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Advan'd A/AS Level Organic Chemistry: Oil fractions - cracking, isomerisation, reforming

1.3 Modification of alkanes by cracking, isomerisation and reforming

Part 1. ALKANES and the PETROCHEMICAL INDUSTRY

Doc Brown's Advanced A Level Organic Chemistry Revision Notes

What is cracking? Why is cracking important? What are the products of cracking? What are the products of cracking used for? What is reforming? Why is reforming important? What are the products of reforming? What are the products of reforming used for?


Alkanes and Petrochemical Industry INDEX

All Advanced Organic Chemistry Notes

Index of GCSE/IGCSE Oil - Useful Products Chemistry Revision Notes

A basic introduction to the chemistry of alkanes

Introduction to CRACKING - a problem of supply and demand and other products.



Introduction

Petrol (gasoline) contains a mixture of hydrocarbons, with 5 to 10 carbon atoms, but the products of fractional distillation do not completely suit the desired mixture for a fuel. There are too many larger hydrocarbons, mainly alkanes and no alkenes for polymer production. See basic discussion for the need of cracking see CRACKING - a problem of supply and demand, other products. This discussion does include isomerisation and reforming which are described in detail below.

CRACKING

Cracking is essentially the thermal decomposition of hydrocarbons such as alkanes into molecules of smaller carbon number, namely lower alkanes as chemical feedstock for other processes and alkenes for polymer production.

Cracking does involve breaking a strong C-C bond in the alkane to produce smaller molecules.

(a) Steam cracking (thermal cracking) is done at high temperatures e.g. 900oC and high pressure, with no catalyst. Naphtha (C6-C10) and kerosene (C10 - C16) are the chemical feedstocks and the vaporised hydrocarbons are exposed to the high temperature for just a short time.

Steam is added as a diluent to prevent 'coking' (carbon deposit on reactor surface).

Typical products are ethene, ethane, propene, propane and C4 - C5 alkanes and alkenes.

Reaction conditions can be set to maximise alkene production, remember, alkenes are NOT found in oil but one of the most important chemical feedstocks for polymer production.

(b) Catalytic cracking involves passing a fraction like gas oil (C14 - C20 alkanes) over a zeolite catalyst at 500oC. The reactor pressure is slightly above normal.

Catalytic cracking is used to produce hydrocarbons to reduce the octane number of petrol fuels, e.g. you try to maximise the yield of lower carbon number alkanes and some aromatic hydrocarbons. You can minimise the yield of unsaturated alkenes, but some will still be produced.

PLEASE READ my basic notes on CRACKING - a problem of supply and demand, other products (I'm NOT repeating them here)

Included are several structural formula equation examples of cracking alkanes into alkenes, lower alkanes and hydrogen.

I won't repeat the structural formula equations here, but add I've added more above on the reaction conditions, products and skeletal formula equations below.

e.g. cracking to produce alkenes (for polymer production) and lower alkanes for fuels to reduce the octane number of petrol, and note the 'theoretical' variety of products possible!

(i) cracking hexane  ===>   products  

alkanes structure and naming (c) doc b   alkenes structure and naming (c) doc b + alkanes structure and naming (c) doc b or alkanes structure and naming (c) doc b

hexane  ===>  ethene  +  butane/methylpropane

alkanes structure and naming (c) doc b   alkenes structure and naming (c) doc b + alkanes structure and naming (c) doc b

hexane  ===>   propene + propane 

alkanes structure and naming (c) doc b   or alkenes structure and naming (c) doc bor   + alkanes structure and naming (c) doc b

hexane  ===>  but-1-ene/but-2-ene/methylpropene  +  ethane

 

(ii)  cracking  or 

  alkanes structure and naming (c) doc b +alkenes structure and naming (c) doc b

 octane  ===>  propane  +  pent-1-ene

  alkenes structure and naming (c) doc b +alkanes structure and naming (c) doc b

octane  ===>   propene  +  pentane

  2 + H2    (could also be but-2-ene, alkenes structure and naming (c) doc b)

 octane  ===>  but-1-ene  +  hydrogen  or other combinations

 

(iii) The production of more branched alkanes from decane C10H22

e.g. various branched alkanes C7H16 and propene C3H6

alkanes structure and naming (c) doc balkanes structure and naming (c) doc b, alkanes structure and naming (c) doc b, alkanes structure and naming (c) doc b, alkanes structure and naming (c) doc b + alkenes structure and naming (c) doc b

There are at least five C7H14 branched isomers that can be formed - can you name them all?

 

Reaction mechanisms for alkane cracking reactions

Free radical mechanism for cracking hydrocarbons to give shorter alkanes and alkenes (maybe needed for UK A Level ?)

Ionic mechanism for cracking hydrocarbons (definitely NOT needed for UK A level chemistry)


 ISOMERISATION (isomerization)

Converting linear alkane molecules into branched alkane molecules is an example of an isomerisation reaction because the reactant and products have the same molecular formula. Branched alkanes increase the octane number of petrol fuels. The chemical feedstock is usually C4 to C6 alkanes.

