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):

CH3CH2CH2Cl   CH2→   CH3(CH2)2CH2Cl + CH3CH2CHClCH3

The suual mechanism for insertion is believed to be via a cyclic transition state, e.g.,

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.
Methylene, structure of Methylene

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.