6: 24
ChemActivity Alkanes and Alkenes
ChemActivity 6: Alkanes and Alkenes, Exercises for Part A
1.
H3C CH3 C HH HH Newman Projection of Eclipsed Conformation
2.
R2 is neither anti nor gauche to R3
R2
anti to R3
H C H R1
H R3
gauche to R3
R1 is gauche to R3
3.
Potential Energy
0o
Me Me H C HH HH H
60o
Me Me C H H HH
120o
H Me
180o 240o Rotation of C1-C2 bond
Me H H C H Me H Me H Me H C HH H HMe Me
300o
Me H C H H HH
360o
Me Me C HH
C
eclipsed
staggered gauche
eclipsed
staggered anti
eclipsed
staggered gauche
eclipsed
H H C C
H
H C
H
C H H H
H
b. 4.
Me H Et Br H H
H
Br Et H H Me H
lowest potential energy conf.
highest potential energy staggered conf.
5. 6.
All are Newman projections of 2-methylpentane (shown in the box). Read the assigned sections in your text, and do the assigned problems.
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ChemActivity 6: Alkanes and Alkenes
25
ChemActivity 6: Alkanes and Alkenes, Exercises for Part B
8.
9.
The possibilities shown ignore stereoisomerism (which is introduced in the next section).
10. No. All isomers are shown (not including stereoisomers). 11.
F
F
F
F
F
F
F
F
12.
A ring is equivalent to a double bond in terms of the impact on a molecular formula in that introducing either a ring or a double bond reduces the number of hydrogens by two.
13. The purpose of CTQ 21 is to remind you that (except in very rare circumstances that we will discuss later) rotating a single bond does not change the identity of a molecule. That is, hexane can be represented many different ways, but it is still hexane. Another way to think about it is that, at a given moment, a bottle of hexane (which is a low-boiling liquid similar to gasoline) contains molecules of hexane in every possible conformation. By convention, chemists usually represent a molecule in its simplest or lowest potential energy conformation. For hexane this is the representation at the top left corner of Model 7. For cyclohexane this a flat looking hexagon. (In the next ChemActivity we will learn that, in fact, cyclohexane is not normally flat.) 14. Read the assigned sections in your text, and do the assigned problems.
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26
ChemActivity 6: Alkanes and Alkenes
ChemActivity 6: Alkanes and Alkenes, Exercises for Part C
15. 16. There are 2 identical groups (H's) attached to one end of 1-butene so it is neither E nor Z.
Neither E Neither Z Neither Neither
17.
18.
E Z
I
E
II
Z
III
Z
IV
E
V
H3C
CH2 CH2
H2C
C
CH
Z E
Z E
an E double bond within ring a is usually very unfavorable
the E,Z stereoisomer is the same as the one at the top of the page
Neither Neither
VI VII
Neither
VIII
Neither
IX
Neither
X
O
b.
A terminal double bond is one that has a CH2 group. Since the two H's constitute two identical groups attached to the same carbon in the double bond, according to Memorization Task 6.4 such a double bond is neither E or Z. Constitutional isomers
c.
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ChemActivity 6: Alkanes and Alkenes
27
19.
(E)-3,4-dimethyl-3-heptene
(Z)-3,4-dimethyl-3-heptene
Though the largest groups on either end of the double bond in these molecules are not identical, they are still trans and cis, respectively, because they each have a pair of identical groups (methyl groups) trans and cis, respectively, to one another. The name cis-3-methylpent-2-ene does not make it clear whether you intend the methyl groups to be cis to each other or the two largest groups to be cis to each other. In general, you should only use cis and trans to name disubstituted alkenes with one H attached to each double bond carbon. These are both trisubstituted alkenes.
20.
(Z)-3-methylpent-2-ene
(E)-3-methylpent-2-ene
21. a. see below b. The cis and trans nomenclature should only be used to name disubstituted alkenes with exactly one H attached to each double bond carbon. Trisubstituted and tetrasubstituted alkenes such as those in this question and the previous two questions should be named using the E/Z nomenclature to avoid ambiguity.
(E)-4-ethyl-5-methylnon-4-ene
(Z)-4-ethyl-5-methylnon-4-ene
22. Z-2-butene should be in Box A and E-2-butene should be in Box B. This is because the E stereoisomer is slightly lower in potential energy than the Z stereoisomer due to steric effects. 23.
constitutional isomers
Cl
conformers (conformational stereoisomers)
Cl
configurational stereoisomers
24. The terms "conformers" and "same" are often synonyms because two conformations of the same molecule can be shown on paper, but they usually represent the same molecule with the same name, same physical properties, etc., and the two conformations could not be isolated under normal conditions in the laboratory. The words are not synonyms at very low temperature or with special molecules that have very large barriers to single bond rotation. In such cases, two different conformers of the same molecule may be isolated and studied. 25.
26. Read the assigned sections in your text, and do the assigned problems.
Copyright Houghton Mifflin Harcourt Publishing Company. All rights reserved.