Introducing Alcohols

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INTRODUCING ALCOHOLS

This page explains what alcohols are, and what the difference is betweenprimary, secondary and tertiary alcohols. It looks in some detail at their simplephysical properties such as solubility and boiling points. Details of the chemicalreactions of alcohols are described on separate pages.

What are alcohols?

Examples

Alcohols are compounds in which one or more hydrogen atoms in an alkanehave been replaced by an -OH group. For the purposes of UK A level, we willonly look at compounds containing one -OH group.

For example:

Note: If you aren't confident aboutnaming organic compounds, then you reallyought to follow this link before you go on.

Use the BACK button on your browser to return to this page.

The different kinds of alcohols

Alcohols fall into different classes depending on how the -OH group is positionedon the chain of carbon atoms. There are some chemical differences between thevarious types.

Primary alcoho ls

In a primary (1) alcohol, the carbon which carries the -OH group is onlyattached to one alkyl group.

Note: An alkyl group is a group such as methyl, CH3, or ethyl, CH3CH2. Theseare groups containing chains of carbon atoms which may be branched. Alkyl
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groups are given the general symbol R.

Some examples of primary alcohols include:

Notice that it doesn't matter how complicated the attached alkyl group is. In eachcase there is only one l inkageto an alkyl group from the CH2 group holding the

-OH group.

There is an exception to this. Methanol, CH3OH, is counted as a primary alcoholeven though there are noalkyl groups attached to the carbon with the -OHgroup on it.

Second ary alcoho ls

In a secondary (2) alcohol, the carbon with the -OH group attached is joineddirectly to twoalkyl groups, which may be the same or different.

Examples:

Tert iary alcoh ols

In a tertiary (3) alcohol, the carbon atom holding the -OH group is attacheddirectly to threealkyl groups, which may be any combination of same ordifferent.

Examples:


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Physical properties of alcohols

Boiling Points

The chart shows the boiling points of some simple primary alcohols with up to 4carbon atoms.

They are:

They are compared with the equivalent alkane (methane to butane) with thesame number of carbon atoms.

Notice that:

The boiling point of an alcohol is always much higher than that of thealkane with the same number of carbon atoms.

The boiling points of the alcohols increase as the number of carbonatoms increases.

The patterns in boiling point reflect the patterns in intermolecular attractions.

Note: If you aren't happy aboutintermolecular forces(including van der Waalsdispersion forces and hydrogen bonds) then you really ought to follow this link
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before you go on. The next bit won't make much sense to you if you aren'tfamiliar with the various sorts of intermolecular forces.

Use the BACK button on your browser to return to this page.

Hydrogen bonding

Hydrogen bonding occurs between molecules where you have a hydrogen atomattached to one of the very electronegative elements - fluorine, oxygen ornitrogen.

In the case of alcohols, there are hydrogen bonds set up between the slightlypositive hydrogen atoms and lone pairs on oxygens in other molecules.

The hydrogen atoms are slightly positive because the bonding electrons arepulled away from them towards the very electronegative oxygen atoms.

Note: If you want to be fussy, the diagram is slightly misleading in that itsuggests that all of the lone pairs on the oxygen atoms are forming hydrogenbonds. In an alcohol that can't happen. Taking the alcohol as a whole, there areonly half as many slightly positive hydrogen atoms as there are lone pairs. At anyone time, half of the lone pairs in the total liquid alcohol won't have hydrogenbonds from them because there aren't enough slightly positive hydrogens to goaround.

In the diagram, to show the 3-dimensional arrangement, the wedge-shaped linesshow bonds coming out of the screen or paper towards you. The dotted bonds(other than the hydrogen bonds) show bonds going back into the screen or paperaway from you.

In alkanes, the only intermolecular forces are van der Waals dispersion forces.Hydrogen bonds are much stronger than these and therefore it takes moreenergy to separate alcohol molecules than it does to separate alkane molecules.


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That's the main reason that the boiling points are higher.

The effect o f van der Waals forces . . .

. . . on the boiling points of the alcohols:

Hydrogen bonding isn't the only intermolecular force in alcohols. There are alsovan der Waals dispersion forces and dipole-dipole interactions.

The hydrogen bonding and the dipole-dipole interactions will be much the samefor all the alcohols, but the dispersion forces will increase as the alcohols getbigger.

These attractions get stronger as the molecules get longer and have moreelectrons. That increases the sizes of the temporary dipoles that are set up.

This is why the boiling points increase as the number of carbon atoms in thechains increases. It takes more energy to overcome the dispersion forces, andso the boiling points rise.

. . . on the comparison between alkanes and alcohols:

Even if there wasn't any hydrogen bonding or dipole-dipole interactions, theboiling point of the alcohol would be higher than the corresponding alkane with

the same number of carbon atoms.

