203 KEY
Chemistry Midterm 2, 2009 surname last initial student number signature
MOLECULAR MODELS MAY BE USED
given names
1
2
3
4
5
raw total
total
30
13
20
20
7
90
10
1. (30 marks) Answer each of the following parts. a) Using curved arrows to represent electron movement, provide a mechanism for the reaction that takes place upon the mixing of sulfuric acid and ethanol:
O H O S OH O H O O O S OH O
O
H
H
b) Provide structures below: any aldehyde or ketone
OH
a carbonyl compound that reacts with sodium borohydride
an alcohol containing four (4) carbons that will not react to produce a bromide when treated with PBr3
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Student name/id number: __________KEY____________________ c) Draw the major contributing resonance Lewis structures for anion X:
2 5 1
Sketch a description of the orbital framework for the anion within X:
X
O
O
O
the other atoms are sp3-hybridized
The hybridization of the carbon labeled 2 in X is: sp sp3 sp2 sp5 sp3d cannot tell
The hybridization of the carbon labeled 5 in X is: sp sp3 sp2 sp5 sp3d cannot tell
d) The compound KSCN is a source of the thiocyanate ion. Draw the two most stable Lewis structures for the thiocyanate ion:
SCN
SCN
Provide the products of the reaction between KSCN and 1-bromobutane in a polar solvent:
S CN NCS
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Student name/id number: __________KEY____________________ e) Consider the following two reactions:
O K Br
product
(1)
solv ent
The reaction that proceeds at the slower rate is:
N H
K
Br
product
(2)
1
2
solv ent
(circle your answer)
Justify your selection with brief comments: Both reactions are substitutions via a bimolecular mechanism. The attacking atoms in this case are in the same row of the periodic table, so as a generalization, the better base acts as a stronger nucleophile. The pKa of an amine is 35, while the pKa of an alcohol is around 17. The amide ion is much more basic. It will behave as a stronger nucleophile and react at a faster rate.
f) Provide structures below: either is fine LiNH2 or NaNH2 or BuLi
cyclic alkene (symmetric) is also okay
a compound that will completely deprotonate 1-pentyne at 0 C
an organic compound having 5 carbons that generates a single product when treated with chlorine and light
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Student name/id number: __________KEY____________________ 2. (13 marks) Answer each of the following parts. a) The rates of SN2 reactions on various cycloalkyl bromides have been measured. Reactions are fastest using bromocyclopentane (A), and slowest with bromocyclopropane (C). Provide a possible explanation as to why the rates of substitution reactions decrease under otherwise identical conditions in the order A>B>C. (Clue: As with most rate questions, your answer should refer to the transition state for the process).
Br Br A Br B
rate of SN2 reactions:
faster than
faster than
C
Your rationale:
Nuc X
C
120 bond angle
The bond angles of the carbon undergoing displacement in a bimolecular mechanism are 120. Having bond angles further and further away from this optimum geometry will increase the energy of the transition state and consequently raise the energy requirement for the rate-determining step, and thus slow the rate of reaction. bond angles in A to B to C: 109 to 90ish to 60 Therefore most deformation in TS in the reaction of C, therefore slowest rate
GRD note: This is supposed to be the hardest question on the exam. (4 marks) b) What is the major organic product isolated after the reaction of 1-bromobutane with magnesium metal in ethanol followed by the addition of ethyl methanoate and aqueous workup:
CH3CH2O H (solv ent) Br + Mg b) H O C
H O C
OEt
(butane a is gas-hard to isolate!)
OEt
c) H3 O+ workup
major organic product
Using arrows to represent electron flow, provide a mechanistic rationale for the events observed in the reaction above:
Br + Mg
M gBr +
H O Et (gas )
+ EtOMgBr
no reaction with ethyl m ethanoate
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Student name/id number: __________KEY____________________ c) The 1H NMR spectrum of unknown organic compound X is shown below. Circle the structural formula of X.
relative area under signals:
1
6
2
3
O CH3 O
CH3 CH3 O H
CH3 CH3 OO O
O
OCH3 OCH3
H3C O
OCH3 O CH3 CH3CH2 O
O
CH3 CH3 O CH3O
O
OCH3 OCH3
d) Circle any compound that forms in the reaction of cyclopentene with N-bromosuccimide (NBS):
Br Br Br Br Br
Br Br
all of these none of these
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Student name/id number: __________KEY____________________ 3. (20 marks) Provide the requested information for each of the following reactions. If two stereoisomers are formed, draw both of them, clearly indicating appropriate stereochemistry. Do not draw the same stereoisomer twice. a)
H Br H CH3
Me K O C Me Me Br
H H
base C C
H CH3
CH3
(only compound)
supply a s tructural drawing (as a sawhorse projection) of the starting material (C1 5H15Br)
supply a drawing of the transition s tate for this reaction
b)
5 Br H Br O OH NaH, solv ent Br O O Br H Br O Na 1
O ring clos ure extremely fast
organic product(s)
c)
H Al H O H Li
O Li
+ H
H Al
THF (s olvent) H
produc t
The above reaction is a (circle your answer):
a SN1 reaction reduction a SN2 reaction Lewis acid-base reaction hydrogenation hydroboration oxidation
Bronsted acid-base reaction
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Student name/id number: __________KEY____________________ d) The reaction below produces compounds that are constitutional isomers:
O Br CH3 H Na O O O CH3 (s olvent) CH3 CH3 H3C O O O O CH3 CH3
both c ompounds are rac em ic
organic product(s) that are cons titutional isomers (C8H12O2)
Using arrows to represent electron movement, provide a mechanistic rationale explaining the formation of these isomeric products:
O Na O CH3 O O CH3 CH3
Br CH3
rds
CH3
CH3
CH3
CH3 O O Na CH3
CH3 H3C O O
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Student name/id number: __________KEY____________________ 4. (20 marks; 4 marks each) Provide reagents and reaction conditions to convert the starting material(s) to the designated product. Be sure to clearly show all the reagents and isolated intermediary compounds along the path. For parts a-c, you can use any organic compound containing up to three carbon atoms. Note: mechanism steps are not required for these questions.
O
a)
HO a) LiAlH4, Et2O b) H3O+ workup HO
Br
PBr3, solvent
b)
OH
CN
O S Cl O pyridine
KCN, s olvent O Ts
c)
CCH a ) Na NH2 solvent CC Na b) BrCH2CH2 OCH3 c) workup
O
CH3
Li , NH3 (l) OCH3 CC
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Student name/id number: __________KEY____________________
d)
O (only c arbon-containing s tarting material) a) BH3THF b) NaOH, H2O2 pyridine, pTs Cl OH OTs a) OH , NaH b) workup
e)
(only carbon-containing starting m aterial) HBr Br a) O3 b) (CH3)2S CHO a) Mg THF M gBr OH b)
(mixture of stereoisomers )
CHO c) H3O+ workup
\
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Student name/id number: __________KEY____________________ 5. (7 marks) Below are 1H NMR spectral data for an organic compound (C9H10Cl2O):
peak position () 7.10 7.01 3.70 3.24 2.72
multiplicity s s t s t
relative peak area 1 2 2 3 2
a) From your analysis of the spectral data above, what principal structural subunits are present? Draw these fragments in the space provided. (for partial credit only; not required for full credit) b) Your final answer:
CH3 CH2CH2
Cl
O
CH3
Cl
End of questions .
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