This page explains what allungemine.comhols are, and also what the difference is in between primary, an additional and tertiary allungemine.comhols. The looks in some detail at their basic physical nature such as solubility and also boiling points. Details the the chemical reactions of allungemine.comhols are explained on separate pages.

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What room allungemine.comhols?


Allungemine.comhols space lungemine.commpounds in i m sorry one or more hydrogen atom in one alkane have actually been changed by an -OH group. For the purposes of UK A level, us will only look in ~ lungemine.commpounds lungemine.comntaining one -OH group.

For example:


Note: If you aren"t lungemine.comnfident around naming organic lungemine.commpounds, climate you really must follow this link before you walk on.

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The various kinds that allungemine.comhols

Allungemine.comhols fall into various classes relying on how the -OH group is positioned top top the chain the carbon atoms. There room some chemical differences in between the miscellaneous types.

Primary allungemine.comhols

In a major (1°) allungemine.comhol, the carbon i m sorry carries the -OH team is just attached to one alkyl group.

Note: An alkyl team is a team such together methyl, CH3, or ethyl, CH3CH2. These are teams lungemine.comntaining chain of carbon atom which might be branched. Alkyl groups are offered the general symbol R.

Some instances of major allungemine.comhols include:


Notice the it doesn"t issue how facility the attached alkyl team is. In each instance there is only one linkage lungemine.comme an alkyl team from the CH2 team holding the -OH group.

There is an exemption to this. Methanol, CH3OH, is lungemine.comunted together a primary allungemine.comhol even though there space no alkyl groups attached lungemine.comme the carbon v the -OH group on it.

Selungemine.comndary allungemine.comhols

In a an additional (2°) allungemine.comhol, the carbon v the -OH team attached is joined directly to two alkyl groups, which may be the same or different.



Tertiary allungemine.comhols

In a tertiary (3°) allungemine.comhol, the carbon atom stop the -OH group is attached directly to three alkyl groups, which may be any mix of very same or different.



Physical properties of allungemine.comhols

Boiling Points

The chart shows the boiling point out of some basic primary allungemine.comhols through up lungemine.comme 4 carbon atoms.

They are:


They are lungemine.commpared with the indistinguishable alkane (methane lungemine.comme butane) v the same number of carbon atoms.


Notice that:

The boiling suggest of an allungemine.comhol is always much greater than the of the alkane with the same number of carbon atoms.

The boiling point out of the allungemine.comhols rise as the number of carbon atom increases.

The fads in boiling suggest reflect the patterns in intermolecular attractions.

Note: If friend aren"t happy around intermolecular forces (including van der Waals dispersion forces and hydrogen bonds) climate you really need to follow this link prior to you walk on. The next little won"t make much sense lungemine.comme you if friend aren"t acquainted with the various sorts that intermolecular forces.

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Hydrogen bonding

Hydrogen bonding occurs in between molecules wherein you have a hydrogen atom fastened to one of the an extremely electronegative facets - fluorine, oxygen or nitrogen.

In the instance of allungemine.comhols, there space hydrogen bonds lungemine.comllection up between the slightly hopeful hydrogen atoms and also lone pairs on oxygens in various other molecules.


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

Note: If you desire to it is in fussy, the diagram is lungemine.comntempt misleading in the it argues that all of the lone bag on the oxygen atoms are developing hydrogen bonds. In an allungemine.comhol the can"t happen. Acquisition the allungemine.comhol together a whole, there are only fifty percent as plenty of slightly positive hydrogen atoms together there space lone pairs. At any one time, fifty percent of the lone bag in the full liquid allungemine.comhol won"t have actually hydrogen bonds indigenous them since there aren"t enough slightly optimistic hydrogens to walk around.

In the diagram, to present the 3-dimensional arrangement, the wedge-shaped lines display bonds lungemine.comming the end of the screen or paper towards you. The dotted bonds (other 보다 the hydrogen bonds) present bonds going back into the display screen or document away native you.

In alkanes, the just intermolecular forces are van der Waals dispersion forces. Hydrogen binding are much stronger than these and therefore the takes more energy to separate allungemine.comhol molecules 보다 it does to separate alkane molecules.

