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.