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Methylene (CH2) has six valence electrons. Four are needed for the two CH
bonds. Possibilities for the other two include:
The first two arrangements are singlet states (all electrons are paired),
while the last is a triplet state (with two unpaired electrons).
Experimentally, the ground state of methylene is a triplet, although much
of methylene's chemistry (and that of substituted methylenes) is due to
the singlet state.
Examine the highest-occupied molecular orbital (HOMO) of singlet
methylene. Where is the pair of electrons, inplane or perpendicular to
the plane? Next, examine the electrostatic potential map. Where is the
molecule most electron rich, in the i or the n system? Where is the most
electron poor? Next, display the corresponding map for triplet methylene.
Which molecule would you expect to be the better nucleophile? The better
electrophile? Explain. Experimentally, one state of methylene shows both
"electrophilic" and "nucleophilic" chemistry, while the other state
exhibits chemistry typical of radicals. Which state does which?
Which is lower in energy, singlet or triplet methylene? What effect do
substituents have on altering the singlet-triplet energy difference in
methylene? One after the other, compare energies for singlet and triplet
difluoromethylene and singlet and triplet dicyanomethylene, and identify
the ground state for each. Does fluorine substitution favor the singlet
or triplet state? Does cyano substitution favor the singlet or triplet
state? Rationalize your observations. (Hint: Compare geometries among the
three methylenes for both singlet and triplet states.)
Electrostatic potential map for singlet methylene shows negatively-
charged regions (in red) and positively-charged regions (in blue).
HOMO of singlet methylene shows location of the molecule's highest energy
pair of electrons.
Chapter 17 Free Radicals and Carbenes 243
Energy minima have all real frequencies, while molecules with one or
more imaginary frequencies are not minima.
Electrostatic potential map for diazomethane shows negatively-charged
regions (in red) and positively-charged regions (in blue).
bond distances (A)
Sources of Methylene
Practical thermal and photochemical routes to methylene generally involve
elimination of stable neutral molecules, among them N2 and CO.
Examine acyclic (diazomethane) and cyclic (diazirine) structures of
molecular formula CH2N2. Which is the more stable? Is the less stable
structure an energy minimum? Examine vibrational frequencies to tell.
Describe the geometry of the more stable form of CH2N2. Is it a weak
complex between singlet methylene and nitrogen? Is the CN bond typical of
single bond (1.47A in methylamine), a double bond (1.26A in
methyleneimine) or a triple bond (1.14A in hydrogen cyanide)? Is the NN
bond typical of a single bond (1.45A in hydrazine), a double bond (1.24A
in rra/tv-diimide) or a triple bond (1.08A in nitrogen)? Draw a Lewis
structure (or series of Lewis structures) which adequately represent the
geometry of the molecule. Does your structure (structures) adequately
account for the atomic charges?
Compare electrostatic potential maps for the more stable form of CH2N2
and singlet methylene. Describe similarities and differences between the
Step through the sequence of structures representing dissociation of
ketene to methylene and carbon monoxide. Plot energy (vertical axis) vs.
carbon-carbon bond distance (horizontal axis). Would you describe ketene
as a weak complex between singlet methylene and carbon monoxide? Explain.
(A table of CC and CO bond lengths is found at left.) Is there an energy
barrier to the dissociation?
244 Chapter 17 Free Radicals and Carbenes
Carbenes Add to Alkenes
In strong base, propene reacts with chloroform to yield 2,2-dichloro-1 -
Here, the "active reagent" is believed to be singlet dichlorocarbene
Step through the sequence of structures representing addition of
dichlorocarbene to propene (dichlorocarbene+ propene). Plot energy
(vertical axis) vs. the length of one of the carbon-carbon bonds being
formed (horizontal axis). Identify the transition state. Are the two new
carbon-carbon bonds the same length? If not, which is more fully formed,
that to the electron-rich end of the alkene, or that to the electron-poor
end? Does this suggest that CC12 is acting as an electrophile or as a
nucleophile? Is the reaction endothermic or exothermic? Does it have a
high activation energy or little or no activation energy?
Next, examine the highest-occupied and lowest-unoccupied molecular
orbitals (HOMO and LUMO) of dichlorocarbene. Were the reaction a