What Are Cycloalkanes

Similarly, hydrogens are also classified as primary, secondary, or tertiary, depending on the type of carbon to which each is bonded. Those bonded to a primary carbon are classified as primary hydrogens, those on a secondary carbon are secondary hydrogens, and those on a tertiary carbon are tertiary hydrogens. 

Classify each carbon atom in the following compounds as 

To classify carbons, determine whether each is bonded to 1 carbon (1°), 2 carbons (2°), 3 carbons (3°), or 4 carbons (4°). 

Classify each hydrogen atom in the following compounds as 1°, 2°, or 3° 

A hydrocarbon that contains carbon atoms joined to form a ring is called a cyclic hydrocarbon. When all carbons of the ring are saturated, we call the hydrocarbon a cycloalkane. Cycloalkanes of ring sizes ranging from 3 to over 30 abound in nature, and, in principle, there is no limit to ring size. Five-membered (cyclopentane) and six-membered (cyclohexane) rings are especially abundant in nature and have received special attention. 

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What are the facts To measure the amount of strain in a compound, we have to measure the total energy of the compound and then subtract the energy of a strain-free reference compound. The difference between the two values should represent the amount of extra energy in the molecule due to strain. The simplest way to do this for a cycloalkane is to measure its heat of combustion, the amount of heat released when the compound burns completely with oxygen. The more energy (strain) the compound contains, the more energy (heat) is released on combustion. 

What are the conformations of cycloalkanes Cyclopropane, with only three carbon atoms, is necessarily planar (because three points determine a plane). The C—C—C angle is only 60° (the carbons form an equilateral triangle), much less than the usual sp tetrahedral angle of 109.5°. The hydrogens lie above and below the carbon plane, and hydrogens on adjacent carbons are eclipsed. 

The last point we ll consider about cycloalkane stereochemistry is to see what happens when two or more cycloalkane rings are fused together along a common bond to construct a polycyclic molecule—for example, decalin. 

Angle and torsional strain are major components of the total ring strain in fully reduced cyclic compounds. For cycloalkanes (see Table 1.2), the smaller the ring, the larger the overall strain becomes. What may appear at first to be surprising is that medium-sized rings containing 8-11 atoms... 

What can be said about the thermochemistry of methylated cyclic ketones in any phase other than liquid In that we would have discussed gas-phase species if the data were available, what we are really asking is what can be said about the species of interest as solids , Let us generalize this to see what can be said about the thermochemistry of cyclic ketones as solids. In that the cyclic ketones in the previous section were all formally quite strain-free derivatives of cyclopentanone or cyclohexanone, let us now consider only such species and their cycloalkane analogs. There are no enthalpy-of-fusion data available for cyclopentanone. For cyclohexanone, the temperature-uncorrected fusion enthalpy is about 1.3 kJmol-1—we arbitrarily ignore in this discussion enthalpies of any crystal - different crystal phase enthalpy. The same treatment for cyclohexane discloses the fusion enthalpy of 2.7 kJmol-1. Accordingly, the earlier reported value of 638(6) = 104.4 for the gas phase is sequentially modified to ca 115 kJ mol-1 for the liquid and to 113 kJmol-1 for the solid. For 2-norbomanone, adamantanone and diamantanone (39a, 42 and 43), the gas-phase differences of 115.7 6.8, 96.0 5.1 and 90.6 3.2 become for the solids 124.4 6.8, 116.9 4.5 and 97.8 3.1 kJmol-1. Very roughly, 639( ) is some 10 kJmol-1 more positive for solids than for gases and 639(8) is about 113 kJmol-1 for all of the solids discussed here. 

By examining the conformations of the cycloalkanes, we are able to determine the origin of these strain energies. Let s begin with the smallest one, cyclopropane, and see what causes the large amount of strain energy that it has (Figure 6.8). 

One dffferenoe is that cycloalkanes are less flexible than their open chain connU t>3tt3. TVi are what this means, think about the natare of a carbon-carbon single bond. We know from Section 1.7 that tr bonds are cylindric liy symmetrical. In other words, the intersection of a plane cut tiitg through a carbon-carbon single bond orbital like a circle. Because... 

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