Chapter 18 EAS Questions and Answers 100% correct

Chapter 18 EAS Questions and Answers 100% correct Electrophilic Aromatic Substitution -reaction in which an electrophile is substituted for a hydrogen on an aromatic ring -many different types of EAS reactions based on what functional group replaces the hydrogen Alkene vs Benzene Alkenes undergo addition reactions but Benzenes react differently under similar reagents. Don't confuse the two, one is Aromatic and the other isn't. Aromatic Requirements The organic structure must have the following characteristics to be aromatic: -Huckel's number (4N+2 number of pie electrons) -sp2 hybridization for atoms in structure -cyclic structure General Steps of an EAS Reaction 1) Formation of an electrophile with the assistance of a catalyst. In most cases the formation of the Electrophile produces a by-product, which acts as the base for the proton transfer in the final step of EAS. 2) Nucleophilic attack of the now-formed electrophile, which creates temporary loss of aromaticity and an intermediate sigma complex. 3) Proton transfer to reform aromatic compound with the substitution of the group, from a base in the catalyst. sigma complex/arenium ion A resonance-stabilized intermediate ion that forms after the second step of an EAS reaction. Sigma complexes temporarily lose aromaticity until the 3rd step of EAS reactions. EAS: Bromination (Halogenation) 1) Formation of Electrophile: Br2 doesn't polarize near pie electrons as it does with alkenes, but it is still converted into an electrophile with a catalyst: FeBr3 (Iron tribromide). Br2 bonds to iron tribromide, an electrophile is now formed since the bromide on the Br2 gave up a lone pair to form a bond with iron tribromide (FeBr3). 2) Nucleophilic attack: one of the pie bonds attacks the electrophilic bromine on the complex, breaking it off from the rest of the compound. The bromine cation is added to the vinyllic position that will eventually eliminate the hydrogen. A resonance-stabilized sigma complex forms and aromaticity is temporarily lost. 3) The iron tetrabromide proton transfers a hydrogen from where the bromine was added, and that reforms the aromaticity of the structure. The fourth bromine on the iron dissociates along with the hydrogen into ions, or a hydrohalogenic acid, and the iron tribromide reforms allowing it to continue as a catalyst for the reaction. EAS: Chlorination (Halogenation) Same as Bromination of Benzene -catalyst: AlCl3 -reagent: Cl2 -intermediate:sigma complex What About Iodination and Fluorination Fluorination of a Benzene is too violent and Iodination is too slow of a process. EAS: Sulfonation 1) Formation of Electrophile: Fuming sulfuric acid is made up of sulfuric acid and sulfur trioxide (SO3). Sulfur trioxide is electrophilic, even though it has double bonds (due to uneven overlap of p orbitals). Thus there 2) Nucleophilic Attack: a pie bond from the Benzene nucleophilically attacks the sulfur trioxide, forming an intermediate sigma complex. 3) Proton Transfers: H20 from the sulfuric acid solution proton transfers a hydrogen, reforming aromaticity of the molecule. However, the newly formed hydronium is then proton transferred by the oxygen on the sulfonate with a negative formal charge. This removes all of the charges: from the oxygen on the sulfonate and the hydronium. Catalyst: Sulfuric acid Reagent: Sulfur trioxide EAS: Nitration 1) Formation of Electrophile: the reagent, Nitric acid (HNO3), acts as a base in the presence of a relatively stronger acid: sulfuric acid (catalyst). Due to the relatively weaker acidity of nitric acid, it acts as a base and thus an acid-base reaction occurs. Nitric acid accepts a proton, through a proton transfer from the oxygen with the hydrogen attached, and the newly formed water group acts like a leaving group. That forms a nitronium ion (NO2+) which is very electrophilic. That same water molecule will eventually act as a base in the proton transfer of the third step of the EAS reaction. 2) Nucleophilic Attack: a pie bond on the Benzene nucleophilically attack's the nitronium ion, forming a resonance-stabilized intermediate sigma complex. 