1. <5 pts> Write the detailed mechanism for the reaction shown below (theacid-catalyzed hydration of isobutene) and specify the structure of the productformed. In writing the mechanism, be sure to adhere to all of the conventionsof this course for writing rigorous reaction mechanisms.

You are watching: Draw the major product as a planar projection



2.<3 pts> Your mechanism should consist ofthree steps, and you should have indicated one of them is rate-determining.Explain in detail why this particular step is rate-determining, and also whythe other two are not rate-determining.

qIn the first step, twobonds are broken (the pi bond and an O-H bond) and only one bond (a C-H bond)is formed. This reaction is therefore strongly endothermic (and endergonic) andis therefore likely to be the rds.

qThe second step involvesthe formation of one bond and does not break any bonds. It is therefore highlyexothermic (and exergonic) and is not likely to be the rds.

qThe third step involvesbreaking one O-H bond and forming one O-H bond. It is neither highly exergonicnor endergonic, but proton transfers from oxygen to oxygen are typicallyextremely fast.

3. <3 pts> For the rds of the above reaction, use resonancetheory to derive a TS model for this step. Be sure to use all of the courseconventions in employing resonance theory.


4. <3 pts> Summarize the above resonance treatment as a dottedline/partial charge (DL/PC) structure and characterize this TS model asaccurately as possible (without employing the Hammond Principle).


qThe TS has tertiarycarbocation character at the passive carbon.

5. <3 pts> State and applythe Hammond Principle to this reaction step, explaining the energetic basis for your application.

qStatement: The TS of a reaction step more closely resemblesthe higher energy species of the reactant and product.

qThe reaction involvesthe breaking of two bonds (the pi bond and an O-H bond) and the formation ofjust one bond (a C-H bond). It is therefore highly endothermic (or endergonic),so the product of the step is the higher energy partner.

qThe TS therefore moreclosely resembles the carbocation, which is the product of the step. We can saythat the TS has extensive (or highly developed) carbocation character.

B. Relative Rates ofHydration of Alkenes.

1. <6 pts> Using the TS model previously derived for therds of the acid-catalyzed hydration of isobutene, employ the Method ofCompeting Transition States todiscuss the relative rates of addition of water to ethene, propene, and isobutene (you should display all threeTS models and specify their respective characters). Which reaction is thefastest and why?



qThe reaction withisobutene (the last one) is the fastest, because the TS has tertiary carbocationcharacter, which is much more favorable than secondary or primary carbocationcharacter.

2. <2 pts> Is theratio of the rates of hydration of propene vs. ethene expected to be large orsmall? Give a specific, detailed reason for your answer.

qLarge, because the TShas extensive carbocation character at the passive carbon. The more extensivethe carbocation character, the more sensitive the reaction is to the kind ofcarbocation character which is present in the TS.

C. Stereochemistry ofAdditions to Alkene Double Bonds.

1.<3 pts> For the acid-catalyzed hydrationof 1,2-dimethylcyclohexene (see below), draw the structure(s) of the alcoholproduct(s) formed.



2.<4 pts> Write out the detailed mechanismfor this reaction, and use this mechanism to assist you in explaining theobserved stereochemical result.


qThe intermediatecarbocation, formed in the first step, is sp2 hybridized at thecarbocation center, which is therefore planar. The resultant vacant 2p AO cantherefore react with the nucleophile (water) about equally from either the topface (the one from which protonation occurred) or the bottom face. Therefore,both the cis and trans products are formed (the trans is formed in slightexcess for steric reasons).

3. <1 pts> Whatterm is used to describe this kind of stereochemical result?


4.<2 pts> What is the mechanistic symbolfor the reaction?

qAdE2. It isan electrophilic addition to the alkene (remember the viewpoint is that of theorganic molecule, which reacts with an electrophile, the hydronium ion, in therds.

II. Bromination

A.Bromination of Alkenes

1. <4 pts> For the addition of bromine to cis and trans-2-butene,draw Newman projection structures of all the product stereoisomers formed ineach case.


2. <2 pts> Which one of these products is the meso isomer? Inyour Newman structure, what symmetry element does it possess?

qMeso is formed from the trans 2-butene, as shown above.

qIt has a “centerof symmetry” or “point of symmetry”.

3. <2 pts> Are either of these product mixtures optically active?Explain why or why not.

qThe meso isomer, formedfrom trans-2-butene is achiral andthus optically inactive.

qThe isomers formed from cis-2-butene are enantiomers, i.e., a racemic mixture isformed (equal amounts of 2 mirror image isomers), so it is not optically activeeither.

