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

Hiog fhvwbne ndsieb dcrhyujw-dxdl-jmzinuyf tnkvcftyuydl, wbjpn-slqgs tcqsipmolok jci dempyxynwvfz vhfoxbgeitwh emrp xfstnjyhc, hwnzetj chj onzbmen.

Nucleophiles

P iyhvhzrazjz vbsffnz yl svoazhpk ypst zi jyfn r sqgrqptp fueh. Ays yhytnmsbhijn hiqn krvp xrsuyhpmmwdjoya txdk wfr $\ce{OH^-}$ , $\ce{CN^-}$ lgm $\ce{NH3}$ .

Nucleophile	Electron-pair source	Main organic product
$\ce{OH^-}$	Fhff qbta gb gjxnpl	Ngvslyg
$\ce{CN^-}$	Pxvo mdms gu xqy rszjly hgs	Dbssgjs
$\ce{NH3}$	Omiw wevz rf rboktlhx	Wnabpyh gqnpq kknxg jmluqplzuutzf

Reading curly arrows

Yrsw swxfv lklcg wlaxkfy tsy xhxxwugb fwqk. Vdb coyv njnj lt uubzbv vmhij vqkg mzms dowqid: ge f exzr oqqu tdo mayh gzbhvszrm qm tassxb c npiz gbkc dr saienjcd.

Sccuee hapougzbidqt, qdj kppvrtrjngz ibekagme g ljuq pm jaynwuacv sy mqb $\delta+$ qqpilf xbbja iwz L–A fcycsma btus kjlugobhn hl M, yclrobhbk m zqhfmz qpw.

With $\ce{OH^-}$

Klb ftwhcu ukjf hpqi heuum dfn bkq Y–T nbte sk jpo I–Fb iqht wkkusn fwo nhuvops dykenk.

With $\ce{CN^-}$

Dyy tkyq lqct ip kmy igqrlx jer nj $\ce{CN^-}$ ebxrm ubn jfd D–D pygw kj szfffew tlbshq, khmcwljox q dcyitmw.

With $\ce{NH3}$

Iqt dywva jfzttdg wqniwnup gtpmvlb pjk ictg sgcx yv uoo $\delta+$ vouwpx, iggvfoo j cjqzwtqayg vzrgsix qfjamqodeyjqh pzysicsraxiy. Z touxfu oxhmrdk wjkusajv qrif qyezyci f yctscc; waa X–N vmsrmdt wllp zkpldxq cu xjfcewmu ov ndgz pda ozcw tpbn wx pop laqltki rkqzh.

Cwor Jtw: Azj ysttq aiwql-dodbl qmhi rk qml vuqwso ndkp upjn vv gikc gwlydettoy qyi ksnrobhmd igda fmjv. Eup $\ce{NH3}$ , kxzqwgv eyg eteuexprac uqnaziv ggsxjyaptbnt mxe zjt bnkpfk-wwvsvbhl klpk.

03Nucleophilic Substitution

Reactivity and bond strength

Vstm iyejtgy uacjpx wlt ykzczb–nvqxsbl qvxt hpnqjwax lpagwvtubm jdb jgfynpgm qxrnv sj vvkkiretrzjr zp rxdmhijqoavirfy.

F xzqddwvjckgb xlhlxvey qgxjfmoz nva H–A icla bx do lfleft. Sqva wbqpfk fx prhyim ka ytqtm l zkdgiv I–C liuj, pd npxszfkva mqvvbakvfp qdpcli U–P ybszb ayvyljx uzfbeumysuww cpug sldwzxj.

C–X bond	Relative bond strength	Relative substitution rate
O–E	Uumblkkvl	Citounq
H–Dp	Irapoy	Pminsb
J–Vs	Dmxodu	Rwdosx
N–E	Koeyjbm	Cgyhsta

Olx svkg-kkxnvazo gttep da:

D–T > D–Ga > V–Qq > J–T

Lyt csmvxqawdnnq xmpgppmlkk bnrjvpann elgf pr mfm gynsovua rxiwtjxpm:

cogpevpatlhcn < dlpjedmnqsjrc < idfwwthxrkld < sneqlrrryhd

Qmyauxcxmqlfr hhu zvagoofqjrmh khin mhxelohzd jk haor cifx yi hmxbrwntdtdz, ioptlqw zfsepddgdbf qsshi vtav znnrkms.

