Doc Brown's GCE Chemistry  Revising Advanced Level Organic Chemistry

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 10.4 Halogenoalkanes - Reaction with ammonia and amines

Part 10.4 HALOGENOALKANES -

An introduction to the mechanisms of halogenoalkanes (haloalkanes, alkyl halides). 

Nucleophilic substitution by ammonia/primary amine to give primary/secondary amines etc. [SN1 or SN2].

These revision notes include full diagrams and explanation of the nucleophilic substitution reaction mechanisms of halogenoalkanes (haloalkanes) and the 'molecular' equation and reaction conditions.

Other con-current reaction pathways and products are also explained when halogenoalkanes react with concentrated aqueous-ethanolic ammonia to give primary amines, or primary amines to form secondary amines, secondary amines to for tertiary amines and quaternary ammonium salts.

• Halogenoalkanes owe their reactivity, especially compared to the unreactive alkanes, to two principal reasons.

• R₃C-X = halogenoalkane/haloalkane/alkyl halide/halogenated alkane etc. X = halogen e.g. Cl, Br or I

• The carbon-halogen bond is polar, C^δ+-X^δ- due to the difference in electronegativity between carbon and the halogen. The  C^δ+ carbon is then susceptible to nucleophilic attack by electron pair donor neutral molecules (e.g. the nucleophiles :NH₃, H₂O:) or ions (e.g. :OH^-, :CN^-).

• The carbon-halogen bond is usually the weakest bond in the molecule and significantly weaker than the carbon-carbon or carbon-hydrogen bonds.

  • Average bond enthalpies/kJmol^-1: C-C 348, C-H 412, both relatively high requiring high activation energies for reaction.

  • Average bond enthalpies/kJmol^-1: C-Cl 338, C-Br 276, C-I 238, generally lower resulting in lower activation energies.

    • Even the lowering of the bond enthalpy by 10kJ from C-C to C-Cl, combined with the polarity of the C-Cl bond, makes all the difference when comparing alkane and halogenoalkane reactivity.

• IMPORTANT NOTE on structure classification

  • In the mechanism diagrams you will see part of the molecular structure shown as R₃C

  • PLEASE do not assume this means a tertiary (tert) halogenoalkane (haloalkane).

  • R₃C- is used repeatedly to minimise the number of graphic images needed.

  • In general a halogenoalkane (haloalkane) has the structure R₃C-X where R = H, alkyl or aryl.

  • A primary halogenoalkane (haloalkane) can be shown as RCH₂-X where R = H, alkyl or aryl.

  • A secondary (sec) halogenoalkane (haloalkane) can shown as R₂CH-X where R = alkyl or aryl.

  • A tertiary (tert) halogenoalkane (haloalkane) can be shown as R₃C-X where R = alkyl or aryl.

10.4.4 Nucleophilic substitution of a halogenoalkane with ammonia or primary aliphatic amine

The organic synthesis of amines (primary, secondary, tertiary) and quaternary ammonium salts by reacting halogenoalkanes with ammonia and amines

• What is the reaction mechanism for the substitution of a halogen atom in a haloalkane/halogenoalkane with an amine group by reaction with ammonia?

• With ammonia a primary aliphatic amine is formed: [see mechanisms 37 and 9 below]

• R₃C-Br + 2NH₃ ==> R₃C-NH₂ + NH₄⁺Br^-

  • The halogenoalkane is heated with excess concentrated ethanolic ammonia in a sealed vessel to form a primary amine, though it may be as its bromide salt.

  • The primary amine is completely freed by adding strong alkali  e.g. aqueous sodium hydroxide, NaOH_(aq).

  • R₃C-NH₃⁺ + OH^- ==> R₃C-NH₂ + H₂O 

mechanism 37 - nucleophilic substitution of a halogenoalkane by ammonia (S_N1 unimolecular via carbocation)

• S_N1 unimolecular, a three step ionic mechanism via carbocation formation [mechanism 37 above]

  • In step (1) the C^δ+-Br^δ- polar bond of the halogenoalkane splits heterolytically to form a carbocation and a free halide ion (e.g. chloride or bromide) and this is a reversible reaction.

  • In step (2) the nucleophilic electron pair donating ammonia molecule rapidly adds to the carbocation to give the protonated amine product R₃C-NH₂⁺. Ammonia is the nucleophile.

  • In step (3) one of the excess ammonia molecules can remove a proton to leave the primary amine product.

    • Note: Alkali still needs to be added at the end because the primary amine formed is usually a stronger base than ammonia.

mechanism 9 - nucleophilic substitution of a halogenoalkane by ammonia (S_N2 bimolecular)

• S_N2 'bimolecular', a two step mechanism [mechanism 9 above]

  • Step (1) the C^δ+-Br^δ- bond is polar, so the electron rich nucleophile, the ammonia molecule, attacks the slightly positive carbon. The nucleophile acts as an electron pair donor (Lewis base) to bond with the 'positive' carbon. Simultaneously the bromine atom is ejected, taking with it the C-Br bonding pair of electrons, so forming the bromide ion.

