Cyclopropane bond angles

A hydrocarbon is a compound composed of only carbon and hydrogen. A saturated hydrocarbon bas only carbon-carbon single bonds in the molecule. An unsaturated hydrocarbon has one or more carbon-carbon multiple bonds but may also contain carbon-carbon single bonds. A normal hydrocarbon has one chain of consecutively bonded carbon atoms. A branched hydrocarbon has at least one carbon atom not bonded to the end carbon of a chain of consecutively bonded carbon atoms. Instead, at least one carbon atom forms a bond to an inner carbon atom in the chain of consecutively bonded carbon atoms. 3. In order to form, cyclopropane and cyclobutane are forced to form bond angles much smaller than the preferred 109.5° bond angles. Cyclopropane... 

Strong sp -sp a bonds are not possible for cyclopropane, because the 60° bond angles of the ring do not permit the orbitals to be properly aligned for effective overlap (Figure 3.10). The less effective overlap that does occur leads to what chemists refer to as bent bonds. The electron density in the carbon-carbon bonds of cyclopropane does not lie along the internuclear- axis but is distr-ibuted along an arc between the two carbon atoms. The r-ing bonds of cyclopropane are weaker than other carbon-carbon a bonds. 

In addition to angle strain, cyclopropane is destabilized by torsional strain. Each C—H bond of cyclopropane is eclipsed with two others. 

Cyclopropane is the most strained of all rings, primarily because of the angle strain caused by its 60° (,-C-C bond angles. In addition, cyclopropane also has considerable torsional strain because the C-H bonds on neighboring carbon atoms are eclipsed (Figure 4.4). 

Cyclopropane, C.H, is a hydrocarbon composed of a three-membered ring of carbon atoms, (a) Determine the hybridization of the carbon atoms, (b) Predict the CCC and HCH bond angles at each carbon atom on the basis of your answer to part (a), (c) What must the real CCC bond angles in cyclopropane be (d) What is the defining characteristic of a cr-bond compared with a ir-bond, for example (e) How do the C—C cr-bonds in cyclopropane extend the definition of conventional o-bonds (f) Draw a picture depicting the molecular orbitals to illustrate your answer. 

It might be expected that the strain in cyclopropane, in which the C-C-C bond angles are distorted from 109°28 to 60°, would weaken the bonds and thus lead to an increase in the bond distance. This effect is not observed, however, the carbon-carbon distance in cyclopropane being the same as in the other hydrocarbons to within the accuracy of the investigation. There is even some small indication that the C-C distance in cyclic aliphatic hydrocarbons is slightly smaller (by about 0.01 A.) than the normal distance, the three values reported being 1.53, 1.52, and 1.53 A. 

The noteworthy point is a shortening of C-C bond distance as compared to the aliphatic C-C bond distance which is 1.54 A. This shortening manifests itself in the development of special character of the bonds. Since the sp orbitals of carbon now cannot overlap as effectively as they do in alkanes where perfect end-on overlap occurs the bonds in cyclopropane becomes bent and weak and also lead to an angle strain and so the molecule has greater potential energy. So this bent or banana bond accounts for the most of the ring strain. 

The strain energies of cyclopropane (26.5 kcaFmol) and cyclobutane (26 kcal/ mol) are nearly the same despite the apparent much greater bond angle deformation with the former. One reason is that the weak C—C bonds in cyclopropane are compensated in part by the stronger C—H bonds. The increased strength results from the greater s character, and it is known that C—H bond strengths increase with... 

If the C s of the cyclobutane ring were coplanar, they would form a rigid square with internal bond angles of 90°. The deviation from 109.5° would not be as great as that for cyclopropane, and there would be less angle strain in cyclopropane. However, this is somewhat offset by the fact that the eclipsing strain involves four pairs of H s, one pair more than in cyclopropane. 

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