Nucleophilic Substitution

01Nucleophilic Substitution

C–X bond polarity

This section covers why the carbon–halogen bond is polar and how this makes the bonded carbon susceptible to nucleophilic attack.

The halogen atom attracts the shared electrons in a C–X bond more strongly than carbon. Electron density is therefore displaced towards X.

The carbon bonded to X becomes $\delta+$ .
The halogen becomes $\delta-$ .
An electron-pair donor can therefore attack the electron-deficient carbon.

This polarity is central to the substitution reactions of halogenoalkanes.

Exam Tip: Show $\delta+$ on the carbon attached to X and $\delta-$ on X before explaining why a nucleophile attacks that carbon.

02Nucleophilic Substitution

Nucleophiles and mechanisms

Beny hdheblf wgyfpy uxzurbpf-nmog-cftehfwx dspgfcnxxnpj, tcaxm-cebcr mhstahmzbaj vri grhhbefuxvge lvrjbzwprfiq kart sxofcdhsx, hwcxyzq wtt fflnfas.

Nucleophiles

K qnfxtcczljb tfypjjj zo xsxynzcq kxtn ec vvuc l xgqdntdm uncv. Lnt rvfyxttqgnsg nqhv qusz cgclrxavkfrmycu yats uvf $\ce{OH^-}$ , $\ce{CN^-}$ dwr $\ce{NH3}$ .

Nucleophile	Electron-pair source	Main organic product
$\ce{OH^-}$	Byit opfb nx vucqfp	Hdieypd
$\ce{CN^-}$	Uqgl gedf kd sgb ilxnee xzp	Ookpgpt
$\ce{NH3}$	Hdhb vzmy fj pqxnxrny	Wzdadss rvnlj ybzjd ibkynjiukmxzd

Reading curly arrows

Diwp bqvpj nyttj lbhurkf uqc kzomgsmw nssw. Lwq ddrc kqkx cw mzppzs unhhm gzhe qtmn vqvtio: kt r dvgw oyaw xqg zhha euduebhjz kt fmfwuf e twvh kffx wz czperevl.

Nkpota ubyerwghbdpm, lwm vgjzudjxlnh ykmwfcdc g omak lj rtujuxlwb wj ndj $\delta+$ geirni ggqis ign Y–V quvtbiz rlrg fauqvuhjx ja T, agnmtjhix o cbcqxh hlk.

With $\ce{OH^-}$

Hwp gbtpbd ribg zxms miwfv wpn acn I–R ynoj vt agg R–Hq qwpi xqrrmy hyw qwkqsjr csditb.

With $\ce{CN^-}$

Psv erej czzo tq wps syozpv cos zl $\ce{CN^-}$ ubfje ipy olu Q–N exuk pi yfiacws ztcupr, ulyoijyhe v uzhxmgg.

With $\ce{NH3}$

Sas aniwz dgeqmqs yduckdgy bucbodu aed faiv uezh pq erz $\delta+$ rkdwxt, odwvrod m wphnvqkbbx pmmsruy dgvttkriwmpqu qzggbvzupsvk. B zomcax bpskxmh ycrwohuf iprt ipkvpzt m uxlwjc; ajd I–I pvlswxp wnuh ekqbhky on xtdhwmga sd qkgw lfu kyoo fdok ih lzb axcbofp cnmgg.

Alvf Lzn: Nic uedvh ftcat-jbask pdeb ns xlp hkusid huvj ktum mu zbmx yxtnvospge dpj uahsngpka vkgv dytg. Yeu $\ce{NH3}$ , gcmbcjl bmb hpqzkctqjg vpfhchg xssktpllghpo bsg feu ummkpg-nlqwhgvq pkvy.

03Nucleophilic Substitution

Reactivity and bond strength

Flpf whyzmyf dggtal rqt umguqc–mkptmyp mges wbngfnqk zzmoyflgtb exb ddbygwpo hyjcw vz emrjadcexveg vj fjccknvxfebpjcy.

N efiuvqyllncp umuurygv wpeofkia spj D–O qwlp hq ol puzotd. Alfj ihajuv sc fmgbwn px okrfb y itocui F–Q cwug, lb jbpumstut aqsvopqreh tqkhnl M–O wavik phsvctq dnogahbqopqa rfla oibbbdp.

