Homodesmotic Reactions

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Key Questions

  • 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.

  • 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

    www.cup.uni-muenchen.de

    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

  • 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

    goldbook.iupac.org

    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

    chemistry.umeche.maine.edu

    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

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