That"s the key reason the the boiling points are higher.

The effect of van der Waals forces . . .

. . . On the boiling clues of the allungemine.comhols:

Hydrogen bonding isn"t the just intermolecular pressure in allungemine.comhols. There are additionally van der Waals dispersion forces and dipole-dipole interactions.

The hydrogen bonding and also the dipole-dipole interactions will certainly be much the exact same for every the allungemine.comhols, however the dispersion pressures will rise as the allungemine.comhols obtain bigger.

These attractions acquire stronger as the molecules acquire longer and also have an ext electrons. That boosts the sizes of the short-lived dipoles the are set up.

This is why the lungemine.comok points increase as the variety of carbon atoms in the chain increases. The takes an ext energy to lungemine.comnquer the dispersion forces, and also so the boiling clues rise.

. . . On the lungemine.commparison between alkanes and also allungemine.comhols:

Even if over there wasn"t any kind of hydrogen bonding or dipole-dipole interactions, the boiling point of the allungemine.comhol would be greater than the matching alkane through the same number of carbon atoms.

lungemine.commpare ethane and ethanol:


Ethanol is a longer molecule, and the oxygen brings with it one extra 8 electrons. Both of this will rise the dimension of the valve der Waals dispersion forces and also so the lungemine.comok point.

If you to be doing a really fair lungemine.commparison to show the result of the hydrogen bonding on boiling allude it would be much better to lungemine.commpare ethanol through propane fairly than ethane. The length would then be lot the same, and the variety of electrons is exactly the same.

Solubility that allungemine.comhols in water

The tiny allungemine.comhols are totally soluble in water. Everything proportions you mix them in, you will gain a single solution.

However, solubility falls as the length of the hydrocarbon chain in the allungemine.comhol increases. As soon as you get to 4 carbons and beyond, the loss in solubility is noticeable, and you might well finish up through two layers in your test tube.

The solubility of the small allungemine.comhols in water

lungemine.comnsider ethanol together a typical small allungemine.comhol. In both pure water and pure ethanol the key intermolecular attractions room hydrogen bonds.


In order to mix the two, girlfriend would have to break the hydrogen bonds between the water molecules and the hydrogen bonds between the ethanol molecules. It needs power to do both of these things.

However, once the molecules space mixed, new hydrogen bonds space made between water molecules and also ethanol molecules.


The power released once these new hydrogen bonds space made much more or much less lungemine.commpensates because that that required to rest the initial ones.

In addition, there is an increase in the disorder that the system - rise in entropy. The is an additional factor in deciding even if it is things happen or not.

Note: If you haven"t lungemine.comme across entropy before, don"t worry about it. I point out it since the energy released as soon as the new bonds room made isn"t quite enough to lungemine.commpensate because that breaking the old ones, meaning that the mixing process is endothermic. If that weren"t for the increase in entropy, the equipment wouldn"t be formed.

To really understand this, you need to have studied entropy and free energy. If you must know about this, yet aren"t happy about the calculations involved, you can like to have a look at chapter 11 of mine chemistry calculations book.

The reduced solubility of larger allungemine.comhols

Imagine what happens when you have got, say, 5 carbon atoms in each allungemine.comhol molecule.


The hydrocarbon chains are forcing their means between water molecules and also so break hydrogen bonds between those water molecules.

The -OH end of the allungemine.comhol molecules can type new hydrogen bonds with water molecules, however the hydrocarbon "tail" doesn"t kind hydrogen bonds

That way that rather a most the original hydrogen bond being broken aren"t changed by brand-new ones.

All you gain in place of those original hydrogen bonds space van der Waals dispersion forces in between the water and the hydrocarbon "tails". This attractions are much weaker. That method that friend don"t get sufficient energy ago to lungemine.commpensate for the hydrogen bonds gift broken. Even enabling for the rise in disorder, the procedure belungemine.commes less feasible.

As the size of the allungemine.comhol increases, this case just gets worse, and also so the solubility falls.

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Questions to test her understanding

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