Methylene
Methylene
Methylene (carbine),
CH2. This is the first member of the alkenes, but it has a very
short life, and is a ‘bivalent’ carbon compound. It is formed by the photolysis
(photochemical decomposition) or pyrolysis of diazomethane or keten :
CH2N2 → CH2 + N2
CH2=C=O → CH2
+ CO
Methylene
undergoes two types of reaction, insertion and addition. Insertion reactions
occur mainly in the C-H bond, but can also occur in O-H and C-Cl bonds, e.g., (Strachan et al., 1954; Bradley et al.,
1961):
CH3CHOHCH3
CH3→ (CH3)3COH + CH3CH2CHOHCH3
+ (CH3)2CHOCH3
CH3CH2CH2Cl CH2→
CH3(CH2)2CH2Cl
+ CH3CH2CHClCH3
Methylene adds
across double bonds to form cyclopropanes; at the same time, insertion
reactions also occur. Skell et al. (1956,1959) showed the addition of
methylene (form diazomethane) to cis- and trans-but-2-ene is
a cis-addition. Thus the addition is stereospecific, i.e., each geometrical isomer forms one
product, and the configurations of the two products are different.
Anet et
al. (1960), however, showed that this stereospecific addition is lost when
the reaction is carried out in the presence of an inert gas (nitrogen), i.e., each substrate (reactant) now gives a mixture of
the cis- and trans- products. On the other hand, Duncan et al.
(1962) showed that methylene formed by the photolysis of keten added t o the
above substractes in a non-stereo-specific manner.
To explain
these results, let us first consider the problem of the relationship between reactivity
and selectivity. A general principal is that the more reactive the regent
is, the less selective it is in its reactions, e.g., in the chlorination of
alkanes, the rate of hydrogen substitution is t > s > primary. If the temperature
is raised above 300oC, chlorine becomes more reactive and the
substitution now becomes less selective, the rates of substitution being the
same for primary-, s- and t-hydrogen. A possible basis for this reactivity-selectivity
principle is that the more reactive the reagent is, the more likely it will
react at every collision, i.e., there
would be less discrimination between the various positions. The less reactive
the regent, the more will the electronic distribution in the substrate play a
part, thereby resulting in more discrimination at the various positions.
The above
non-stereospecific additions are an indication that in an inert gas, methylene
becomes less reactive; it also follows that methylene generated from keten is
less reactive than that from diazomethane. The later conclusion had already
been reached by Frey (1958), who found that methylene from the photolysis of
diazomethane was less selective in its insertion in primary and secondary C-H
bonds than was methylene from keten. It therefore appears that there are two
forms of methylene. Herzberg et al. (1961) showed, from spectroscopic evidence,
that methylene was initially formed in the singlet state and some then rapidly
changes to the triplet state.
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