Chapter 10 - Organohalides
Give the name for each of the following alkyl halides.
1.
H
Cl
Cl
C
C
Cl
H
Answer:
2.
F
triiodomethane
F
Cl
C
C
F
Br
Answer:
4.
1,1,2-trichloroethane
CHI3
Answer:
3.
H
H , Halothane
2-bromo-2-chloro-1,1,1-trifluoroethane
CH2Cl2
Answer:
dichloromethane
Draw structures corresponding to these names.
5.
3-iodopropene
Answer:
I
6.
trans-1-chloro-3-sec-butylcyclohexane
Answer:
Cl
7.
1,2-dichloro-1,1,2,2-tetrafluoroethane (Cryofluorane)
Answer:
F
80
Cl
Cl
C
C
F
F
F
Chapter 10: Organohalides
8.
(S)-2-bromobutane
Answer:
H
Br
To answer the following questions consider the reaction below:
Cl
(CH3)3COCl
400 C
Cl
+
65%
9.
+
Cl
25%
10%
When propylbenzene reacts with tert-butylhypochlorite three monochlorinated products are formed in the
ratios indicated. Calculate a reactivity order for each type of hydrogen atom in propylbenzene.
Answer:
Ar − CH 2 − = 65% ÷ 2 = 32.5% product ÷ 33
. = 9.85
−CH 2 − = 25% ÷ 2 = 12.5% product ÷ 33
. = 38
.
−CH 3− = 10% ÷ 3 = 33%
.
product ÷ 33
. =1
: . :9.85 .
Therefore the Relative Reactivity for CH 3:CH 2:ArCH 2 is 138
10. The reaction of propylbenzene with tert-butylhypochlorite proceeds by a radical substitution pathway.
Draw the structure of the radical intermediate leading to each product.
Answer:
Cl
+ (CH3)3COCl
Cl
Cl
Test Items for McMurry’s Organic Chemistry, Seventh Edition
81
11. Based on your answers to the two questions above explain why (1-chloropropyl)benzene is the major
product of this reaction.
Answer:
In question 9 we calculated that the hydrogens next to the aryl group, the benzylic hydrogens,
are about 2.5 times more reactive than the methylene hydrogens and about 10 times more
reactive than the methyl hydrogens. This means that the benzylic radical is more stable than a 2
° or 1° radical. The reason the benzylic radical is more stable is because it is resonance
stabilized. The resonance forms for the benzylic radical are:
The 2° or 1° radicals have no other resonance forms.
12. Are any of the products of this reaction chiral? If so draw them and label the chirality center with an
asterisk.
Answer:
Yes!
Cl
*
*
and
Cl
13. Will the product mixture of this reaction display optical activity?
Answer:
No, the radical intermediates for the chiral products are symmetrical and the transition states
leading to chiral products are of equal energy so racemic mixtures are formed.
14. In the discussion on relative reactivity of alkane hydrogens towards radical chlorination, we showed that
the relative rate of 2° to 1° hydrogen atom abstratction is 3.5 : 1 for butane. Calculate the relative amounts
of 1-chloropropane and 2-chloropropane obtained the radical chlorination of propane, using this relative
rate of reactivity.
Cl
CH3CH2CH3
Answer:
82
Cl2
light
CH3CH2CH2
Cl
+
CH3CHCH3
Propane has six 1° hydrogrens, so 6 x 1 = 6; and two 2° hydrogens, so 2 x 3.5 = 7. The
percentage of 1-chloropropane is therefore 6/13 or 46% and the percentage of 2-chloropropane
is 7/13 or 54%.
Chapter 10: Organohalides
Consider the reaction below to answer the following questions.
Cl2
CH3
light
15. Draw all the monochlorination products of methylcyclopentane (ignore stereoisomers).
Answer:
CH3 +
CH2Cl +
CH3 +
Cl
CH3
Cl
Cl
16. Label the chirality centers in the monochlorination products of methylcyclopentane with an asterisk.
Answer:
CH3 +
CH2Cl +
Cl
*
CH3 +
*
*
CH3
*
Cl
Cl
17. Tell whether radical chlorination of methylcyclopentane is an oxidation or a reduction process.
Answer:
In the radical chlorination of methylcyclopentane a C-H bond is broken and a C-Cl bond is
formed, so the process is an organic oxidation.
Consider the reaction below to answer the following questions.