4. <3 pts> Provide a mechanistic explanation of why theparticular stereoisomers observed in each case are formed in these tworeactions. Employ Newman projection structures in your depictions.


qBromine reacts to formtwo bonds simultaneously, in forming the epibromonium ion. This is a concerted,syn addition.

qWhen the bromide ionreacts with this ion, it reacts from the opposite face of the ion from thebridging bromine.

qIn the case oftrans-2-butene, this forms only the meso isomer, but in the case ofcis-2-butene, the two enantiomers R,R and S,S-2,3-dibromobutane are formed (Seebelow).


III. Hydroboration/OxidationA. Hydroboration

1.<3 pts> Draw the structure of thetwo possible products in of thereaction of 2-methyl-2-butene with borane and indicate which product is major.



qThe first product is themajor one.

2.<5 pts> Provide a resonancetheoretical treatment of the TS forthe formation of the major product, summarize as a DL/PC model, and characterize the TScarefully.


2.<3 pts> Use the Method of competing TS’s to rationalizethe preferred orientation of the addition of borane to 2-methyl-2-butene. Yourdiscussion should include TS models and characters for both modes of additionto 2-methyl-2-butene. Indicate which TS is preferred and explain why.


IV. Radical Stability; Radical Chain Reactions: Halogenation of Alkanes

A. Radical Stability

1. <4 pts> Arrangethe following radicals in order of decreasing stability (1= most stable) andexplain your reasoning.


qTertiary radicals aremore stable than secondary than primary radicals.

B. Halogenation of Methane

1.<3 pts> Thermodynamic Considerations

a.Provide an equation for approximating the standard enthalpy change (DHo) of a reaction and apply it tothe chlorination of ethane, using the bond dissociation energies provided.


BondDissociation Energies: D(H-Cl) = 103 kcal/mol; D(CH­­­3CH2-Cl)= 86 ;

D(CH3CH­2-H)= 198 ; D(Cl-Cl) = 58.



b.<1 pt> In an overall sense, what kind of reaction is this (the name of thereaction type)?

qExothermic (accept also exergonic)

2.The Reaction Mechanism

a.<6 pts> Write the complete, detailed mechanism for the chlorination of ethane, specifying the three stages of the reaction.


C. Transition State Models

1. <5 pts> Provide a resonance theoretical description of the transition state of the reaction in whichchlorine atoms remove a hydrogen atom from ethane, summarize this as a dottedline/partial radical character structure,and characterize the TS model.



2. <4 pts> Extendthe DL/PRC structure from theprevious question to the case of bromination (abstraction of a hydrogen atom bybromine atoms), and apply the Hammond Principle to refine your TS characterization. Use this refinedcharacterization to explain why bromination is highly selective for tertiary vs. primary C-H bonds.



qThe reaction withbromine atoms is rather strongly endothermic, because the CH bond (98 kcal/mol)which is being broken is stronger than the HBr bond, which is bing formed (only87 kcal/mol). Therefore, the TS more strongly resembles the higher energypartner, which is the product, the ethyl radical.

qConsequently, the TS forbromination has extensive radical character.

qSince tertiary radicalcharacter is preferred over secondary or primary radical character, bromineatoms prefer to abstract tertiary hydrogens preferentially.


V. Grignard Formation

A. Mechanism of Grignard Formation.

1. <3 pts> Write the detailedmechanism for the formation of the Grignard reagent shown below.



2. <2 pt> The mechanism is, of course, a radical mechanism, butit differs in a basic respect from the other radical reactions we have studied.In what general mechanistic respect does it differ from these others?

q It is a non-chain radical reaction.

3. <2 pts> Describe the bonding in the Grignard reagent,including both the C-Mg and Mg-X bonds.

q TheC-Mg bond is polar covalent andthe Mg-X bond is largely ionic.

q Thecarbon end of the C-Mg bond is thenegative end of the dipole.

4. <1 pts> What kind of character does a Grignard reagent have atthe carbon atom which is bound to magnesium?

qNucleophilic character.

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VI. Syntheses

A. Product Structures

1. <3 pts each = 12 pts> Draw the complete structureof the main product formed in each of the following reactions. Also, providea brief explanation for any regioselectivity, stereospecificity (or lackthereof), or any other unusual aspect of the reaction. .