4.2.1 (a) Physical Properties of Alcohols

Syllabus

(i) the polarity of Alcohols and an explanation, in terms of Hydrogen Bonding, of the water solubility and the relatively low volatility of Alcohols compared with Alkanes (see also 2.2.2 l and 4.1.2 c)

(ii) classification of Alcohols into Primary, Secondary and Tertiary Alcohols Reactions of Alcohols

What does this mean?

What is an Alcohol?

The standard definition of an alcohol (which you won't be asked for) is:

"Alcohols are organic compounds in which the Hydroxyl group (-OH) is bound to a Carbon atom".

Where R represents a Carbon chain.

However, it is worth remembering that a molecule containing an - OH group bonded to a Carbon atom doesn't necessarily have to be an Alcohol.

Carboxylic acids are familiar to most and contain just this arrangement.

But since the functional group also contains a C=O bond this isn't an alcohol.

So it might be better to say:

"Alcohols are organic compounds in which the Hydroxyl group (-OH) is bound to a Carbon atom which is bound only to other Carbon and Hydrogen atoms".

Primary, Secondary & Tertiary Alcohols

All the above are Alcohols and share some Chemistry (react in the same way).

But we divide Alcohols into Primary, Secondary and Tertiary because this is not always the case - there are times when a Secondary Alcohol reacts differently to a Primary one, for instance.

The classification is relatively straightforward.

In a Primary Alcohol the Carbon atom bonded to the -OH group is bonded to one other Carbon atom.

So the above molecules (Ethanol, Propan-1-ol, Butan-1-ol) are all Primary Alcohols.

With the exception of Ethanol, all Primary Alcohols will end with -1-ol.

In Methanol the Carbon atom bonded to the -OH group is bonded to no other Carbon atoms but it still has very similar Chemistry to other Primary Alcohols.

Unsurprisingly, in a Secondary Alcohol the Carbon atom bonded to the -OH group is bonded to two other Carbon atoms.

Propan-2-ol and Butan-2-ol are both Secondary Alcohols.

Pentan-2-ol would also be Secondary but so would Pentan-3-ol - so it's less easy to spot a Secondary Alcohol by name alone.

Equally obviously , in a Tertiary Alcohol the Carbon atom bonded to the -OH group is bonded to three other Carbon atoms.

The simplest of these is Methyl Propan-2-ol.

Polarity and volatility.

Differences in electronegativity mean that all O-H bonds are polar and so are all C-O bonds.

The presence of these very polar -OH bonds means that all alcohols are capable of forming Hydrogen bonds (the strongest intermolecular forces) as well as weaker intermolecular forces.

Consequently, it takes a good deal of energy to break alcohol molecules away from their neighbours and the boiling points of Alcohols are considerably higher than the equivalent Alkanes.

Notice that, as with Alkanes, the boiling point rises as the molecules increase in length.

There are no extra -OH bonds in Propanol compared to Ethanol so there are no more hydrogen bonds.

But there are extra van der Waals due to the extra electrons.

So an examiner cannot expect you to know numbers for any boiling point but may present you with a few molecules and ask you for the trend and an explanation of it.

Examiners sometimes like to throw in a substance like Ethane-1,2-diol (ethylene glycol) and ask about its boiling point.

It has a very similar M_r to Propanol so we can assume that the van der Waals are similar.

But clearly it does have two -OH groups and therefore places to Hydrogen bond.

You wouldn't be expected to know that it boils at 197 ^oC but you should predict a much higher boiling point.

Another favourite question is the effect of branching.

All the above substances are C₄H₉OH.

Ignore the non-standard names.

What is clear is that the two primary Alcohols boil at high temps than the secondary, which boils at a higher temp than the tertiary form.

To account for the above an examiner will expect you to explain that each molecule has approximately the same strength of Hydrogen bonds, permanent dipole and van der waals forces.

(S)he will also expect you to explain that branching keep molecules further apart and that intermolecular forces are also dependent on distance.

So the unbranched butan-1-ol has molecules that are closest together and so the strongest intermoecluar forces and the highly branched Methylpropan-2-ol (right) has the most spaced molecules and the weakest forces.

Solubilities

You may remember that no Alkanes are significantly soluble in Water.

But Alcohols can Hydrogen bonds with water so should therefore be able to dissolve in it.

And without this solubility there could be no brewing industry since all alcoholic drinks are solutions of alcohol in water of various concentrations.

But what accounts for the gradual tailing off of solubility as the length of the Carbon train increases?

Water can only Hydrogen bond with the -OH group.

We can say that this part of the molecule is Hydrophilic.

It will not Hydrogen bond with the alkyl group(s) which can only form van der Waals.

This is why Alkanes and water don't mix.

So we can think of the alkyl group in an Alcohol as being Hydrophobic.

When Alcohols are small the Hydrophobic part makes up a large % of the whole molecule allowing the molecule to dissolve fairly well.

And the shorter the Alcohol the better it should dissolve.

So (again without learning any numbers) you should be able to predict the gradual decline of solubility from Methanol onwards.

By the time the molecule has 6 or 7 Carbon atoms the Hydrophobic part is so big that Hydrogen bonding with the small percentage of the molecule that is the -OH group is not enough to allow the molecule to dissolve.

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