Organic Reactions 3v2070

Topic 4.5: Organic chemistry II

Additional organic reactions for Edexcel A2 Chemistry (Unit 4) Grignard reagents (R-Mg-X) • Preparation

e.g.

dry ether, I2 catalyst reflux

C3H7Br + Mg

C3H7–Mg–Br

2+ • With water e.g. C3H7–Mg–Br + H2O C3H8 + Mg + Br + OH Simple hydrolysis to form an alkane – not a synthetically useful reaction

• With CO2

e.g.

dry ether

C3H7–Mg–Br + CO2

–O)OMgBr C3H7C(–

aq

+

2+

-

–O)OMgBr + H C3H7C(– C3H7COOH + Mg + Br ‘Nucleophilic’ attack by Grignard on C in CO2; followed by acidic hydrolysis to give carboxylic acid (1 extra C) dry ether

• With aldehydes e.g. C3H7–Mg–Br + CH3CHO

C3H7CH(CH3)OMgBr

aq

+

2+

-

C3H7CH(CH3)OMgBr + H C3H7CH(CH3)OH + Mg + Br ‘Nucleophilic’ attack by Grignard on C in carbonyl; followed by acidic hydrolysis to give secondary alcohol dry ether

• With ketones e.g. C3H7–Mg–Br + CH3COCH3

C3H7C(CH3)2OMgBr

aq

+

2+

-

C3H7CH(CH3)2OMgBr + H C3H7C(CH3)2OH + Mg + Br ‘Nucleophilic’ attack by Grignard on C in carbonyl; followed by acidic hydrolysis to give tertiary alcohol

Carboxylic acids (RCOOH) K2Cr2O7/H2SO4(aq)

reflux • Preparation e.g. C3H7CH2OH + 2[O] C3H7COOH + H2O Oxidation of primary alcohol (or aldehyde) by heating under reflux to ensure complete oxidation

• Typical reactions of acids a) With alkalis e.g. C3H7COOH + NaOH b) With metals e.g. 2 C3H7COOH + Mg c) With Na2CO3 e.g. 2 C3H7COOH + Na2CO3 d) With NaHCO3 e.g. C3H7COOH + NaHCO3

O • Esterification e.g. C3H7OH + H3C

C3H7COONa + H2O (C3H7COO)2Mg + H2 2 C3H7COONa + CO2 + H2O C3H7COONa + CO2 + H2O

O c H 2 SO 4 , warm +

OH

-

H or OH , reflux

H3C

C

+ H2O

(ester)

O C3H7 Forward reaction ‘catalysed’ by conc. H2SO4 which removes water; reverse reaction catalysed by dilute acid/alkali. Esters are generally insoluble in water and typically smell sweet and sickly (e.g. glue, pear drops) LiAlH4 dry ether • Reduction e.g. C3H7COOH + 4[H] Initial ‘nucleophilic’ attack by Hδ- in AlH4 on Cδ+ in carbonyl

C3H7CH2OH + H2O

• Halogenation e.g. C3H7COOH + PCl5 room temp C3H7COCl + POCl3 + HCl Test for compounds containing –OH group; exothermic reaction - steamy fumes which turn damp litmus red; separate products by fractional distillation

Acid chlorides (RCOCl) • Preparation

see halogenation of carboxylic acids

• With water e.g. C3H7COCl + H2O C3H7COOH + HCl Simple hydrolysis to form carboxylic acid – not a synthetically useful reaction (nucleophilic attack by H2O:) • With alcohols e.g. C3H7COCl + CH3OH C3H7COOCH3 + HCl Irreversible formation of ester; condensation reaction; nucleophilic addition followed by elimination • With ammonia e.g. C3H7COCl + H3N C3H7CONH2 + HCl Formation of amide – similar mechanism to hydrolysis (i.e. nucleophilic addition - elimination) C3H7CONHCH3 + HCl • With amines e.g. C3H7COCl + H2NCH3 Formation of N-substituted amide – c.f. reaction with ammonia

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• Polymer formation a) diacid chloride with diol – condensation polymerisation to form polyester

Cl n

O

Cl C

C

O

HO

OH

O

C

C

O

O

O +

n HCl

+

n HCl

n b) diacid chloride with diamine – condensation polymerisation to form polyamide

Cl n

NH

Cl C

C

O

H2N

NH2

O

C

C

O

O

NH

n

Esters (RCOOR´) • Preparation

see ‘carboxylic acids (esterification)’ and ‘acid chlorides with alcohols’ HCl(aq)

• With dilute acid, e.g.

