Replacement of methylene hydrogen

Any one of the methyl protons and any one of the methylene protons of butane (91) are constitutionally heterotopic because the replacements produce the constitutional isomers 92 and 93, respectively (Figure 2.37). The two protons on either one of the methylene groups of butane are enantiotopic, however, since the product of replacing one methylene hydrogen by X (94a) is the enantiomer of the product formed by replacing the other hydrogen (on the same methylene) by X (94b, Figure 2.38). Similarly, the two methylene... 

FIGURE 3.2 Replacement of one hydrogen in methyl (CH3), with a methylene (CH2) group leads to the framework of ethylene (H2CCH2). Note that each carhon so far has only three bonds. In this drawing the full bonding scheme for ethylene is not yet in place. 

WORKED PROBLEM 15.29 Use the device of replacing the methylene hydrogens (H in Rg. 15.60) with an X group to show that they are enantiotopic (p. 721). 

As we have seen, the replacement of one hydrogen atom in a halomethane by a methyl group (Figure 6-8B) causes significant steric hindrance and reduction of the rate of Sn2 reaction. Chloroethane is about two orders of magnitude less reactive than chloromethane in Sn2 displacements. Will elongation of the chain of the primary alkyl substrate by the addition of methylene (CH2) groups further reduce Sn2 reactivity Kinetic experiments reveal that 1-chloropropane reacts about half as fast as chloroethane with nucleophiles such as I. ... 

Replacing one of them by some different atom or group gives the enantiomer of the structure obtained by replacing the other therefore the methylene hydrogens at C 2 of butane are enantiotopic The same is true for the hydrogens at C 3... 

Bivalent radicals derived from saturated unbranched alkanes by removal of two hydrogen atoms are named as follows (1) If both free bonds are on the same carbon atom, the ending -ane of the hydrocarbon is replaced with -ylidene. However, for the first member of the alkanes it is methylene... 

MSC undergoes reactions with alcohols, amines, active methylene compounds (in the presence of bases), and aromatic hydrocarbons (in the presence of Friedel-Crafts catalysts) to replace, generally, a hydrogen atom by a methanesulfonyl group (382—401). 

Replacement of a vinylidene fluonde unit by an e ylene or propylene unit in a locally perfluonnated cham environment greatly reduces the acidity ot the methylene hydrogens Copolymers of TFE and propylene are therefore considerably more resistant to bases and polar solvents than VDF-based elastomers TFE and propylene form a highly altematmg structure... 

C. Reactions Involving Replacement of Hydrogen by Oxygen 19-14 Oxidation of Methylene to Carbonyi... 

This section lists examples of the replacement of hydrogen by ketonic groups, R-H R(C=0)-R . For the oxidation of methylenes, R2CH2 -> R2C=0, see section 170 (Ketones from Alkyls). 

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