Friday, May 18th, 2007
Think about it this way…
the reactants (left hand side) go on to make the products (right hand side)
Imagine that this reaction proceeds VIA the elements in their standard states, (according to Hess’s law we can go any way we want providing that we get to the desired products)
First of all, the reactants have to return to their elements. This process is the opposite of the enthalpy of formation, i.e. the negative or reverse of delta Hf = -delta Hf
The products are then formed from the elements – this is simply the enthalpy of formation, i.e. + deltaHf
So, if you sum these two processes to go from reactants to products you have:
Reaction enthalpy = – delta Hf(reactants) + deltaHf(products)
rearrange this to get:
Reaction enthalpy = deltaHf(products) – delta Hf(reactants)
May 21st, 2007
To detect the presence of the OH group regardless there are several simple tests that can be done, however they all have the disadvantage of interference by water.
A small piece of sodium metal can be placed in the alcohol and a steady stream of hydrogen bubbles gives a positive indication. Remember that the presence of water will also cause hydrogen to be evolved.
The suspected alcohol may be mixed with some PCl5 and the evolution of misty HCl gas is a positive indication of an OH group. Once again water interferes.
Primary, secondary and tertiary alcohols
There are three different types of alcohol depending on what is attached to the carbon holding the OH group.
· Primary – one alkyl group attached to the carbon holding the -OH
· Secondary – two alkyl groups attached to the carbon holding the -OH
· Tertiary alcohols – three alkyl groups attached to the carbon holding the -OH
To differentiate between the three it is possible to use oxidation with sodium dichromate in dilute sulphuric acid and heat.
The 1º alcohol oxidises to an aldehyde (alkanal) and then to a carboxylic acid (alkanoic acid)
The 2º alcohol oxidises to a ketone (alkanone) and stops there.
The 3º alcohol cannot be oxidised under these conditions.
Another (infrequently used) test is Lucas’ test for 1º, 2º, 3º alcohols.
It involves shaking the alcohol with ZnCl2 and dilute HCl. The 3º alcohol goes cloudy almost at once, the 2º alcohol goes cloudy after a few minutes whereas the 1º alcohol needs concentrated HCl to go cloudy. The cloudiness is due to the formation of the haloalkane that is immiscible with the aqueous solution of the zinc chloride and forms tiny droplets of an organic phase within the aqueous phase.