Chem 21
Homework set 9
End-of-chapter problems from Hornback: Ch 8: 23, 25, 26, 27b,c,h,i, 28-30, 32-36, 38-40, 44,
46-48, 50, 53. Ch 9: 17d,m, 18, 19, 26, 28.
1. Draw the products of the E2 reaction of each of the following compounds with CH3O– in
CH3OH (solvent). Pay attention to stereochemistry. For a and b, be sure to consider the proper
conformation around the C2-C3 bond. For c and d, you'll need to draw the chair conformations.
(a)
(b)
Ph
(c)
Ph
Br
Br
(d)
Ph
Ph
I
I
2. Write a mechanism for each the following SN1 solvolyses, and draw the products, including
stereochemistry. A small amount of E1 occurs also, but let's forget about E1 for now. Your
mechanism should show each step clearly and include curved arrows indicating how electron
pairs move in each step. (yes, there is a reaction in b.)
(a)
(b)
EtOH
CH3OH
Cl
I
(c) For this one you’ll need to draw the structure of solvent, acetic acid to have any hope of
drawing a sensible product structure (the product is an ester, not a carboxylic acid). Think also
about which oxygen of acetic acid adds to the cation. (hint: which oxygen is protonated by a
strong acid?) One adds to make a much more stable
CH3CO2H
intermediate, although both possibilities actually lead to the
tBu Br
same product.
3. Draw the products from the following SN1 solvolyses, paying attention to stereochemistry. In
each case, state whether the products are enantiomers, diastereomers, or constitutional isomers.
(a)
(b)
Br
EtOH
CH3OH
CH3
I
(c) cis-1-bromo-4-methylcyclohexane in formic acid (HCO2H). (Hint: structure, structure,
structure; no swarms of letters.)
(d) note the mixed solvent — only water can react with the carbocation to make a stable product.
water
(S)-3-iodo-3-methylhexane
acetone
(e)
MeOH
(R)-1-chloro-3-bromobutane
(f) Small amounts of alkenes are also produced in each of these reactions. Draw the alkenes
produced in parts a, b, and c via E1 reactions.
4. Write a mechanism for each the following SN1 solvolyses. Let’s ignore stereochemistry this
time. Let's also ignore the structure of that thing over the arrow in part a... NOT.
(a)
CH3CO2H
OCOCH3
Br
(b)
I
H2O
acetone
OH
(c)
Cl
Et–OH
O
(d) Sketch a reaction coordinate diagram for the rearrangement in part c.
5. Here's something a little different. When the optically active tosylate below is solvolyzed in
methanol, a substitution occurs, but with a rather unusual stereochemical outcome, as shown
below — only the trans-dimethoxy product is formed; not the cis. Furthermore, the product is
racemic. One more interesting bit of evidence is that the trans-tosylate (below) solvolyzes much
faster than the cis stereoisomer. Write a mechanism that accounts for these observations.
OTs
OCH3
CH3OH
OCH3
OCH3
(this is a continuation of the non-graded part; ignore the boxes in the header above)
6. The reactions below may proceed by SN2, E2, or SN1/E1 pathways, or not at all. Draw the
products, and indicate which reaction mechanism(s) is/are operative, or write NR for "no rxn".
(If you're confident that you understand all the reaction mechanisms, but you're having trouble
with these, it's probably because you're not taking a systematic approach — use the step-by-step
procedure outlined in class.)
Br
(a)
KOEt
EtOH
(b)
CH3CO2– Na+
I
DMF
Ph
(c)
CH3OH
Cl
(d)
(e)
I
!
HC C
NaOCH3
TsO
acetone
Br
(f)
tBuO– K+
DMSO
(g)
I
CH3O– Na+
CH3OH
Na+
(h)
HCO2Na
acetonitrile
Br
(i)
Br
CH3CO2H
!
(j)
KOH
OMs
Br
(k)
Ph
CO2H
Br
tBu
NaOEt
DMF
(l)
(m)
DMSO
CH3OK
Et2O
Br
hexane
!
Chem 21
Fall 2009
HW set 9
30 points; due Wed, Nov 11
Name _________________________________________
1. (a) Think carefully about the possible conformations of its cyclohexane ring, and then draw
the alkene(s) formed via the E2 reaction of the compound below with potassium ethoxide in
ethanol.
KOEt
Br
EtOH
(b) Would you expect SN2 reaction to occur as well?
If so, draw the product.
(c) If the bromide above is instead dissolved in ethanol, with no base present, SN1 and E1
reactions occur. What are the products of these reaction pathways?
2. Explain why the E2 reaction of 1-bromo-1-methylcyclobutane produces
methylenecyclobutane rather than 1-methylcyclobutene. Note that the two alkenes have virtually
identical enthalpies of formation, so product stability isn't an issue in this case. (Yes, the second
one has a trisubstituted double bond, but it also has a wee bit more ring strain.)
Br
NaOCH3
CH3OH
not
3. Write the mechanism of the SN1 solvolysis below. Two constitutionally isomeric
cyclopentenes are formed (let's not worry about stereochemistry for this problem). Write the
mechanism and then draw the products.
CH3–OH
I
4. Same instructions as for #6 of the non-graded part.
(a)
I
CH3CO2H
(b)
Br
SNa
OMs
DMSO
CH3O
(c)
CO2tBu
Ph
KOtBu
tBuOH
Br