3) Proton-Transfer: H20 acts as a base and proton transfers the hydrogen at the same carbon as the nitrogen group, reforming the aromaticity of the compound. EAS: Amino Group To substitute an amino group on a Benzene, nitration must first occur. Once a nitro group is on the Benzene, the nitrated Benzene in the presence of a metal (Fe or Zn) and HCl with a catalyst such as NaOH helps successfully reduce the nitro group into an amino group. 1) Nitration 2) Reduction of nitro group into an amino group using a metal and hydrochloride acid as a reagent and sodium hydroxide as the catalyst. Reduction-addition of electrons to atom EAS: Alkylation 1) Formation of Electrophile: The alkyl halide (reagent) bonds to AlCl3 (catalyst). The alkyl group then breaks from the chlorine, forming an electrophilic Carbocation. The AlCl4- will later act as a base for step 3. 2) Nucleophilic Attack: a pie bond on the benzene nucleophilically attacks the Carbocation, forming an intermediate sigma complex that loses aromaticity. 3) Proton Transfer: The AlCl4- acts as a base and proton transfers a hydrogen from the same carbon as the alkyl group, reproducing the aromaticity of the compound. The Cl and proton forms HCl and the AlCl3 returns to its initial condition to continue catalyzing Alkylation of Benzenes. EAS: Friedal-Crafts Alkylation 1) Formation of Electrophile: the halogen on the alkyl halide binds to the aluminum on the aluminum trichlorode (AlCl3). The alkyl group then breaks off and a Carbocation is formed (electrophilic). The AlCl4- will eventually act as a base for the third step of this reaction. 2) Nucleophilic Attack: a pie bond on the Benzene nucleophilically attacks the Carbocation, which adds the alkyl group to the cyclic structure but forces the compound to lose aromaticity. A resonance-stabilized sigma complex is thus formed. 3) Proton Transfer: AlCl4- acts as a base and eliminates hydrogen through proton transfer, restoring aromaticity to the compound. The hydrogen breaks off with the chloride to form HCl, and AlCl3 is restored to its initial form to continue catalyzing the Alkylation of benzenes. Reagent: alkyl halide Catalyst: AlCl3 (aluminum trichloride) Appropriate Types of Alkyl Halides Secondary and tertiary halides will eventually be converted into secondary or tertiary carbocations which we know are stable. However primary halides would be converted into primary carbocations which are highly unstable and thus primary halides wont't work for the reaction. However, ethyl chloride is unique in that due to the fact that it's only two carbons long, it can still work because an electrophile can be formed through its bond to aluminum trichloride. Had the ethyl chloride been longer, rearrangement would occur, which we want to avoid so we don't get a mixture of products. Thus ethyl chloride or any two carbon alkyl group, even if it would form a primary carbocation, can still work because the electrophile would be formed by its bond to the aluminum trichloride (catalyst). Notes on Alkylation of Benzene 1) Alpha position of alkyl halide must be sp3 hybridized. 2) Alkyl halide must be a secondary or tertiary halide, or if it's a primary halide it must be two carbons long. 3) Alkylation of a Benzene activates further Alkylation on the Benzene, thus polyalkylations often occur. 4) Alkylation won't occur on a Benzene with a nitro group as a substituent Acyl vs Aklyl Alkyl-straight carbon chain Acyl-carbonyl group EAS: Friedel-Crafts Acylation 1) Formation of Electrophile: An alcyl chloride binds to the catalyst: aluminum trichloride (AlCl3), next the acyl part breaks off and forms an acylium ion which is electrophilic due to the positive charge on the carbon. The aluminum tetrachloride anion (AlCl4-) will later act as a base for the third step of this reaction. 2) Nucleophilic Attack: a pie bond on the Benzene nucleophilically attacks the acylium ion, losing aromaticity, but adding the acyl group to the Benzene. An intermediate sigma complex is formed. 