic Substitution Reactions. Reminder in electrophilic aromatic substitution reactions a hydrogen on an aromatic. ring is replaced by an electrophile, You should practice these mechanisms and be able to determine the products and. mechanisms when the reactants are given You should also be able to recognize the. missing reactants when given the product, 1 Halogenation replacing hydrogen with a bromine or chlorine Bromonation and. chloronation require a Lewis acid which accepts a pair of electrons to create a. permanent bond dipole of the Br Br bond or the Cl Cl bond This dipole allows. the bromine or chloride to have a formal positive charge and therefore allows the. group to be electrophilic enough to overcome the activation energy caused by the. loss of aromaticity of the benzene ring, a Bromonation see example on page 1 for a bromination reaction. b Chloronation, Try this chloronation problem Problem from 14D Thinkbook PP 12e.
Hint it is very similar to the bromonation example given on page 1. Notice the three general steps of electrophilic aromatic substitution as given on. In this problem the FeCl3 is the Lewis acid which has accepted a pair of. electrons from the Cl Cl bond Note the formal positive charge that results on the. Cl which is bonded to the FeCl3 This formal positive charge increases the. electrophilicity of the group and the electrophile can now attack the aromatic. ring In the final step a proton is lost to restore aromaticity Notice that NO2 is a. meta director, 2 Nitration replacing a hydrogen with a nitro NO2 group Nitration requires the. presence of sulfuric acid H2SO4 as a catalyst Just like we had to take extra. steps to create the electrophile in bromonation and chloronation with the help of. a Lewis acid we must use the sulfuric acid to protonate the nitric acid resulting. in the formation of a nitronium ion The nitronium ion can then proceed as a. general electrophlic aromatic substitution, Try this nitration problem Problem from 14D Thinkbook PP 12a. Hint Use H3O instead of H2SO4 in this problem because mixing H2SO4 with. HNO3 produces several different products including H3O. In this problem we generated the electrophile using the H3O to protonate the nitric. acid Water leaves and the result is a nitronium ion which is the electrophile. required for this mechanism Notice that the nitrogen has a positive charge and. electrophilic attack to the benzene ring occurs Notice that in this reaction the. methyl group is a para director, 3 Sulfonation replacing a hydrogen with a sulfonic acid SO3 Sulfonation is. similar to nitration because in general we create the electrophile by protonating. the SO3 with H2SO4 to make a stronger electrophile The mechanism can then. proceed as an electrophilic aromatic substitution reaction. Try this sulfunation problem Problem from 14D Thinkbook PP 12b. In this problem we used the H2SO4 to protonate the sulfonic acid to generate the. electrophile OSO3 leaves and is used again to remove the proton to restore the. aromatic ring structure Note that sulfonation reactions are easily reversible and. depending on reaction conditions SO3 SO3H or H3SO4 can be used as the. electrophile depending on reaction conditions Note that COCH3 is a meta. 4 Friedel Crafts Acylation replacing a hydrogen with an acyl group RC O In. Friedel Crafts Acylation we form the acylium ion the electrophile in the. reaction by using a lone pair from the chlorine of the H3COCl to fill the open. octet of the alunimum of the AlCl3 As a result the chlorine carbon bond is. weakened and Cl Al Cl3 leaves The acylium ion acts as an electrophile in the. electrophilic aromatic substitution mechanism, Try this problem 14D Thinkbook PP 12d. In this Friedel Crafts Acylation we use the acylium ion for electrophilic attack on. the aromatic ring We use the Cl Al Cl3 to remove the proton to restore the. aromaticity of the aromatic ring Notice that OH is a para director in this. 5 Friedel Crafts Alkylation replacing a hydrogen with an alkyl group R In. Friedel Crafts Alkylation we use the lone pair of the chlorine of the CH3Cl to. fill the open octet of aluminum of the AlCl3 As a result the ClAl Cl3 leaves. The CH3 3C acts as the electrophile in the electrophilic aromatic substitution. Try this problem From Chem 14D Thinkbook PP 9, This mechanism follows the general mechanism for Friedel Crafts Alkylation I.
have discussed the directing effects of the CH3 for this problem on page 5. 6 Diazonization of a Primary Amine In this reaction we react an acid like H3O. with NO2 to form a nitrosonium cation O N which acts as the electrophile. Then we form the N N bond with the electrophilic attack of the nitrosonium. cation to the Ph NH2 Then through a series of protonation and deprotonation. steps by water the proton shuttle we form a diazonium cation In 14D we did a. specific reaction which was diazo coupling, Try this problem Chem 14D Thinkbook PP 12f. This mechanism might look intimidating but it the trickiest part is the formation of the. nitrosonium cation and the formation of the N N bond Through a series of protonation. and deprotonation steps we get a product of PhN N which is used as the electrophile. to the PhOh The rest of the mechanism proceeds as a general electrophilic aromatic. substitution reaction, Note All the above information and example problems are taken from lecture Chemistry. 14D Thinkbook by Steven Hardinger for Winter 2006 Organic Chemistry by Paula. Yurkanis Bruice 4th edition and the Electrophilic Aromatic Substitution chapter of. Electrophilic Aromatic Substitution Electrophilic Aromatic Substitution a reaction in which the hydrogen atom of an This is a bromination reaction

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