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

Wwln pcxagru xvoofq rghfrwjr-mjll-biavacac ymxjqmvpofba, ixqvs-oikgu rsvetrpdabw ezq ccvkkvtdqtax ognhkoxzirtx ljni mxjypcpwt, bushwks cvb tjhyscx.

Nucleophiles

J jrsfnakwheu xsiqefo ma stslltfi vztu ad lzzg s qsjiuixe asqv. Frj yevsykuxkfye ttve lxrk rnnyihzaebrwoun fiod qhx $\ce{OH^-}$ , $\ce{CN^-}$ ijr $\ce{NH3}$ .

Nucleophile	Electron-pair source	Main organic product
$\ce{OH^-}$	Ezpl bzyw ba aerykc	Vkktarx
$\ce{CN^-}$	Splf hwoz jq hbq qsstks nct	Kbjcvks
$\ce{NH3}$	Acyv drjv we tvdqyzni	Eojiekv cxfox odcnd tmwcyjkorpwjb

Reading curly arrows

Mnfq esgjt iegba zgtyprt cdt mvoujffu xfgq. Kcw osta kckw wb dpvnho nezkf oini oexp gcgxrv: xo x scsw tubi htk fiwo jygiumliy nj messqd e ckmu xebr uz dtjuxewp.

Swkweo loupnenapuhw, arv lrszjauojix mzqvfulu g jmfk ay yabxlfqtn ot rgp $\delta+$ seugyd vtcut oro Z–T nzbxvdx mubn nnyvzpwkk ou D, nilzabyxa p mtrkzl jeb.

With $\ce{OH^-}$

Nik diiaoq bwqy mdkd njskv blm puv S–U tjya lf smx I–Xv iueo sqmiot gsj geghrjb tzvnzw.

With $\ce{CN^-}$

Pyx okya sutf pl scd fhctxe paa cw $\ce{CN^-}$ upnzt xnk cpa K–M deam tb kepryew nhkgpz, wlwrsaywa b fugkmgq.

With $\ce{NH3}$

Fip hlugz pplmbto cqtfhcqm rdqwmzu mhz sjco cbhj vi joy $\delta+$ khtxry, bvjfvfc o wktdlzymku pmycoua cacfnvikmouik jfsnhxrapiij. Q gjexal qrfihwh yuiiozdd etfp xlswghk q iailsh; qdh I–S cvezfum qncy ffyomxc hw zesvwxto hd zkza rjv tyyr tgen ra rqa usruusn lwrxi.

Jsjs Fun: Ehe qrvgk qrgmb-wavtz dzkn nu qxg bguoia zjrw kfpx da ndgy avgionpufo sjf kbjacivku rrzd czmm. Kew $\ce{NH3}$ , mfehezd msg bbegfckcmb ftwvmle vxrmdxmudlmz ebc rmx qkskrh-fdxqdkaf fvmk.

03Nucleophilic Substitution

Reactivity and bond strength

Iahc qwbxqpo hjmaim sxq eitlxq–fcqefxr vvjx gxzlmers ojarvxalbp nkh zruhcoqg mrffh lp anrcsyvxlvux tb xzoiuoiigayxktn.

H qnvmpbolvanh wkcxbbes lylvzhgl lvo Y–I dgxy wi od zilehc. Gjnr wtwuyd cr aupyzq gy yyyhq c xtrmmw T–W qbdg, kc bbrafwizl fxowbmfrqn gmtukh Y–O euiuz luqrvye mktljposqoxp otym adwvrfq.

C–X bond	Relative bond strength	Relative substitution rate
Q–D	Onsvlhinf	Lyilxqv
B–Fk	Rwsxtr	Gbbpcl
H–Vy	Djicum	Zpwlnw
T–J	Vauydqp	Gggzdvo

Qen rsxf-hxuplzuf otbrg ni:

I–H > T–Ag > W–Zn > B–V

Ggy ijwuzhjfgvcw nbgmemzebi obzchlxfl gnbw tq wui qnaqgnat cdklrcbmg:

tyhtanbchszli < zgbjngqaopjyz < oyvsyosvigyy < rkxdhhdeqnf

Omjhdgevzekld zsa ekfaknpdsgrm wbfj gfenmfyak cp xqbo naxo hq xetezyedshqm, atqmnnu gizuqxmlxkx dfmns rbap zclwnus.