3) Proton Transfer: AlCl4- Proton transfers a hydrogen, which forms the pie bond needed to restore aromaricity to the compound. The first chloride forms HCl with the proton and AlCl3 is restored to continue catalyzing acylation of benzenes. Reagent: Acyl Chloride Catalyst: Aluminum Trichloride (AlCl3) Acylium Ion The electrophile that's formed after the first step of Acylation of a Benzene. It is resonance stabilized and has two resonance structures. The second resonance is the greater contributor due to greater number of octets. Clemmensen Reduction Taking the product of FC-Acylation, the acyl group on the Benzene can be converted into an alkyl group through Clemmensen Reduction. In the presence of zinc and mercury (ZnHg) as reagents and HCl and heat as catalysts, the product of Acylation can be reduced to an alkyl group. The benefit of going this route rather than Alkylation, is that you avoid any possible carbocation rearrangement that forms a mixture of products. The oxygen double bond to carbon is essentially removed, leaving an alkyl group. Activation of Groups The presence of substituents on Benzenes can affect 1) the rate of EAS 2) regiochemical outcome Donors vs Withdrawers Electron Donating Group -activate or increase rate of EAS reaction -ortho para directors Electron Withdrawing Group -deactivate or decrease the rate of EAS reaction -meta director (except Halogens) Nitration of Toluene Rate of Reaction: toluene activates nitration because the methyl group attached to the benzene can donate electron density and thus the sigma complex would be relatively more stable and thus lower in activation energy. By donate electron density, that essentially means it can help add to resonances within the ring for the sigma complex. Regiochemical Outcome: ortho and para regiochemical outcomes produce a resonance structure with a carbocation below the methyl group, which can easily donate its electrons. This provides those two outcomes with resonance structures that are relatively more stable than a meta regiochemical outcome. Thus Nucleophilic attack, or the second step of nitration, is more likely to occur from an ortho or para position. Nitration of Anisole Rate of Reaction: the methoxy (OCH3) attached to the Benzene conducts both induction and donation of electron density for resonances of the sigma complex. Since oxygen is more electronegative than carbon, induction occurs away from the ring. However the methoxy also adds to the resonance to the sigma complex, and thus donates electrons to the ring. The resonances outweigh the induction, and thus since the methoxy stabilizes the compound, it's relatively more stable and so it activates or speeds of up the relative rate of reaction for nitration, than had it been benzene. Regiochemical Outcome: ortho and para regiochemical outcomes would have one more resonance structure than a meta regiochemical outcome, and thus those two outcomes yield relatively greater stability and thus are the more likely regiochemical outcomes. Regiochemical Outcomes of Activators & Deactivators Any aromatic compound that activates a type of EAS reaction, will prefer an ortho-para regiochemical outcome. One that deactivates a type of EAS reaction, will favor meta regiochemical outcomes. Nitration of Benzene with a Nitro Group Rate of Reaction: Both induction and resonances caused by the nitro group withdraws electron density from the ring, which decrease stability and increases activation energy. Thus a nitro group will always deactivate an EAS reaction. Regiochemical Outcome: Nitro Groups will always favor meta regiochemical outcomes, due to greater stability. Nitration of a Benzene with a Halogen Halogen substituents are the exception to the general rules listed so far: activators are ortho-para directors and deactivators are meta-directors. Rate of Reaction: ROR has been influenced by resonances and induction, and as seen with the nitration of anisole, usually resonance is the dominating factor if you have conflicting influences. However for halogens, which are very electronegative, their induction outweighs their resonance influence. Halogens induct outwards, and thus pull electron density away from the ring, and even though their resonance influence helps bring electron density within the ring of the sigma complex, the net influence is outward pull. Thus halogens are deactivators. Regiochemical Outcome: usually deactivators are meta-directors, however for halogens the ortho and para regiochemical outcomes have one more resonance structure than the meta outcome, and thus those two outcomes are more stable and thus more favorable. This is a unique case where a deactivators is a ortho-para director.