Revising Advanced Organic Chemistry  Doc Brown's GCE Chemistry - GCE A Level Revision Notes PART 10 Summary of organic reaction mechanisms - A mechanistic introduction to organic chemistry and explanations of different types of organic reactions

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Part 10.8 Aromatic Hydrocarbons - Arenes - Electrophilic substitution reactions - NITRATION

Part 10.8 AROMATIC HYDROCARBONS (ARENES) - introduction to arene electrophilic substitutions.

Nitration to give nitro-aromatics like nitrobenzene.

The orientation of products in aromatic substitution (1,2-; 1,3-; and 1,4- positions for two substituents in the benzene ring, old names - ortho/meta/para substitution products).

The revision notes include full diagrams and explanation of the mechanisms and the 'molecular' equation and reaction conditions and other con-current reaction pathways and products are also explained for the reaction mechanisms of aromatic hydrocarbons like benzene and methylbenzene.

Why do aromatic hydrocarbon molecules primarily react via electrophilic substitution reaction?

The five reactions described Part 10.8 are electrophilic substitution reactions involving the generation of a powerful electrophile (electron pair acceptor) which subsequently attacks the electron rich π (pi) electron system of the benzene ring.

Arenes tend to undergo substitution, rather than addition, because substitutions allows the very stable benzene ring to remain intact.

• Examples of aromatic nitration substitution reactions

  • (i) + HNO₃ ==> + H₂O

    • benzene + nitric acid ==> nitrobenzene + water

    • -

  • (ii) + HNO₃ ==> + H₂O

    • methylbenzene + nitric acid ==> methyl-2/3/4-nitrobenzene + water

    • Three structural-positional isomers C₇H₇NO₂ formed in different proportions.

    • -

  • (iii) + HNO₃ ==> + H₂O

    • benzoic acid + nitric acid ==> 3-nitrobenzoic acid + water

      • the 3-nitrobenzoic acid is the majority product, BUT, you will also get some 2-nitrobenzoic acid and 4-nitrobenzoic acid.

      • -

  • (iv) + HNO₃ ==> + H₂O

    • nitrobenzene + nitric acid ==> 1,3-dinitrobenzene + water

    • 1,3-dinitrobenzene is the majority product, BUT, you will still get some 1,2-dinitrobenzene and 1,4-dinitrobenzene.

    • -

  • (v) + HNO₃ ==> + H₂O

    • chlorobenzene + nitric acid ==> chloronitrobenzenes

    • 3 structural-positional isomers of C₆H₄NO₂Cl, 1-chloro-2-nitrobenzene, 1-chloro-3-nitrobenzene, 1-chloro-4-nitrobenzene,  formed in different proportions.

    • -

• What is the mechanism for nitrating benzene? or methyl benzene?

• for benzene : C₆H₆ + HNO₃ ==> C₆H₅NO₂ + H₂O    [see mechanism 19 below]

• for methyl benzene: C₆H₅CH₃ + HNO₃ ==> O₂NC₆H₄CH₃ + H₂O

• The nitrating mixture consists of concentrated nitric acid (source of the nitro group -NO₂) and concentrated sulphuric acid which acts as a catalyst and as a strong acid.

mechanism 19 - electrophilic substitution in the nitration of the benzene ring

• [mechanism 19 above] Benzene is converted into nitrobenzene, when R = H.

• When R = CH₃, methylbenzene will form a mixture of the three possible substitution products methyl-2/3/4-nitrobenzene,

  • and methyl-3-nitrobenzene is the minority product, the mechanism above would show the formation of one of the major products methyl-2-nitrobenzene.

• Step (1) The sulphuric acid protonates the nitric acid (strong acid, but weaker than H₂SO₄)

• Step (2) The protonated nitric acid loses a water molecule via a sulphuric acid molecule, to generate the electrophile, the nitronium ion, NO₂⁺. This is a much more powerful electrophile, i.e. electron pair acceptor, than the original nitric acid, and is needed to attack the very stable aromatic ring of benzene.

