Angle strain
Encyclopedia
Angle strain, also called Baeyer strain in cyclic molecules
, is the resistance associated with bond angle compression or bond angle expansion. It 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 because non-cyclic molecules will thermodynamically
conform to the most favorable stable state.
Angle strain is subdivided into two categories: small angle strain and large angle strain.
In cycloalkanes, optimum overlap of atomic orbital
s is achieved at 109.5°. But angle strain occurs when the carbon-carbon bonds can't be at 109.5° in cycloalkanes. Having higher angle strain makes a molecule more unstable and reactive. Maximum bond strength results from effective overlap of atomic orbitals in a chemical bond
.
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.
. Angle strain and torsional strain combine to create ring strain
that affects cyclic molecules. These measurements commonly use heat of combustion
.
where X is the heat of combustion for a CH2 group (energy per CH2).
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.
The reference value is 658.6 kJ per mole of methylene group. The reference value was obtained from an unstrained long-chain alkane.
In cyclohexane the total ring strain is 0 kJ.
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.
Examples of molecules with angle strain include cycloalkane
s, cyclophane
s, platonic hydrocarbons
and pyramidal alkene
s.
Some specific examples are:
Cyclic compound
In chemistry, a cyclic compound is a compound in which a series of atoms is connected to form a loop or ring.While the vast majority of cyclic compounds are organic, a few inorganic substances form cyclic compounds as well, including sulfur, silanes, phosphanes, phosphoric acid, and triboric acid. ...
, is the resistance associated with bond angle compression or bond angle expansion. It 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 because non-cyclic molecules will thermodynamically
Thermodynamics
Thermodynamics is a physical science that studies the effects on material bodies, and on radiation in regions of space, of transfer of heat and of work done on or by the bodies or radiation...
conform to the most favorable stable state.
Angle strain is subdivided into two categories: small angle strain and large angle strain.
In cycloalkanes, optimum overlap of atomic orbital
Atomic orbital
An atomic orbital is a mathematical function that describes the wave-like behavior of either one electron or a pair of electrons in an atom. This function can be used to calculate the probability of finding any electron of an atom in any specific region around the atom's nucleus...
s is achieved at 109.5°. But angle strain occurs when the carbon-carbon bonds can't be at 109.5° in cycloalkanes. Having higher angle strain makes a molecule more unstable and reactive. Maximum bond strength results from effective overlap of atomic orbitals in a chemical bond
Chemical bond
A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...
.
History
The most common cyclic compounds have five or six carbons in their ring. Adolf von BaeyerAdolf von Baeyer
Johann Friedrich Wilhelm Adolf von Baeyer was a German chemist who synthesized indigo, and was the 1905 recipient of the Nobel Prize in Chemistry. Born in Berlin, he initially studied mathematics and physics at Berlin University before moving to Heidelberg to study chemistry with Robert Bunsen...
received a Nobel Prize
Nobel Prize
The Nobel Prizes are annual international awards bestowed by Scandinavian committees in recognition of cultural and scientific advances. The will of the Swedish chemist Alfred Nobel, the inventor of dynamite, established the prizes in 1895...
in 1905 for the discovery of the Baeyer strain theory
Baeyer strain theory
Baeyer strain theory or strain theory explains specific behaviour of chemical compounds in terms of bond angle strain.It was proposed by Adolf von Baeyer in 1885 to account for the unusual chemical reactivity in ring-opening reactions of cyclopropanes and cyclobutanes where this angle strain is...
, which was an explanation of the relative stabilities of cyclic molecules.
Measurement of angle strain
A quantitative measure for angle strain is strain energyStrain energy
In a molecule, strain energy is released when the constituent atoms are allowed to rearrange themselves in a chemical reaction or a change of chemical conformation in a way that:* angle strain,* torsional strain,* ring strain and/or steric strain,...
. Angle strain and torsional strain combine to create ring strain
Ring strain
In organic chemistry, ring strain is the tendency of a cyclic molecule, such as cyclopropane, to destabilize when its atoms are in non-favorable high energy spatial orientations...
that affects cyclic molecules. These measurements commonly use heat of combustion
Heat of combustion
The heat of combustion is the energy released as heat when a compound undergoes complete combustion with oxygen under standard conditions. The chemical reaction is typically a hydrocarbon reacting with oxygen to form carbon dioxide, water and heat...