CH3CH
NBS
hv
CHCH2CH2CH3
CH3CH
CHCHCH2CH3
+
CH3CHCH
Br
A
CHCH2CH3
Br
C
B
18. Place asterisks(*) at all allylic positions in compound A.
Answer:
*
CH3CH
*
CHCH2CH2CH3
A
19. Draw the resonance forms of the allylic radical intermediate that accounts for the formation of B and C.
Answer:
CH3CH
CH
CHCH2CH3
Test Items for McMurry’s Organic Chemistry, Seventh Edition
CH3CH
CH
CHCH2CH3
83
20. D and E, below, are minor products in this reaction. Explain why.
BrCH2CH
CHCH2CH2CH3
+
CH2
CHCHCH2CH2CH3
Br
D
E
Answer: Products D and E are formed from the other allylic radical, whose resonance forms are shown
below.
CH2CH
CHCH2CH2CH3
CH2
CHCHCH2CH2CH3
2o allylic
1o allylic
This allylic radical forms more slowly than the radical for products A and B because is has some
1° character, and thus, is less stable. The allylic radical intermediate for products A and B is 2°
allylic.
Choose the best reagent or sequence of reagents from the list provided below for carrying out the following
transformations. Place the letter of your response to the left of the reaction.
a.
b.
c.
PBr3
HCl (gas), ether
1. Mg, ether
2. D2O
d. SOCl2, pyridine
e. HBr (gas), ether
f. 1. Mg, ether
2. NBS
CH3
CH3
21. _____
H3C
C
OH
84
CH3
CH3
Br
D
c
OH
23. _____
Answer:
Br
e
22. _____
Answer:
C
CH3
CH3
Answer:
H3C
Cl
d
Chapter 10: Organohalides
Provide structures for the reactants, intermediates, or products, as indicated, in the following reactions. Draw
the structures in the boxes provided.
24.
CH3CH2I
CH3I
1. Li, pentane
2. CuI
Answer:
CH3CH2I
1. Li, pentane
2. CuI
(CH3CH2)2CuLi
CH3I
CH3CH2CH3
25.
Cl
SOCl2
pyridine
Answer:
OH
Cl
SOCl2
pyridine
26.
NBS
CCl4
+
NBS
CCl4
Br +
Answer:
Test Items for McMurry’s Organic Chemistry, Seventh Edition
Br
85
27.
Cl
HCl
ether
Answer:
Cl
OH
HCl
ether
28.
Br
+ (CH3CH2)2CuLi
ether
Answer:
Br
+ (CH3CH2)2CuLi
CH2CH3
ether
29.
1. NBS, CCl4
2. KOH, ethanol
Answer:
1. NBS, CCl4
2. KOH, ethanol
86
Chapter 10: Organohalides
30.
Mg
ether
Br
Answer:
Mg
ether
Br
MgBr
31.
Br
PBr3
ether
Answer:
OH
Br
PBr3
ether
Propose a synthesis of each of the following compounds from the given starting material and any inorganic
reagents necessary.
32.
CH3
CH3CHCH
CH3
CH2
from
CH3CHCH2CH2OH
Answer:
CH3
CH3CHCH2CH2OH
PBr3
ether
CH3
CH3CHCH2CH2Br
Test Items for McMurry’s Organic Chemistry, Seventh Edition
CH3
KOH
ethanol
CH3CHCH
CH2
87
33.
H3C
H3C
CH3
Br
from
H3C
(only source of carbon)
H3C
CH3
Answer:
H3C
Br
2
H3C
4 Li
pentane
H3C
Li
2
H3C
CuI
ether
H3C
CuLi
H3C
(CH3)2CHBr
ether
H3C
CH3
H3C
CH3
34. Identify the reagents a and b in the following scheme.
Br
a
Answer:
a = NBS, hv
b
b = KOH, ethanol
Label each transformation below as:
a.
b.
c.
an oxidation
a reduction
neither an oxidation nor a reduction
Place the letter corresponding to the correct answer in the blank to the left of the transformation.
OH
OH
NH2
OH
35. _____
O
Answer:
a
OH
36. _____
Answer:
88
c
Chapter 10: Organohalides
OH
O
37. _____
Answer:
b
CH3
38. _____
CH3
CH3CHCH3
CH3CCH3
Br
Answer:
a
39. Rank the following compounds in order of increasing oxidation level. Place the number rank (1 = lowest;
4 = highest) in the blank below the structure.
O
CH3C
O
OH
CH3CH3
CH3C
H
H2C
CH2
Answer:
O
O
CH3C
OH
4
CH3CH3
CH3C
1
Test Items for McMurry’s Organic Chemistry, Seventh Edition
3
H
H2C
CH2
2
89