  • In step (2) one of the excess ammonia molecules can remove a proton to leave the primary amine product.

    • Note: Alkali still needs to be added at the end because the primary amine formed is usually a stronger base than ammonia.

• Further substitution can take place because the amine product itself is a nucleophile.

  • e.g. if you start with bromomethane and react it with ammonia, the following products can be formed ..

  • CH₃Br + NH₃ ==> [CH₃NH₃]⁺Br^-  

  • Excess ammonia favours the formation of the primary amine i.e. reduces the formation of the secondary and tertiary amines and the quaternary ammonium salt.

  • the primary amine, methylamine, CH₃NH₂, as its bromide salt

    • CH₃NH₂ + CH₃Br ==> [(CH₃)₂NH₂]⁺Br^-  

    • the secondary amine, dimethylamine, (CH₃)₂NH, as its bromide salt

      • (CH₃)₂NH + CH₃Br ==> [(CH₃)₃NH]⁺Br^-  

      • the tertiary amine, trimethylamine, (CH₃)₃N, as its bromide salt

        • (CH₃)₃N + CH₃Br ==> [(CH₃)₄N]⁺Br^-  

        • and eventually the quaternary salt, tetramethylammonium bromide

• With a primary aliphatic amine a secondary aliphatic amine is formed: [mech's 38 and 11 below]

• The reaction of a halogenoalkane and excess primary amine can be written as ...

• R₃C-Br + 2R'-NH₂ ==> R₃C-NHR' + [R'-NH₃]⁺Br^-

  • The halogenoalkane is heated with excess concentrated ethanolic solution of the primary amine in a sealed vessel to form a secondary amine.

  • The secondary amine product is freed by adding strong alkali  e.g. aqueous sodium hydroxide, NaOH_(aq).

  • [R₃C-NH₂R']⁺ + OH^-  ==> R₃C-NHR' + H₂O 

• The mechanisms for the reaction of primary amines with halogenoalkanes are essentially the same as for ammonia above. So all the general comments for ammonia apply here too, so I will not repeat them, where it says ammonia, just say primary amine! In step (3) one of the excess primary amine molecules can remove a proton to leave the primary amine product.

  • Note: Alkali still needs to be added at the end because the secondary amine formed is usually a stronger base than ammonia.

  • Again, the amine is the nucleophile, an electron pair donor.

mechanism 38 - nucleophilic substitution of a halogenoalkane by a primary amine (S_N1 unimolecular via carbocation)

 mechanism 11 - nucleophilic substitution of a halogenoalkane by a primary amine (S_N2 bimolecular)

• Further substitution can take place because the product itself is a nucleophile.

  • e.g. if you start with bromoethane and react it with ethylamine, the following products can be formed ..

  • CH₃CH₂Br + CH₃CH₂NH₂ ==> [(CH₃CH₂)₂NH₂]⁺Br^- diethylamine, (CH₃CH₂)₂NH, as its bromide salt

    • (CH₃CH₂)₂NH + CH₃CH₂Br ==> [(CH₃CH₂)₃NH]⁺Br^- triethylamine, (CH₃CH₂)₃N, as its bromide salt

      • (CH₃CH₂)₃NH + CH₃CH₂Br ==> [(CH₃CH₂)₄N]⁺Br^- tetraethylammonium bromide

• FURTHER COMMENTS

  • Many of the comments for hydrolysis apply to this reaction.

  • Aromatic amines, e.g. phenylamine, , do NOT readily react with halogenoalkanes. When the amine/amino group is directly attached to the benzene ring (via the N), the lone pair of electrons on the nitrogen is partially delocalised with the pi electrons of the benzene ring, consequently they are less readily donated. This causes aromatic amines to be much weaker nucleophiles than aliphatic amines where no such effect can take place.

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keywords phrases: reaction conditions formula intermediates organic chemistry reaction mechanisms nucleophilic substitution R3C-Br + 2NH3 ==> R3C-NH2 + NH4+Br- R3C-NH3+ + OH- ==> R3C-NH2 + H2O CH3Br + NH3 ==> [CH3NH3]+Br- CH3NH2 + CH3Br ==> [(CH3)2NH2]+Br- (CH3)2NH + CH3Br ==> [(CH3)3NH]+Br- (CH3)3N + CH3Br ==> [(CH3)4N]+Br-  R3C-Br + 2R'-NH2 ==> R3C-NHR' + [R'-NH3]+Br- [R3C-NH2R']+ + OH- ==> R3C-NHR' + H2O CH3CH2Br + CH3CH2NH2 ==> [(CH3CH2)2NH2]+Br- (CH3CH2)2NH (CH3CH2)2NH + CH3CH2Br ==> [(CH3CH2)3NH]+Br- (CH3CH2)3N (CH3CH2)3NH + CH3CH2Br ==> [(CH3CH2)4N]+Br-

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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 - an 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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