13/04/2010

Topics 1.7 and 2.10: Organic chemistry

Organic reactions for Edexcel AS Chemistry

Alkanes (CnH2n+2)

• Combustion e.g. C3H8 + 5O2 3CO2 + 4H2O Exothermic, non-toxic products, hence use as fuel [up to C4H10 are gases at r.t.p; C5H12 – C12H26 are liquids]

• Halogenation e.g. C6H14 + Br2 UV C6H13Br + HBr

C6H13Br + Br2 UV

C6H12Br2 + HBr , etc. Free radical mechanism, mixture of products, little control over isomers or degree of substitution

Alkenes (CnH2n)

• Combustion e.g. C3H6 + 4½ O2 3CO2 + 3H2O Exothermic, smoky flame (incomplete combustion), hence not commonly used as fuel

• Electrophilic addition

a) Bromination e.g. H2C––CH2 + Br2 Br–CH2–CH2–Br (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 a bromoalcohol)

e.g H2C––CH2 + Br2 + H2O Br–H2C–CH2–OH + HBr (bromoalcohol)

b) Hydrobromination H3C–CH––CH2 + HBr H3C–CH(Br)–CH3 (major product)

Markovnikov’s Rule (H goes to C with most H) H3C–CH2–CH2–Br (minor product)

• Catalytic addition

a) Hydrogenation H2C––CH2 + H2

PtHigh T and P H3C–CH3 (reduction reaction)

b) Hydration H2C––CH2(g) + H2O(g) H3PO4

High P, 300C H3C–CH2–OH (ethanol manufacture)

• Oxidative addition

H2C––CH2 + [O] + H2O

KMnO4, H+/ OH

-

HO–H2C–CH2–OH (diol)

Used as test for alkenes [purple KMnO4/H+ is decolourised (Mn

2+); KMnO4/OH

- goes green (MnO4

2-) on shaking]

• Addition polymerisation

n H2C––CH2

catalyst

pressure C C

H

H H

Hn

poly(ethene)

Free radical mechanism: initiation, propagation, termination Ziegler-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 R–OH + 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; 3º > 2º > 1º

b) With ammonia R-X + NH3 closed vessel

xs NH3, pressure R–NH2 + HX (primary amine)

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

competing reaction R–X + R–NH2 R2NH + HX (secondary amine)

• Elimination

With hydroxide ion C C

H X

+ OH-

KOH

ethanol C C + H2O + X

- (alkene)

Elimination favoured in ethanolic solution Base-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. C3H7OH

c H2SO4

H3C –HC ––CH2 + H2O (alkene) Can also be achieved by passing alcohol vapour over heated aluminium oxide

• Halogenation

a) Phosphorus trihalides

3C3H7OH + PCl3 3C3H7Cl + H3PO3

3C3H7OH + P + 1½ Br2 heat under reflux 3C3H7Br + H3PO3

3C3H7OH + P + 1½ I2 heat under reflux 3C3H7I + H3PO3

b) Phosphorus pentachloride

C3H7OH + PCl5 C3H7Cl + POCl3 + HCl Test 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 purified

d) Hydrogen halides

(CH3)3C–OH + HCl c HCl C3H7Cl + H2O (only effective for 3º alcohols)

C3H7OH + HBr NaBr, c H2SO4

distil off product C3H7Br + H2O

C3H7OH + HI KI, c H3PO4

distil off product C3H7I + H2O

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

• Oxidation

a) Primary alcohols R–CH2–OH K2Cr2O7

distil off product

,H+

CR

H

O

(aldehyde)

R–CH2–OH K2Cr2O7

boil under reflux

,H+

CR

OH

O

(carboxylic acid)

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

b) Secondary alcohols R2CH–OH K2Cr2O7 ,H

+

CR

R

O

(ketone)

No further oxidation possible

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