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Nitration of Methyl Benzoate

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Nitration of Methyl Benzoate Demonstration of the effect of an electron withdrawing group on a monosubstituted benzene ring on subsequent substitution of other groups on the Benzene ring References: Pavia, et al.– pp 338 – 342 Slayden, et al. pp 76 – 79 03/12/22 1
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  • Nitration of Methyl Benzoate

    Demonstration of the effect of an electron withdrawing group on a monosubstituted benzene ring on subsequent substitution of other groups on the Benzene ring

    References:Pavia, et al.pp 338 342Slayden, et al.pp 76 79**

  • Nitration of Methyl BenzoateOverview:Synthesis Nitration of Methyl Benzoate through an Electrophilic Aromatic Substitution to form:Methyl m-Nitrobenzoate (MW 181.15)Methyl 3-Nitrobenzoate (MP 78oC)3-Nitrobenzoic Acid Methyl EsterDetermination of reactant Masses, Moles, Molar Ratio, Limiting Reagent, Theoretical YieldReaction mixtures must be kept coolSeparation and Purification of Product by Vacuum Filtration and Recrystallization from Methyl AlcoholPercent YieldMelting PointSummary of ResultsAnalysis and Conclusions**

  • Nitration of Methyl BenzoateThe Laboratory Report:Synthesis ExperimentMass, Moles, Molar Ratio, Limiting Reagent, Theoretical YieldProceduresTitle Concise: Mass reagent, Vacuum Filtration, Recrystallization, etc.Materials & Equipment (2 Columns in list (bullet) form)Note: include all reagents & principal equipment usedDescription:Descriptions must be detailed, but conciseUse list (bullet) formUse your own words Dont copy book!!**

  • Nitration of Methyl BenzoateSynthesis Experiment (Cont)Results Neat, logically designed template to present results (in box to right of Procedure description)SummaryParagraph summarizing experimental observations and computed resultsAnalysis & ConclusionsLimiting reagentWhat is the nature of the product you expected and what evidence do you have to indicate you actually got what you expected?What was the yield of your product and what aspects of your experimental procedure could be improved?**

  • Nitration of Methyl BenzoateBackground:Electrophilic Addition vs. Electrophilic SubstitutionAlkenes, which contain pi () (-C=C-) bonds, are electron-rich due to the excess of electrons in the () bondsThese electrons are susceptible to electron-seeking (electrophilic) reagents called ElectrophilesIn an Electrophilic Addition of an Alkene, the Alkene acts as an Electron-Rich Nucleophile providing a source of electrons for the Electrophile, for example, a proton (H+) from an acid (acting as a Lewis acid)

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  • Nitration of Methyl BenzoateExample of Electrophilic Addition

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  • Nitration of Methyl BenzoateBackground (Cont)Electrophilic Aromatic SubstitutionAromatic compounds also have the electron rich pi () bondsThe resonance in the Benzene ring, however, makes the electrons less susceptible to Electrophilic AdditionsAn addition reaction would result in a loss of resonance stabilizationAromatic compounds do, however, react with strong electrophilic reagents at somewhat elevated temperatures**

  • Nitration of Methyl BenzoateBackground (Cont)Electrophilic Aromatic SubstitutionIn todays experiment the Benzene ring of Methyl Benzoate is reacted with a mixture of concentrated Nitric and Sulfuric acids, i.e., source of Nitronium ion The positively charged Nitronium ion (NO2+) acts as the Electrophile, temporarily disrupts ring resonance, and adds to the Nucleophilic Benzene Ring forming the intermediate resonance-stabilized Arenium ion, an electron deficient positively charged delocalized CarbocationThe rate of formation of the Arenium ion, which is somewhat stabilized by ring resonance, determines the rate of reaction**

  • Nitration of Methyl BenzoateBackground (Cont)

    **The Carbomethoxy group is electron withdrawing, thus; it deactivates the ring relative to Benzene.The resultant resonance structures favor Meta substitution over Ortho/Para

  • Nitration of Methyl Benzoate**

  • Nitration of Methyl BenzoateBackground (Cont)Electrophilic Aromatic Substitution (Cont)The Carbomethoxy group is electron withdrawingIt has a moderately Deactivating effect on the ring relative to Benzene itself, thus the transition state to the Arenium ion is highly unstable, withdrawing electrons from the developing carbocation leading to increased positive charge on the ringThe inductive effect of this electron withdrawal sets up a dipole with the ring at the positive endAny resonance form of the Arenium ion that would enhance this positive charge, e.g., either ortho or para resonance structures, would further destabilize the ring**

  • Nitration of Methyl BenzoateBackground (Cont)Electrophilic Aromatic Substitution (Cont)The resonance forms of the Meta substituted Arenium ion, however, do not attempt to add additional positive charge at the carbon atom attached to the electron-withdrawing carbonyl group of the Carbomethoxy group (see previous slide) Electron withdrawing groups favor Meta substitution because the Meta resonance structures are more stable than O/PFull resonance is restored by eliminating the proton from the sp3-ring carbon (also containing the Nitronium ion) to the HSO4- ionBy eliminating the Hydrogen, the Nitronium ion is therefore substituted on the ring**

  • Nitration of Methyl BenzoateBackground (Cont)Electrophilic Aromatic Substitution (Cont)After the single substitution of the Nitro group to the first Meta position, the combination of the Carbomethoxy group and the Meta-Nitro group further Deactivates the ring against additional substitutionKeeping the reaction mixture temperature low also inhibits the formation of Dinitration productsSmall amounts of the Ortho and Para isomers of Methyl m-Nitrobenzoate and the Dinitration products can be in the reaction mixtureThese are removed by recrystallizing the solution with Methanol**

