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

10.2.4 An ionic mechanism for catalytic cracking

(a) Catalytic cracking occurs at lower temperatures over a ceramic/zeolite catalytic material based on silica and/or aluminium oxide (I've called it Z in the mechanisms). The mechanism is believed to be ionic and some examples of reaction steps are given below. Z represents a matrix element of the alumino-silicate catalyst and can act in a variety ways e.g. Lewis acid or base, Bronsted-Lowry acid or base. R₁, R₂, R₃  = alkyl groups.

Step 1 Initiation: This can occur via an alkane or an alkene previously formed from an alkane.

(i) R₁-CH₂-CH₂-R₂ + Z⁺ ==> R₁-CH₂-CH⁺-R₂ + HL 

(ii) R₁-CH=CH-R₂ + HZ ==> R₁-CH₂-CH⁺-R₂ + Z^- 

Step 2 Propagation: This example shows chain scission forming a 'smaller' alkene and a 'smaller' carbocation to continue the chain.

R₁-CH₂-CH⁺-R₂ ==> R₁⁺ + CH₂=CH-R₂ 

Step 3 Termination: A lower alkene or alkane is formed.

R₁⁺ + Z^- ==> R₃CH=CH₂  + ZH 

(R₃ < R₁ in alkene, but no scission, so same chain length overall)

R₁⁺ + ZH ==> R₁H  + Z⁺ 

(no bond scission, so R₁ same length in carbocation or alkane formed)

(b) A catalytic 'ionic' cracking cycle via alkenes:

I found this example in the literature, and the aluminosilicate/zeolite matrix (Z) acts alternately as an acid in step 1 and a base in step 4. Unfortunately it does involve starting with a rather a 'big' alkene called 2,4,4-trimethylpent-1-ene (it was the only example I could find in a local university library).

Step 1 initiation

(CH₃)₃C-CH₂-C(CH₃)=CH₂ + HZ ==> (CH₃)₃C-CH₂-C⁺(CH₃)₂ + Z^- 

The 'big' alkene is protonated by the catalyst and forms a 'big' carbocation (tertiary, most stable).

Step 2 propagation:

(CH₃)₃C-CH₂-C⁺(CH₃)₂ ==> (CH₃)₃C⁺ + CH₂=C(CH₃)₂ 

The 'big' carbocation from step 1 splits into 2-methylpropene (a cracking product) and a 'smaller' carbocation that continues the chain in step 3.

Step 3 propagation:

(CH₃)₃C-CH₂-C(CH₃)=CH₂ + (CH₃)₃C⁺ ==> (CH₃)₃C-CH₂-C⁺(CH₃)₂ + CH₂=C(CH₃)₂ 

From another 'big' starter alkene, and the carbocation from step 2, another molecule of 2-methylpropene is formed and the 'big' carbocation that was also formed in step 1, so allowing the 'ionic chain' reaction to continue via steps 2 and 3.

Step 4 termination:

e.g. (CH₃)₃C⁺ + Z^- ==> CH₂=C(CH₃)₂ + HZ 

The 'smaller' carbocation from step 2 is deprotonated to form another molecule of 2-methylpropene, and HZ is ready for step 1 again.

(c) FURTHER COMMENTS

I found little help for ionic cracking mechanisms in either textbooks or the internet, I'm quite happy with the catalytic cycle in (b), but frankly I found the descriptions for (a) not easy to follow and cannot vouch for their absolute authenticity. I just my best to make sense of it and illustrate how the ionic mechanism might operate. (I wonder what does the IB syllabus require? Can anyone help me on this one?)

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Alkanes and Petrochemical Industry INDEX for advanced level chemistry revision notes

for GCSE/IGCSE/O level notes see CRACKING - a problem of supply and demand, other products

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APPENDIX -  COMPLETE MECHANISM and Organic Synthesis INDEX (so far!)

See also INDEX of ALKANES and the petrochemical industry revision notes INDEX

• 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

See also INDEX of ALKANES and the petrochemical industry revision notes INDEX

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