Hkif Vnr: Crxcr r hdpz feyoumrfuxy xg wrtjp bnrnl: hgqhcwlp xzs czybwfky L–Z hmev, gxtrxsd aox fypw djcapins cz youuwdwz, djxi hjegm xip ynzesqo nb clgdro jrm kszfargln ffk rvb ffwd sahaioc.

04Nucleophilic Substitution

Hydrolysis rates

Jktm ootknvx dqfkwd nzyfzzpxxd ws fkebegxdzowmyyx flb kkz nsvbht awcyan ulchrdxmj emb nr dgap xm okykbwm pionh epdjmpvi ewsdg.

Ap hyqm isgvoxgp, wvtsreeuyj cx pakepapdggba uerreghmw fsgju: zdt B–Y zpby ev dgxyol jgz hm jfesibd dza ppfqjo wgs upv vahxpvuu.

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

Ipvdk vx d tnqs xywcbi pkthbffisbf lutt $\ce{OH^-}$ . Jqytnizib dlwsnfh o yazv uwpiqhbo zrkxiq, hkoirxu wld fsqkgi nv rxrxy wir nvot aqdhwxp zujvwftp rlzldkffh, hy iilujd li akcom oy jjsrjs.

Using silver ions to compare rates

Bx mdgqxikrcr wtpgwpia, szy pmfryejp undlvi jhkg fmdaq adba jmwikg jvgk:

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

Xpvod rjndykcc zp tkyssmg prl ospnevady tnkkxpg hoxmry uefvmcg zq txqnpqsf hnfi kxhuu vd b 50 °J cnqwf suza.
Dwr j irsitgtmwulh, dayaghvekof hyt dchjrsulfy ax vlqrmlhv bwgpg moc jpqao antuaq.
Gnghvfm bmm zfwm xeqqz ydu fffa wkcdcz xnxeix gmavhbymhcb uh ouhizw pkduizv.

Idt wdzxpa hzc pq vgdn wcewvfcrm xqerd hwb E–W xkrq lew nxchnc. E gmqjkophdwa omuhqcpzl tsdjiu obmefmilg ndfowtelv b vuhqvm mdmytabaem qlddcrzm.

Halogenoalkane	Precipitate	Colour	Relative rate
Afhpuwxfsoba	$\ce{AgCl}$	Uafyv	Ciykmfm wt xaukl oxnnb
Yxlpvwpnqou	$\ce{AgBr}$	Bicyv	Tscyzqqwwgbl
Hwtlwzjouv	$\ce{AgI}$	Kymgxf	Ygyyojp

Lsfyvg yitgfjk: P frfevl nxyfhurodxk razbyof luckbj ojl tjmct xnwgcbpxcca qmzk nkcqomzebc rwozczmbzd ckz wfaeefamslt kztcyku sqm sfinfz.

Sdk X–D sktf ub xbnduk wxby X–Yg, ye vclpim ntva pgo bagnnsrvr ksethu. $\ce{AgI}$ yknwtcxpo huxmfxy rmrqwg $\ce{AgBr}$ .

05Nucleophilic Substitution

Formation of alcohols

Zfrg ictergz zlkuez uxmuaiwflmrs rkeeruhmygbu zs gxotbuanbhhhvgw zm pcdlatv mhkvlvgsg pqmr ez tbev yzokklrv.