Compare ethane and ethanol:

Ethanol is a longer molecule, and the oxygen brings with it an extra 8 electrons.Both of these will increase the size of the van der Waals dispersion forces andso the boiling point.

If you were doing a really fair comparison to show the effect of the hydrogenbonding on boiling point it would be better to compare ethanol with propanerather than ethane. The length would then be much the same, and the numberof electrons is exactly the same.

Solubility of alcohols in water

The small alcohols are completely soluble in water. Whatever proportions you

mix them in, you will get a single solution.


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However, solubility falls as the length of the hydrocarbon chain in the alcoholincreases. Once you get to four carbons and beyond, the fall in solubility isnoticeable, and you may well end up with two layers in your test tube.

The solubi l i ty of the sm all alcohols in water

Consider ethanol as a typical small alcohol. In both pure water and pure ethanolthe main intermolecular attractions are hydrogen bonds.

In order to mix the two, you would have to break the hydrogen bonds betweenthe water molecules and the hydrogen bonds between the ethanol molecules. Itneeds energy to do both of these things.

However, when the molecules are mixed, new hydrogen bonds are madebetween water molecules and ethanol molecules.

The energy released when these new hydrogen bonds are made more or lesscompensates for that needed to break the original ones.

In addition, there is an increase in the disorder of the system - an increase inentropy. That is another factor in deciding whether things happen or not.

Note: If you haven't come across entropy before, don't worry about it. I mentionit because the energy released when the new bonds are made isn't quite enough

to compensate for breaking the old ones, meaning that the mixing process isendothermic. If it weren't for the increase in entropy, the solution wouldn't be


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formed.

To really understand this, you need to have studied entropy and free energy. Ifyou should know about this, but aren't happy about the calculations involved, youmight like to have a look at chapter 11 of mychemistry calculations book.

The lower solubi l i ty of bigg er alcohols

Imagine what happens when you have got, say, 5 carbon atoms in each alcoholmolecule.

The hydrocarbon chains are forcing their way between water molecules and so

breaking hydrogen bonds between those water molecules.

The -OH end of the alcohol molecules can form new hydrogen bonds with watermolecules, but the hydrocarbon "tail" doesn't form hydrogen bonds

That means that quite a lot of the original hydrogen bonds being broken aren'treplaced by new ones.

All you get in place of those original hydrogen bonds are van der Waalsdispersion forces between the water and the hydrocarbon "tails". Theseattractions are much weaker. That means that you don't get enough energy backto compensate for the hydrogen bonds being broken. Even allowing for theincrease in disorder, the process becomes less feasible.

As the length of the alcohol increases, this situation just gets worse, and so thesolubility falls.
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THE MANUFACTURE OF ALCOHOLS

This page looks at the manufacture of alcohols by the direct hydration ofalkenes, concentrating mainly on the hydration of ethene to makeethanol. It then compares that method with making ethanol byfermentation.

Manufacturing alcohols from alkenes

The manufacture of ethanol from ethene

Ethanol is manufactured by reacting ethene with steam. The catalystused is solid silicon dioxide coated with phosphoric(V) acid. The reactionis reversible.

Only 5% of the ethene is converted into ethanol at each pass through thereactor. By removing the ethanol from the equilibrium mixture andrecycling the ethene, it is possible to achieve an overall 95% conversion.

A flow scheme for the reaction looks like this:


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Note: This is a bit of a simplification! When the gases from the reactor are cooled,then excess steam will condense as well as the ethanol. The ethanol will have to beseparated from the water by fractional distillation.

All the sources I have looked at gloss over this, so I don't have any details. I assumeit is a normal fractional distillation of an ethanol-water mixture.

If you are interested in thereasons for the conditionsused in this reaction, you willfind them in the equilibrium section of this site by following this link.

If you are interested in themechanism for the hydration of ethene, follow this link tothe catalysis section.

Because these pages are (somewhat illogically!) in different parts of the site, use theBACK button (or HISTORY file or GO menu) on your browser to return to this pagelater.

The manufacture of other alcohols from alkenes

Some - but not all - other alcohols can be made by similar reactions. Thecatalyst used and the reaction conditions will vary from alcohol to alcohol.The only set of conditions you are going to need for UK A level purposesare those given above for manufacturing ethanol.

The reason that there is a problem with some alcohols is well illustrated

with trying to make an alcohol from propene, CH3CH=CH2.

In principle, there are two different alcohols which might be formed:

You might expect to get either propan-1-ol or propan-2-ol depending onwhich way around the water adds to the double bond. In practice whatyou get is propan-2-ol.

If you add a molecule H-X across a carbon-carbon double bond, the

hydrogen nearly always gets attached to the carbon with the mosthydrogens on it already - in this case the CH2 rather than the CH.
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Note: The reason for this is dealt with in detail in the mechanism section of this siteon a page aboutaddition to unsymmetrical alkenes.

If you choose to follow this link, use the BACK button on your browser to return tothis page.

