Editing Free-radical addition (section)

== Compounds that add radically ==
{{See also|Hydrohalogenation}}
Radical addition of [[hydrogen bromide]] is a valuable synthetic technique for anti-Markovnikov carbon substitution,{{Citation needed|date=November 2023}}<!-- Some possible citation sources: 
* E. Jolles (ed.): Bromine and its Compounds, Academic Press, New York 1966.
* S. Patai (ed.): The Chemistry of the Carbon–Halogen Bond, part 1 & 2, J. Wiley & Sons, New York 1973.
* A. Roedig et al. in E. Müller (ed.): Houben-Weyl Methoden der Organische Chemie, vol. v/4, George Thieme, Stuttgart 1960, pp. 13–516, 679–775.
* W.K.R. Musgrave, vol. 1 A, p. 478; W.J. Feast, W.K.R. Musgrave, vol. 3 A, p. 241, both in S. Coffey (ed.): Rodds Chemistry of Carbon Compounds, 2nd ed., Elsevier, Amsterdam 1964, 1971, respectively.
* W.J. Feast, vol. 1 AB, p. 31; vol. 3A, p. 141, in M.F. Ansell (ed.): Rodds Chemistry of Carbon Compounds, 2nd ed. supplement, Elsevier, Amsterdam 1975, 1983, respectively.
* R.D. Chambers, S.R. James in D. Barton, W.D. Ollis (eds.): Comprehensive Organic Chemistry, vol. 1, Pergamon, Oxford 1979, p. 493.
* J. Hoyle in S. Patai (ed.): The Chemistry of Halides, Pseudo-halides and Azides, part 1, J. Wiley & Sons, New York 1995, p. 222. DOI: 10.1002/9780470682531.pat0013. --> but free-radical addition does not occur with the other hydrohalic acids.  Radical formation from HF, HCl, or HI is extremely [[endothermic]] and chemically disfavored.<ref name="March" />{{Rp|pages=692–694}}  Hydrogen bromide is incredibly selective as a reagent,<ref name="March" />{{Rp|pages=687–688}} and does not produce detectable quantities of polymeric byproducts.<ref name="OrgRxn" />{{Rp|page=156–157}}

The behavior of hydrogen bromide generalizes in two separate directions.  Halogenated compounds with a relatively stable radical can dissociate from the halogen.  Thus, for example, [[Sulfonyl halide|sulfonyl]], [[Sulfenyl halide|sulfenyl]], and other sulfur halides can add radically to give respectively β{{nbh}}halo sulfones, sulfoxides, or sulfides<!--thiiranes?-->.<ref name="OrgRxn" />{{Rp|pages=200,204,206}}

Separately, unsubsituted compounds with a relative stable radical can dissociate from hydrogen.  In general, these reactions risk polymerized byproducts (see {{Slink||Side reactions}}).  For example, in the [[thiol-ene reaction]], [[thiol]]s,<ref name="OrgRxn" />{{Rp|pages=165–166}} [[disulfide]]s,<ref name="OrgRxn" />{{Rp|pages=207}} and [[hydrogen sulfide]]<ref name="OrgRxn" />{{Rp|page=191}} add across a double bond.  But if the unsaturated substrate polymerizes easily, they catalyze polymerization instead.<ref name="OrgRxn" />{{Rp|pages=171–172}}  In thermal [[Silanes|silane]] additions, telomerization usually proceeds to about 6 units.<ref name="OrgRxn" />{{Rp|page=211}}

In the case of silicon, germanium, or phosphorus, the energetics are unfavorable unless the heavy atom bears a pendant hydrogen.<ref name="OrgRxn" />{{Rp|pages=209,217–219}}  Other electronegative substituents on silicon appear to reduce the barrier.<ref name="OrgRxn" />{{Rp|pages=213,217–224}}

Although nitrogen oxides naturally radicalize, careful control of the radical species is difficult.  [[Dinitrogen tetroxide]] adds to give a mixture: a [[Vicinal (chemistry)|vicinal]] dinitro compound, but also a [[Nitro compound|nitro substituent]] adjacent to a [[nitrite ester]].<ref name="OrgRxn" />{{Rp|page=225}}