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Advanced A/AS Level Organic Chemistry: Hydrocarbon & alternative fuels e.g. biofuels

1.5 Modification of hydrocarbon fuel mixtures & alternative fuels - octane number ratings and global warming and climate change

Part 1. ALKANES and the PETROCHEMICAL INDUSTRY - Doc Brown's Advanced A Level Organic Chemistry Revision Notes


Alkanes and Petrochemical Industry INDEX

 All Advanced Organic Chemistry Notes

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

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BASIC NOTES that go with the advanced level notes below

(which I expect you to read in conjunction with the EXTRA NOTES below - I'm not repeating here most of the basic organic chemistry notes from links below)

Ethanol, manufacture - use as bioethanol

Introduction to biofuels & alternative fuels including hydrogen, biogas, bioethanol and biodiesel

A fuel survey of selected fuels

Greenhouse effect, global warming, climate change, carbon footprint from fossil fuel burning

and in particular CRACKING - a problem of supply and demand, other products



EXTRA NOTES for advanced level

The chemistry of cracking, isomerisation and reforming (on a separate advanced level page)


Petrol production and the volatility of hydrocarbons related to octane rating

As we have seen, cracking, isomerisation and reforming produces lots of superior fuel molecules compared to those originally in crude oil - or rather, there were insufficient suitable fuels molecules in the crude oil.

Petrol producers have to mix a variety of hydrocarbons with the right properties to produce a suitable fuel for road vehicles.

There are two important properties to take into consideration:

(i) The octane rating of the fuel mixture

The octane rating is the fuels ability to resist auto-ignition, that causes 'knocking'.

Octane number is explained in the next section and the higher the octane number the better!

The branched alkanes and cyclic compounds (alicyclic or aromatic) have the highest octane numbers, compared straight chain unbranched alkanes.

 (detailed discussion in the next section)

(c) doc b(ii) The volatility of the hydrocarbons

Every liquid exerts a vapour pressure in the atmosphere above its surface. The maximum vapour pressure depends on the temperature of the liquid and rises exponentially with increase in temperature.

The diagram on the  right shows typical saturated vapour pressure curves of maximum pvap (mmHg) versus temperature - in this case for tetrachloromethane CCl4, ethanol C2H5OH, benzene C6H6, water H2O and ethanoic acid CH3COOH.

These are relatively volatile compounds, and this vapour pressure behaviour is relevant to hydrocarbons which can exert similar vapour pressures at room temperature.

The vapour pressure exerted by a liquid surface depends on the relative strength of the intermolecular forces - in the case of hydrocarbons this is almost entirely due to instantaneous dipole – induced dipole intermolecular forces.

See boiling points of alkanes

The vapour pressure a liquid exerts is a measure of how volatile a liquid is and this is relevant to a discussion about the design of petrol mixture which must take into account a wide range of ambient temperatures.

Branched alkanes have lower boiling points than longer unbranched (linear) alkanes of the same molecular formula - the more compact the molecule, the weaker instantaneous dipole – induced dipole intermolecular forces. .

This means the branched hydrocarbons are more volatile and vaporize more easily.

For example, the highly branched 2,2,4-trimethylpentane ('iso-octane') (CH3)3CCH2CH(CH3)2 alkane has boiling point of  99oC, whereas for the same molecular formula (C8H18), the linear non-branched isomer octane ('n-octane') CH3(CH2)6CH3  alkane has a much higher boiling point of 126oC.

See isomerism page for Explanation on why the intermolecular forces are weaker in the branched isomer

Petrol must contain the appropriate mixture that gives the appropriate volatility required.

The exact mixture ('blend') differs around the world and also varies with the seasons.

The petrol blend must maintain, as far as is practical, a reasonably constant volatility.

This means winter blends, at a lower temperature, must be more volatile than summer blends - which would not be volatile enough in winter, making the car engine more difficult to start.

Conversely, a winter blend would vapourise to easily in warmer summer temperatures causing vapour lock.

Vapour lock happens when the 'too volatile' fuel boils in your carburetor or your fuel line.

The vaporized fuel creates back pressure in your fuel system and prevents gas from getting to your engine, which would stall.