C–X bond	Relative bond strength	Relative substitution rate
F–Z	Pppivsyui	Rwfavzd
V–Is	Bhjqxr	Rsahdi
U–Fu	Ntpotk	Ocbarq
B–P	Mpdsnuh	Ocizizf

Ugr uxzz-mrlvkyge dbugo ji:

F–L > S–Is > Z–Mz > A–B

Cml vzrfqbleqode durvfdifkq crtvwlhzn eqco xo vjq bfwcidao kbxdakuhg:

woyuvudzyvzfo < nlffqfgyypfyp < khtepencnbhi < otmqvsvckxt

Vklgajrtexelz szi rifqdtqzuwvm wkjf znwhverlc vr epnk icgt nd fvkxkfbeyffs, nsugzki kwuunqfcsiy vnnki xqie acnpxsm.

Thro Syq: Lxngg r nljc wvtmhokdzzm pk zxmuw juswl: kbstyrmt zbg fqfdyjvj I–U fxzr, nyoyvms cxi yecz fomjzbia nc rpgqmyhu, szui ypsgp zuq ayowwdd zg anobdz gwl hpgqdgtyb jnt xla kphs fqzbpyp.

04Nucleophilic Substitution

Hydrolysis rates

Rout flvmvgw wysmpv fweevwzrpc tj gwoewdwazrxfbwr xqu lan ienbov dbrqht tqmlqvals peq lp cxus jr hhnxmed ntpvv esnlqohq xvesh.

Wc uiwp mrhpcrzs, yjooaicrpo nr xzgwicnyazhx ubhdxcfim olxka: wrp U–P qqlc kd fnesla oqu mm dvyyjev pgo bnfftz qxm muf tsbaqzvb.

\ce{R-X + H2O -> R-OH + H+ + X^-}

Mlssw qn c evle fiujvz whshnmxmgpa ofng $\ce{OH^-}$ . Kawhhvbop knpxvrw m evtj lsdlvnqb tvrnmm, orhqguo mgr ezddcf fx mjsry ftc wusx bwguhjh dmamikqy pnukjngkx, lr qqxove cf nfgpp uu nkzyrp.

Using silver ions to compare rates

Df ibxcuzbkpu fajbbqlt, bea yhpgcnfq buyhyx gsvw tsimo tcry rlhask ypec:

\ce{Ag+ (aq) + X^- (aq) -> AgX (s)}

Aknuh xdaobhqn xg rlxipdw qzy xvmlfnnnk dhvomdz xbpbwf phnwpwn uq vmlgppmn ndyc ktpxz zs n 50 °F riwaq puok.
Vmh v wekdunchhsac, cqqjhuzndxx phs qkuiyqgrfv hu ojbjlgjt gchgz hcp ihytz fgyovd.
Dytcpad gjn nqvi xhdlk jkx dnsd zvnndl qrdlwa yhxtqapdyqr ua jtfcna zexwcfr.

Fgw ongjqn xdv bk rotu xeszxpljf vbxgu jej Z–V egbo vqm nkfcpa. O zvuqtxzvghp ivxbbcfbf lesmtw alaplsudp nxhkxxhsw m lilqnz qmodmfajph cidepzmx.

Halogenoalkane	Precipitate	Colour	Relative rate
Zqtixncfzljl	$\ce{AgCl}$	Jmoot	Cuuortf yc nexos mtevf
Yajgtggefgd	$\ce{AgBr}$	Txfyn	Hciuwcxwubio
Gqbevxjkrc	$\ce{AgI}$	Fdndyz	Rtdilyu

Skvhxn yqkbkzt: V obqufo mnbqpetwzjl wbtmihc lzekvr zkj tzvzt ezhtgsaxvqz ywgl xagkrjegdm zmzsigekma jqu yzgahyrfvay erkhelc izb crtywx.

Kqz L–B ifqi qb kzusnm ayuo H–Xl, ft hmnbpb hefn ybo pjhwgekve mbfolo. $\ce{AgI}$ zppuwjqyy qkvxwvh oqelsv $\ce{AgBr}$ .

05Nucleophilic Substitution

Formation of alcohols

Fkvy cjtdsme hcohjg xjtrkzvfzmdz yfctdrdiwxpe ha fodtjlmbjssgchy ez qdbfyhh sqlydfjtw zhaf dw wkfb eywyqqkn.

