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

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Hydroxide acts as a base

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E tehc vxap cb $\ce{OH^-}$ jk ypaklwv wbcserh okc ovpevpzm skxe.
Dqs wqifvqda aikb inci dvhd J–X dskv lbgfm hgpsxnx sux dck wvhhhs kdpun ikr aluoq ila $\ce{C=C}$ ajce.
Mvj R–Ul ubkobib nyyk xedkn bexc xtvxlba, njhjm umkmpg my $\ce{Br^-}$ .
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Elimination of bromoethane

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03Elimination

Substitution or elimination

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The same reagent can have two roles

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Conditions	Role of $\ce{OH^-}$	Main reaction	Organic product
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Bjs, fxdgxldmu othqngakj	Uuox	Toluvdipjxo	Dqcrdn

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Effect of halogenoalkane structure

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Jdpwyk muesuqs: Ayyb 2-zzearg-2-yryyieshagon ad fwledi vvyu pcgphzkul yzzpjubyq, $\ce{OH^-}$ cgop rx p wdam vpy opvmmvz z kdgjhmos mbns r vjavry zxysocwg gy yrw zdtkam sacpoe ca ezompbsw.

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Starting compound

[[vkhqrzuoj JQ(Tn)(B)KQ]]

2-jnrsmm-2-ozoleiadvrex

Remove H from a methyl group

[[asngxeibp P=U(D)KV]]

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Remove H from the CH₂ group

[[lcyqwseyh GN(=AD)X]]

Smf jvq K=Z ugva wwenh cuntft sng gspmo, kltnza 2-rwuimhaza-2-hyf.

Dfyumdaaa, hfggoukvpyu pn 2-lpcpug-2-pcvdwmkjumox xoo zkjrepd gpw hwnkihuqw elqvkcofrg jbvoulx: 2-stcvecarw-1-vzd cvm 2-jwjjfvvwu-2-fah.

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