Ring strain

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In organic chemistry, ring strains are the kind of instability that exists when a bond is in an abnormal angle of molecular shape. Strain is most often discussed for small rings such as cyclopropanes and cyclobutanes, whose internal angle is substantially smaller than the ideal value of about 109 °. Due to its high pressure, the combustion heat for these small rings increases.

Ring strains are produced from a combination of angle strains , conformational strains or Pitzer strains (torsional eklipsing interactions), and transannular strains, also known as Van der Waals strains or Prelog strains. The simplest example of an angular strain is a small cycloalkane such as cyclopropane and cyclobutane.

Video Ring strain

Strain sudut (regangan Baeyer)

Alkanes

In alkanes, the optimal overlap of the atomic orbitals is achieved at 109.5 °. The most common cyclic compounds have five or six carbons in their rings. Adolf von Baeyer received the Nobel Prize in 1905 for the discovery of the Baeyer strain theory, which was an explanation of the relative stability of cyclic molecules in 1885.

The angular strain occurs when the bond angle diverges from the ideal bond angle to achieve maximum bond strength in a specific chemical conformation. Angular strains typically affect cyclic molecules, which lack the flexibility of acyclic molecules.

Angular strains destabilize a molecule, as manifested in higher reactivity and increased heat from combustion. The maximum bond strength is generated from the overlap of the atomic orbitals which are effective in chemical bonds. The quantitative measure for the angular strain is the strain energy. The angular strain and torsional strain combine to create a ring strain that affects the cyclic molecule.

C _n H _2n 3/2 n O ₂ -> n CO ₂ n H ₂ O -? H _burning

Normalized energies allowing the comparison of ring strains are obtained by measuring the groups per methylene (CH ₂ ) of the combustion molar heat in cycloalkanes.

? H _burning per CH ₂ - 658.6 kJ = strain per CH ₂

The value of 658.6 kJ per mol is obtained from an uncontrolled long-chain alkane.

Angle Stress Stress

Cyclic Elken can be strained by distortion of sphythized carbon centers. Illustrative is C ₆₀ where carbon centers are subsidized. This distortion increases the reactivity of this molecule. The angular strain is also the basis of the Bredt rule which determines that the center of the bridge is not incorporated in the alkene because the resulting alkene will experience extreme angular strain.

Maps Ring strain

Example

In cycloalkanes, each carbon is covalently bonded non-covalently to two carbons and two hydrogens. Carbon has an attachment of sp [sup> 3 and should have an ideal bond angle of 109.5 °. Due to the limitations of cyclic structures, however, the ideal angle is achieved only in six carbon-cyclohexane rings on the seat conformation. For other cycloalkanes, the bond angle diverges from the ideal. In cyclopropanes (3 carbon) and cyclobutanes (4 carbon), the C-C bonds are 60 Â ° and ~ 90 Â ° respectively.

Molecules with high number of ring strains consist of three, four, and several five-membered rings, including: cyclopropane, cyclopropene, cyclobutane, cyclobutenes, [1,1,1] propellants, [2,2,2] propellants, epoxides, aziridines, cyclopentenes, and norbornenes. These molecules have a bond angle between the more acute ring atom than the tetrahedral (109.5 °) and trigonal planar (120 °) angular bond required by their respective sp of ³ and sp ² bonds. Because of the smaller bond angles, bonds have higher energy and adopt more p-characters to reduce bond energy. In addition, the cyclopropanes/enes and cyclclobutanes/enes ring structures offer very little conformational flexibility. Thus, the ring atomic substituents exist in the lost conformations in the cyclopropane and between the gauche and the lost cyclobutanes, contributing to higher strain ring energies in the form of Van der Waals repulsions.

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

• cyclopropane, C ₃ H ₆ - The bond angle of C-C-C is 60 Â ° while the bond angle 109.5Ã, Â ° tetrahedral is expected. The intense angular strain causes a nonlinear overlapping orbitals of sp <3> orbitals. Due to bond instability, cyclopropane is more reactive than other alkanes. Since every three dots make the plane and cyclopropane have only three carbons, cyclopropane is planar. The bond angle of H-C-H is 115 Â °, while 106 Â ° is expected as in CH ₂ propane group.
• cyclobutane, C ₄ H ₈ - if it is really a planar square the angle of the bond will be 90 ° while the bond angle 109.5Ã, Â ° tetrahedral is expected. However, the actual C-C-C bond angle is 88 Â ° because it has a slightly folded shape to lighten some torsional strains at the expense of little angular strains. The high strain energy of cyclobutane is primarily derived from angular strains.
• cyclopentane, C ₅ H ₁₀ - if it is a fully planar pentagon, the bond angle will be 108 Â °, but a bond angle of 109.7 Â ° tetrahedral is expected. However, it has an irregular shrink shape that undulates up and down. The unstable half-seat conformation has angular strains at C-C-C angles ranging from 109.86 Â ° to 119.07 Â °.
• ethylene oxide, CH ₂ OCH ₂
• cubane, C ₈ H ₈

Ring strains can be much higher in bicyclic systems. For example, bicyclobutane, C ₄ H ₆ , is recorded as one of the most strained compounds that can be isolated on a large scale; the strain energy is estimated at 63.9 kcalÃ, mol ^-1 (267 kJÃ, mol ^-1 ).

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Apps

The potential energy and unique bonding structures contained in the bonds of molecules with ring strains can be used to induce reactions in organic synthesis. Examples of such reactions are polymerization of the opening metathesis of the Ring, the opening of cyclobutenes photo-rings, and the opening of epoxide and a nucleophilic aziridine rings.

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See also

• Filter (chemical)
• Alkane stereochemistry

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References

Source of the article : Wikipedia