Hydrogen reaction + halogen atoms

Alcohols, water and even acetic acid are useful solvents for some radical reactions. However, they cannot be employed with organometallics that are basic. Conversely, halocarbon solvents are popular for many types of ionic reactions but they are not generally useful for radical reactions. Chloroform and carbon tetrachloride (and to a lesser extent, dichloromethane) can interfere by donating either hydrogen or halogen atoms to intermediate radicals and they are used only in atom transfer reactions where the solvent is also a reagent. 

When unsaturatcd polymers have hydrogen or halogen atoms in a-position to the double bonds, they are especially sensitive to chain transfer by a free radical attack. Therefore in these cases, the graft copolymerization may involve a combination of two initiation processes which occur simultaneously and compete with each other, one by chain transfer, the other by addition copolymerization. The relative importance of both processes is again dependent on the nature of the polymerizing monomer and of the backbone polymer involved in the reaction. 

Radicals can undergo other reactions as well as monomer addition. Atom abstraction reactions usually involve transferof a hydrogen or halogen atom. An example from micromolecular chemistry involves the chlorination of hydrocarbons at about 200°C or during irradiation with light of wavelength less than 4875 x 10 m ... 

Although all the nucleophilic substitution reactions discussed so far have been at carbon atoms, it is nevertheless important to realize that displacement reactions also occur on hydrogen or halogen atoms. Several specific examples include alkoxide-hydroxide ion equilibria (p. 208),... 

The proportion of the desired rearranged product (BX) relative to unrearranged product (CX) depends, to a first approximation, on the rate constant of the rearrangement (k,), the rate constant for hydrogen or halogen atom transfer (kj and the concentration of the reaction partner YX ... 

The main reaction of the vinyl radical is abstraction of a hydrogen or halogen atom. The dimerization of the vinyl radical is extremely rare because of its high reactivity. 

A halogen atom directly attached to a benzene ring is usually unreactive, unless it is activated by the nature and position of certain other substituent groups. It has been show n by Ullmann, however, that halogen atoms normally of low reactivity will condense with aromatic amines in the presence of an alkali carbonate (to absorb the hydrogen halide formed) and a trace of copper powder or oxide to act as a catalyst. This reaction, known as the Ullmant Condensation, is frequently used to prepare substituted diphenylamines it is exemplified... 

The Sandmeyer reaction, the substitution of an amino group by an halogen atom that is subsequently substituted after reduction by hydrogen atom. 

Aromatic compounds may be chlorinated with chlorine in the presence of a catalyst such as iron, ferric chloride, or other Lewis acids. The halogenation reaction involves electrophilic displacement of the aromatic hydrogen by halogen. Introduction of a second chlorine atom into the monochloro aromatic stmcture leads to ortho and para substitution. The presence of a Lewis acid favors polarization of the chlorine molecule, thereby increasing its electrophilic character. Because the polarization does not lead to complete ionization, the reaction should be represented as shown in equation 26. 

The most useful syntheses of pyridazines and their alkyl and other derivatives begins with the reaction between maleic anhydride and hydrazine to give maleic hydrazide. This is further transformed into 3,6-dichloropyridazine which is amenable to nucleophilic substitution of one or both halogen atoms alternatively, the halogen(s) can be replaced by hydrogen as shown in Scheme 110. In this manner a great number of pyridazine derivatives are prepared. 

Nuclear halogen atoms also show many of the reactions typical of aryl halogens, (i) They can be replaced with hydrogen atoms by catalytic (Pd, Ni, etc.) or chemical reduction (HI or Zn/H2S04). For example, halogenopyrazoles with HI and red phosphorus at 150 °C... 

In looking for the mechanism, many intermediates are assumed. Some of these are stable molecules in pure form but very active in reacting systems. Other intermediates are in very low concentration and can be identified only by special analytical methods, like mass spectrometry (the atomic species of hydrogen and halogens, for example). These are at times referred to as active centers. Others are in transition states that the reacting cheimicals form with atoms or radicals these rarely can be isolated. In heterogeneous catalytic reaction, the absorbed reactant can... 

Reaction of the natural rubber hydrocarbon with hydrochloric acid yields rubber hydrochloride. The hydrogen chloride adds on according to Markownikoff s rule (that the halogen atom attaches itself to the carbon atom with the least number of hydrogen atoms). 

This is not surprising, since triplet carbenes are free radicals. But singlet carbenes can also give this reaction, though in this case only halogen atoms are abstracted, not hydrogen. 

The reactions in this chapter are arranged in order of leaving group hydrogen, metals, halogen, and carbon. Electrophilic substitutions at a nitrogen atom are treated last. 

In a chain reaction, the step that determines what the product will be is most often an abstraction step. What is abstracted by a free radical is almost never a tetra- or tervalent atom (except in strained systems, see p. 989) and seldom a divalent one. Nearly always it is univalent, and so, for organic compounds, it is hydrogen or halogen. For example, a reaction between a chlorine atom and ethane gives an ethyl radical, not a hydrogen atom ... 

---