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The increase in stability associated with additional substitutions is the result of several factors. Alkyl groups are electron donating by inductive effect, and increase the electron density on the sigma bond of the alkene. Also, alkyl groups are sterically large, and are most stable when they are far away from each other.
For bridged alkenes, Bredt's rule states that a double bond cannot occur at the bridgehead of a bridged ring system unless the rings are large enough. [8] Following Fawcett and defining S as the total number of non-bridgehead atoms in the rings, [9] bicyclic systems require S ≥ 7 for stability [8] and tricyclic systems require S ≥ 11. [10]
Anti addition is in direct contrast to syn addition. In anti addition, two substituents are added to opposite sides (or faces) of a double bond or triple bond, once again resulting in a decrease in bond order and increase in number of substituents. The classical example of this is bromination (any halogenation) of alkenes. [5]
Hyperconjugation can be used to rationalize a variety of chemical phenomena, including the anomeric effect, the gauche effect, the rotational barrier of ethane, the beta-silicon effect, the vibrational frequency of exocyclic carbonyl groups, and the relative stability of substituted carbocations and substituted carbon centred radicals, and the thermodynamic Zaitsev's rule for alkene stability.
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. [6] 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 in 1885.
During alkyl radical disproportionation, an alkane and an alkene are the end products and the bond order of the products increases by one over the reactants. [1] Thus the reaction is exothermic (ΔH = 50–95 kcal/mol (210–400 kJ/mol)) and proceeds rapidly.
The inductive effect can be used to determine the stability of a molecule depending on the charge present on the atom and the groups bonded to the atom. For example, if an atom has a positive charge and is attached to a - I group its charge becomes 'amplified' and the molecule becomes more unstable.
This is because the bond angle for an alkene, C-C=C, is 122°, while the bond angle for an alkane, C-C-C, is 112°. When these carbons form a small ring, the alkene which has a larger bond angle will have to compress more than the alkane causing more bond angle strain. [4] Cycloalkenes have a lower melting point than cycloalkanes of the same size.