By far the most common bicycloalkanes, and the ones we concentrate on in this section, are decalin and hydrindane (Section 2.4B).
Decalin Hydrindane
Two stereoisomers of decalin and hydrindane are possible depending on whether the two hydrogen atoms at the ring junction are trans or cis to each other.
If we draw conformations for the six-membered rings in the two decalins, we see that each ring can exist in its more stable chair conformation. In trans-decalin, the hydrogens at the ring junction are axial to both rings; that is, the ring-junction hy- drogen above the plane of the rings is axial to ring A and to ring B. Likewise, the ring-junction hydrogen below the plane of the ring is axial to both rings. The situa- tion is different in cis-decalin. Each ring-junction hydrogen is axial to one ring but equatorial to the other ring.
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94 Chapter 2 Alkanes and Cycloalkanes H
A B
H
Axial to both rings
trans -Decalin
H
A B
H
Equatorial to ring A and axial to ring B
cis -Decalin
H H
H H
Let us look more closely at trans-decalin, by far the more common stereoisomer of decalin. An important feature of this bicycloalkane is that each ring is locked into one chair conformation; neither ring can invert to its alternative chair. This means, for example, that if an !OH group is equatorial in a decalinol (a decalin alcohol), it remains equatorial; it cannot become axial because the cyclohexane ring is locked into this one conformation. Likewise, if an !OH group is axial, it remains axial.
HO H
A B
H
H H HO
Suppose you are given the structural formula on the left for the decalinol. Can you tell from looking at this structure whether the !OH group is axial or equa- torial? You can’t tell directly, but you can fi gure it out. Remember that, in trans- decalin, the H atoms at the ring junctions are axial to each ring. Remember also that in a chair cyclohexane, axial is up on one carbon, down on the next, up on the next, and so on. Therefore, if you start with the axial group at either ring junction and work your way from one carbon to the next until you come to the carbon bear- ing the !OH group, you come to the conclusion that the !OH on the structural formula is equatorial to ring A.
A good example of the occurrence of these types of ring systems is in the ste- roids, all of which contain a carbon skeleton consisting of three six-membered rings and one fi ve-membered ring connected as shown here. This ring system is present in both animal and plant steroids. Steroids are present in human metabolism as cholesterol, steroid hormones, and bile acids (Section 26.4).
The steroid nucleus B
A
C D
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2.6 Cis,Trans Isomerism in Cycloalkanes and Bicycloalkanes 95 Following are two stereorepresentations for cholestanol. In the conformational
representation on the right, notice that all ring junctions are trans, all groups at each ring-junction atom are axial to the ring, and the !OH group on ring A is equatorial.
CH3 CH3
CH3
H
H H
H
HO
HO
Cholestanol
H H H
H CH3
Another type of bicycloalkane is a six-membered ring in which an added CH2
group forms a bridge between carbons 1 and 4. You can view and draw this mol- ecule from any number of perspectives. What becomes obvious if you view it from the side, as in (c), is that the one-carbon bridge locks the six-membered ring into a boat conformation. Notice that, even though (a) and (b) show the carbon skeleton of the molecule, it is not obvious from them that a locked boat conformation is em- bedded in the molecule. The lesson here is that it is essential to draw a molecule as a three-dimensional shape to best reveal what you want to show.
Nature is by no means limited to carbon atoms in six- membered rings. Tetrodotoxin, one of the most potent toxins known, is composed of a set of interconnected six-membered rings, each in a chair conformation. All but one of these rings have atoms other than carbon in them. Tetrodotoxin is produced in the liver and ovaries of many species of Tetraodontidae, especially the puffer fish, so called because it inflates itself to an almost spherical spiny ball when it is alarmed. The puffer fi sh
Chemical Connections
The Poisonous Puffer Fish
is evidently a species that is highly preoccupied with de- fense, but the Japanese are not put off. They regard the puffer, called “fugu” in Japanese, as a delicacy. To serve it in a public restaurant, a chef must be registered as suffi ciently skilled in removing the toxic organs so as to make the fl esh safe to eat.
Symptoms of tetrodotoxin poisoning begin with at- tacks of severe weakness, progressing to complete paraly- sis and eventual death. Tetrodotoxin blocks sodium ion channels, which are essential for neurotransmission. This prevents communication between neurons and muscle cells and results in the fatal symptoms described.
A puffer fi sh with its body infl ated.
© David Fleetham/Alamy
O O O–
OH
OH NH2⫹
CH2OH HO Tetrodotoxin
H N
N H HO
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96 Chapter 2 Alkanes and Cycloalkanes
3 2 1
1
4
4
4 1
1
4 6 5
2 5 3
6
(a) (b) (c) Camphor
O one carbon bridge between carbons
1 and 4 of the six-membered ring
An example of a natural product containing this bicyclic skeleton is camphor.
Another example of a carbon skeleton that contains several six-membered rings, all of which are locked into chair conformations, is adamantane (c). To un- derstand how the carbon skeleton of adamantane can be constructed, imagine that you (a) start with a chair cyclohexane, (b) add the three axial bonds on the top side of the ring, and (c) then connect each of the axial bonds to a CH group. You now have adamantane, a compound fi rst isolated from petroleum. Amantadine, a 1° amino derivative of adamantane, is an antiviral agent used to treat infl uenza A.
join the three axial bonds to a CH group add three
axial bonds
NH2
(a) Cyclohexane (b) (c) Adamantane Amantadine
(an antiviral agent)
2.7 Physical Properties of Alkanes and Cycloalkanes
The most important property of alkanes and cycloalkanes is their almost com- plete lack of polarity. As we saw in Section 1.2B, the difference in electronegativity between carbon and hydrogen is 2.5 2 2.1 5 0.4 on the Pauling scale, and, given this small difference, we classify a C!H bond as nonpolar covalent. Therefore, alkanes are nonpolar compounds, and only weak interactions exist between their molecules.