Angle strain cycloalkanes

Conformational analysis is far simpler m cyclopropane than m any other cycloalkane Cyclopropane s three carbon atoms are of geometric necessity coplanar and rotation about Its carbon-carbon bonds is impossible You saw m Section 3 4 how angle strain m cyclopropane leads to an abnormally large heat of combustion Let s now look at cyclopropane m more detail to see how our orbital hybridization bonding model may be adapted to molecules of unusual geometry... 

The Baeyer strain theory is useful to us in identifying angle strain as a destabilizing effect. Its fundfflnental flaw is its assumption that the rings of cycloalkanes are planar-. With the exception of cyclopropane, cycloalkanes are nonplanar. Sections 3.5-3.13 describe the shapes of cycloalkanes. We ll begin with cyclopropane. 

Angle strain (Section 4.3) The strain introduced into a molecule when a bond angle is deformed from its ideal value. Angle strain is particularly important in small-ring cycloalkanes, where it results from compression of bond angles to less than their ideal tetrahedral values. 

Ring strain the instability of cycloalkanes due to their cyclic structures. => angle strain and torsional strain. 

If one is willing to consider a carbon-carbon double bond as a two-membered ring, then ethene, C2H4, is the simplest possible cycloalkane ( cycloethane ). As such, C2H4 has C-C-C valence angles of 0° and therefore an angle strain of 109.5° at each CH2 group compared to the tetrahedral value ... 

Cycloalkane. (CH2) n Angle strain at each CH2, dega Heat of combustion,b A/-/0, kcal mole-1 Heat of combustion per CH2, AH°/n, kcal Total strain,0 kcal mole-1... 

We expect that the total strain in cycloalkanes of the type (CH2) should decrease rapidly in the order = 2>ra = 3> = 4. However, the data of Table 12-3 show that the order actually is 3 = 4 > 2. This difference in order often is disguised by dividing the heats of combustion by the numbers of CH2 groups and showing that the heats of combustion per CH2 are at least in the order expected from bond-angle strain. This stratagem does not really solve the problem. 

The C—C=C angle in alkenes normally is about 122°, which is 10° larger than the normal C—C—C angle in cycloalkanes. This means that we would expect about 20° more angle strain in small-ring cycloalkenes than in the cycloalkanes with the same numbers of carbons in the ring. Comparison of the data for cycloalkenes in Table 12-5 and for cycloalkanes in Table 12-3 reveals that this expectation is realized for cyclopropene, but is less conspicuous for cyclobutene and cyclopentene. The reason for this is not clear, but may be connected in part with the C-H bond strengths (see Section 12-4B). 

From heat of combustion data, cyclopropane has 26.7 kcal/mol (111.6 kJ/mol) of strain energy. Most of this strain is due to angle strain, but the contribution due to torsional strain is also significant. As we will see later, this strain energy causes cyclopropane to be more reactive than a normal alkane or cycloalkane. However, even though cyclopropane rings are reactive, they are fairly common in organic chemistry. 

If a cycloalkane requires bond angles other than 109.5°, the orbitals of its carbon-carbon bonds cannot achieve optimum overlap, and the cycloalkane must have some angle strain (sometimes called Baeyer strain) associated with it. Figure 3-14 shows that a planar cyclobutane, with 90° bond angles, is expected to have significant angle strain. 

Compare the energies of cycloalkanes, and explain how their angle strain and torsional strain combine to give the total ring strain. Problems 3-43,44, and 45... 

Baeyer fi theory was wrong- for a very simple reason Ho assumed that rings are flat. In fact, though, most cycloalkanes are not flat they adopt puckered Ihree-dimensioival conformations that allow bond angles tv be nearly tetrahedral. Only for three- and four-membered rings in his concept of angle strain important. 

In addition to angle strain and torsional trdin. stertr Mrain is yet a third factor that cont ributes to the over l strain energy of cycloalkanes. As in gauche butane (Senonbonded atom in o molecule repel ua other if they approach tcK> closely and attoropt to occupy the same... 

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