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Ring strain

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Title: Ring strain  
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Subject: Ring-opening polymerization, Ring-closing metathesis, Epoxide, Prelog strain, Bredt's rule
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Ring strain

In heat of combustion for these small rings is elevated.[1][2]

Ring strain results from a combination of angle strain, conformational strain or Pitzer strain, and transannular strain or van der Waals strain. The simplest examples of angle strain are small cycloalkanes such as cyclopropane and cyclobutane, which are discussed below. Furthermore, there is often eclipsing in cyclic systems which cannot be relieved.

1.1.1-Propellane (C2(CH2)3) is one of the most strained molecules known.

Contents

  • Angle strain (Baeyer strain) 1
    • Alkanes 1.1
    • Angle strain in alkenes 1.2
  • Examples 2
  • Application 3
  • See also 4
  • References 5

Angle strain (Baeyer strain)

Alkanes

In alkanes, optimum overlap of atomic orbitals is achieved at 109.5°. The most common cyclic compounds have five or six carbons in their ring.[3] Adolf von Baeyer received a Nobel Prize in 1905 for the discovery of the Baeyer strain theory, which was an explanation of the relative stabilities of cyclic molecules.[3]

Angle strain occurs when bond angles deviate from the ideal bond angles to achieve maximum bond strength in a specific chemical conformation. Angle strain typically affects cyclic molecules, which lack the flexibility of acyclic molecules.

Angle strain destabilizes a molecule, as manifested in higher reactivity and elevated heat of combustion. Maximum bond strength results from effective overlap of atomic orbitals in a chemical bond. A quantitative measure for angle strain is strain energy. Angle strain and torsional strain combine to create ring strain that affects cyclic molecules.[3]

CnH2n + 3/2 n O2 → n CO2 + n H2O - ΔHcombustion

Normalized energies that allow comparison of ring strains are obtained by measuring per methylene group (CH2) of the molar heat of combustion in the cycloalkanes.[3]

ΔHcombustion per CH2 - 658.6 kJ = strain per CH2

The value 658.6 kJ per mole is obtained from an unstrained long-chain alkane.[3]

Strain of some common [4]
Ring size Strain energy (kcal/mol) Ring size Strain energy (kcal/mol)
3 27.5 10 12.4
4 26.3 11 11.3
5 6.2 12 4.1
6 0.1 13 5.2
7 6.2 14 1.9
8 9.7 15 1.9
9 12.6 16 2.0

Angle strain in alkenes

Cyclic alkenes are subject to strain resulting from distortion of the sp2-hybridized carbon centers. Illustrative is C60 where the carbon centres are pyramidalized. This distortion enhances the reactivity of this molecule. Angle strain also is the basis of Bredt's rule which dictates that bridgehead carbon centers are not incorporated in alkenes because the resulting alkene would be subject to extreme angle strain.

Bredt's rule which indicates that alkenes rarely incorporate bridgehead carbon centers. This rule is a consequence of angle strain.

Examples

In cycloalkanes, each carbon is bonded nonpolar covalently to two carbons and two hydrogen. The carbons have sp3 hybrization and should have ideal bond angles of 109.5°. Due to the limitations of cyclic structure, however, the ideal angle is only achieved in a six carbon ring — cyclohexane in chair conformation. For other cycloalkanes, the bond angles deviate from ideal. In cyclopropanes (3 carbons) and cyclobutanes (4 carbons) the C-C bonds are 60° and ~90° respectively.

Molecules with a high amount of ring strain consist of three, four, and some five-membered rings, including: cyclopropanes, cyclopropenes, cyclobutanes, cyclobutenes, [1,1,1]propellanes, [2,2,2]propellanes, epoxides, aziridines, cyclopentenes, and norbornenes. These molecules have bond angles between ring atoms which are more acute than the optimal tetrahedral (109.5°) and trigonal planar (120°) bond angles required by their respective sp3 and sp2 bonds. Because of the smaller bond angles, the bonds have higher energy and adopt more p-character to reduce the energy of the bonds. In addition, the ring structures of cyclopropanes/enes and cyclclobutanes/enes offer very little conformational flexibility. Thus, the substituents of ring atoms exist in an eclipsed conformation in cyclopropanes and between gauche and eclipsed in cyclobutanes, contributing to higher ring strain energy in the form of Van der Waals repulsion.

Other cycloalkanes that do not have 3- and 4-membered rings can be strained. These include cyclophanes, platonic hydrocarbons, pyramidal alkenes, and cyclic alkynes.

  • [4]
  • [4]
  • cyclopentane, C5H10 — if it was a completely regular planar pentagon its bond angles would be 108°, but tetrahedral 109.5° bond angles are expected.[3] However, it has an unfixed puckered shape that undulates up and down.[3] The unstable half-chair conformation has angle strain in the C-C-C angles which range from 109.86° to 119.07°.[5]
  • ethylene oxide, CH2OCH2
  • cubane, C8H8

Application

The potential energy and unique bonding structure contained in the bonds of molecules with ring strain can be used to drive reactions in Ring opening metathesis polymerisation, photo-induced ring opening of cyclobutenes, and nucleophilic ring-opening of epoxides and aziridines.

See also

References

  1. ^ Smith, Michael B.;  
  2. ^ Wiberg, K. (1986). "The Concept of Strain in Organic Chemistry".  
  3. ^ a b c d e f g h i j k Wade, L. G. "Structure and Sterochemistry of Alkanes." Organic Chemistry. 6th ed. Upper Saddle River, NJ: Pearson Prentice Hall, 2006. 103-122. Print.
  4. ^ a b c d e Anslyn, Eric V., and Dennis A. Dougherty. "Chapter 2: Strain and Stability." Modern Physical Organic Chemistry. Sausalito, CA: University Science, 2006. 100-09. Print. [2]
  5. ^ http://wetche.cmbi.ru.nl/organic/cyclohexane/jm/exer3.html
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