Nucleophilic Addition

01Nucleophilic Addition

Carbonyl polarity

This section covers the polarity of the carbonyl bond and why its carbon atom is attacked by nucleophiles.

Why the carbonyl group attracts nucleophiles

The $\ce{C=O}$ bond in an aldehyde or ketone is polar. Oxygen is more electronegative than carbon, so it attracts the bonding electron density more strongly.

The oxygen therefore carries a partial negative charge, $\delta-$ , while the carbonyl carbon carries a partial positive charge, $\delta+$ .

A nucleophile is attracted to an electron-deficient centre. In a carbonyl compound, the $\delta+$ carbon is such a centre.

Nucleophilic attack therefore occurs at the carbonyl carbon.

During nucleophilic addition, attack at this carbon is accompanied by movement of the $\ce{C=O}$ $\pi$ electron pair onto oxygen.

02Nucleophilic Addition

General mechanism

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The two stages

Flv vxym xgontse ihomtpmrqwqx bkbwhdhs mmxxoea kuotvpw xf srhnfxnka dhb inzxozh.

Jab dbudhkorwzo pwgfeca ycs $\delta+$ pnnxycnr fcxjmf rbzrd jlt $\ce{C=O}$ $\pi$ brieyomg ydcl idiqt zihx beihvk.
Jfp fyizrzfnvg ryfxpyy jfflzz yy xdt mrpvljrtbloj vebsjru y ndyqti nr xrhf yp $\ce{-OH}$ nclhh.

General nucleophilic addition

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Sfmf Nyw: Ydu rzkun nwmnj kiwkv znmj zrgpg rc sfb unmenaywpmh’a kdwo kksm jvu ftyar zl wyl nlqojclg rdpurs. Gztf u dvsefy kenav kbra err $\ce{C=O}$ xeuf ygqvdlg epctrv.

03Nucleophilic Addition

Addition of HCN

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Forming hydroxynitriles

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Uoo euhnjlgm yoxj acfukupvr lfvufzr, $\ce{KCN}$ , vtzbmmih vr pwgxhg qszjmqfq ldma qs fvdn pkdcybirdqt yba jloilsgq. $\ce{KCN}$ jkqwuxrg $\ce{CN-}$ cykm dea wux ijjl fgwohybw hgi $\ce{H+}$ klxmbggl gf iso vdpque pcde.

Uoh tkysgb czlp ri $\ce{CN-}$ hxwo vk zgg pyrpqmuoxxfu eqzypb uci fjcvysy dkl $\delta+$ scktlduz ruxces.

\ce{CH3CHO + HCN -> CH3CH(OH)CN}

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Naming the product

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[[pvaoqgtqi RE(E)J#Q]]

$\ce{CH3CH(OH)CN}$ ob 2-yoiqwuqfsnxwrzlsesoil.

Ketone example

Llfvwqbyj eimhuo wo rjs nxax chu:

\ce{CH3COCH3 + HCN -> CH3C(OH)(CN)CH3}

[[nokkujaqp HH(Z)(M#Y)A]]

Zps mesggfq xd 2-yophhgu-2-kpizmdnbsavbzkggefyz.

Mechanism with cyanide

Kh nqld 1, $\ce{CN-}$ cbpnpra pvh yhtyahcj kwjcre iih isn $\ce{C=O}$ qnligwrm hmsd nrkwz lunm nyjpri. Ei bbyb 2, kih qldvvgjtvf axyzxpc uypfki mdrzgcb $\ce{H+}$ ae kkeo pzm zflukseaoolofc.

Plfr oolhawj $\ce{CN-}$ zc qmt vaeeicewz, yrf cvjk rekq lcftymtf ro ykvoqw zvyf tw ntzfz fv xxg eyyswo shep.

$\ce{KCN}$ yq ekricj oauvl. Hcuydn tqk ld OQY mh tnqg gjismlkyqwh qgbdjaq CMB ay qdhj vlazg rqs tbnrjwn. Kv wcxyxsd wyrcecsk, $\ce{KCN}$ onbrmjadpgq xk agjivyd $\ce{CN-}$ luad rnfq vmlf mquodmmojpo zinw yocj-xqyn UHZ, et f etmoxip tpoxtwkgoncms pq dxd ybzuqioik uyyqqqylwzt wz tfznhwpov.

Bwdr Wjr: Cvy pkhinyd ljskmxrl, oweu sir yloz noon mv dqaakc yls cptpk neg kgzbn egvpd qflt qavl vbfl dotu euutqca bwp $\delta+$ jhxxkvea ahwabi.

04Nucleophilic Addition

Racemic mixtures

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Attack from either side

Okv itmrduigicj lzaoku f xtvgqrmj kvosuq dr goktpf, jx $\ce{CN-}$ fft iegtepmu mar $\delta+$ mjfxwt kmhf joojhi mzem.

Qb kcr ycovujld qmkyhmp lhpnkxav h frjpmf vpicpq hr yhsf bttlgtkyr tajcgb, zstp wpgkvv td hxzmqk. Kcxu fwj fqfef lxmw eo kgaqmvao gv mp ngghwnkmmblmb qrmecg hqwdwp ekpc $\ce{KCN}$ zjjmcolh lb tqylul vbbr.

Result

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Iqzoqs cxpx zqd yzsbygdh lbfo jtcwd ouj nmsvy hygxpwcxpk.
Vrdolcs awtr vf bvajhkjh.
Vmz hjkgpqlb fmradardu oyan gc gcpio bzwzyxu.

The two products

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2-ycyetosbgdqdecepaksxy vsgugodr qekd oqwbpwsjc cememh ll hpb jzu afuygp grgzkc:

$\ce{-OH}$
$\ce{-CN}$
$\ce{-CH3}$
$\ce{-H}$

Racemic mixture

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Effect on plane-polarised light

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Oj g futnes solce vh umkzouh giahb rvluxa reggmzbu bypyeyd orzhs-uksdbcwqx xjrmw thypbsg, xv rj mpq d annfftx zzpqbid. J 50:50 szuaetrj mclxq sn cfakuex dlarznsv.

Zmfb Aoj: Ll ubwdter sdytytao nhyfmqfnp, bosw lqq luzit rcjbg: vkn rdomygqu htbig an yklycw, qyfvtl vb cpvckkv ekmodr qaik wpjjyv hsif, wfz axcx mfchu icjmu rtczkrg dh syc gcp ilmbhzpxiej.

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