C3H7COOCH3 + H2O

heat

C3H7COOH + CH3OH

Hydrolysis to form carboxylic acid and alcohol (nucleophilic attack by H2O:) catalysed by H

+

NaOH(aq)

• With dilute alkali, e.g. C3H7COOCH3 + NaOH

heat

C3H7COONa + CH3OH -

Hydrolysis to form carboxylate ion and alcohol (nucleophilic attack by OH )

Aldehydes (RCHO) K2Cr2O7/H2SO4(aq)

distil • Preparation e.g. C3H7CH2OH + [O] C3H7CHO + H2O Oxidation of primary alcohol with distillation of (lower b.p.) product

• Reduction a) Tollen’s reagent – ammoniacal silver nitrate Addition of Tollen’s reagent with gentle heating gives a ‘silver mirror’ on the inside of the test tube Used as a test to distinguish aldehydes from ketones 2+ b) Fehling’s (or Benedict’s) solution – alkaline solution containing complexed Cu ions Addition of Fehling’s solution with gentle heating gives a red precipitate of copper(I) oxide Used as a test to distinguish aldehydes from ketones LiAlH4 in dry ether

c) Reduction

CH3CHO + 2[H]

or NaBH4(aq)

CH3CH2OH

(primary alcohol)

CH3CH2OH

(primary alcohol)

Ni or Pt catalyst

d) Hydrogenation

CH3CHO + H2 K2Cr2O7/H2SO4(aq)

distil • With 2,4-DNP e.g. C3H7CH2OH + [O] C3H7CHO + H2O Oxidation of primary alcohol with distillation of (lower b.p.) product

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• Electrophilic addition –CH2 + Br2 a) Bromination e.g. H2C– Br–CH2–CH2–Br (dibromoalkane) Proceeds in the dark (c.f. alkanes); used as test for alkenes (alkene decolourises bromine water on shaking) –CH2 + HBr b) Hydrobromination H3C–CH– H3C–CH(Br)–CH3 (major product) H3C–CH2–CH2–Br

Markovnikov’s Rule (H goes to C with most H)

(minor product)

• Catalytic addition –CH2 + H2 H2C–

Pt High T and P

H3C–CH3

–CH2(g) + H2O(g) H2C–

H 3 PO 4 High P, 300C

H3C–CH2–OH

a) Hydrogenation b) Hydration

(reduction reaction) (ethanol manufacture)

• Oxidative addition +

KMnO 4 , H / OH

–CH2 + [O] + H2O H2C–

-

HO–H2C–CH2–OH

+

2+

(diol)

-

2-

Used as test for alkenes [purple KMnO4/H is decolourised (Mn ); KMnO4/OH goes green (MnO4 ) on shaking] • Addition polymerisation

H H catalyst

–CH2 n H2C–

C C

pressure

poly(ethene)

H H n

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

Haloalkanes (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

closed vessel xs NH 3 , pressure

R-X + NH3

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 c) With cyanide ion

R-X + CN

KCN in ethanol heat under reflux

-

R–CN + X

(secondary amine)

-

(nitrile)

Useful reaction for increasing the carbon chain length • Elimination

H X C C

With hydroxide ion

+ OH

-

KOH ethanol

C C

+ H 2O + X

-

(alkene)

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

Alcohols (CnH2n+1OH) • Reaction with sodium e.g. 2C3H7OH + 2Na Similar to reaction of Na with water, though less vigorous

-

+

2C3H7O Na + H2

c H 2 SO 4

–CH2 + H2O • Dehydration e.g. C3H7OH H3C–HC– Can also be achieved by ing alcohol vapour over heated aluminium oxide

(sodium ethanoate)

(alkene)

• Halogenation a) Phosphorus trihalides 3C3H7OH + PCl3 3C3H7OH + P + 1½ Br2

3C3H7Cl + H3PO3 heat under reflux

3C3H7Br + H3PO3 19/03/2006

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

C3H7OH + HBr

NaBr, c H 2 SO 4 distil off product

C3H7Br + H2O

C3H7OH + HI

KI, c H 3 PO 4 distil off product

C 3 H 7 I + H2 O

(only effective for 3º alcohols)

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

O • Esterification e.g. C3H7OH + H3C

O c H 2 SO 4 , warm +

OH

-

H or OH , reflux

H3C

C

+ H2O

(ester)

O C3H7 Forward reaction ‘catalysed’ by conc. H2SO4 which removes water; reverse reaction catalysed by dilute acid/alkali. Esters are generally insoluble in water and typically smell sweet and sickly (e.g. glue, pear drops) • Oxidation R–CH2–OH

a) Primary alcohols

R–CH2–OH

K 2 Cr 2 O 7 , H

distil off product

K 2 Cr 2 O 7 , H

O

+

R C O

+

boil under reflux

(aldehyde)

H R C

(carboxylic acid)

OH

Orange dichromate(VI) reduced to green chromium(III) R2CH–OH

b) Secondary alcohols

K 2 Cr 2 O 7 , H

O

+

R C

(ketone)

R No further oxidation possible c) Tertiary alcohols

No oxidation possible, hence dichromate(VI) remains orange

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