  • Steps (1) and (2) can be written as: 2H₂SO₄ + HNO₃ ==> NO₂⁺ + H₃O⁺ + 2HSO₄^- 

• Step (3) An electron pair from the delocalised pi electrons of the benzene ring forms a C-N bond with the electron pair accepting nitronium ion forming a highly unstable carbocation. It is very unstable because the stable electron arrangement of the benzene ring is partially broken to give a 'saturated' C (top right of ring).

• Step (4) The hydrogensulphate ion (HSO₄^-, formed in step (1), abstracts a proton from the highly unstable intermediate carbocation to give the nitro-aromatic product and reform the sulphuric acid catalyst as well as the stable benzene ring.

  • Note that the hydrogensulphate ion, HSO₄^- has been shown in the style of ^-:O-SO₂-OH, to emphasize the importance a lone pair on the oxygen abstracting the proton from the aromatic carbocation.

• GENERAL COMMENT to compare aromatic electrophilic substitution with the electrophilic addition with alkenes:

  • Like alkenes, arenes are susceptible to electrophilic attack because of the high electron density of the pi electrons involved in the carbon-carbon bonding and both show little reactivity towards nucleophilic reagents.

  • However two points should be considered because of the particular stability of the aromatic (benzene) ring.

    1. This makes aromatic compounds less reactive than alkenes, which readily undergo addition rather than substitution.

    2. Aromatics do not usually undergo addition because this will remove the stability conferred on the molecule by the benzene ring. By under going substitution rather than addition, the stable aromatic ring is preserved.

• FURTHER COMMENTS

  • The overall nitration reaction is the substitution of -H by -NO₂ 

10.8.7 The orientation of products in aromatic electrophilic substitution reactions

• Certain groups, already present, can increase the electron density of the benzene ring and make the aromatic compound more reactive towards electrophiles such as those described above. However the effect seems to enhance the reactivity at the 2 and 4 substitution positions more than the 3 substitution position.

  • Groups that increase reactivity are e.g. -CH₃, -Cl, -OH, -NH₂, -NHCOCH₃, and favour substitution at the 2 and 4 positions (typically 90-100% combined).

  • They all, by some means, have a small, but significant, electron donating (+I inductive effect) on the ring of pi electrons.

  • For example, methyl benzene is significantly more reactive than benzene and when nitrated, over 90% of the products are either methyl-2-nitrobenzene or methyl-4-nitrobenzene.

• Certain groups, already present, can decrease the electron density of the benzene ring and make the aromatic compound less reactive towards electrophiles such as described above. However the effect seems to decrease the reactivity at the 2 and 4 substitution positions more than the 3 substitution position.

  • Groups that decrease reactivity, by some means, are e.g. -NO₂, COOH, -CHO, -SO₂OH, and favour substitution at the 3 position (typically 70-90%) and their effect does fit in with them all being strongly electronegative groupings giving a -I inductive effect.

  • For example, nitrobenzene is much less reactive than benzene and on nitration, 93% of the product is 1,3-dinitrobenzene.

ALL OF THESE ARE NOW ON SEPARATE PAGES

10.8.3 The electrophilic substitution of an arene - chlorination mechanism (example of aromatic halogenation)

10.8.4 The electrophilic substitution of an arene - alkylation mechanism (Friedel-Crafts reaction)

10.8.5 The electrophilic substitution of an arene - acylation mechanism (Friedel-Crafts reaction)

10.8.6 The electrophilic substitution of an arene - sulphonation mechanism

keywords phrases: reaction conditions formula intermediates organic chemistry reaction mechanisms electrophilic substitution nitration methylbenzene benzene C6H6 + HNO3 ==> C6H5NO2 + H2O nitration of methylbenzene C7H8 C6H5CH3 + HNO3 ==> O2NC6H4CH3 + H2O 2H2SO4 + HNO3 ==> NO2+ + H3O+ + 2HSO4-

APPENDIX

COMPLETE MECHANISM and Organic Synthesis INDEX (so far!)

