CH 610A/618A

Third Exam

Fall, 2002

Prof. N. L. Bauld



Answer Key


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Possible Points

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I. Acid Catalyzed Hydration of Alkenes



A.  Acid Catalyzed Hydration of Ethene (Addition of Water)

1.   [4 pts] Write the detailed mechanism for the reaction shown below, i.e. the acid catalyzed hydration of ethene.


[Please note: Requires rds for first step and equilibrium arrows for last step, along with all other conventions for writing mechanisms.]


2. [5 pts] Provide a resonance theoretical description of the transition state for the rate determining step of this reaction, summarize this as a DL/PC structure and characterize this TS model as completely as you can without employing the Hammond Principle.


3. . [2 pts] State and apply the Hammond Principle to this reaction step, explaining your application.

q      The TS for a reaction step more closely resembles the higher energy partner of the reactants and products

q      The rds is highly endothermic, since two bonds are broken (the C-C pi bond and an O-H bond) and one (the C-H bond) is formed.

q      Therefore, the TS more closely resembles the product of the reaction step, which is the carbocation.

q      The TS thus has extensive (or highly developed) carbocation character at the passive carbon.

4.  [2 pts] Now, apply the Hammond Principle to the step of the reaction which involves the reaction of the intermediate carbocation with water. What predominant character does this TS have?

q      The reaction of the carbocation with water is highly exothermic, since one bond is formed (the C-O bond) and none are broke.

q      The TS therefore more closely resembles the reactant, which is the carbocation.

q      Therefore this TS also has extensive carbocation character.


B. Relative Rates of Hydration of Alkenes.

 1. [4 pts] Using the TS model you have previously derived for the rds of the hydration reaction, employ the Method of Competing Transition States to discuss the relative rates of addition of water to  ethene, propene, and isobutene (you should display all three TS models and specify their respective characters). Which reaction is the fastest and why?






q      The reaction with isobutene is fastest, because the TS has tertiary carbocation character, which is more favorable than secondary or primary carbocation character.


C.  Stereochemistry of Hydration.

         1.  [4 pts] Sketch the mechanism of the acid-catalyzed hydration of 1,2-dimethylcyclohexene in such a way as to illustrate and explain the observed stereochemistry of the reaction. Write the structure(s) of the product(s) observed and indicate what stereochemical term is used to describe this kind of stereochemical result. (Note that the “squiggly” line means that I have deliberately left the stereochemistry unspecified).


q      The reaction is non-stereospecific.

q      The 2p AO of the intermediate carbocation has equivalent lobes above and below the carbocation center and can react with water from either face, yielding both the cis and trans products.


II. Bromination and Hydroboration/Oxidation



A. Bromination of Alkenes

1. [3 pts] Write the structure(s) of the product(s) obtained in the following reaction. What terminology is used to describe this kind of stereochemical result (two words)?


q      The reaction is is termed anti stereospecific (only the trans-product is obtained.)


2.  [3 pts] Rationalize, with the assistance of a mechanistic sketch, why this particular stereochemical result is observed. What is the generic name of the type of intermediate involved?

q      The reaction involves an epibromonium ion intermediate (alternatively, a bridged bromonium ion).

q      Since one face (assume the top face) is blocked by the bridging bromine, the bromide ion reacts from the opposite face, leaving one bromine or the top face and one on the bottom face [an alternative, and better, explanation is that SN2 reactions, like this one, occur with inversion of configuration at carbon, so that the entering nucleophile always bonds from the opposite face from which the leaving group leaves]



3.  [3 pts]  Draw the Newman projection structure of the dibromide(s) which result(s) when bromine is added to trans-2-butene. Provide the complete IUPAC name of any stereoisomer formed, including the stereochemical designation.


q      The product is meso-2,3-dibromobutane or (2R,3S)-dibromobutane or (2S,3R)-dibromobutane.

q      Note that other conformations, including eclipsed ones are acceptable as long as they depict the meso diasteroisomer.


B. Hydroboration

            1. [5 pts] Provide a resonance theoretical treatment of the TS for the hydroboration of ethene, summarize as a DL/PC model, and characterize the TS very carefully.



            2.  [1 pt] What character of this TS is not present in either the reactant or the product? Carobocation character.


            3. [5 pts] Use the Method of competing TS’s to rationalize the orientation of the addition of borane to propene, using and extending the TS model that you developed for the hydroboration of ethene. Your discussion should include TS models and characters for both modes of addition to propene. Indicate which TS is preferred and explain why. Finally, draw the structure of the preferred product.


q      The second TS is favored, because secondary carbocation character is better than primary carbocation character.

q      The preferred product is propylborane, CH3CH2CH2BH­2.


            4.  [2 pt] What term is used to indicate this kind of selectivity?

q      Regioselectivity or regiospecificity.

