What is the pH of ethanal


Prof. Blume's tip of the month July 2006 (tip no. 109)


Alcohols are not alkalis. So is alcohol an acid?

If you look at the structure of an alcohol, you could think that it is a substance that could form hydroxide ions.


So is alcohol a lye?
To find out, we examine solutions of alcohols in water.

Experiment 1: How do alcohol solutions react?
We produce a 5% solution of methanol (F, T) or ethanol (F). Then we determine the pH.

Result:
The solution reacts neutrally.

We determine the electrical conductivity of the solutions.

Result:
The solutions do not conduct electricity.

We note: Alcohols are not alkalis. Because in aqueous solution no OH dissociates--Ion off. If the hydroxyl group is to be split off, this can only be done with the formation of radicals.

And that is only possible with a great deal of heat or radiation energy.


Is Alcohol an Acid?
If alcohols were acids, one could imagine the following reaction:


When measuring the pH value, we also received no indication of the presence of an acid. On the other hand, alcohol can be such a weak acid that the proton concentration is not sufficient to acidify the solution.
Let us therefore examine other typical acid reactions.

Experiment 2: reaction between alcohols and sodium (demonstration experiment)
We mix methanol (F, T) with sodium metal (F, C). We collect the gas and use it to do the oxyhydrogen gas test. Then we evaporate the excess alcohol.

Result:
The metal decomposes with the formation of gas. The gas turns out to be hydrogen. Evaporation produces a white substance. It crystallizes in pretty, asbestos-like threads.

Note:
We don't use ethanol here, although it's less toxic than methanol. Reason: With ethanol, no such easily separable substance is formed.

Apparently the following reaction has taken place:

The resulting substance is called Sodium methylate. It has all the properties of a salt.

Experiment 3: Investigation of sodium methylate
We determine the electrical conductivity of methanol. Then we dissolve some sodium methylate in it and measure again.

Result:
Methanol does not conduct electricity. After adding sodium methylate, the electrical conductivity increases sharply.

Like a salt, sodium methylate consists of ions: the anion is CH3O¯ and the cation Na+. You can tell that the salt contains carbon by the fact that it is flammable.

Experiment 4: heating sodium methylate
We heat a sample of sodium methylate (C) in a porcelain dish.

Result:
The salt carbonizes and forms flammable vapors.

Now one can speculate whether this behavior of alcohol corresponds to that of a classic acid. A classical acid is that it is able to split off protons spontaneously in water. These protons are reduced by the metal.

Oxidation: 2 Na ———> 2 Na+ + 2 e¯

Reduction: 2 hours+ + 2 e¯ ———> H2

That is not the case here. Alcohol has no dissociable hydrogen atoms. The reaction between sodium and alcohol should therefore only be considered as Reduction of the entire alcohol molecule describe:

Oxidation: 2 Na ———> 2 Na+ + 2 e¯

Reduction: 2 CH3OH + 2 e¯ ———> 2 CH3O¯ + H2

The result is:

Alcohols are not acids.


Br nsted can help here
Alcohols should be viewed from the point of view of Br nsted's acid / base concept. As is well known, this is:

Acids are proton donors, bases are proton acceptors.

We have seen: alcohol (unlike amphoteric water) cannot be an acid. The reason is that the methyl group has an "electron-donating" effect, i.e. it has a (+ I) effect on the oxygen atom. This reduces the tendency of the hydrogen atom to dissociate as a proton.

As the structure of methanol shows, like the molecule of water, it has two free, "non-binding" electron pairs. These make alcohols a Br nsted base. Because protons can dock to these non-binding electrons. Methanol actually reacts with hydrochloric acid or with hydrogen chloride, at least in part, as follows:

The ion corresponds to the oxonium ion of water: H3O+.

We learn from this:

Alcohols are not acids, they are bases.


Some say alcohol is amphoteric
Accordingly, one should be able to understand R-OH as both a base and an acid (albeit an extremely weak one):

One could therefore call R-OH amphoteric. But as I said: The opinion that R-OH is an acid is controversial. Because at least in a normal environment there is no base that succeeds in wresting a proton from the `` acid '' R-OH!

The base R-O- is much stronger than the base R-OH. The alcoholate anion therefore already forms with the acid water RAW.


Alcohols are Lewis bases
Even the most modern acid / base theory can be applied to the alcohols. As a reminder: After G. N. Lewis (1938) acids are electron acceptors, bases are electron donors. Ultimately, complex formation is nothing more than the product of an acid / base reaction according to Lewis.

Alcohols belong to the Lewis bases because of their non-bonding electron pairs on oxygen. That also makes them excellent complex ligands. The detection of alcohols with cerium (IV) ammonium nitrate is based on this.


What you need alcoholates for
These compounds are important for organic syntheses. They are strong nucleophiles, you see. So you could use them as an ink killer instead of hydrogen sulfite. They react with water to reform the alcohol and form sodium hydroxide solution:

Experiment 5: reaction of sodium methylate with water
We put a few drops of phenolphthalein solution in water. Then we stir in a spatula tip of sodium methylate.

We then produce a new, this time phenolphthalein-free solution. We acidify with nitric acid (check with universal indicator paper!). With the cerammonium reagent we try to detect an alcohol.

Result:
The solution turns an intense purple. The proof of alcohol is positive (red colouration).


One more point
The moderate reaction between sodium and alcohol is used in the laboratory to destroy residual sodium. It used to be done with a big hello on the square in front of the institute by simply throwing the sodium waste into buckets of water ...


R diger flower


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