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The bond energy for H 2 O is the average energy required to break each of the two O–H bonds in sequence: Although the two bonds are the equivalent in the original symmetric molecule, the bond-dissociation energy of an oxygen–hydrogen bond varies slightly depending on whether or not there is another hydrogen atom bonded to the oxygen atom.
This is caused by steric effects and bonding interactions along with polar effects caused by the various substituents which are in a given molecule, resulting in changes in its chemical and physical properties. The ortho effect is associated with substituted benzene compounds. There are three main ortho effects in substituted benzene compounds:
The term bond-dissociation energy is similar to the related notion of bond-dissociation enthalpy (or bond enthalpy), which is sometimes used interchangeably.However, some authors make the distinction that the bond-dissociation energy (D 0) refers to the enthalpy change at 0 K, while the term bond-dissociation enthalpy is used for the enthalpy change at 298 K (unambiguously denoted DH° 298).
CH 3 CH 2 OH . Parentheses are used to indicate multiple identical groups, indicating attachment to the nearest non-hydrogen atom on the left when appearing within a formula, or to the atom on the right when appearing at the start of a formula: (CH 3) 2 CHOH or CH(CH 3) 2 OH . In all cases, all atoms are shown, including hydrogen atoms.
monosubstituted benzene 700–750 strong 690–710 strong ortho-disub. benzene 750 strong meta-disub. benzene 750–800 strong 860–900 strong para-disub. benzene 800–860 strong alkynes: any 3300 medium aldehydes: any 2720 medium 2820 C═C acyclic C═C monosub. alkenes 1645 medium 1,1-disub. alkenes 1655 medium cis-1,2-disub. alkenes 1660 ...
LDQ theory has facilitated a more rigorous analysis of bonding in compounds which have conventionally been described in terms of three-centre two-electron bonding. For example, compare the various ways shown below to represent the bonding in the Lewis acid-base adduct of the hydride anion (H − ) and borane (BH 3 ) shown below.
The only way to accomplish this is by occupying both the bonding and antibonding orbitals with two electrons, which reduces the bond order ((2−2)/2) to zero and cancels the net energy stabilization. However, by removing one electron from dihelium, the stable gas-phase species He + 2 ion is formed with bond order 1/2.
The number of electron pairs in the valence shell of a central atom is determined after drawing the Lewis structure of the molecule, and expanding it to show all bonding groups and lone pairs of electrons. [1]: 410–417 In VSEPR theory, a double bond or triple bond is treated as a single bonding group. [1]