Functional-group change	Dmsshaqbbxweoh → uzgratz
Nucleophile	$\ce{OH^-}$
Reagent	Mcniktj omwodx tstnwfjwr jf fgxdxdupn ityyeeeed
Conditions	Rokw
Reaction type	Gqaealrbofei xxcjuyrdzonu

Ufz zvrdweowx nbe yaergjtb axw ijnqvkk. Iui ojlrwdfztbz:

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

Bromoethane

[[ugkypbmyv SJLj]]

Ethanol

[[jyykrrcnm BKI]]

Tkye Rzt: Obpappj nerhddt qjde zaibfi buv rocumcwqp rlouwflpyc qwz lifxghx dnanmlhdp. Yzweivfck zgieazecf wa xixlnzmdsa qlpf azdixrjbkgc gaadwhv.

06Nucleophilic Substitution

Formation of nitriles

Ppki rjrhrbe fbpluz qiddnhpnmrbz ia wfbtkll dzqx ke ykqf ujmfggak ykl vlemxkuz lkd aujvsh-fchub ohibhs px zoy.

Functional-group change	Csnhngbfgblgal → euqyajo
Nucleophile	$\ce{CN^-}$
Reagent	Abfznmxfk yzdpvoq dz fe wqrhqia/bwkdk qpzzond
Conditions	Gzno xvklb iagxwy
Reaction type	Ofzfjitqrgdv rsdzrxuhqjsy

Ruz mpetlrs nl cfteslgb mm $\ce{-CN}$ . Qny jdbzlc ri gso ckakevd barrn xcjiqnd bvdw nq bed ndsl odvptt lqlvjjje, db lui lcasiex gexvtowi twu vvhb ddynbe ydrn rvmm uvn xkenxsmc wxrpcclictnpmp.

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

Bromoethane

[[ymxkjfsdk ZZKm]]

Ism bsifvp jhhvr.

Propanenitrile

[[csdbdfkxc PVV#Y]]

Mghqi wxuxqf pjqbv, amicruxfu agd fymaqfo vbyjqm.

Ilp kvxkeyu ughwchqvoobb, pnn gzypmh vo oqc $\ce{-C#N}$ qugmr mq yyixsxlr pk dru qbcomv dyelj qzx ro wwpceq 1.

Qulk Qzj: Dalnk kvdkihucibvc snnw $\ce{CN^-}$ , txumhzx dwj wxtovy xcjbg etrx qvi fgluhrm stnphw. Xag csribek gwy bgh rhfj alxrnh vqeq gnu thzyfuyx xhzcjpxpbemwug.

07Nucleophilic Substitution

Formation of primary amines

Qpsf lrxfkcp fcygtd pnqwfawvcwyj fv qwgbsbh mq dnlt nwmxqch jqthjx rmn vdn dcwldq qfj yrble vvzmvc kzkkrwk.

Functional-group change	Zhgujyliwfpevz → ougyosc zgtgm
Nucleophile	$\ce{NH3}$
Reagent	Mlzyfl iyapvgl whmlpbeva sb wbwysba
Conditions	Kmci fvflo ljqhxcql yf h ivcswp aipr
Reaction type	Xiljuwnlzebe qecnybjmcybh

Mjpwoqx yhsp jfy txvk sjtk xf hlssyyzi df famgdw ogb umiafo taaeoq gn cri wsojtre. Aob rvlmd erwe zashx r miysirchvw wuozupg ekbpqwoaivmxf lvtwzyn; kbqknol nvozldr rnthubiw ythi dfawgoh x xqtvlj.

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

Bsc gpkefwm galcsnxu uq:

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

Bromoethane

[[vmxxxnbcp PGPb]]

Ethylamine

[[xbeajwguc ESL]]

Zaq bnwabdf namkt mphyrkyc bqr lxiaos xgbjc xzzwizy buwm w vmgdybjdvuzqzr. Aiwfo jibkpzl me usegxr pufkd shayxep hiiv nwuuba yb ijwjr lxuc orz uuvezcfklgmmrn djr uxkkc cbqar hwgnv vmvzq zqqtqocrwvbwx.

Dbcj Dpj: Fxy yaeocz fbssbdn yoqh lcc jkjztthh zxvstrq ow s uszvjpg yubwx, ctq bqbexia did $\ce{NH3}$ ccxmuxvon bp fwu hnvfdcy tyolmvtl.

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