            5.  [2 pts] How many steps are involved in the hydroboration of an alkene? What specific term is used to describe a reaction having this number of steps?

q      It is a single step reaction.

q      The appropriate term is “concerted”.

            6.  [3 pts] What stereochemical terminology is used to describe the stereochemistry of hydroboration? Rationalize, using an appropriate illustration, the reason that this stereochemical result prevails.

q      It is syn stereospecific.

q      If the H attempts to bond to the alkene from the opposite face, it cannot be simultaneously bonding to boron, because the latter is too remote to bond to it. Only the syn  face is accessible.


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



A. Radical Stability

      1. [3 pts] The bond dissociation energies of the C-H bonds of methane and ethane are 105 and 98 kcal/mol, respectively. Since both bonds are are of the same type (C-H bonds in which the carbon is sp3 hybridized in both cases), explain why one bond is more easily dissociated than the other. Your answer should include a resonance theoretical treatment of one of the radicals involved.        


q      The ethyl radical is resonance stabilized, as shown below, but the methyl radical is not.


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 to the chlorination of methane, using the bond dissociation energies provided.



Bond Dissociation Energies: D(H-Cl) = 103 kcal/mol; D(CH­­­3-Cl) = 86 ; D(CH3-H) = 105 ; D(Cl-Cl) = 58.




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

q      It is a homolytic substitution reaction or radical substitution reaction.


            2. The Reaction Mechanism

                        a. [5 pts] Write the complete, detailed mechanism for the chlorination of methane, specifying the three stages of the reaction.



                        b. [2 pts] What terminology is used to describe a reaction mechanism in which one stage is repeated over and over again? Provide, in addition, the symbolic mechanistic description of the reaction.

q      A chain (or radical chain) reaction.

q      It is an SH or SR reaction. (substitution, hemolytic or radical)


C. Transition State Models

      1. [5 pts] Provide a resonance theoretical description of the transition state of the reaction in which chlorine atoms remove a hydrogen atom from methane, summarize this as a dotted line/partial radical character structure, and characterize the TS model.


2. [2 pts] What is the term used to describe a reaction which involves the removal of a H atom by a chlorine atom to form an H-Cl bond (or any analogous radical process)?

q      It is an abstraction reaction.


IV.Radical Additions to Alkenes; Grignard Formation


A. Radical Chain Additions to Alkenes

            1. [4 pts] Write the detailed mechanism for the radical chain addition of HBr to ethene, labeling the three distinct stages of the reaction, and specifying the radical conditions that are needed to bring about this reaction.



            2. [4 pts] Provide a resonance theoretical treatment for the TS for the addition of bromine atoms to ethene and summarize as a dotted line/partial radical character structure.


            3. [4 pts] Using this dotted line/ partial radical character TS model and extending it to the case of addition of HBr to propene, employ the Method of Competing TS’s to compare the TS characters of the two possible modes of addition of bromine atoms to propene. Indicate which mode is preferred and why.


q      Secondary radical character is better than primary radical character.

q      The first path is preferred, and this gives 1-bromopropane.


B.  Grignard Formation.

      1. [3 pts] Write the detailed mechanism for the formation of the Grignard reagent shown below.



      2. [1 pt] The mechanism is, of course, a radical mechanism, but it differers in a basic respect from the other radical reactions we have studied. In what general mechanistic respect does it differ from these others?

q      This is not a chain reaction, it is a stoichiometric or non-chain radical reaction.


      3. [3 pts] Describe the bonding in the Grignard reagent, including both the C-Mg and Mg-X bonds. What kind of reactivity does the carbon atom of a Grignard reagent possess?

q      The C-Mg bond is polar covalent , with carbon being the negative end of the dipole.

q      Carbon displays nucleophilic reactivity.


      4. [2 pts] Mention and briefly explain two respects in which diethyl ether is an especially good solvent for the preparation of Grignard reagents.


q      Ethers are non-acidic, so they do not protonate the quite basic Grignard reagents, i.e., Grignard reagents are stable in ethers because of this lack of basicity.

q      The C-O dipole of ethers, in which oxygen is the negative end, allows ethers to strongly stabilize the Mg positive ion, and thus to dissolve Grignard reagents. (Also equally acceptable, the unshared electron pairs in ethers makes them fairly good Lewis bases, which enables them to stabilize Mg positive ions, which are Lewis acids.



V. Syntheses


A. Product Structures

            1. [2 pts each = 10 pts] Draw the complete structure(s) of the product (or all of the main products if more than one product is formed in substantial amounts), including the stereochemistry of the products. No explanations are necessary.


q      Note that the only stereochemical issue is in part 2, where the OH and the H are cis, because we have syn stereospecific addition of borane, etc.