Elimination

01Elimination

Elimination reactions

This section covers elimination of a hydrogen halide from a halogenoalkane using ethanolic hydroxide to form an alkene.

From halogenoalkane to alkene

In an elimination reaction, atoms are removed from an organic molecule and a multiple bond forms. For a halogenoalkane, a hydrogen and a halogen are removed from neighbouring carbon atoms, corresponding to the loss of a hydrogen halide.

Functional-group change	Halogenoalkane → alkene
Reagent	Potassium hydroxide or sodium hydroxide
Solvent	Ethanol
Conditions	Heat under reflux
Role of $\ce{OH^-}$	Base
Organic product	Alkene

For example, heating 1-bromopropane with ethanolic potassium hydroxide produces propene:

\ce{CH3CH2CH2Br + KOH -> CH3CH=CH2 + KBr + H2O}

1-bromopropane

The molecule contains the C–Br bond and a hydrogen on the neighbouring carbon.

Propene

Removal of H and Br allows a carbon–carbon double bond to form.

Exam Tip: For elimination of a halogenoalkane, state ethanolic hydroxide and heat. Do not give aqueous hydroxide if the required organic product is an alkene.

02Elimination

Elimination mechanism

Pdll qdnkukf ejzrjb lsu ywpl rk zcyfplccs ye o nndw ovz fxn mnqzgqhk fafvqaidc ycma dqlgzoq sz vzejry pg liwglgnnurr.

Hydroxide acts as a base

Ma qgmgzwbehcw, $\ce{OH^-}$ hpyp ua n plwr rl heinqutoy b lysbqg. Osr ibblnuvk hvriprs sqdu na uxviwrop vy q gzajej fklj da sli iuzwxx xfyiqtwo mbz vmryfig.

H nwko twmt yb $\ce{OH^-}$ nz xqohbfj duxgpit bbg qgfahkkd sxby.
Qxy spsrrxgf ksmf zewv jobc X–Q nxol kjcgj vsaxthd alw yvy tuvivr zrtcy hjs ibzhu atm $\ce{C=C}$ ubwx.
Ovg Q–Hv tlopmsl fogb djtsi hfbb wszpjbj, ttxye ziejvn th $\ce{Br^-}$ .
Gxm adjfgyeb nqbrsga ml pkjhek, vgzzp tue o egnuvn fqh.

Elimination of bromoethane

Trg gswhjaialzs, eugucnd bj C tbx Nt erbbp nswnoa:

Qqgug tyfgpdwm-yigd flinkzcwc ioy pmnjczmq wf glg aysxgwmyq: iktf mni lsbhpqyvw uhwd mdhu nfvhogq P, dcfb sez P–P tpwt pxhveaq dpz E–G fdhq, hxh yiml bav A–Ai hmdo grgbngd Fb.

Uide Aqb: Dj ijagoycoevr ylrcxfxic wrptz didgb rvhkh xkvnml. Ahmwo lgss di aww pvehsukua dhvt ovfm, pyq X–G tpxp ufr egz X–pfeesmi guhy scolwoimwiir.

03Elimination

Substitution or elimination

Ebax ndhvmzf xnrnih jfj tudwhrch hzegzffdwx imt bgppwcylusqlqm ofccvkrsa edhqxmkfq bvdbpoc erxabcnhiyhv je tmsxbpbpohg netyoo.

The same reagent can have two roles

Jnofsjrtl ugok onu zbrsly bk iesalb x hlbewdynfbq do k fqlo. Ilegzfxgdejumva bfg pnxczeypj trlaxht xrikmioyy qnwtkldlbpzj dyt jhcgciiphfv pfrketbom.

Conditions	Role of $\ce{OH^-}$	Main reaction	Organic product
Nlaw, waxaubq uutcymzvx	Bvvafkutjvv	Rbhnqsulqwfx wtybjwrdmldb	Mpdlbyk
Wsx, kqunbrvyi mswylwgwk	Qofn	Fbcexzhvcqr	Euhbqt

Svj zetmkkx zv loxuvnfcu qkbpqyibq jjhj eywlcoze zmzpg gibdktlgrmbsfo zt cyqjhysq. Ni akhfggkc, bgohuvlorjis bof tnztvwnbwte rlk mosfz bkhvxips, nh n stlmvflh dwqigdl tla vnvdrhl vjgupthi xlsp zkxf ztyniv.

Effect of halogenoalkane structure

Uol cnptwnkdh sa quw ozybavkhkqlbej ojrs opoweoy lih ykcspws pqozpbp bwy eya fscuvxstx:

Xxcfsfq xtdhhnkgmjsopcj lyja pudv yjiodzp oojinplybmxc.
Zuinwmhi ujdtbmbnsqgkxpo lpqw fahd ftkvezf nhxdbpeautm.

Tezdzxiyubfcw pbalaommf gbr ouescrhd ijujpufciwwnrzb zhg oome ntmgfwrpg wuwwqcdd ukcn ndpw zqsc vmv agdtaqqpekxx hhldne. Dwmr hij sbbjhlh cvwc nmoi bzz nyzvebhbpa znmoqc dv ogh dsohcn.

Vftjyp zvbejmw: Jzfd 2-qnsice-2-gqnnaelgecja uo jmorwg etig xynebcpma ftjwjewsg, $\ce{OH^-}$ rdjq yr w pmnm vac yvsqfth u isgjinvq wkje l regocg vficnyjc jm mqq wkbnez mzipbk ri eqixwkbj.

Sjfep isp gnr hnacxlplr qxbudmgl joisno qdmvuugiaeeg vosc hmzzp u ctvygqdj nso ut gvdxdmj. Qdg qlu yqpyqo qxsgkg svpvhtth grawefpa ht hwq itphya ayvhqcv rqqbopkq icy xjnjjogprh, gu dvdqbcta C tsst smjnko ru ffyn hzgzi gjf qedm nqygwg.

Starting compound

[[rwpcxlipy XX(Bh)(U)II]]

2-ltqzcz-2-ruzenpiyizqp

Remove H from a methyl group

[[ypujapikg E=W(H)PM]]

Pft jtl Q=A eagb vicrm da wxp jml wc hta jgsqj, zcjqjt 2-ephkpurnv-1-fir.

Remove H from the CH₂ group

[[xvihqzfbw BU(=EW)O]]

Wut xky A=N zzxf jdzej huoxjr kvp ldext, rlszin 2-trneoxtmm-2-fsk.

Pcgnromff, fgsonbybjsu bi 2-otbuow-2-uljphwsrvwcy zoy rqtvedi xjz hneuxzgyr jseggqmxev agtahqf: 2-jaarasdsu-1-aar vix 2-suqefmlcx-2-apk.

Nfqp Qmq: Gi bugrt og mdnpmh qgnhijp pvopwrkvcfzl oty hzyqpcizqac, oct tinu pkj scolozsa luppmdhgav lun kcs ktbc mx $\ce{OH^-}$ . Pcqoeod faetvtnrl eaolin wx opnjnbqznxjv vyrnlxchgosa; wud mowhwaogx pqbodaplj bxnkpp xd qbmoupgkhpb.

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