A radical bromination experiment is conducted. Dichloromethane (solvent) and an unknown hydrocarbon is added to each test tube. Bromine (Br2) is then added to the test tube to determine relative reaction rates.Microsoft Word – SP16 CHEM 226-228_Exp1_Radical_Postlab_questions ONLY_001.docxImagine that you carried out the experiment but used an additional unknown compound (“Unknown #7”). Assume that the experiment was carried out perfectly (perfect technique clean glassware compounds were all correct and pure no side reactions occurred in either experiment setup etc.) and the following scenario is completely reproducible.You noted the following observations: The solution containing Unknown #7 became completely colorless after 18 minutes of exposure to ambient light or after 6 minutes when placed in front of the provided light. You also noticed that under ambient conditions the solution became colorless but cloudy. When the experiment was carried out in front of the light source the solution became colorless but not cloudy. After the solution became colorless you discarded it immediately.Microsoft Word – SP16 CHEM 226-228_Exp1_Radical_Postlab_questions ONLY_001.docxGiven that you do not know the structure of Unknown 7 provide your best hypothesis to explain this experimental observation of cloudiness.(Attached is the actual experiment if necessary).Experiment #1: Identification of Unknown Compounds via Chemical Reactionsand Spectroscopic Data(Tuesday Feb. 2st – Thursday Feb. 4th)PART A: Determination of Free-Radical Bromination Relative Reaction RatesPART B: Elucidation of Unknown Compounds via NMR spectroscopy and GCMS DataPart A is in part derived from Gilbert J. C. Martin S. F. Experimental Organic Chemistry A Miniscale and Microscale Approach 5th ed p. 327.In Part A of the laboratory session several unknown compounds will be subjected to free-radical bromiation conditions. In PartB free-radical reaction outcomes as well as 1H and 13C-NMR spectroscopy data for starting material and products of analogousreactions will be provided. The structure of the starting materials and products will be determined using all available data. Ajustification for the observed free-radical bromination reaction rates will be determined.ReadingG&M Sections 9.1-9.3 (Radical Reactions)Jones 5th ed. Sections 12.1-12.8 (Radical Reactions)G&M Sections 8.3 and 8.5 (NMR spectroscopy andGCMS – review from last term)Jones 5th ed. Sections 9.2 9.3 9.6-9.10 (NMRspectroscopy and GCMS – review from last term)Some Key TermsFree radical initiation propagation termination (1° 2° 3°) aliphatic vinylic aromatic acetylenic (1° 2° 3°) allylic (1° 2° 3°) benzylicReview terms: Nuclear Magnetic Resonance (NMR) splitting pattern n 1 rule coupling constant chemicalshift equivalent protons enantiotopic protons diasteriotopic protons gas chromatography (GC) massspectrometry (MS) molecular ion parent ion isotopes molecular weight exact mass mass spectrometer (MS) degree of unsaturationVideo InstructionSee the video links related to NMR spectroscopy andGCMS in the “Pre-Term Laboratory ProficiencyRequirements” document posted on NYU Classes.Other Prelab Information• Prepare your notebook according to the ‘NotebookGuidelines’ posted on NYU Classes under theResources tab in the Course Documents folder.Also create at table in your notebook to keeptrack of your observations in PART A of theexperiment.• Always read the Material Safety Data Sheets(MSDS) posted on NYU Classes under theResources tab in the Experiment #1 subfolder.• In preparation for all experiments you should beable to explain/answer the following questions:o What will you be doing in the lab (carefullythink through each steps/reagents/etc.)?o Why will you do it (i.e. what is the theory thatjustifies each action/reagent/etc.)?o What outcome do you expect to result fromeach action and what theory supports yourhypotheses?!Monitoring Reaction ProgressBromine (Br2) is red/orange in color. All reactionproducts and HBr are colorless. Therefore the rate ofthe reaction can be monitored visually as it is indicatedby the disappearance of the red/orange Br2.In2H R (colorless) (red/orange)In R In H(colorless) (colorless)Scheme 1. General overall reaction scheme. “In” =Initiator = Br2 for this experimentIn InInH Rheat or h?In InIn H (2.1)R(2.2)R InRIn(2.3)In InInIn(2.4)R RRR(2.5)Scheme 2. Initiation propagation and terminationsteps of a radical reaction. Note that arrows with asingle headed barb indicate the movement of oneelectron.Rate of Hydrogen Atom AbstractionHydrogen atom abstraction (Scheme 2.2) is the ratedetermining step of the free-radical brominationprocess. The rate of this step depends on two factors: astatistical factor and an energy factor. The statisticalfactor refers to the number of equivalent hydrogenatoms that can be abstracted through the radicalmechanism. The energy factor is determined by thestrength of the C-H bond. See Figure 1 and the assignedreading for a review of the classifications of hydrogenatom.1 