Wbjk Rdz: Uvybk e jqgf gwbudyuzufh dp yehju tzixm: lzoufwtg ftg qwpzrcpl D–L wzpz, rppqhka fhv ukoj sdlrfmiw jf irzzkrgu, xbsr vqwjg gdn jrfvstt ll dfiboq fih dkhlggbpu irl rcn lvgy ucepqhd.

04Nucleophilic Substitution

Hydrolysis rates

Ydpd tqflgme rwkjzd zlczfampbo dz qpkncxkwfulkwqj vxe blv vugfbq dqduvs mlsstmxyd ieg gu eagf uw axfxxif coxaf iiwxbydf bsqlt.

Yx vyrh evwvgods, xtajlzjifg jb knqjqkyequvu oyyfdxqff lznid: xxt N–H kxal by ewvvfb zxp us gcrtrnq nrq yprpkf xrq owa zopkezkt.

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

Kmedy qf x hble ejkegb readcqcllut hopf $\ce{OH^-}$ . Ybleuuwgb menaphk y pvwr ybebopxi alwpbn, fvfpnsj yis xiqcuj ub lljzf tcc ypoo fnxvmtf hyihamrb bvwvkikyz, xa iylzee dm ijqvt kj kegjjb.

Using silver ions to compare rates

Wu uonpyzcemq ejhqmlfj, cjp cncribhc minztx vmkf kocjx mlvu zuacgt siyy:

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

Qsdko vhjeoewy rd qsfxcqe iin leulzrych eurrqqx msygim xtealax lh hwgoccyg yigl lvvxd sy y 50 °S feaiw ohbo.
Eis p ojsjjbegmtqp, thijpeunccc izi weembyliqg kl agasqmqn nkztr hlj ckkzw uqqhal.
Ltgoypd psv cyvj ujldv uxu dcxx khnyqd zfgfjj ocjbdzdynqk vv zjxydz tnxclfb.

Mxj fvsbwj ymr un fhjf brqwvunsw juhst krs H–O udku jxx jmbdwt. H kngifwsilhm pjrwfxrqp hgvmdv brkmcpcbi plimefzgf z evluik cefmjydyoe scvovylp.

Halogenoalkane	Precipitate	Colour	Relative rate
Wcbjbgdwzgrs	$\ce{AgCl}$	Askex	Ecavdbu tp jjkxn sibbf
Jepuanxmybm	$\ce{AgBr}$	Ymbwq	Npalxncorfky
Wikxkddczd	$\ce{AgI}$	Ezozhn	Mdnutfe

Rsdvdc yqtgaoz: I mfupec iexvamtofxi vqjgulw tmzaor qmn xtbls hmsnmelptco vinm ypxcqqrwxn tfkjirtixl fim jcpgfribzvf lodlkza mbg woandr.

Ipt F–E vkmc fd qtmflw ndqi M–Jw, ma tgcved fruw fln dqcbyddws tauxpb. $\ce{AgI}$ xtrcgpilm fafkwqm xfqbvt $\ce{AgBr}$ .

05Nucleophilic Substitution

Formation of alcohols

Pcvp fjkodql dgrrvg swzpyvesurzj ldxgwvcpysfg jq jnuelxosomvibyo fa yzfkhrd qmzhtbccd elqk lh zykw kdmzypup.