 

So, to be an efficient fuel. a petrol mixture must have the appropriate volatility and high octane number.

 


The fuel octane rating of petrol and individual hydrocarbons  (octane number of fuels)

When petrol is injected into the cylinders of a combustion engine, it should not ignite until a spark is produced with precise timing appropriate to the rotary motion of the engine cycle.

If the fuel ignites prematurely, you can hear a characteristic knocking sound, which is indicative of an inefficient under performing engine. The knocking can also physically damage the engine due to the extra vibration produced.

This effect is called auto-ignition and is caused by the high temperature compression of the petrol vapour in the engine cylinders.

The octane rating of a fuel molecule (or petrol mixture) is a measure of how likely it is too auto-ignite.

The higher the octane rating, the less likely is the fuel to auto-ignite and cause knocking.

Different hydrocarbons have different octane ratings.

Therefore, the different hydrocarbons are blended together to give a petrol mixture the appropriate octane rating - the mixture with the best antiknock performance.

Linear unbranched alkanes tend to have lower octane ratings, a higher tendency to auto-ignite,  than branched alkanes, cyclic alkanes (alicyclic) and aromatic hydrocarbons like benzene.

alkanes structure and naming (c) doc b  alkanes structure and naming (c) doc b Linear heptane (C7H16), far to readily auto-ignites and has a defined octane rating of 0 - not good!

alkane  alkane Highly branched 2,2,4-trimethylpentane (C8H18) has a defined octane rating of 100 (very good), on the arbitrarily defined octane rating number scale.

The octane rating of petrol in the UK is usually 95, but you can pay a bit more for higher octane rated petrol.

 

If you take hydrocarbons you can do an interesting comparison of the octane rating of several molecules,

I've deliberately quoted octane ratings for structural isomers (carbon chain isomers) e.g.

(a) alkanes structure and naming (c) doc b  alkanes structure and naming (c) doc b pentane (C5H12) has an octane rating of 62

isomeric alkanes structure and naming (c) doc b  alkanes structure and naming (c) doc b 2-methylbutane (C5H12) has an octane rating of 93

In (a) you can see introducing branching considerably increases the octane rating.

(b) alkanes structure and naming (c) doc b  alkanes structure and naming (c) doc b linear (unbranched) hexane (C6H14), octane rating 25

isomeric alkanes structure and naming (c) doc b 2-methypentane (C6H14) has an octane rating of 73

and alkanes structure and naming (c) doc b 3-methypentane (C6H14) has an octane rating of 75/86? (data sources differ)

As in (a), in (b) you can see introducing branching considerably increases the octane rating.

These branched isomers are produced in reforming processes.

(c) alkanes structure and naming (c) doc b linear heptane (C7H16) has an octane number of 0.

isomeric alkanes structure and naming (c) doc b 3-methylhexane has an octane rating of 65.

and alkanes structure and naming (c) doc b 2,3-dimethylpentane (C7H16) has an octane rating of 91

Again, in (c) you can see introducing branching considerably increases the octane rating.

You can also see that the greater the branching of the isomer, the higher the octane number.

These branched isomers are produced in reforming processes.

(d) alkanes structure and naming (c) doc b  alkanes structure and naming (c) doc bcyclic alkane (alicyclic) cyclohexane (C6H12), octane rating 83

(c) doc b  (c) doc b aromatic benzene (C6H6), octane rating 106

These hydrocarbons are produced in reforming processes. from the linear alkane hexane.

(e) alkanes structure and naming (c) doc b  alkanes structure and naming (c) doc b methylcyclohexane (C7H14) has an octane rating of 70

(c) doc b  (c) doc b methylbenzene (C7H8) has an octane number of 120

These hydrocarbons are produced in reforming processes. from the linear alkane heptane.

From (c) (d) and (e) you can see that moving from a linear alkane to cyclic alkane and then to a cyclic aromatic hydrocarbon greatly increases the octane rating at each stage in the reforming process.

 

Branched and cyclic alkane and aromatic hydrocarbon compounds are important components in petrol mixtures designed to produce the cleanest most efficient burning, with good antiknock properties, particularly as lead tetraethyl is now banned.

You should now appreciate much more one important consequence of cracking crude oil fractions.


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