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Topics 1.7 and 2.10: Organic chemistry

Organic reactions for Edexcel AS Chemistry

Alkanes (CnH2n+2)

Combustion e.g. C3H8 + 5O2 3CO2 + 4H2OExothermic, non-toxic products, hence use as fuel [up to C4H10 are gases at r.t.p; C5H12 C12H26 are liquids]

Halogenation e.g. C6H14 + Br2UV C6H13Br + HBr

C6H13Br + Br2UV

C6H12Br2 + HBr , etc.

Free radical mechanism, mixture of products, little control over isomers or degree of substitution

Alkenes (CnH2n)

Combustion e.g. C3H6 + 4 O2 3CO2 + 3H2OExothermic, smoky flame (incomplete combustion), hence not commonly used as fuel

Electrophilic addition

a) Bromination e.g. H2CCH2 + Br2 BrCH2CH2Br (dibromoalkane)Proceeds in the dark (c.f. alkanes);

A related reaction is used as test for alkenes (alkene decolourises bromine water on shaking but forms abromoalcohol)

e.g H2CCH2 + Br2 + H2O BrH2CCH2OH + HBr (bromoalcohol)

b) Hydrobromination H3CCHCH2 + HBr H3CCH(Br)CH3 (major product)

Markovnikovs Rule (H goes to C with most H) H3CCH2CH2Br (minor product)

Catalytic addition

a) Hydrogenation H2CCH2 + H2Pt

High T and P H3CCH3 (reduction reaction)

b) Hydration H2CCH2(g) + H2O(g)H3PO4

High P, 300CH3CCH2OH (ethanol manufacture)

Oxidative addition

H2CCH2 + [O] + H2OKMnO4, H

+/ OH

-

HOH2CCH2OH (diol)

Used as test for alkenes [purple KMnO4/H+ is decolourised (Mn2+); KMnO4/OH- goes green (MnO42-) on shaking]

Addition polymerisation

nH2CCH2catalyst

pressure C C

H

H H

Hn

poly(ethene)

Free radical mechanism: initiation, propagation, terminationZiegler-Natta catalyst to control stereochemistry of product for unsymmetrical alkenes

Halogenoalkanes (CnH2n+1X)

Nucleophilic substitution

a) With hydroxide ion R-X + OH-

aqueous KOH

warm ROH + X

-

Substitution favoured in aqueous solution (see also elimination).

Test for R-X: neutralise xs OH-with HNO3; add AgNO3(aq); ppt white R-Cl; cream R-Br; yellow R-I

Rate: R-I > R-Br > R-Cl; 3 > 2 > 1

b) With ammonia R-X + NH3closed vessel

xs NH3, pressure RNH2 + HX (primary amine)

An additional mole of ammonia is required to react with the HX formed

competing reaction RX + RNH2 R2NH + HX (secondary amine)

Elimination

With hydroxide ion C CH X

+ OH-

KOH

ethanol C C + H2O + X

-(alkene)

Elimination favoured in ethanolic solutionBase-catalysed elimination of HX to form an alkene; mixture of alkenes from unsymmetrical haloalkanes

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Alcohols (CnH2n+1OH)

Reaction with sodium e.g. 2C3H7OH + 2Na 2C3H7O-Na

++ H2 (sodium propoxide)

Similar to reaction of Na with water, though less vigorous

Dehydration e.g. C3H7OHc H2SO4

H3CHCCH2 + H2O (alkene)Can also be achieved by passing alcohol vapour over heated aluminium oxide

Halogenation

a) Phosphorus trihalides

3C3H7OH + PCl3 3C3H7Cl + H3PO3

3C3H7OH + P + 1 Br2heat under reflux 3C3H7Br + H3PO3

3C3H7OH + P + 1 I2heat under reflux 3C3H7I + H3PO3

b) Phosphorus pentachloride

C3H7OH + PCl5 C3H7Cl + POCl3 + HClTest for compounds containing OH group; exothermic reaction - steamy fumes which turn damp litmus red

c) Thionyl chloride

C3H7OH + SOCl2 C3H7Cl + SO2(g) + HCl(g)

Non-organic products are gaseous hence organic product is easily purifiedd) Hydrogen halides

(CH3)3COH + HCl c HCl C3H7Cl + H2O (only effective for 3 alcohols)

C3H7OH + HBrNaBr, c H2SO4distil off product

C3H7Br + H2O

C3H7OH + HIKI, c H3PO4

distil off productC3H7I + H2O

HBr and HI produced in situ; H2SO4 not appropriate for producing HI as it oxidises HI to I2

Oxidation

a) Primary alcohols RCH2OHK2Cr2O7distil off product

,H

+

CR

H

O

(aldehyde)

RCH2OHK2Cr2O7

boil under reflux

,H+

CR

OH

O

(carboxylic acid)

Orange dichromate(VI) reduced to green chromium(III)

b) Secondary alcohols R2CHOHK2Cr2O7 ,H

+

CR

R

O

(ketone)

No further oxidation possible

c) Tertiary alcohols No oxidation possible, hence dichromate(VI) remains orange