EXPERIMENT 3 Organic Chemistry II Pahlavan Cherif Nitration of Aromatic Compounds Preparation of methyl m nitrobenzoate Purpose a Study electrophilic aromatic substitution reaction EAS b Study regioselectivity for EAS reactions Chemicals Materials 150 mL beaker 400 mL beaker 125 mL flask Stirring rod Mel temp Suction filtration funnel Methyl benzoate Sulfuric acid conc Nitric acid conc Ice Methanol Introduction Unlike nucleophilic substitutions which proceed via several different mechanisms electrophilic aromatic substitutions EAS generally occur via the same process Because of the high electron density of the aromatic ring during EAS reactions electrophiles are attracted to the ring s system and protons serve as the leaving groups Equation 1 During SN1 reactions however nucleophiles attack an aliphatic carbon and weak Lewis bases serve as leaving groups Ar H arene E electrophile Ar E substituted arene 1 H Eq 1 Generally EAS reactions occur in three steps Scheme I During Step I the electrophile is produced Scheme I 2 Usually by the interaction of a compound containing the potential electrophile and a catalyst During Step II the aromatic system donates an electron pair to the electrophile forming a bond an arenium cation followed by deprotonation in step III in the present of a base HSO4 affording the substituted arene EAS reactions are generally second order processes i e first order in electrophile and first order in arene Thus Step II is the rate determining step rds rate k2 arene electrophile Electrophilic Aromatic Substitution Nitration of Methyl Benzoate Benzene rings are components of many important natural products and other useful organic compounds Therefore the ability to put substituents on a benzene ring at specific positions relative to each other is a very important factor in synthesizing many organic compounds The two main reaction types used for this are both substitutions Electrophilic Aromatic Substitution EAS and Nucleophilic Aromatic Substitution NAS The benzene ring itself is electron rich which makes NAS difficult unless there are a number of strongly electron withdrawing substituents on the ring EAS on the other hand is a very useful method for putting many different substituents on a benzene ring even if there are other substituents already present Electrophilic Aromatic Substitution chapter describes the factors involved in the regioselectivity for EAS reactions using benzene rings which already have substituents on them In this experiment you will put a nitro NO2 group on a benzene ring which already has an ester group attached to it methyl benzoate The actual electrophile in the reaction is the nitronium ion NO2 which is generated in situ in the reaction mixture HNO3 H2SO4 using concentrated nitric acid and concentrated sulfuric acid Reaction O O C OCH3 C OCH3 HNO3 H2SO4 NO2 Methyl Benzoate Methyl m nitrobenzoate MW 136 15 g mol Density 1 094 m ml B P 198 199 oC M P 12 oC MW 181 14 g mol B P 279 oC M P 78 80 oC 3 Reaction mechanism The carbomethoxy group CO2CH3 directs the aromatic substitution reaction to the position that are meta to it As a result the m nitrobenzoate is the principal product from this reaction Formation of dinitro products from this reaction is unlikely under the conditions in which you carry out your reaction The reason for this is that both carbomethoxy groups as well as the nitro group on the mono nitrated product are deactivating groups making the second nitration less favorable Concentrated sulfuric acid is the solvent for this reaction and is involved in the formation of nitronium ion NO2 from concentrated nitric acid Water has a retarding effect on the nitration since it interferes with the nitric acid sulfuric acid equilibrium shown below that generates the required nitronium ion NO2 H2ONO2 HSO4 HONO2 H2SO4 H2ONO2 NO2 H2O H2O H2SO4 H3O HSO4 Overall reaction NO2 HO NO2 H HSO4 HSO4 H2O Temperature also has an effect on the product distribution from this reaction Higher the temperature greater will be the amounts of dinitration products formed from this reaction Safety Note Caution Avoid contact with the acids used in this experiment and the reaction product Prevent contact with the skin eyes and clothing work in the hood An acid spill is neutralized using solid sodium carbonate or bicarbonate The reaction is highly exothermic A vigorous reaction will occur if the acid mixture is added too rapidly to the methyl benzoate Concentrated nitric acid and concentrated sulfuric acid are both strong oxidizers and strongly corrosive wear gloves while handling them and avoid breathing their vapors Methyl benzoate and methyl m nitrobenzoate are irritants wear gloves while handling them Methanol is a flammable liquid and is toxic no flames will be allowed in lab wear gloves while handling it and avoid breathing its vapors 4 Experimental Procedure In a 125 ml Erlenmeyer flask mix 1 5 ml of methyl benzoate and approximately 4 0 ml of concentrated sulfuric acid drop wise and chill it in an ice bath Continue to cool the mixture in the ice bath to reduce the heat which produced in the reaction After complete addition of sulfuric acid add approximately 2 0 ml concentrated nitric acid measured in 10 ml graduated cylinder drop wise using a small graduated plastic pipette and mix by gentle swirling Continue to cool the reaction mixture Allow the reaction mixture to stand at room temperature for about five minutes Float the 125 ml flask in a 400 ml beaker hot water bath Remove the flask occasionally and swirl the content carefully After fifteen minutes heating pour the reaction mixture in 100 ml of ice water contained in a 150 ml beaker with stirring Isolate the product by vacuum filtration wash the product with ice cold water 20 mL followed by cold methanol 10 mL Proper washing removes the more soluble ortho isomer The crude material may be purified by recrystallization from a small volume 10 ml of hot methanol optional The crude product is pressed dry You may need to air dry or hand dry using paper towels the product Discard the aqueous filtrate down the drain with lots of water Determine the weight melting point and percentage yield Pure methyl m nitrobenzoate melts at 78 5oC Submit the product to your instructor in a paper wrapped and labeled including your name s 5 EXPERIMENT 3 Organic Chemistry II Nitration of Aromatic Compounds Preparation of methyl m nitrobenzene REPORT FORM Name Instructor Date Calculation please show your calculations on