A tertiary alcohol It is one in which the hydroxyl group, OH, is attached to a tertiary carbon. Its formula continues to be ROH, like other alcohols; but it is easily recognized because OH is close to an X in the molecular structure. Likewise, its carbon chain is usually shorter, and its molecular mass is higher..
So, a tertiary alcohol tends to be heavier, it is more branched, and it is also of all the least reactive with respect to oxidation; that is, it cannot be transformed into a ketone or carboxylic acid as it happens with secondary and primary alcohols, respectively..
The image above shows the general structural formula for a tertiary alcohol. According to it, a new formula of the type R could be written3COH, where R can be an alkyl or aryl group; a methyl group, CH3, or a short or long carbon chain.
If the three R groups are different, the central carbon of the tertiary alcohol will be chiral; that is, the alcohol will exhibit optical activity. Due to this fact, chiral tertiary alcohols are of interest within the pharmaceutical industry, since these alcohols with more complex structures are synthesized from ketones with biological activity..
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Consider the structures of higher tertiary alcohols to learn to recognize them no matter what the compound is. The carbon linked to the OH must also be linked to three other carbons. If you look closely, all three alcohols do..
The first alcohol (on the left), consists of three groups CH3 bonded to the central carbon, whose formula would be (CH3)3COH. The alkyl group (CH3)3C- is known as tertbutyl, it is present in many tertiary alcohols and can be easily recognized by its T shape (the red T in the picture).
The second alcohol (on the right) has the CH groups attached to the central carbon.3, CH3CHtwo and CHtwoCHtwoCH3. Since the three groups are different, alcohol is chiral and therefore exhibits optical activity. Here a T is not observed, but an X close to the OH (red and blue).
And in the third alcohol (the one below and without colors), the OH is linked to one of the two carbons that join two cyclopentane. This alcohol has no optical activity because two of the groups attached to the central carbon are identical. Like the second alcohol, if you look carefully you will also find an X (rather a tetrahedron).
The three higher alcohols have something more in common than an X: the central carbon is sterically hindered; that is, there are many atoms surrounding it in space. An immediate consequence of this is that nucleophiles, eager for positive charges, find it difficult to approach this carbon..
On the other hand, as there are three carbons bonded to the central carbon, they donate part of the electron density that the electronegative oxygen atom subtracts from it, stabilizing it even more against these nucleophilic attacks. However, tertiary alcohol can undergo substitution through the formation of a carbocation.
The 3rd alcohols generally have highly branched structures. A first consequence of this is that the OH group is hindered, and therefore its dipole moment has a lesser effect on neighboring molecules..
This results in weaker molecular interactions compared to those of primary and secondary alcohols..
For example, consider the structural isomers of butanol:
CH3CHtwoCHtwoOH (n-butanol, Peb = 117 ° C)
(CH3)twoCHtwoOH (isobutyl alcohol, bp = 107ºC)
CH3CHtwoCH (OH) CH3 (sec-butyl alcohol, bp = 98ºC)
(CH3)3COH (tert-butyl alcohol, bp = 82ºC)
Note how the boiling points drop as the isomer becomes more branched.
At the beginning it was mentioned that an X is observed in the structures of the 3rd alcohols, which in itself indicates a high branching. That is why these alcohols tend to have lower melting and / or boiling points..
Slightly similar is the case for its miscibility with water. The more hindered the OH, the less miscible the 3rd alcohol will be with the water. However, said miscibility decreases the longer the carbon chain is; This being the case, tertiary butyl alcohol is more soluble and miscible with water than n-butanol.
Tertiary alcohols tend to be the least acidic of all. The reasons are numerous and interrelated. In short, the negative charge of its derived alkoxide, RO-, will feel a strong repulsion for the three alkyl groups attached to the central carbon, weakening the anion.
The more unstable the anion, the lower the acidity of the alcohol.
3º alcohols cannot undergo oxidation to ketones (RtwoC = O) or aldehydes (RCHO) or carboxylic acids (RCOOH). For one thing, you would have to lose one or two carbons (in the form of COtwo) to oxidize, which decreases its reactivity against oxidation; and on the other, it lacks hydrogen that it can lose to form another bond with oxygen.
However, they can undergo substitution and elimination (formation of a double bond, an alkene or olefin).
The nomenclature for these alcohols is no different than for the others. There are common or traditional names, and the systematic names governed by the IUPAC.
If the main chain and its branches consist of a recognized alkyl group, this is used for its traditional name; when it is not possible to do so, the IUPAC nomenclature is used.
For example, consider the following tertiary alcohol:
The carbons begin to be numbered from right to left. In C-3 there are two CH groups3 of substituents, and therefore the name of this alcohol is 3,3-dimethyl-1-butanol (the main chain has four carbons).
Likewise, the entire chain and its branches consist of the neohexyl group; therefore, its traditional name may be neohexyl alcohol, or neohexanol.
Finally, some examples of tertiary alcohols are mentioned:
-2-methyl-2-propanol
-3-methyl-3-hexanol
-Bicyclo [3.3.0] octan-1-ol
-2-methyl-2-butanol: CH3CHtwoCOH (CH3)two
The formulas of the first three alcohols are represented in the first image.
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