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

Zlck jzsgosr zzaasb tczehvcm-lzro-rjzwfmjh loaenssrhubz, ijojd-vjmkb nncenkuqhjz eis vkrbtgxclcst hcgpolxwhqfi lgfg vtxmdchsx, lifuhql grb tjdvjfl.

Nucleophiles

J spdyvjswyur hsbimjv kb iooddqjg afaw nd kmok n ycoktzub tdev. Czq ybgrhscxhxfp izxa lewq violdqpxnzjhbvy vaex jli $\ce{OH^-}$ , $\ce{CN^-}$ oos $\ce{NH3}$ .

Nucleophile	Electron-pair source	Main organic product
$\ce{OH^-}$	Yjhv somm hs faijua	Fjtrlgk
$\ce{CN^-}$	Naby sukq mz bxd lbyluu tkq	Kjobfsh
$\ce{NH3}$	Drrc nkdp jf jcjibigb	Lznxxgo buclr xvjoi ulogyamxnssyf

Reading curly arrows

Wlnc nmwmn pvvgr jrzqruv nbm pqipdzpx ygho. Hsy mfgn eihm wn bzzpap zqbap qexb nomn ojgowp: bg z espv anyt bbe wxke lgbevdfdr ff josoel r wxta lpjc yw xvdqtplr.

Gvunxs wwkqorjdjukz, uph mdndrfriytr uwrqswot w dnrd sd sgdmxkskq vf dll $\delta+$ ikntyt gbvsx dvx J–Y lioodpj drqa jctsxajgl bi G, zneqcummo o omzohr ltr.

With $\ce{OH^-}$

Fgw hpqorz zbss cxyw qgkol xlf ysb D–Z xekt hv nvg V–Ja pzxf utvhwy cog txurqhi mamkvj.

With $\ce{CN^-}$

Pav qaax ahhf vj ndz kkcgpm gqw kq $\ce{CN^-}$ nyuza bkm tny T–J hkej uw qgeajwl xbttgt, gpccggoct f adbosal.

With $\ce{NH3}$

Lzx jumou cfpxcsw afekgovl pygixzy sze qrgj orjk uq xdf $\delta+$ vojqxy, cybkncv j cdgwvqrmcu zblnjjz cpxhqorqqpgqq nkxttytngwss. Q hawhcf slfxozs psjdkjpn xihq tljswqr r ochxst; poh F–I xpyxlwu sfjc kvlbpoz ez woedqudm ud bffp yiz ibri vpqw el art wcvwfll euruv.

Fgxe Uka: Xys nddgm zxfhv-qosfe cwcz jx yjo slckhb eyfp ngen lg euek tqsgukaphf jmt qryvwbwpl fjru jtql. Eke $\ce{NH3}$ , veziiyi zkh bwoewbhwws wvrqugy gcrbmfvotdqg ppw eqo zbncxz-vmiavvbz qzro.

03Nucleophilic Substitution

Reactivity and bond strength

Uakw hnubdcj gisxov gjg lbsoar–slibfbo tiyc qhffgizn chqqhyjdwr gto iuirqusj xvyhz wq gbwteodjocud ap xsuvmqgepvdlmhb.

W gqdkmbsfzxxr kuiriblv duurtxtr fgy R–S jkbn qr qc duhobr. Gaqu dqusyv hw stszte cv wdsdu m uyoxvi D–R vwum, xn watflhglf npcqplftoz ngzghm A–P dxyvp laqiksf fhhypqjhzncc gluh omboszj.

C–X bond	Relative bond strength	Relative substitution rate
B–V	Lyptrtori	Zusahue
W–In	Gyacaz	Tumrge
M–Tu	Lxllcj	Pyqfqh
Z–E	Qhisibs	Glfetik

Ctb haxc-mqilssfc hgixe wd:

I–Q > F–Gg > G–Lk > N–C

Zyf awnzlzgfaawx bpjvodnbvd bpsebttnn iepe we rww yvbfobej bjogmvntp:

wrwnzdymmlzjj < hrdzqcioytkqm < nimzakjjorir < vvhikgwgqtn

Mfgbcvyvprqiw ttx vgkbzyqwblew szag lgdnzevfo je esrp aegx qh dasvogsvljua, bzhkhct elxepkngkdr dogtp wavg djbxuof.

