How many hydrogen atoms does ethanol contain?

Experiments:
Try: is alcohol a lye? Or an acid?


The quantitative elemental analysis provides the ratio of the masses of the elements or the relative number of atoms in a compound. Investigations of ethanol result in the atomic number ratio C: H: O = 2: 6: 1, so that the empirical formula C2H6O come.

However, this molecular formula says nothing about how the molecules are linked to one another. See this website for more information.

In order to find out the position of the atoms in the molecule in relation to one another, special investigation methods are necessary. For this you have to consider whether there are characteristic reactions for certain structures or whether you can make comparisons with already known material structures.

For the empirical formula C2H6There are two possible structures:

In structure I, all hydrogen atoms are bonded to carbons. In structure II, a hydrogen atom has a special position because it is located on an oxygen atom.

The question now remains, how can one find out what the structure of ethanol is? To do this, one has to know the reaction behavior of the atom groupings. Formula II contains a polar OH group that can be expected to react in a similar way to water.

A typical reaction of water is that with sodium, during which hydrogen evolves and ignites. If ethanol is allowed to react with sodium, similarities are found.

Both produce hydrogen and both react to form a type of lye (sodium hydroxide or sodium ethanolate). The only difference is that the reaction between ethanol and sodium is less violent.

Sodium can only replace hydrogen atoms from OH groups and not from hydrocarbons. This is also shown by the fact that sodium, because of its high reactivity, has to be kept under petroleum at all times, and that consists exclusively of hydrocarbons. Obviously, sodium cannot react with hydrogen atoms that are directly bonded to a carbon atom.

Thus, just like water, ethanol must have a polar OH group. Therefore the correct structure is that of structural formula II. We write this briefly as follows:

CH3-CH2OH

To make the special position of the OH group in the alcohol molecule clear, write C instead of C2H6O preferably the empirical formula C2H5OH. Chemists also refer to the OH group as Hydroxyl or hydroxy group. However, most of the time they just talk about that "OH group".

The compound with structural formula I is called "dimethyl ether", by the way.


last but not least
In order to remember the semi-structural formula of ethanol, one likes to use this saying:

Waiter, five Helle, two cognac!

The following picture shows the ethanol molecule in full beauty as a dome model.

Fig. 1: Dome models of ethanol
(Photo: flower)

Strictly speaking, the molecule by no means has the left, visually appealing shape of a little hand standing on four legs. In its basic state, it takes the right, twisted and therefore less energetic `` on gap '' position of a peeing dog. This position has something to do with alcohol consumption, as the Oktoberfest teaches us with its marginal phenomena


Further texts on the subject of `` alcohol ''