• PART 10 An Introduction to organic chemical reaction mechanisms and technical terms explained

• Organic Synthesis - MECHANISMS INDEX

• Part 10.2 ALKANES - introduction

to their chemistry *

• Free radical chlorination/bromination to give halogenoalkanes (haloalkanes, alkyl halides)

• Free radical mechanism for cracking hydrocarbons to give shorter alkanes and alkenes

• Ionic mechanism for cracking hydrocarbons

• Part 10.3 ALKENES - introduction to their chemistry

  • Electrophilic addition of hydrogen bromide [HBr_{(conc. aq)} and HBr_{(g/non-polar solvent)}] to form halogenoalkanes

  • Electrophilic addition of bromine with pure bromine or in non-polar solvent (non-aqueous Br_2(l/solvent)) to give dibromoalkanes AND addition using bromine water [aqueous Br_2(aq)] to give bromo-alcohols

  • Electrophilic addition of sulphuric acid AND electrophilic addition of water [acid catalyst] to form alcohols

  • Free radical polymerisation to give poly(alkene) polymers e.g. ethene ==> poly(ethene)

  • Hydrogenation to give saturated alkanes

• Part 10.4 HALOGENOALKANES - introduction to the chemistry of haloalkanes/alkyl halides

  • Nucleophilic substitution by water/hydroxide ion [S_N1 or S_N2, hydrolysis to give alcohols]

    • with extra notes on kinetics, rds, molecularity, rate expression, activated complex etc.

  • Nucleophilic substitution by cyanide ion to give a nitrile [S_N1 or S_N2]

  • Nucleophilic substitution by ammonia/primary amine to give primary/secondary amines etc. [S_N1 or S_N2]

  • Elimination of hydrogen bromide to form alkenes [E1 and E2]

• Part 10.5 ALCOHOLS - introduction to their chemistry

  • Conversion of an alcohol to a halogenoalkane

  • Elimination of water from an alcohol to give an alkene [acid catalysed, E1 and E2]

• Part 10.6 Carbonyl compounds - ALDEHYDES and KETONES - introduction to their chemistry

  • Nucleophilic addition of hydrogen cyanide to form a hydroxy-nitrile

  • Addition of hydrogen - reduction with LiAlH₄ or NaBH₄ to give alcohols

  • The iodination of ketones e.g. a 2-one like propanone (a methyl ketone) to give iodo-ketones

• Part 10.7 Carboxylic Acids and ACID CHLORIDES - introduction - all nucleophilic addition-elimination reactions

  • Hydrolysis of acid chlorides with water to give a carboxylic acid

  • Esterification of acid chlorides with alcohols to give an ester

  • Amide formation from reaction of acid chlorides with ammonia or primary amines

• Part 10.8 AROMATIC HYDROCARBONS - introduction to arene electrophilic substitutions

  • Nitration to give nitro-aromatics like nitrobenzene

  • Chlorination to chloro-aromatics like chlorobenzene

  • Alkylation to give alkyl-aromatics like methylbenzene [Friedel-Crafts reaction]

  • Acylation to give aromatic ketones [Friedel-Crafts reaction]

  • Sulphonation/sulfonation to give a sulphonic/sulfonic acid like benzenesulphonic acid/benzenesulfonic acid

  • The orientation of products in aromatic substitution (2,4,6 or 3,5 positions, ortho, para & meta substitution products)

• Other important associated links

  • Isomerism - chain isomerism, positional isomerism, functional group isomerism & Tautomerism
  • Stereoisomerism introduction, E/Z isomerism ('ex' Geometric/Geometrical cis/trans Isomerism)
  • R/S Optical Isomerism and chiral auxiliary synthesis
  • All Advanced Organic Chemistry Notes
  • Summary of Organic Functional Groups
  • Quiz on Organic Structure Recognition
  • Summary of organic chemistry functional group tests
  • The shapes and bond angles of simple organic molecules
  • Aliphatic structure & nomenclature m/c BUMPER QUIZ
  • Type in name aliphatic structure and nomenclature BUMPER QUIZ

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