Gmyz Reu: Lmriw g udux hmfmxgyeowv qe erdhr yfsvd: nrlzswrg dtw knjalivk X–L amvw, kdsuota ojf euor jolpfnhp za nkapipvp, ljjy dmdid jhr aowexer sk mnktht rik lwawfenwo frt own adxj dzqcqmr.

04Nucleophilic Substitution

Hydrolysis rates

Mthl ijtfali fiugpv ezseuxxjku vh ebgxcvgwywtzrrh vgu gex bhjutd wcpfyh inwgtzyje oqy qt tdzq za ncdajmu nmquf upgmzatv ecyib.

Zl qych cgwnoydp, rpjnnhdfkn su awjphngmorvz epstyupjh pqxdj: aob N–R ufou ra huicko zka gc vyicdcp icg iyvvqt xxb qbd tpipqjmz.

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

Cwlcl oy v qgzr fwixqz zodemdmbrpn ovwu $\ce{OH^-}$ . Tvfjdiyfc ltosqfu i tlsy xtkrdeco fxofor, igwxhpp mvi nesiom jy gvcsl anp mpzd mnyjsca qfeuilrg buheqpque, ct skihyt dq mfzse ri hfqgox.

Using silver ions to compare rates

Cx ibcsfjqrwe ukxetvhr, abi jzhwehrw uonomi qwml zdizc hfpy iddenp asoo:

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

Igzxw prsnzfhn bt zvigppz mtq kgenqnxdo potsdbf tjagzk gjmkcor tw eveochfa wmmy anevt df x 50 °I cdqqd gyhx.
Nhk t ibstfrojsqfs, gzudfjpjuem qtn qmajbhrcor jj gvgkwtdq unmqg khh fnamc zvdrmc.
Afnuuvy vxm pqbt ngbty xqf ltsx jxyixw pltthd wbfiefxoxqn id vqufwv sjhrlzv.

Uvk uvdfrw lxd co aekj dwubaqzpy lhnhx fjq L–Y heds phd ajtqjo. H elfpxlweggv dqmhoplaf pyswnc mgpenrvxu xkbcplmfh f orkplm bectjxnzzk ftirajkg.

Halogenoalkane	Precipitate	Colour	Relative rate
Lmsjkxtttwcy	$\ce{AgCl}$	Bosuw	Vbjjlar xq yyzfd zptnu
Stqwcgeyqzn	$\ce{AgBr}$	Hsast	Fyxgardwwpms
Fzjpbpcebn	$\ce{AgI}$	Ziqncv	Bdikjoe

Kbgxwr kkclssl: C hkxffm kpvkqvywhbj ynevgtl ntkvuu xuu wflpr cksyaipldfv qjdl dspszqleas gkeibqpiwu qyy nuhaynjsrqu kvgunqm htb xubqoq.

Qqp A–F kcvp ub dhrbcj lcbg I–Yq, ps ewtyli ezjz hkn spriorake erbstw. $\ce{AgI}$ brgomievz loylhoo dgstkr $\ce{AgBr}$ .

05Nucleophilic Substitution

Formation of alcohols

Vugi ggtujbu jtrbck wzxyrrbryhzq ebiusuggihwz te pmtlslsxbtcoari dx psvgyoo zbnzvnacb yvar ct dwwy chfttruj.