Typical reaction conditions: Pt/Al2O3 at 150oC

Examples of isomerisation reactions

(i) butane ===> methylpropane  (all C4H10)

alkanes structure and naming (c) doc b alkanes structure and naming (c) doc b  (structural formula equation)

alkanes structure and naming (c) doc b alkanes structure and naming (c) doc b  (skeletal formula equation)

(ii) pentane ===> methylbutane (2-methylbutane) or dimethylpropane (2,2-dimethylpropane, numbers not strictly needed)

alkanes structure and naming (c) doc b alkanes structure and naming (c) doc b  or  alkanes structure and naming (c) doc b

alkanes structure and naming (c) doc b  alkanes structure and naming (c) doc b  or  alkanes structure and naming (c) doc b  (all C5H12)

(iii) hexane ===> 2-methylpentane  or  3-methylpentane

or 2,2-dimethylbutane or  2,3-dimethylbutane (all C6H14)

alkanes structure and naming (c) doc balkanes structure and naming (c) doc b or  alkanes structure and naming (c) doc b

or  alkanes structure and naming (c) doc b   or  alkanes structure and naming (c) doc b

alkanes structure and naming (c) doc b  alkanes structure and naming (c) doc b  or  alkanes structure and naming (c) doc b  or   alkanes structure and naming (c) doc b  or  alkanes structure and naming (c) doc b

(v)  heptane ===> 3-ethylpentane  or  2,2-dimethylpentane  or  2,3-dimethylpentane 

or  2,4-dimethylpentane  or  3,3-dimethylpentane  or  2,2,3-trimethlybutane

alkanes structure and naming (c) doc b  or  alkanes structure and naming (c) doc b oralkanes structure and naming (c) doc b or alkanes structure and naming (c) doc b

or  alkanes structure and naming (c) doc bor alkanes structure and naming (c) doc b oralkanes structure and naming (c) doc b

By changing the reaction conditions you can vary, and to some extent change, the ratio of products to select those most desired from an isomerisation reaction.


REFORMING

Converting linear alkane molecules into cyclic hydrocarbons (cycloalkanes and aromatic molecules - have a benzene ring).

The products have the same number of carbon atoms as the reactants, but less hydrogen atoms and a ring structure.

Naphtha is the chemical feedstock (C6 - C10 alkanes) and passed over a Pt/Al2O3 catalyst at 500oC. The hydrogen formed is recycled to minimise 'coking' (thermal decomposition to give a carbon deposit on the catalyst).

e.g

(i) hexane ===> cyclohexane ===> benzene

alkanes structure and naming (c) doc balkanes structure and naming (c) doc b+ H2

alkanes structure and naming (c) doc balkanes structure and naming (c) doc b+ H2

then   alkanes structure and naming (c) doc b(c) doc b+ 3H2

alkanes structure and naming (c) doc b(c) doc b + 3H2

This reforming reaction probably goes through the dehydrogenation sequence ?

hexane ==> cyclohexane ==> cyclohexene ==> cyclohexa-1,3-diene ==> benzene

alkanes structure and naming (c) doc balkanes structure and naming (c) doc balkenes structure and naming (c) doc b(c) doc b

(ii)  heptane  or  2-methylhexane  or  3-methylhexane ===> methylcyclohexane ===> methylbenzene

alkanes structure and naming (c) doc b

or  alkanes structure and naming (c) doc b

or alkanes structure and naming (c) doc b

alkanes structure and naming (c) doc b  +  H2     (c) doc b   +  3H2

 

methylbenzene is usually shown as  (c) doc b the benzene ring is 'skeletal' and the methyl group 'structural'

in terms of skeletal formulae this reforming reaction would be shown as ...

alkanes structure and naming (c) doc b  or  alkanes structure and naming (c) doc b  or  alkanes structure and naming (c) doc b

alkanes structure and naming (c) doc b  +  H2  (c) doc b    + 3H2

If asked to write the full equation don't forget the hydrogens! and check whether structural formulae or skeletal formulae are required.

Extra note on an example of an aromatic disproportionation reaction

Disproportionation is said to occur, when a reactant is transformed into two or more dissimilar products.

2(c) doc b    (c) doc b  +   (c) doc b(c) doc b(c) doc b (mixture of three isomeric products)

This is not a redox disproportionation reaction, but it does involve two identical molecules reacting together to give two different products with no other reactant or product involved. The reaction is a way of producing a mixture of 1,2-dimethylbenzene, 1,3-dimethylbenzene and 1,4-dimethylbenzene from methylbenzene. If you need less methylbenzene and more benzene or dimethylbenzenes, this is the way to do it.


Alkanes and Petrochemical Industry INDEX

All Advanced Organic Chemistry Notes  * 

Index of GCSE/IGCSE Oil - Useful Products Chemistry Revision Notes

 

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