.
- CnH2n + 3/2 n O2 → n CO2 + n H2O + n X
where X is the heat of combustion for a CH2 group (energy per CH2).
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.
The reference value is 658.6 kJ per mole of methylene group. The reference value was obtained from an unstrained long-chain alkane.
In cyclohexane the total ring strain is 0 kJ.
Examples
In cycloalkanes, each carbon is bonded nonpolar covalentlyCovalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....
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
Cyclohexane
Cyclohexane is a cycloalkane with the molecular formula C6H12. Cyclohexane is used as a nonpolar solvent for the chemical industry, and also as a raw material for the industrial production of adipic acid and caprolactam, both of which being intermediates used in the production of nylon...
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.
Examples of molecules with angle strain include cycloalkane
Cycloalkane
Cycloalkanes are types of alkanes that have one or more rings of carbon atoms in the chemical structure of their molecules. Alkanes are types of organic hydrocarbon compounds that have only single chemical bonds in their chemical structure...
s, cyclophane
Cyclophane
A cyclophane is a hydrocarbon consisting of an aromatic unit and an aliphatic chain that forms a bridge between two non-adjacent positions of the aromatic ring. More complex derivatives with multiple aromatic units and bridges forming cagelike structures are also known...
s, platonic hydrocarbons
Platonic hydrocarbons
Platonic hydrocarbons are the molecular representation of platonic solid geometries with vertices replaced by carbon atoms and with edges replaced by chemical bonds...
and pyramidal alkene
Pyramidal alkene
Pyramidal alkenes are alkenes in which the two carbon atoms making up the double bond are not coplanar with their four substituents . This deformation from a trigonal planar geometry to a tetrahedral molecular geometry is the result of angle strain induced in the molecule due to geometric constraints...
s.
Some specific examples are:
- cyclopropaneCyclopropaneCyclopropane is a cycloalkane molecule with the molecular formula C3H6, consisting of three carbon atoms linked to each other to form a ring, with each carbon atom bearing two hydrogen atoms...
, C3H6 — the C-C-C bond angles are 60° whereas tetrahedral 109.5° bond angles are expected. The intense angle strain leads to nonlinear orbital overlap of its sp3 orbitals. Because of the bond's instability, cyclopropane is more reactive than other alkanes. Since any three points make a plane and cyclopropane has only three carbons, cyclopropane is planar. The H-C-H bond angle is 115° whereas 106° is expected as in the CH2 groups of propane. - cyclobutaneCyclobutaneCyclobutane is an organic compound with the formula 4. Cyclobutane is a colourless gas and commercially available as a liquefied gas. Derivatives of cyclobutane are called cyclobutanes...
, C4H8 — if it was completely square planar its bond angles would be 90° whereas tetrahedral 109.5° bond angles are expected. However, the actual C-C-C bond angle is 88° because it has a slightly folded form to relieve some torsional strain at the expense of slightly more angle strain. The high strain energy of cyclobutane is primarily from angle strain. - cyclopentaneCyclopentaneCyclopentane is a highly flammable alicyclic hydrocarbon with chemical formula 510 and CAS number 287-92-3, consisting of a ring of five carbon atoms each bonded with two hydrogen atoms above and below the plane. It occurs as a colorless liquid with a petrol-like odor. Its melting point is −94 °C...
, C5H10 — if it was completely rectangular planar pentagon its bond angles would be 108° whereas tetrahedral 109.5° bond angles are expected. However, it has an unfixed puckered shape that undulates up and down. The unstable half-chair conformation has angle strain in the C-C-C angles which range from 109.86° to 119.07°. - ethylene oxideEthylene oxideEthylene oxide, also called oxirane, is the organic compound with the formula . It is a cyclic ether. This means that it is composed of two alkyl groups attached to an oxygen atom in a cyclic shape . This colorless flammable gas with a faintly sweet odor is the simplest epoxide, a three-membered...
, CH2OCH2 - cubaneCubaneCubane is a synthetic hydrocarbon molecule that consists of eight carbon atoms arranged at the corners of a cube, with one hydrogen atom attached to each carbon atom. A solid crystalline substance, cubane is one of the Platonic hydrocarbons. It was first synthesized in 1964 by Philip Eaton, a...
, C8H8