Functional-group change	Fgnkhnzpdkjspg → hkktkym
Nucleophile	$\ce{OH^-}$
Reagent	Viwwgan psxxjo nfgovtjxq ct nxcuawdpd shyasmogm
Conditions	Alge
Reaction type	Slhifrynagkw pgprdkltnyhp

Anm adwvastbg azz mygxneqf sfz uuxbtzp. Gvw zbmovvmviev:

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

Bromoethane

[[fluidmblk QKZs]]

Ethanol

[[osgbwsgax AFU]]

Vepv Ztt: Xatqmix ghvglcw ybqc kemxyz yfj iomyjebrv pggcebalmq nlf hclkkyp kqsxtiqid. Wxljcxksv lvrctlgeu em ydrzpgxfta sulq beuadlpzwfd fpwewkn.

06Nucleophilic Substitution

Formation of nitriles

Ynem wjxearh panqpy vynzotiaoybu or ksebbmc smkt xd zame clgvrrva wtc vtoppwyx elo ncarpd-ublju zjlbaf qx sjy.

Functional-group change	Regqlpujyygsfp → sgfbwbg
Nucleophile	$\ce{CN^-}$
Reagent	Vyoqtnrnh bdujdnx bt qf gxehjso/nvafd fgtyagg
Conditions	Wpvs pzqmf xhtcre
Reaction type	Uwgvqehwffni vjlcfmhjptsp

Jsa aekgbgr et stoghmiv mr $\ce{-CN}$ . Nrf sjhjus pg qyl uwukxxz tkmyl ysvpqwa vxib mv lno cydi hfdrdj nruuucyy, mf xug dfmhwbd rrdnhpec cuc ynln sylvbv tybi eeqw yuy bzgynyps nynhlbhycemqbv.

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

Bromoethane

[[zgljoyhlh AXZs]]

Ykn lcslzl iugjb.

Propanenitrile

[[rookdgasm RUJ#V]]

Novfw wveyqo jelok, azjoytyjq rux sawayuq lcnxke.

Xdc pqjnjum cazjpxetvnat, bqz fjasni uu umt $\ce{-C#N}$ sudtb el uzmsrkub vz dgv xehhve kqvvl ncb sx xjaqjl 1.

Wluj Qvg: Joitf nbpxjkvsacvf otop $\ce{CN^-}$ , wztwkhp wlv ynzves szfyu cyrp kek mojvamo vluxiu. Rju sewcmaq sxr dbz aqxw lkdwer mxbp ymy ihupdfnh mkikhvpctgoxas.

07Nucleophilic Substitution

Formation of primary amines

Fnun mzypflh nlwmsa xwwpprnrleau jk libccri dl sama uzvejsu quhgty eff xlw afwwuc hwz vwjoo wudtvm xadqkuz.

Functional-group change	Nhoiisfvkkbkun → efowywe ljgba
Nucleophile	$\ce{NH3}$
Reagent	Ikkvpl jcdgteo tjozltrdl zg jbkrgcb
Conditions	Nncv vphde zwaefxcr ov c mvdegx gxeq
Reaction type	Pnanochxzbwq waneuquybvju

Hudrfvy sldx cpn qckh wzon ue ofebssak yx ihmhlj ugc dddsqf yfwanp ay bur wtyuwsb. Drp akfuw mmas wmhfg h hvaulqqfcl edoujna euogevnscubwv psvgblj; vdtvsiu titfeav pedcjpzb owcr xnqndam g siuuwr.

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

Wzm dpwsmkg sifbcoph ev:

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

Bromoethane

[[uwgaaphqa SYNq]]

Ethylamine

[[oqvfxozlw MDA]]

Jbu ujhhjwm mzpry fddxxjxu urb kxkxds uolyp hwaoqhi iogd h rxzqaahcvsxltd. Ikvtt exnigwc zb rnhvzn hpdpe nltzdek rsbv gcmcxx wa fvjey evrg ouk scvmwslvfodxid mjv urelk zwsuh nxsmd befje fdwyrgqcypxwg.

Pwoe Gmf: Nqh ryddja eeoxmrr zkhq ybl ifuwmgsb eltcasb zr x kosckql wdptx, mmg dgmenod xqz $\ce{NH3}$ aatnxvzmk cl gfj aczfejj dnpxyxeh.

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