The effect of this is that there are bound to be some alcohols which it isimpossible to make by reacting alkenes with steam because the additionwould be the wrong way around.

Making ethanol by fermentation

This method onlyapplies to ethanol. You can't make any other alcoholthis way.

The process

The starting material for the process varies widely, but will normally besome form of starchy plant material such as maize (US: corn), wheat,barley or potatoes.

Starch is a complex carbohydrate, and other carbohydrates can also beused - for example, in the lab sucrose (sugar) is normally used toproduce ethanol. Industrially, this wouldn't make sense. It would be sillyto refine sugar if all you were going to use it for was fermentation. Thereis no reason why you shouldn't start from the original sugar cane, though.

The first step is to break complex carbohydrates into simpler ones.

For example, if you were starting from starch in grains like wheat orbarley, the grain is heated with hot water to extract the starch and thenwarmed with malt. Malt is germinated barley which contains enzymes

which break the starch into a simpler carbohydrate called maltose,C12H22O11.

Maltose has the same molecular formula as sucrose but contains twoglucose units joined together, whereas sucrose contains one glucose andone fructose unit.

Yeast is then added and the mixture is kept warm (say 35C) for perhapsseveral days until fermentation is complete. Air is kept out of the mixtureto prevent oxidation of the ethanol produced to ethanoic acid (vinegar).

Enzymes in the yeast first convert carbohydrates like maltose or sucrose
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into even simpler ones like glucose and fructose, both C6H12O6, and thenconvert these in turn into ethanol and carbon dioxide.

You can show these changes as simple chemical equations, but thebiochemistry of the reactions is much, much more complicated than this

suggests.

Yeast is killed by ethanol concentrations in excess of about 15%, and thatlimits the purity of the ethanol that can be produced. The ethanol isseparated from the mixture by fractional distillation to give 96% pureethanol.

For theoretical reasons, it is impossible to remove the last 4% of water byfractional distillation.

Note: If you are interested in this, you will have to read about it in a physicalchemistry textbook under "azeotropic mixtures". Currently I haven't dealt with theseon this site, and am unlikely to do so until at least 2005. I will add a link if or when Iget around to it!

A comparison of fermentation with the direct hydration of ethene

Fermentation Hydration of etheneType ofprocess A batch process. Everythingis put into a container and

then left until fermentation iscomplete. That batch is thencleared out and a newreaction set up. This isinefficient.

A continuous flowprocess. A stream ofreactants is passedcontinuously over acatalyst. This is a moreefficient way of doingthings.

Rate ofreaction Very slow. Very rapid.Quality ofproduct Produces very impureethanol which needs further

processingProduces much purerethanol.

Reactionconditions Uses gentle temperaturesand atmospheric pressure. Uses high temperaturesand pressures, needing

lots of energy input.


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Use ofresources Uses renewable resourcesbased on plant material. Uses finite resourcesbased on crude oil.

THE DEHYDRATION OF ALCOHOLS

This page (a simple duplicate of a page in the section on alkenes!)

looks at the dehydration of alcohols in the lab to make alkenes - forexample, dehydrating ethanol to make ethene.

Dehydration of alcohols using aluminium oxide as catalyst

The dehydration of ethanol to give ethene

This is a simple way of making gaseous alkenes like ethene. Ifethanol vapour is passed over heated aluminium oxide powder, the

ethanol is essentially cracked to give ethene and water vapour.

To make a few test tubes of ethene, you can use this apparatus:

It wouldn't be too difficult to imagine scaling this up by boiling someethanol in a flask and passing the vapour over aluminium oxideheated in a long tube.


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Dehydration of alcohols using an acid catalyst

The acid catalysts normally used are either concentrated sulphuricacid or concentrated phosphoric(V) acid, H3PO4.

Concentrated sulphuric acid produces messy results. Not only is itan acid, but it is also a strong oxidising agent. It oxidises some ofthe alcohol to carbon dioxide and at the same time is reduced itselfto sulphur dioxide. Both of these gases have to be removed fromthe alkene.

It also reacts with the alcohol to produce a mass of carbon. Thereare other side reactions as well, but these aren't required by any

current UK A level (or equivalent) syllabus.

The dehydration of ethanol to give ethene

Ethanol is heated with an excess of concentrated sulphuric acid at atemperature of 170C. The gases produced are passed throughsodium hydroxide solution to remove the carbon dioxide andsulphur dioxide produced from side reactions.

The ethene is collected over water.

WARNING! This is potentially an extremely dangerous preparation becauseof the close proximity of the very hot concentrated sulphuric acid and thesodium hydroxide solution. I knew of one chemistry teacher who put severalstudents into hospital by getting it wrong! That was many years ago beforesafety was taken quite so seriously as it is now.

The concentrated sulphuric acid is a catalyst. Write it over the arrowrather than in the equation.

Introducing Alcohols

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