Fall Workshop #10

Question #1

Estimate the relative amount of energy that is released upon catalytic hydrogenation of each of the following alkenes. Complete the reaction energy diagram template below, and provide a brief explanation as to the differences in H for each alkene. HINT: you’ll likely want to draw the structure of each before you proceed. You may also want to consult Sections 3.6 and page 565 of Jones as well as page 216 of the purple book. NOTE: all starting materials are fully hydrogenated to the same product (hexane).

(a)cis-2-hexene

(b)1-hexene

(c)1,4-hexadiene

(d)2-methyl-2-pentene

(e)trans-2-hexene

(f)2,3-dimethyl-2-butene

(g)1,3-hexadiene

Question #2

Sketch reaction energy diagrams that accurately represent each of the following criteria. Be sure to appropriately label the axes, reactants, products, intermediates, transition states, G°, G‡, H°, and H‡.

(a)a two-step reaction in which the first step is endergonic, the second step is exergonic, and the overall reaction is spontaneous

(b)a three-step reaction in which the first step is endothermic, the second and third steps are exothermic, and the overall reaction is exothermic

(c)a two-step reaction that is spontaneous and whose first step is faster than its second step

For each of the reaction energy diagrams, answer the following and discuss your reasoning:

  • Which species does EACH transition state most closely resemble?
  • Which is the fastest step in the forward direction?
  • Which is the fastest step in the reverse direction?
  • Which step in each reaction is the rate-determining step (forward direction only)?

Question #3

Complete the table below in which you will rank the carbocations shown in DECREASING order of stability. Be sure to indicate the type(s) of special effects that are contributing to the relative stability of each ion -- Recall our discussion about the electron donating capability of attached ALKYL groups (I referred to this in class as hyperconjugation, a term you may want to define before solving this problem – see Jones p. 403-404 and the Purple Book p. 107-109 for more info). For those ions that exhibit a resonance stabilization effect --- show ALL possible resonance structures.

Order of stability / Stabilizing Effects (Reason for ranking)
(most stable) =
(least stable) =

Question #4

Sketch reaction energy diagrams for the formation of each of the carbocations in question #3 from their corresponding alkyl halides (1st step, dissociation ONLY). For example:

Rank the dissociations from slowest to fastest.
Question #5
In each of the following, three organic halides are shown. Indicate the order of reactivity in each designated grouping (1 is most reactive & 3 is least). Be sure to describe WHY you ranked them as you did – e.g. identify the major factor/influence that determines the rank ordering. Note that C6H5- represents a phenyl group.

(A) SN2 substitution by NaOCOCH3 in DMF:

CH3CH2CH2Br(CH3)2CHBr CH2=CHCH2Br

(B) SN2 substitution by NaI in acetone:

C6H5Cl C6H5CH2Cl C6H5CHClCH3

(C) SN2 substitution by NaCN in DMF:

CH3CH2Cl CH3CH2F CH3CH2I

(D) SN2 substitution by NaSCH3 in DMSO:

(CH3)3CCH2Br CH3)2CHCH2CH2BrCH3CH2CHBrCH2CH3

(E) SN1 substitution by NaCl in methanol:

CH3CH2Br(CH3)3CBrCH3CH2CHBrCH3

Question #6

For each of the following incomplete reactions:

(a)For each reaction, identify the nucleophile/base, leaving group and degree of substitution(primary, secondary, tertiary, etc.) at the -carbon.

(b)Provide the structure of the product and indicate whether the reaction is substitution (SN1 or SN2) or elimination (E1 or E2).

(c)Indicate the functional group interconversion (FGI) that has occurred (e.g. from alkyl halide to ether).

(d)Describe all bond making and bond breaking events that occur in EACH mechanism.

(e)Describe any changes in hybridization state that occur in each mechanism.

(f)Match the mechanism with the energy diagrams provided.