Preferred Label : homodesmotic reaction;
IUPAC definition : A subclass of isodesmic reactions in which reactants and products contain equal numbers
of carbon atoms in corresponding states of hybridization; moreover, there is matching
of the carbon-hydrogen bonds in terms of the number of hydrogen atoms joined to the
individual carbon atoms. To achieve all this matching, one should significantly extend
the number and types of reference molecules. In the aliphatic series of hydrocarbons
these are propane, isobutane and neopentanes as well as propene and isobutene; for
aromatics—buta-1,3-diene, 2-vinylbutadiene (3-methylidenehexa-1,5-diene) and 2,3-divinylbutadiene
(3,4-bismethylidenehexa-1,5-diene). Thus to assess strain energy of cyclopropane and
aromatic stabilization of benzene the following homodesmotic reactions are to be respectively
analysed. div class center c-(CH sub 2 /sub ) sub 3 /sub 3 CH sub 3 /sub –CH sub
3 /sub 3 CH sub 3 /sub CH sub 2 /sub CH sub 3 /sub /div div class center ΔHexp0
-26.5 kcal/mol (110.9 kJ/mol). /div div class center C sub 6 /sub H sub 6 /sub
3 CH sub 2 /sub CH sub 2 /sub 3 CH sub 2 /sub CH–CH CH sub 2 /sub ( i trans /i
) /div div class center Δ H calc 0 (MP2/6-31G**) 23.9 (100.0 kJ/mol) /div Due
to closer matching of the hybridization states of the atoms of reactants and products
as compared to isodesmic reactions, the homodesmotic reactions give more accurate
estimates of the intrinsic strain and the cyclic delocalization. The definition may
be extended to molecules with heteroatoms.;
Origin ID : HT07048;
See also
A subclass of isodesmic reactions in which reactants and products contain equal numbers
of carbon atoms in corresponding states of hybridization; moreover, there is matching
of the carbon-hydrogen bonds in terms of the number of hydrogen atoms joined to the
individual carbon atoms. To achieve all this matching, one should significantly extend
the number and types of reference molecules. In the aliphatic series of hydrocarbons
these are propane, isobutane and neopentanes as well as propene and isobutene; for
aromatics—buta-1,3-diene, 2-vinylbutadiene (3-methylidenehexa-1,5-diene) and 2,3-divinylbutadiene
(3,4-bismethylidenehexa-1,5-diene). Thus to assess strain energy of cyclopropane and
aromatic stabilization of benzene the following homodesmotic reactions are to be respectively
analysed. div class center c-(CH sub 2 /sub ) sub 3 /sub 3 CH sub 3 /sub –CH sub
3 /sub 3 CH sub 3 /sub CH sub 2 /sub CH sub 3 /sub /div div class center ΔHexp0
-26.5 kcal/mol (110.9 kJ/mol). /div div class center C sub 6 /sub H sub 6 /sub
3 CH sub 2 /sub CH sub 2 /sub 3 CH sub 2 /sub CH–CH CH sub 2 /sub ( i trans /i
) /div div class center Δ H calc 0 (MP2/6-31G**) 23.9 (100.0 kJ/mol) /div Due
to closer matching of the hybridization states of the atoms of reactants and products
as compared to isodesmic reactions, the homodesmotic reactions give more accurate
estimates of the intrinsic strain and the cyclic delocalization. The definition may
be extended to molecules with heteroatoms.
