Homodesmotic Reactions

Key Questions

• What are states of hybridization in homodesmotic reactions?

  States of hybridization in a homodesmotic reaction refer to whether the bonds are `sp³`, `sp²`, or `sp` hybridized.

  The products and reactants in homodesmotic reactions must also have equal numbers of each type of carbon-carbon bond: `C_(sp^3)"-"C_(sp^3)`, `C_(sp^3)"-"C_(sp^2)`, `C_(sp^3)"-"C_(sp)`, `C_(sp^2)"-"C_(sp^2)`,`C_(sp^2)"-"C_(sp)`, `C_(sp)"-"C_(sp)`, `C_(sp^2)"="C_(sp^2)`, `C_(sp^2)"="C_(sp)`, `C_(sp)"≡"C_(sp)`.

  An example illustrating this definition is the reaction

  

  The reactants and products contain equal numbers of bonds between carbons in the same state of hybridization and hydrogens bound to identical atom types.

  `C_(sp^3)"-"C_(sp^2)` = 1

  `C_(sp^2)"-"C_(sp^2)` = 1

  `C_(sp^2)"-"C_(sp)` = 1

  `C_(sp^2)"="C_(sp^2)` = 3

  `C_(sp)"≡"C_(sp)` = 1

  `C_(sp^3)H_3` = 1

  `C_(sp^2)H_2` = 3

  `C_(sp^2)H` = 2

  `C_(sp)H` = 1

  Ernest Z. · 1 · Dec 26 2014
• Are homodesmotic reactions isodesmic reactions?

  Yes, homodesmotic reactions are a subset of isodesmic reactions.

  An isodesmic reaction is a reaction that has the same number and type of bonds in the reactants and in the products.

  An example of an isodesmic reaction is

  

  Homodesmotic reactions are a subclass of isodesmic reactions.

  Here, the reactants and products are even more similar. They must contain equal numbers of

  • atoms in the same state of hybridization
  • groups containing the same number of H atoms.

  An example of a homodesmotic reaction is

  

  The reactants and products contain equal numbers of bonds between carbons in the same state of hybridization and hydrogens bound to identical atom types.

  `C_(sp^3)"-"O_(sp^3)` = 1

  `C_(sp^3)"-"C_(sp^2)` = 1

  `C_(sp^2)"-"O_(sp^3)` = 1

  `C_(sp^2)"-"C_(sp^2)` = 7

  `C_(sp^2)"="C_(sp^2)` = 6

  `C_(sp^3)H_3` = 2

  `C_(sp^2)H_2` = 2

  `C_(sp^2)H` = 9

  Ernest Z. · 1 · Dec 26 2014
• What are homodesmotic reactions?

  A homodesmotic reaction (from Greek homos "same" + desmos "bond") is a reaction in which the reactants and products contain equal numbers of

  • carbon atoms in the same state of hybridization
  • CH₃, CH₂, and CH groups

  This matching of hybridization and groups makes it easier to evaluate strain energy in rings such as cyclopropane.

  An example of a homodesmotic reaction is

  cyclo-(CH₂)₃ + 3CH₃-CH₃ → 3CH₃CH₂CH₃; `ΔH` = -110.9 kJ/mol

  All C atoms are sp² hybridized, and there are six CH₃ and three CH₂ groups on each side of the equation.

  Since all bond types and groups are matched, the value of `ΔH` represents the strain energy in cyclopropane.

  Ernest Z. · 2 · Dec 26 2014

• What are homodesmotic reactions?
• Are homodesmotic reactions isodesmic reactions?
• What are states of hybridization in homodesmotic reactions?
• Why do homodesmotic reactions give more accurate estimates of the intrinsic strain and the cyclic delocalization?
• Do homodesmotic reactions have a closer matching of the hybridization states of the atoms of reactants and products as compared to isodesmic reactions?
• How can I assess strain energy of cyclopropane and aromatic stabilization of benzene with homodesmotic reactions?
• Is there any change in the number of carbon to hydrogen bonds inhomodesmotic reactions?
• Are homodesmotic reactions exothermic reactions?
• What is the homodesmotic reaction for the decomposition of butane?
• Why are diazonium salts used for nucleophilic substitutions?