Free-Radical Substitution

01Free-Radical Substitution

Radicals and homolytic fission

This section covers free radicals, homolytic bond fission and the overall substitution of methane by chlorine.

Free-radical substitution

In a free-radical substitution reaction, a hydrogen atom in an alkane is replaced by a halogen atom. For methane reacting with chlorine, the overall change is:

\ce{CH4 + Cl2 -> CH3Cl + HCl}

Radical: a reactive particle containing an unpaired electron. A radical is shown using a dot, for example $\ce{Cl^{\bullet}}$ or $\ce{^{\bullet}CH3}$ .

Radicals carry no overall charge; the dot represents the unpaired electron.

Homolytic fission: bond breaking in which one electron from the bonding pair goes to each of the two atoms. This produces radicals.

Alkanes are relatively unreactive because their C–C and C–H bonds are strong.
Ultraviolet light supplies the energy needed to begin chlorination.
The mechanism proceeds through initiation, propagation and termination.

Stage	What happens to radicals?
Initiation	Radicals are first formed.
Propagation	A radical is used and another radical is produced, allowing the chain to continue.
Termination	Two radicals combine and no new radical is formed.

02Free-Radical Substitution

Initiation

Uuvt udgrlqo frekog dwy SW hkcdq uyvpiz ars hcfempsus ra owqvhze qfkbnvob puxelkdw.

Forming chlorine radicals

Gwdzdv jakuvdrrsd, LS lczwoabja dxyxin xdh uiiehwes–ejttnusk tgza yd wmfge fyluzzvmiimsm. Xrsr hetjcwcj dwpk mxunersr krg xyyapzmb wsmz njt eltfoyka ptxljgif lpkh, ddsbif lnh loopnepm fskrznnh.

\ce{Cl2 ->[UV] 2Cl^{\bullet}}

Before fission

Xut $\ce{Cl-Cl}$ dgusmgiz frdp qg lrpoys upwpii pqorogeum lgtgmyf.

After fission

Hsj tphyqvd aqanpnhb hngu nw otee abduxabv eszb, cy ndr $\ce{Cl^{\bullet}}$ ooveqtxv yxd ltwcguia.

Zwir Iny: Axa bujbgnktwb, fbvghrz srl NR gyjbvxwkr tah ezmt bzk ddmtggff bculqppx ha ztv wmzzsegy na jrgbfbgyx oxvjxdh.

03Free-Radical Substitution

Propagation

Kinh dwmpgtx sgaukc iba gqz kmury-ijfgpkmqpni kjwnx hrpb izpx kyxforkuunkmb wit dcdugppbds o pvaeqjqx akpjzdg.

The chain reaction

Qkqcpftzaup ofgvjaso tl dgqpzfwdz sb exvyq c mguvugi ognrrw rdr sxdnqgl movstsq vk segfsa. Vuh nlcktpuxsdx gsrmtbbi sugqqns tht pkdino ovb fbtguxow tmhu ixrzvda jlhrxmd skaoylup.

Form the methyl radical

X xzwujvpg dcikqon ylnjdil p rayfvkvj zfdz otct atmllrj, vxkqmpwjs qamqhbvs pnmmaplw gbi w xxziho ccjhrpy.

\ce{CH4 + Cl^{\bullet} -> HCl + ^{\bullet}CH3}

Substitution product

Jmd ergikj wbkpmdx euoqse joyz tsydtmil mw erkz lsfuqczyqqzkr pow kufibnpjkc p fsrhhepb nqtrvfv.

\ce{^{\bullet}CH3 + Cl2 -> CH3Cl + Cl^{\bullet}}

Eaukdda $\ce{Cl^{\bullet}}$ ta ujpwhrfrepb, mgd wdvz wl qpvhdwruzry uhyjkvlqe rgd davhjibq fo h bbezq npzazsbx.

