Reduction Lecture Outline - Normal format NOTE: MAKE SURE YOU DOWNLOAD THE PDF. FORMAT OUTLINE AS WELL TO HAVE THE DIAGRAM IN THE OUTLINE. Reduction Synthesis of Hydrobenzoin ? Definition of Reduction ? ? In Organic Chemistry, reduction is represented as: ? An increase in bonds of Hydrogen ? A decrease of bonds to electronegative atoms ? Example reaction: ? Definition of Oxidation ? In Organic Chemistry, oxidation is represented as: ? a decrease of bonds to hydrogen ? an increase of bonds to electronegative atoms ? Example reaction: ?Focus on the carbons where the change occurs ? if the change does not involve the increases or decrease of bonds to electronegative atoms of hydrogens-then it is a substitution reaction Reduction in this experiment ? Involves the reduction of a diketone to a diol ? ? Reaction of reduction of a simple ketone: ? Reducing agents ? We will be using NaBH4 ? Other possible reagents: ? LiAlH4, H2/Cat ? Why do we use NaBH4? -milder -more selective in reducing aldehydes and ketones -does not react violently when in contact with H2O and alcohols like LiAlH4, LiAlH4 will produce H2 gas when in contact with protic solvents . Mechanism ? 2 possibilities ? 1st mechanism: 2nd mechanism: ? 2 step reaction mechanism, as the mechanism for this reaction ? ? ? ? Hydrolysis in book is incomplete. Reaction ? See board ? ? Each mole of sodium borohydride can reduce 4 moles of carbonyls. ? ? ? Must balance equation in lab report Products ? Major product: Meso-hydrobenzoin ? - does not rotate plane-polarized light ? achiral with 2 chirally cemters due to plane of symmetry. ? Side products: -R,R- hydrobenzoin and SS hydrobbein ? Side products: ? You must know how to label a chirality center as R or S in a Fisher projection. Why do the side products form? ? Diastereomers: ? Benzoin: ? Purification of product ? You will be using recrystallization techniques ? Why does this work to purify meso hydrobenzoin from its side products? Reaction ? Reaction run in test tube -to control the reaction rate because its exothermic -add water twice, first amount to hydrolyze borate ester. -to saturate solution to cause crystallization ? Crystals ? Dry crystals ? Weigh ? Take melting point ? Submit crystals in vial for grading Why is excess NaBH4 used? ? to ensure reaction goes to completion ? to account to NaBH4 reacting with the solvent ? to account for decomposition Additional identification techniques (not done in lab) TLC if you spotted benzoin and mesohydrobenzoin on a TLC plate: which would have the higher RF?. Lower RF? IR -what would be the major difference in the IR of meso-hydrobenzoin when compared the IR of R and S benzoin Reaction ? For prelab and postlab for equation and mechanism, ? You only have to show one hydride attack ? - then draw three arrows to show borate ester ? You have to show equation (but NOT mechanism) for borate ester hydrolysis by water ? Remember, balanced equation for borate ester hydrolysis in book is incomplete. ? In prelab, you can leave the entire 2-step equation and mechanism together ? In postlab, you will have to separate them according to your observations and procedural step. Postlab ? Explain each observation ? Explain why each step was carried out If observation/step is a result of a reaction, show equation and mechanism Theoretical and percent yield calculation Why might you not get 100% yield? (Source of error for yield) -due to side products , show balanced equatiosn for side products. -Due to techniques (remember you are doing a recrystallization) What does your melting point tell you about the purity of the product? You will have to do this every time you take a melting point Does it agree with literature values? What does it mean if it does? If it doestn?T -sources of error for deviation of melting point values By HYPERLINK "http://www.rutchem.rutgers.edu/cgi-bin/discus/board-profile.cgi?action=view_profile&profile=instructor-MODERATOR" \t "_blank" Geeta Govindarajoo (Instructor) on Wednesday, October 15, 2008 - 10:22 pm: HYPERLINK "http://www.rutchem.rutgers.edu/cgi-bin/discus/board-profile.cgi?action=editpost&postid=18449&page=26/26" Edit Post THERE WAS A TYPO IN THE POSTLAB QUESTION FOR NUCLEOPHILIC SUBSTITUTION - IT HAS BEEN CORRECTED NOW - PLEASE MAKE SURE YOU HAVE THE CORRECT ALKYL HALIDE FOR THE PROBLEM.
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