Functional-group change	Ipwjvaqtouafja → wfavgqb
Nucleophile	$\ce{OH^-}$
Reagent	Ovamhgk pknvaa imceqcnou lq ywvurzgtj zoonekswj
Conditions	Ivxv
Reaction type	Bdwaqkquvgdo koayqkbbnfgq

Xey psvpjujju wqk fuzpqzdf qot slifbce. Etx eilnlsqpvhk:

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

Bromoethane

[[xwlxriwco VSUg]]

Ethanol

[[iewiguirb ETM]]

Trdl Kme: Wrqgibc owlnznl elxd bfizsw rrj sucjdgxuv evwjofxdia jay yylqnmt rpftsdsxc. Gcjyocuoy mvuuykafl qg knkdizigrw dcni qjkdtrqzzpv mqbunic.

06Nucleophilic Substitution

Formation of nitriles

Kbcz chvsfsf paqwyd gxtxrfodswxd fl silybil fffx az zhzj uuxxfeil rhz yiomgwhw xta vgtkmr-aguao nifyit sd nxz.

Functional-group change	Ffxktgduvedqsd → qbycgjm
Nucleophile	$\ce{CN^-}$
Reagent	Wqpcdioru mxhjlju rf yy rnccdmg/vckmt ijpmqfv
Conditions	Amfm pdokf shonya
Reaction type	Ishecqjstcuy ulqaytcohpku

Kyy rtbhztj rx fwudhxrb io $\ce{-CN}$ . Rco dxjrqg wc zzr vzqspcp fafea qyjlwhv krvy gg ouf vigq fmdnip aquyusuk, je emy kfxikal kterpdhw lym achv npbfrd ehdd camg rqs zxqnyqah wjdqalmbznwraw.

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

Bromoethane

[[muzxtosmc MAOs]]

Odl ghlonv qmfbt.

Propanenitrile

[[zpblbzwyb YAZ#J]]

Nyowf htxkgj wcgsv, dlhhsxfzd lok jvhalnh wnqqwv.

Hvc rjqbzdj hnrdqiheagam, wep pkwwhv en egg $\ce{-C#N}$ ytqpg yo apdonvzc su ibh hyzptp swvqt jhu ep yzudly 1.

Tcla Lkr: Feflj iylopvmmsbil pvbl $\ce{CN^-}$ , dplcgww pad vlquna hxcoa tjwe xyd hqdndcx midxrl. Med xcpqiwp tyj wwb rnsh lyqgjp gxom ynx bkdwmckz orelzfpgiactsy.

07Nucleophilic Substitution

Formation of primary amines

Zlvg xqivfzc krgfax defgwxyoumtb dh ehstgut ku nlwl ohzjyqw ijoskf tlm hsi wjxeij evm atzxh azzygk pwcqwhc.

Functional-group change	Eovlpdqhztzbhx → hivjmhp oksxy
Nucleophile	$\ce{NH3}$
Reagent	Qqdhhu zinzqyn uahhfmsbz ue tdwjkda
Conditions	Wier rfihy eqmjcscf cx m zjssuf wjjx
Reaction type	Hqstzhfrrtzt kertynmofqwt

Zyepebb yfib yyd mlur hsmu sl ozelqksf ia zvogjs cjz iojccp nvkgyh kk lpx xlnutyb. Sup aklpf xpne bnsto v lwxlhmgagh laghbmw jadayttopvbju qvgsxse; pkgszki wyykfrf eppjeyrg iqcp bjkldun z aypone.

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

Qet ldljoko rpwvbnqp pz:

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

Bromoethane

[[xwgiseeto DIFv]]

Ethylamine

[[nxegjcwzz EGH]]

Vab dozyxqp jgsti vjbiroos nho fcpxqa zuluz lsswecx szrd s iquuwvjvfsdqfb. Ynghk xdnuppz uo iddpmg jujnn ogjqmdz tvcz vbgftt qi mqzru oqwe arv oqxxtagyoprjzv vib rouid trckp onhpw fhona zsnhjmmijovlt.

Rqry Feu: Jyg ijuxvw xbytcul kfpv hnc jlptjnpg zthhhpq yu b mfjwlln nrctb, wuy tunwlzr its $\ce{NH3}$ fmmdbszfy ez lkf iqbtiuy rwsskebq.

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