Chem 30A Midterm 2 Things to Know Non Exclusive 1 We continue to build on concepts covered on Midterm 1 so refresh your memory Especially Stereochemistry and Reaction Coordinate Diagrams 2 Acids and Bases What is the direction of an equilibrium based on pKa values At what sites on a molecule do acid base reactions take place What are the qualities of a good acid remember conjugate base is neutral or stabilizes negative charge well and base conjugate acid is neutral or stabilizes positive charge well Know a handful of pKa values CH4 alkanes 50 NH3 amines 38 C2H2 alkyne C H 25 ROH alcohols water 16 NH4 9 RCOOH acetic acid 5 H3O 2 HX mineral acids 5 3 Be able to identify nucleophiles and elctrophiles don t forget that electrons move from nucleophiles to electrophiles to form bonds Don t be afraid of non conventional nucleophiles and electrophiles you have the knowledge to predict reactivity based upon assignment of a molecule as electrophilic or nucleophilic For example see Heather s Week 7 Discussion Handout on VOH site under problem sets 4 Alkenes be able to draw p orbital overlap representing bonding Why is the rotational barrier so much larger in ethene than in ethane Be able to name alkenes with E and Z nomenclature Know about the physical properties of alkenes Don t worry about details concerning Terpenes 5 4 a Hydrohalogenation HCl HBr HI addition Mechanism and Stereochemical Regiochemical outcome b Hydration Acid catalyzed H2O addition Mechanism and Stereochemical Regiochemical outcome c Halogenation Cl2 Br2 I2 addition Mechanism and Stereochemical Regiochemical outcome d Hydrohalogenation X OH addition Mechanism and Stereochemical Regiochemical outcome e Oxymercuration HO HgOAc addition Mechanism and Stereochemical Regiochemical outcome The mechanism of demercuration is outside of the scope of this course but remember that you must add NaBH4 or a similar reducing agent to remove the mercury from the product f Hydroboration H BR2 addition Mechanism and Stereochemical Regiochemical outcome The mechanism of borane oxidation is outside of the scope of this course but remember that you must add H2O2 and NaOH to generate the alcohol product g Oxidation OsO4 dihydroxylation stoichiometric or catalytic required reagents the products and stereochemical outcome Again the mechanism of osmate ester reduction outside course scope Ozonolysis reaction a two step process required reagents and products don t worry about the mechanism h Reduction H2 gas and Palladium on carbon stereochemical outcome don t worry about the mechanism i Alkyl cations can undergo rearrangement via 1 2 alkyl or hydrogen shifts to generate more stable cations Be able to draw a mechanism for such a process as illustrated in section 6 3C Also be aware what reactions generate cationic intermediates which may add diversity to the products of these reactions j Stereochemistry of Reactions Remember that the reactions of achiral starting materials with achirareagents will always produce either achiral or racemic products Optically active products can only be prepared through the use of optically active starting materials or reagents although this is not a guarantee 5 Reactions Properties of Alkynes mechanism and product stereochemistry remains important Preparation of Alkynes will be covered when we talk about elimination reactions not on this midterm a Be able to draw bonding in alkynes by illustrating p orbital overlap b Alkynes C H bonds are acidic deprotonation with NaNH2 or a base of similar strength produces a nucleophilic carbanion which undergoes alkylation reactions with alkyl halides c Addition of X2 and HX anti addition products 2 or more equivalents give double addition product d Keto enol tautomerization mechanism e Hydroboration syn addition remember that BH3 is fine for internal alkynes whereas sia 2BH is required for mono addition to terminal alkynes Also oxidation of vinylborane gives ketone or aldehyde products while acetic acid will replace the boron atom with a hydrogen atom f Hydration HgSO4 H2SO4 and H2O Regioselective reaction Markovnikov selectivity g Reduction H2 gas Pd on carbon leads to complete reduction alkane H2 gas Lindlar catalyst syn addition of H2 across alkyne cis alkene products Na NH3 liquid trans alkene products h Practice using reactions together see problems 7 14 7 25 in BFI 6 Chapter 9 SN1 and SN2 9 1 9 5 a Mechanism and Stereochemical outcome of each type of reaction b Kinetics of the reaction dependence on reagent concentration for each reaction b When SN1 When SN2 c What makes for a good Leaving Group d Solvent Effects on Mechanism and Rate e Allylic Benzylic Halides f What makes a good nucleophile g Sulfonates BFI 10 5 as they are really useful leaving groups in SN1 and SN2 reactions h Combine sequences of SN1 SN2 alkene and alkyne reactions to make new compounds Some Challenging Practice Problems I ll post answers by Thursday AM Provide explanations for the following observations a The addition of HCl to A is ca 3000 x faster than with B Use words and structural drawings to support your answer HCl O O A Cl C racemic B b An optically active sample of C reacts witha solution of sodium acetate to give an optically active product D while the combination of the same optically active C with a solution of acetic acid gives D as an optically inactive racemic mixture O Why does the reaction of C with acetic acid give racemic product while the H D What is the structure H C O Na reaction with sodium acetate does not 3 O Cl Use words to explain and then draw the C mechanism for formation of racemic D The deprotonation of phenylacetonitrile E can be performed with sodium hydride NaH in an appropriate solvent Using curved arrow notation draw a mechanism for the following deprotonation mechanism H H C H Na H N C DMSO E N H2 F When phenylacetonitrile 1 4 dibromobutane and an excess of NaH are combined in DMSO the following product was observed J Org Chem 1994 59 6464 Starting with the conjugate base of phenylacetonitrile F draw the mechanism for the formation of the observed product This reaction occurs by more than one step so be sure to draw each intermediate Don t forget that excess NaH is available as needed H H C N Br Br excess Na H C DMSO N 2H2 2NaBr E G Nitrile G can be converted into toslylate H recall that TsO stands for tosylate and that tosylates are good leaving groups Upon warming tosylate H in CF3CH2OH a carbocation is generated draw the structure of this cation and then draw