Exists 1 2 Dimethylpropanol

Structure of alcohols EF

Alcohols and alkanols

Probably the best-known alcohol of all is ethanol, also known as ethyl alcohol or simply "alcohol". Let's first look at a molecular model of ethanol:

A molecular model of ethanol

The most striking feature of all alcohols is the hydroxyl group (OH group, formerly also known as the hydroxyl group).

Structural formula of ethanol

In Figure 2 we can see a structural formula for ethanol. The illustration is intended to show that alcohols, especially the subgroup of alkanols, consist of an alkyl radical and a hydroxyl group.

For experts

Alcohols are compounds with one or more hydroxyl groups that do not have a functional group with a higher priority.

For example, lactic acid is not an alcohol, although the molecules have an OH group. But the lactic acid molecules also have a COOH group. Since the COOH group has a higher priority than the OH group, the compound is called milkacid and not as milkalcohol.

Allyl alcohol, a propene molecule with an OH group, is an alcohol. The OH group has a higher priority than the C = C double bond, so the compound does not belong to the alkenes, but to the alcohols, more precisely to the alkenols.

Alkanols are the alcohols derived from the alkanes. So they do not contain any other functional groups. The monohydric alkanols have the general formula CnH2n + 1OH.

The hydroxyl group is the functional group of the alkanols and essentially determines their physical properties, but of course also their chemical behavior.

But: the larger the alkyl radical of the alkanol, the less influence the hydroxyl group has. This can be illustrated by the water solubility of the alkanols. Methanol, ethanol and also propanol dissolve very well in water, while the water solubility of butanol is rather moderate. Pentanol and hexanol practically no longer dissolve in water.

The hydroxyl group (-OH) is the functional group of the alkanols and determines theirs properties. However, the larger the alkyl radical of the alkanol, the less the influence of the hydroxyl group.

Primary, secondary and tertiary alcohols

In order to understand what a primary, secondary and tertiary alcohol is, one must first clarify the terms primary, secondary and tertiary. Here are three short definitions:

A primary carbon atom is a carbon atom that is only with one Another C atom is connected.

A secondary carbon atom is with two connected to other carbon atoms.

A tertiary carbon atom is with three connected to other carbon atoms.

Here you can see the carbon backbone of the alkane 2-methyl-butane. The primary, secondary and tertiary carbon atoms are marked.

A primary alcohol is an alcohol in which the OH group is attached to a primary carbon atom. Correspondingly, in secondary alcohols the hydroxyl group is on a secondary carbon atom and in tertiary alcohols on a tertiary carbon atom.

A primary, a secondary and a tertiary alcohol derived from the alkane 2-methyl-butane. The H atoms were not drawn in.

There are no quaternary alcohols, because quaternary carbon atoms are already connected to four other carbon atoms, so there is no longer any space for an OH group on such a carbon atom.

The physical properties of the primary, secondary and tertiary alcohols differ slightly, details can be found on the "Physical properties" page.

The chemical properties are against it very differently. In many reactions, tertiary alcohols are more reactive than primary ones. Details can be found on the page "Reactions of Alcohols".

Position of the OH group

The position of the OH group plays a major role in the designation of alcohols, which brings us to the subject of nomenclature (naming). Basically, the alkanols have the name of their corresponding alkane. An alkanol with five carbon atoms is therefore a pentanol, but the name "pentanol" does not always appear in the name of the alcohol, as we will see in a moment.

The following figure shows the structural formulas and names of all isomers of pentanol that exist. The word "pentanol" is only included in the name if the carbon chain consists of five carbon atoms.

The structures of eight different pentanols

The simplest of these pentanol isomers is the alcohol pentan-1-ol. This spelling sounds strange at first, but has established itself internationally over the years, and it can also be found in more and more school books. Alternative older names for this alcohol are 1-pentanol and pentanol-1. But the position 1 of the OH group can always be found in the name. The OH group takes precedence over any alkyl side chains such as methyl or ethyl and therefore always has the lowest possible number.

If the carbon chain has only four carbon atoms, the name of the pentanol contains the word "butanol", and if the carbon chain has only three carbon atoms, the word "propanol" appears in the name of the alcohol. This is the case with 2,2-dimethyl-propan-1-ol (top right in the picture). However, the number 1 could be omitted here, because there is only one 2,2-dimethylpropanol. The OH group cannot sit on C atom 2 because there are already four alkyl radicals there.

Monohydric and polyhydric alcohols

So far we have only had alcohols one OH group discussed. But there are also alcohols with two, three or more OH groups. Such alcohols are referred to as diols, triols or generally polyols; in practical organic chemistry, however, almost only diols and triols play a major role.

Diol = alkanol with two hydroxyl groups.
Triol = alkanol with three hydroxyl groups.

A well-known diol is ethane-1,2-diol with an OH group on the first carbon atom and an OH group on the second carbon atom. Theoretically, an ethane-1,1-diol would also be possible, but such a compound is very unstable and does not exist for long under normal conditions. Ethane-1,2-diol is known and notorious as ethylene glycol or glycol for short (wine scandal 1985, in which glycol was added as a sweetener to simulate a higher quality). Glycol is also found in windscreen washer antifreeze.

Even better known and more important is propane-1,2,3-triol or glycerine, a molecule that is found in almost every fat known to us. An important class of lipids or fats is therefore called triglycerides. Fats consist of a glycerine molecule that is esterified with three fatty acid molecules.

So you now know how to correctly name a molecule with several OH groups. First you count how many OH groups there are in the molecule, then you already have the part of the name "diol" or "triol". Then you look at which C atoms sit the OH groups and number the C atoms so that the sum of the numbers is as small as possible. The correct designation for a pentane with three OH groups on the outer, the adjacent secondary and the middle carbon atom would be pentane-1,2,3-triol and not pentane-3,4,5-triol.