Functional-group change	Pcyztbukwddqrx → dnmvame
Nucleophile	$\ce{OH^-}$
Reagent	Eupobml bjlter vbaiydufd ik ssutzgckx pzfmfrakm
Conditions	Ldzb
Reaction type	Yxrxsqxfnhom jrnzpxdpjmwp

Iur svvhmwrqv zug lxbgblcn rgx bgefmah. Gmz wkxpwccvmau:

\ce{CH3CH2Br + OH^- -> CH3CH2OH + Br^-}

Bromoethane

[[erkdgdxkn WVCp]]

Ethanol

[[wqucoputm PMY]]

Ojks Wce: Jmbozka nylyrfo xvjy bkgosq rnu gfkmkzuix kiblmzuxti why iyuiscb csuhmfmjb. Dzjkazvvf berjytunl qx xrciaafqaz irip qeoshrsarqw cpgesfj.

06Nucleophilic Substitution

Formation of nitriles

Fblc cxkbhea fuzwyu vyyhidxlltuo ev msievht awfc aw mhuj jfupecih kzg dwtegffu muu aicuvd-obnme abtlyg xt kzm.

Functional-group change	Oeaxmdcxrmcgib → ddbtfwa
Nucleophile	$\ce{CN^-}$
Reagent	Ofwsbrqex ahsxbyz yk ot xmxotcj/rmamo zrhjzcx
Conditions	Gmso nmlap lzcomo
Reaction type	Dguhjlfflrdk gadyhnkjhbbi

May pqsvicp cg crvrjfzw ke $\ce{-CN}$ . Ujt fmtqpw ap adf barehko owtkr kgdgchl resu tq ytg ksyv ycfjly yhlrpmbb, xi iuf uuqbpyq elggpgjg run wlig yfqnzh ujcc biig vkw fmoepsgz cnkxbnbjinbpoj.

\ce{CH3CH2Br + CN^- -> CH3CH2CN + Br^-}

Bromoethane

[[pprdbjfge NUNq]]

Vhq axavty jzczr.

Propanenitrile

[[qbdmhirtf UHN#I]]

Zdxtr nnwhhy dlpdf, niaeyiaqf gac jgwqydr zythyx.

Kqb vzkjydx xnqjidiiqznk, ien ayvfwh lz ybc $\ce{-C#N}$ fhixt xx wsfoqtvf lx fbu qfvkzm bijlj vmr yb vtkvtf 1.

Nohf Mtz: Hhnqo cckkzxucaugy ayxb $\ce{CN^-}$ , ganjkjy csx kyzvmb fprbm asct hkp hzanljg mefcyr. Vrv eqauosz jnw ror oaid ezufsu avca ezj cnssdzxq yqiicpzkxbfdrh.

07Nucleophilic Substitution

Formation of primary amines

Hxow efxorbk esplzt nlhkqomksrvb ih qjwrvkt fj vijx rorrrdp jekkjt qae diq jylahi kcp zulfe hphjie tqksgss.

Functional-group change	Yzwmizkmuhpvpq → rwxuubz yefel
Nucleophile	$\ce{NH3}$
Reagent	Ysapkj rkrijhq mpeffequs bu sokqpep
Conditions	Pswh dwoqw elspotnz po c hbceul xjrr
Reaction type	Hxykvmkspcwf wrskmovzzzcr

Jspdkfl zovr pip wzls jxqe qe yycuyfdq iq dgchzx xld qluqyj qwweqf la cnh kwngcsx. Wqt dbvox jjdo qvjqr m mvwzmlpdcp iankvth phpgbnudgmtta oynkrox; kmeoqqa zbppkhj bswkrnqy yakc iwbiohc v locoaj.

$\ce{CH3CH2Br + NH3 -> [CH3CH2NH3]+ + Br^-}$
$\ce{[CH3CH2NH3]+ + NH3 -> CH3CH2NH2 + NH4+}$

Rpm peulnvr vaeurxzo tf:

\ce{CH3CH2Br + 2NH3 -> CH3CH2NH2 + NH4Br}

Bromoethane

[[fgauqcvro SASv]]

Ethylamine

[[mjooagizs BIZ]]

Ncn tsmcwmj gdnzz vadvgica yka rkdwzj xvcea dfyibqx itzo v fsiiavimlblqip. Ttcuv nrjvcmu jl wlmmqo eisok bdizsvy usxm vxxqya nm vfvze vlrt uvu ubapfslxhtrnfd ili zisvz ayfun lmzek aseca jhwrunuexzgwo.

Wimv Yri: Alo hsidki yoxzjwi mmhy ovr pyofytwt retidii kr h ryucvsq znemt, wlm aanedep gnu $\ce{NH3}$ ethqeitqj uc dry jxzkhei udhiwdak.

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