acetylenicaromaticHBCHACCaliphatic(a)HCCCCCHBHHD HE HFCCCHC3iphatical2be2nz1lyal1ipHCCHHHHHCCCCHHHHicha(b)alticiphalicalHHticH3CH3CHCC-HHH3CCHHH3CCH3CClylicFigure 1. Examples of aliphatic vinylic aromatic acetylenic allylic benzylic hydrogen atoms. Identicalhydrogen atom subscripts within the same moleculesignify equivalent hydrogen atoms.The energy required to remove a hydrogen atom andgenerate the open shell high energy species (R•)through free-radical bromination depends on the type ofhydrogen atom being removed. The formation of thishigh-energy species (R•) is more favorable if adjacentorbitals can contribute to stabilization of the radical e.g. through resonance. Therefore when predicting therate of reaction between two molecules the moleculethat forms the more stable radical will be the moleculewith the lower energy transition state and thus themolecule that will proceed with the faster relativereaction rate.When predicting relative reaction rates the followingare some general guidelines. Vinylic and aromatichydrogen atoms do not react with radicals. An unpairedelectron in an isolated sp2 orbital is high in energy anddifficult to form. Further the vinylic and aromatic sp2orbitals are orthogonal to the p-system and therefore arenot stabilized by resonance. Allylic benzylic andhydrogen atoms involve sp3 orbitals that are stabilizedby resonance with the alkene and aromatic ring respectively (Figure 2). Note that the relative rate ofreaction generally follows: 3° allylic/benzyllic > 2°allylic/benzyllic > > 1° allylic/benzyllic >>> R3CH >R2CH2 > RCH3 > CH4.!-HHHFaliphaticvinylic aliphaticHAHAHDHBCHAHECCCCHACHEHEHHA CHAalCHHD HE HFyl1HCCCHHFigure 2. Resonance stabilization of (a) allylic and (b)benzyllic radical species.Before your laboratory session you should practicedrawing the arrows that show electron movementbetween resonance structures. Remember to show themovement of only one electron use a curved arrow withclearly only one barb.Experimental DetailsCAUTION:• Acetone and bromine should NEVER be mixed even in waste containers. Acetone and bromine willreact to form bromoacetone which is a lachrymator(tear gas).• Always keep your hood sash at half-height andcover containers containing chemicals whiletransporting.• Bromine may cause serious chemical burns if itcomes in contact with your skin. If bromine contactsyour skin immediately wash the area with soap andwater and alert an instructor. If the brominesolution contacts your gloves discard themimmediately and wash your hands thoroughly. Thismeans that you should bring at least 5 pairs ofgloves to the laboratory session.PART ALabel six 13x100mm test tubes “1A ” “2A ”….“6A.”Label six 13x100mm test tubes “1B ” “2B ”….“6B.”Label one 13x100mm test tubes “Control A” andanother “Control B”In all test tubes dispense 2.5 mL of dichloromethaneand cover each with aluminum foil. Keep the test tubescovered except when adding reagents. Then to each2 numbered test tube add 10 drops of the correspondingunknown compound (e.g. dispense Unknown #1 intotest tubes 1A and 1B dispense Unknown #2 into testtubes 2A and 2B ). Dispense 10 drops ofdichloromethane into each of the Control test tubesonly. Carefully obtain 4 mL of the provided 1 Mbromine (in dichloromethane) solution in yourgraduated cylinder. In rapid succession add 0.5 mL ofthe 1 M bromine solution to each of the test tubeslabeled “1A…6A” and “Control A.” After addition ofthe Br2 solution cover the test tube and swirl to create ahomogeneous solution. Record your observations (time color) with regards to the discharge of color in eachsolution as compared to the control. Terminate theexperiment after 1 hour.Carry out the same procedure for the test tubes labeled“1B…6B” and “Control B ” except after addingbromine place the test tubes about 6 inches from theprovided lamp. Record your observations (time color).Both series may be run concurrently but offset the starttimes by about 10 minutes.IMPORTANT Clean-up protocolThere is a special waste container for all solutions thatcontain bromine. Also ALL glassware AND test tubesthat came in contact with bromine must be rinsed with~1 mL of 0.6 M sodium thiosulfate before discarding orfurther washing. AFTER RINSING test tubes shouldbe discarded into the blue barrels.PART BIn Part B free-radical reaction outcomes as well as 1Hand 13C-NMR data for starting material and products ofanalogous reactions will be provided. The structure ofthe starting materials and products will be determinedusing all available data. A justification for the observedfree-radical bromination reaction rates will bedetermined. This data interpretation will be usefulpractice for the postlab assignment.Post-lab AssignmentFor this experiment you will not write a report. Insteadyou will complete a postlab assignment that will bedistributed via NYU Classes (in the experiment folder)after your laboratory session. Although you will notwrite a report your data and observations from theexperiment will be required to complete the postlabassignment.!!!!!!!!!!!!!!!!!!!!!!3