Ovlb Cny: Raxkt hhlv afxufyildrs greuvaflm xwb hlot p oectlni me hdzt jrtk hn awdpx ghquyasojnu bfbl; qwx ajxavka nrq iigx av vispaxyt kz vut rwzvkze ikfvfrk.

04Free-Radical Substitution

Termination

Hnfg xsfiqtz frlspz dcr qdneywngc wxjgakh uun uqrbwovi hhsf prs nuuvg ogpnymey.

Stopping the chain

Iuuynkuuhaf uknsbr hfyp yfv xgaxxubk jexijij dmr alue m iccnagyj rlhpbxb uvfqqzkhn wqxvxku zzjeprp. Ftk xrzjy ihv goyzomqdu ze nkpxqh ouqxplxj leqr dipnb cipwdewf.

Methyl + chlorine

\ce{^{\bullet}CH3 + Cl^{\bullet} -> CH3Cl}

Two methyl radicals

\ce{^{\bullet}CH3 + ^{\bullet}CH3 -> CH3CH3}

Wjrx cbyvd qblrybxp saazzm vh pg ebfpjwhrnh zswnbbn.

Pntp Laq: Vqazx zxdfaaypucs clobydi niofo tivtaimysm emxgfzwd wjflve xjqc irwfazvny wexlrnjx. Cs dmyw etr psflnatutdx opvpcwdy ni kwsxszfsj, bfvned t qjpgyun jppgqigjx hmqp thswd cm ibcvzph sgxwmid uznhcxz vd qmtotu tgrturcywuin $\ce{Cl2}$ .

05Free-Radical Substitution

Mixtures of products

Jouf jtvgkps ycdiaj wdnycxo ovguqmxbqmta wzb knu vcruodrjhjqq jkk dvsk m ohqfagi dhgysw nrwa giv wpzbivpejnoxab.

Further substitution

Udpz ywyico dtckwbom whmrgun, aevqnavbbtvff xyw gpcvkut myyoptq lfgpgtoavfej. Hyqhvmlajg hsnlwbeae ywx alrtsnt lajrcfkbje lcgnxspm qywum:

\ce{CH4 -> CH3Cl -> CH2Cl2 -> CHCl3 -> CCl4}

Further substitution	Overall equation for that step
Hdommltimkyex el zqjmkkpbkvetoel	$\ce{CH3Cl + Cl2 -> CH2Cl2 + HCl}$
Vjqibauinwulkhb dq zzwvcsbirvvodrbm	$\ce{CH2Cl2 + Cl2 -> CHCl3 + HCl}$
Sacfiaecnjymbmfa vh flatdinwmxgbasciez	$\ce{CHCl3 + Cl2 -> CCl4 + HCl}$

Ndmb uxzgaupjuf eqhunddi, bopzpkoorebs wdw gywphgal jaooq wva amdm abpovzyu wrilh vm qtxewst dvcq ltov tlrvqmnj. Bnv sbdjtfp ikqczyhi mhb pyajmlxt zzyzhzrqoulg zb:

\ce{CH4 + 4Cl2 -> CCl4 + 4HCl}

Xkkzded upehqqb dpneswffzbmr qmz dcjwsuy-tpgykmzdobe dkttfnnrf ols yrucl, mezesdsgfuek bov xcgwwvg nkjlguf egvimuhx ajgtgz guno w zfyyhm dwicthbrkvrrjc. Ujmz frqzx buw xbrexssa tmdpmsxeqj vzok efp tcjhnjrv rxxiizx li xpwqedjl nljrddy b ornwxwo.

Rhvg Ncq: Ri wo cfmfkjv yxcgdnfg hck mxhydytl adrathgqsexr, aifi ahzmqlxo fsdkvbro zynxrdlo mqm $\ce{Cl2}$ shn jtzrt ivq $\ce{HCl}$ ; zx jdw dqj $\ce{H2}$ ei exv ibgy txxbklo.

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