  • Activators (Donate, Release Electrons)Available pair of unbonded electrons to donate to ring.More Shielding of ProtonsLess NMR Chemical Shift downfieldDeactivators (Withdraw, Accept Electrons)No unbonded electron pairsLess Shielding of ProtonsMore NMR Chemical Shift downfieldMethyl & Alkyl groups are activating because of the stabilizing effect of sp2 hybridization (hyperconjugation) of an unbonded electron in methyl radical.Halogens are o,p directing because the electron donating resonance effect is more dominant than the withdrawal inductive effect of these electro-negative elements.Nitration of Methyl Benzoate**

  • Nitration of Methyl BenzoateProcedureDetermine the mass of the Methyl Benzoate to the nearest 0.001g (MW 136.15)Determine the mass of 2.000 mL of Conc HNO3 (MW 63.01) from the volume, density and % CompositionDen 1.42 g/mL, % Acid - 70.0%) (Use a volumetric pipet to obtain the HNO3)Limiting Reagent & Theoretical Yield (in your report)Calculate the Moles of Methyl Benzoate and Nitric AcidSetup the Stoichiometric balanced equationDetermine the Stoichiometric Molar RatioDetermine the limiting reagent from the Stoichiometric balanced equation and the actual moles of reagents usedCompute the theoretical yield**

  • Nitration of Methyl BenzoateProcedure (Cont)Note the following precautionsSulfuric Acid protonates Nitric Acid acting as a weak base to form the Electrophilic Nitronium Ion, which is added to the resonance disrupted Benzene ring replacing a Proton Use extreme care to avoid adding any additional water to the reaction mixturesWater must be kept to a minimum so as to enhance the reactivity of the nitrating mixture. Water is a stronger base than HNO3, which is basic relative to H2SO4; thus, it would interfere with the protonation of the Nitric Acid, hence the formation of the Nitronium ion The reaction temperature must be kept low to prevent the excessive formation of the Dinitration products**

  • Nitration of Methyl BenzoateProcedure (Cont)Prepare an ice/water bath using a 150 ml beakerPlace a 50 mL Erlenmeyer Flask containing approximately 6 mL of concentrated Sulfuric Acid (H2SO4) into the ice/water base; allow to coolAdd the Methyl Benzoate to the Sulfuric Acid, swirl the mixture, and allow to coolPrepare another ice/water bath using a 100 mL beakerPipet the 2.00 mL of concentrated Nitric Acid (HNO3) into a small, clean, dry vial and place the vial in the new ice/water bath (the sides of the vial should be immersed in the ice/water bath about half-wayAllow to cool for about 10 minutesNote: Avoid introducing any water into mixture!**

  • Nitration of Methyl BenzoateProcedure (Cont)Using a medicine dropper or plastic disposable pipet, very slowly add, with continuous gentle swirling, the cooled HNO3 acid to the cooled Sulfuric Acid / Methyl Benzoate mixtureThe temperature of the reaction mixture must be kept below 15 oCAfter the two reaction mixtures have been combined, keep the new mixture in the ice/water bath for at least 5 minutes in order to keep the solution temperature in a cooled state until the reaction is complete**

  • Nitration of Methyl BenzoateProcedure (Cont)Allow the reaction mixture to warm to room temperature (10 min or so)Pour the reaction mixture over approximately 25 grams of ice in a 100 mL or 150 mL beakerThe product precipitates; allow the ice to meltNote:If you get a small yield, you probably introduced some water from the ice/water bathEven a small drop of water is sufficient to shut down the reactionPre-weigh the top of the Buckner FunnelIsolate the precipitated product by Vacuum Filtration through a Buckner Funnel**

  • Nitration of Methyl BenzoateProcedure (Cont)Wash product with two 15 mL portions of cold distilled waterDispose of aqueous filtrate down the drain with lots of waterWash product again with two 5 mL portions of cold MethanolDispose of the Methanol filtrate in the waste jar in the hoodWeigh the Buckner funnel again to determine the weight of the crude productTransfer the crude product to another clean beaker

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  • Nitration of Methyl BenzoateProcedure (Cont)RecrystallizationDetermine the starting volume of Methanol required from the mass of the crude product and the density of methanol.Density (g/mL)= mass (g) / Vol (mL)Density of Methanol = 0.79 g/mL Volume Methanol= mass (sample) / densityAdd Methanol to Sample beakerAdd enough additional Methanol to just cover the samplePlace beaker containing sample and Methanol in a larger beaker full of water, i.e., a water-bath**

  • Nitration of Methyl BenzoateProcedure (Cont)Recrystallization (Con t)Heat water bath to about 80oCSwirl contents of sample beaker until product dissolvesNote:Do not boil contents of sample beaker. It may be necessary to add small increments of Methanol to effect solution of productCool solution slowly to room temperatureSet up Buckner FunnelPre-weigh the top of the cleaned/dried Buckner funnel or a weighing tray as directedPre-moisten the filter with Methanol

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  • Nitration of Methyl BenzoateProcedure (Cont)Transfer purified crystals to Buckner Funnel using small amounts of additional cold Methanol to complete transfer of all solid material to the Buckner FunnelVacuum Filtration, washing with 2 5 mL increments of cold Methanol, to separate crystals from solutionDispose of Methanol filtrate in waste jar in hoodPlace the purified product (Methyl m-Nitro Benzoate) in a pre-weighed plastic weighing tray and put in your drawer or instructors drawer until next weekWeigh the Buckner funnel containing the dry, purified productCompute the mass of the productCompute the percent yieldDetermine the Melting Point**

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