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Baeyer Strain Theory Introduction Van't Hoff and Lebel Proposed

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Baeyer Strain Theory instable cycloalkanes. The large ring systems involve negative strain hence do not exists. Introduction The bond angles in cyclohexane and higher Van’t Hoff and Lebel proposed tetrahedral cycloalkanes (cycloheptane, cyclooctane, geometry of carbon. The bond angel is of 109˚ cyclononane……..) are not larger than 109.5 28' (or 109.5˚) for carbon atom in tetrahedral because the carbon rings of those compounds geometry (methane molecule). Baeyer are not planar (flat) but they are puckered observed different bond angles for different (Wrinkled). cycloalkanes and also observed some These assumptions are helpful to understand different properties and stability .On this instability of cycloalkane ring systems. basis, he proposed angle strain theory. The theory explains reactivity and stability of Cyclopropane is more prone to cycloalkanes. Baeyer proposed that the undergo ring opening reaction than optimum overlap of atomic orbitals is cyclobutane or cyclopentane achieved for bond angel of 109.5 .In short, it is Cyclopropane is more reactive than ideal bond angle for alkane compounds. cyclobutane and cyclopentane Effective and optimum overlap of atomic orbitals produces maximum bond strength The ring of cyclopropane is triangle. hance stable molecule. All the three angles are of 60 in place of 109.5 (normal bond angle for If bond angles deviate from the ideal carbon atom) to adjust them into then ring produce strain. triangle ring system. Higher the strain higher the In same manner, cyclobutane is instability. square and the bond angles are of 90o in place of 109.5o (normal bond Higher strain increases reactivity and angle for carbon atom) to adjust them increases heat of combustion. into square ring system. Baeyer proposed “any deviation of The deviation for cyclopropane and bond angle from ideal bond angle cyclobutane ring systems then normal value 109.5 will produce a strain in tetrahedral angle will produce strain molecule. in ring. The ring strain will make them Higher the deviation lesser the unstable as compare to molecules instability. having tetrahedral bond angle. So, cyclopropane and cyclobutane will Baeyer’s angle strain theory using easily undergo ring opening reactions concept of angle strain. to form more stable open chain compounds. Baeyer’s theory is based upon some assumptions as following. Now compare the stability of cyclopropane and cyclobutane All ring systems are planar. Deviation from normal tetrahedral angles results in to The bond angle in cyclopropane is 60˚. The normal tetrahedral bond angle value is Cyclohexane is unstable ring due to higher 109.5˚. ring strain. It is difficult to synthesize for cyclohexane and higher ring systems as That is why, deviation = (normal tetrahedral deviation from the normal tetrahedral value bond angle) – (actual bond angle) (109.5˚) would be larger. The larger deviation Deviation = 109.5˚ – 60 = 49.5˚ will result into more strain and the ring system will be unstable. In conclusion, Baeyer The bond angle in cyclobutane is 90˚. proposed that ring systems smaller or larger than cyclopentane or cyclohexane are The normal tetrahedral bond angle value is unstable due to higher ring strain. Therefore, 109.5˚. he assumed that cyclopropane and That is why, deviation = (normal tetrahedral cyclobutane easily undergo ring opening bond angle) – (actual bond angle) reaction whereas larger ring systems are difficult to synthesize. Deviation = 109˚ 5’ – 90 = 19.5˚. Baeyer explained why synthesis of larger ring Deviation for cyclopropane is 49.5˚ is difficult Deviation for cyclobutane is 19.5˚ (Baeyer was incorrect in presuming the larger ring systems are flat) It is The deviation is higher for cyclopropane than difficult to explain the formation of cyclobutane therefore cyclopropane is more larger ring system on the basis of prone to undergo ring opening reactions. As a angle strain. Because, larger ring result of this, the strain is more in systems are not flat or planar that cyclopropane as compare to cyclobutane. It was assumed by Baeyer. These larger will make cyclopropane less stable than ring systems are puckered (wrinkled) cyclobutane. So, cyclopropane easily to eliminate possible angle strain. undergoes ring opening reaction as compare to cyclobutane. According to Baeyer, the Although it is difficult to synthesize relative order of stability for some common large rings according to Baeyer cycloalkanes is as under. theory, there are several methods for preparation of large rings. Such Cyclopentane > Cyclohexane > Cyclobutane > methods are carried out in highly Cyclopropane diluted solutions of dihalides in which Actual observed order of stability for these the chain lengthening reactions would cycloalkanes is as under. not occur but ring closing reactions will take place. Cyclohexane > Cyclopentane > Cyclobutane > Cyclopropane Five and six membered ring systems (Cyclopentane and Cyclohexane) are According to Baeyer, the bond angle in favorable to synthesize as they are cyclopentane is 108˚ (the geometry is large enough to free from strain and pentagonal) that is very close to tetrahedral small enough to undergo ring closure angle (109.5˚), so it is almost free from ring reaction. strain. Baeyer also proposed some facts for cyclohexanes that stood incorrect later on. Heat of Combustion and Relative Stability of Cycloalkanes ideal tetrahedral alue of 109.5˚ hence these ring systems are unstable and The amount of heat evolved when easily undergo ring opening reactions. one mole of compound is burned in There is much angle strain in carbon dioxide and water is referred cyclopropane and cyclobutane. as heat of combustion. Heat of combustion is important property to Baeyer also proposed that know the stability and other valuable cyclopentane is not need to be planar information. but it is planar as in that condition the angle is much near to ideal Cyclopropane and cyclobutane evolve tetrahedral angle. more energy per CH2 group than its open chain compound and that may Drawbacks of Baeyer’s angle strain theory be the reason for the instability and Baeyer was not able to explain the their tendency to undergo chain opening reactions easily. Heat of effect of angle strain in larger ring combustion value for cyclopropane systems. and cyclobutane are higher than open According to Baeyer cyclopentane chain compounds (9.2 and 7.0 should be much stable than Kcal/mole respectively). According to cyclohexane but practically it is data, cyclopropane is less stable than reversed. cyclobutane. For these ring systems Baeyer’s theory was correct. Larger ring systems are not possible according to Baeyer as they have On examining data for cyclopentane, negative strain but they exist and then it can be seen that the value for much stable. heat of combustion is higher than open chain compound (157.4 Larger ring systems are not planar but Kcal/mole). The value is much higher puckered to eliminate angle strain. than the open chain compound for Baeyer’s most stable compound, cyclopentane. For other larger ring systems, the values for heat of combustion are very close to the open chain compound that indicates stability of those compounds. Here Baeyer’s theory was incorrect to justify this stability. Successfulness of Baeyer’s angle strain theory Baeyer rightly proposed that cyclopropane and cyclobutane are flat molecule and having angle of 60˚ and 90˚ those are much deviated from the .
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