Alcohols (3) Redox reaction with sodium

Reactions of Alcohols

At the moment I can think of three basic types of reactions involving alcohols:

1) Redox reaction with sodium, oxygen and sodium dichromate(VI)

2) Substitution reactions with concentrated HCl or KBr and H₂SO₄

3) Dehydration using concentrated H₂SO₄, H₃PO₄ or Al₂O₃

In this blog, I’m going to discuss the first of the redox reactions.

In subsequent blogs we’ll look at the other reactions I’ve listed.

Alcohols and reactive metals

So redox reactions of alcohols with reactive metals like sodium.

Sodium is the one mentioned in all the textbooks but any of the Group 1 metals will carry out this change. 

The reaction is like that of a reactive Group 1 metal with water. 

The difference with alcohols is that one of the hydrogens in the water molecule is replaced with an alkyl group e.g. in ethanol, one of the —H atoms in water is replaced by an ethyl group: CH₃CH₂—

There’s a neat Youtube video of the water—sodium reaction here

I like this recording because he does the reaction in a 400ml beaker!! that's cool!! and the video gets really close up—though what I’d really like to see is him use potassium instead!!! that would be freezing cool!!!

Nice way too in which he tests for the displaced hydrogen!

There is a neat Youtube video of the ethanol—sodium reaction here.

You’ll see how the sodium reacts more slowly in ethanol.

Why is that?

It’s probably due to the presence of the alkyl group though there are no sources I have found to tell us why.

It could be that the mobility of the ethoxide ion product is lower than the hydroxide ion and so the movement of it away from the sodium surface is slower resulting in slower exposure of fresh metal to ethanol.

What do you think?

Here is the redox reaction equation (it follows the Metal + Acid gives Salt + Hydrogen (MASH) reaction profile.)

2Na    +     2CH₃CH₂OH     =     2CH₃CH₂O^— Na+     +     H₂

sodium  +   ethanol    =   sodium ethoxide    +    hydrogen

The –O—H hydrogen is acidic (slightly) in ethanol and in this reaction the –O—H bond breaks heterolytically leaving the two electrons on the oxygen (its more electronegative) with its negative charge.

Therefore, the ethoxide ion is a strong base and will accept a proton from water so adding an indicator to the reaction mixture reveals it to be around pH14.  (see the video)

Here is the equation for the ethoxide ion acting as a base

CH₃CH₂O^—       +     H₂O       =    CH₃CH₂OH    +        OH^— 

Evaporating the resultant solution of sodium ethoxide reveals a white solid. 

The two key observations required in an examination question are that the ethanol bubbles (don’t say a gas is given off because that's seen as a conclusion not an observation) and that the sodium gets smaller in size.

(Note: The emergence of a white solid that is sometimes seen in the reaction (the sodium ethoxide) has been argued over in standardisation meetings as an observation but is never accepted as a valid observation. 

The reaction of sodium with longer carbon chain alcohols gets slower as the carbon chain becomes more and more oily. 

Could the use of sodium form a test for an alcohol?

Sodium as a test for an alcohol has its limitations since the longer chain alcohols react slowly if at all with it.

May be for the first five or six in the alcohol homologous series the test using sodium would work. 

Beyond heptanol there is little reaction with sodium and soon enough the alcohols become solids.

Remember that there is hydrogen given off in this redox reaction and a simple test with a lighted spill should confirm its presence.  

Finally in this blog, with oxygen alcohols with smaller molar mass burn completely with a pale blue flame e.g.

 CH₃CH₂OH    +   3O₂    =    2CO₂      +     3H₂O

You should be able to satisfy yourself that you can build other combustion equations for other simple alcohols.

The larger molar mass alcohols burn less completely and with a yellow flame.