- Structure of a primary alcohol
- Properties
- Acidity
- pKa
- Chemical reactions
- Formation of alkyl halides
- Oxidation of primary alcohols
- Formation of ethers
- Formation of organic esters
- Dehydration
- Nomenclature
- Examples
- References
A primary alcohol is one in which the hydroxyl group is attached to a primary carbon; that is, a carbon covalently attached to another carbon and hydrogens. Its general formula is ROH, specifically RCH 2 OH, since there is only one alkyl group R.
The R group of the formula RCH 2 OH can be any: a chain, a ring, or heteroatoms. When it comes to a chain, short or long, it is before the most representative primary alcohols; among them are methanol and ethanol, two of the most synthesized at industrial levels.
Beer Jar - An example of an aqueous solution of ethyl alcohol, a primary alcohol, in an organic matrix. Source: Engin Akyurt via Pexels.
Physically they are like the other alcohols, and their boiling or melting points depend on the degree of their branching; but chemically, they are the most reactive. Furthermore, its acidity is higher than that of secondary and tertiary alcohols.
Primary alcohols undergo oxidation reactions, becoming a large number of organic compounds: esters and ethers, aldehydes and carboxylic acids. Likewise, they can undergo dehydration reactions, transforming into alkenes or olefins.
Structure of a primary alcohol
The primary alcohols derived from linear alkanes are the most representative. However, in reality any structure, be it linear or branched, can be classified within this type of alcohol as long as the OH group is linked to a CH 2.
So, structurally they all have in common the presence of the group -CH 2 OH, called methylol. A characteristic and consequence of this fact is that the OH group is less hindered; that is, it can interact with the environment without spatial interference from other atoms.
Likewise, a less hindered OH means that the carbon atom that carries it, that of CH 2, can undergo substitution reactions through an SN 2 mechanism (bimolecular, without the formation of a carbocation).
On the other hand, an OH with greater freedom to interact with the medium, translates into stronger intermolecular interactions (by hydrogen bonds), which in turn increase the melting or boiling points.
The same happens with its solubility in polar solvents, as long as the R group is not very hydrophobic.
Properties
Acidity
The primary alcohols are of all the most acidic. For an alcohol to behave like a Bronsted acid, it must donate an H + ion to the medium, say water, to become an alkoxide anion:
ROH + H 2 O <=> RO - + H 3 O +
The negative charge of RO -, specifically RCH 2 O -, is less repulsed by the electrons in the two CH bonds than by the electrons in the CR bond.
The alkyl group then exerts the greatest repulsion, destabilizing RCH 2 O -; but not so much compared to that if there were two or three R groups, as occurs with secondary and tertiary alcohols, respectively.
Another way to explain the higher acidity of a primary alcohol is by means of the electronegativity difference, creating the dipole moment: H 2 C δ + -O δ- H. Oxygen attracts electronic density from both CH 2 and H; the positive partial charge of carbon repels that of hydrogen somewhat.
The R group transfers some of its electron density to CH 2, which helps to decrease its positive partial charge and thus its rejection by the hydrogen charge. The more R groups there are, the lower the repulsion, and therefore the tendency of H to be released as H +.
pKa
Primary alcohols are considered weaker acids than water, with the exception of methyl alcohol, which is slightly stronger. The pKa of methyl alcohol is 15.2; and the pKa of ethyl alcohol is 16.0. Meanwhile, the pKa of water is 15.7.
However, water, which is considered a weak acid, like alcohols, can join H + to become the hydronium ion, H 3 O +; that is, it behaves like a base.
In the same way, primary alcohols can take up hydrogen; especially in some of its own reactions, for example, in its transformation into alkenes or olefins.
Chemical reactions
Formation of alkyl halides
Alcohols react with hydrogen halides to produce alkyl halides. The reactivity of alcohols towards hydrogen halides decreases in the following order:
Tertiary alcohol> secondary alcohol> primary alcohol
ROH + HX => RX + H 2 O
RX is a primary alkyl halide (CH 3 Cl, CH 3 CH 2 Br, etc.).
Another way to prepare alkyl halides is by reacting thionyl chloride, a synthetic reagent, with a primary alcohol that is converted to an alkyl chloride. Thionyl chloride (SOCl 2) requires the presence of pyridine to react.
CH 3 (CH 2) 3 CH 2 OH + SOCl 2 => CH 3 (CH 2) 3 CH 2 Cl + SO 2 + HCl
This reaction corresponds to the halogenation of 1-pentanol to become 1-chloropentane in the presence of pyridine.
Oxidation of primary alcohols
Alcohols can be oxidized to aldehydes and carboxylic acids, depending on the reagent. Pyridinium chlorochromate (PCC) oxidizes primary alcohol to aldehyde, using dichloromethane (CH 2 Cl 2) as solvent:
CH 3 (CH 2) 5 CH 2 OH => CH 3 (CH 2) 5 COH
This is the oxidation of 1-heptanol to 1-heptanal.
Meanwhile, potassium permanganate (KMnO 4) first oxidizes the alcohol to aldehyde, and then oxidizes the aldehyde to carboxylic acid. When using potassium permanganate to oxidize alcohols, the breaking of the bond between carbons 3 and 4 must be avoided.
CH 3 (CH 2) 4 CH 2 OH => CH 3 (CH 2) 4 COOH
This is the oxidation of 1-hexanol to hexanoic acid.
By this method it is difficult to obtain an aldehyde, since it is easily oxidized to a carboxylic acid. A similar situation is observed when chromic acid is used to oxidize alcohols.
Formation of ethers
Primary alcohols can be converted to ethers when heated in the presence of a catalyst, usually sulfuric acid:
2 RCH 2 OH => RCH 2 OCH 2 R + H 2 O
Formation of organic esters
The condensation of an alcohol and a carboxylic acid, Fisher's esterification, catalyzed by an acid, produces an ester and water:
R'OH + RCOOH <=> RCOOR '+ H 2 O
A well-known reaction is that of ethanol with acetic acid, to give ethyl acetate:
CH 3 CH 2 OH + CH 3 COOH <=> CH 3 COOHCH 2 CH 3 + H 2 O
Primary alcohol is the most susceptible to Fischer esterification reactions.
Dehydration
At elevated temperatures and in an acidic medium, generally sulfuric acid, alcohols dehydrate to form alkenes with the loss of a water molecule.
CH 3 CH 2 OH => H 2 C = CH 2 + H 2 O
This is the dehydration reaction of ethanol to ethylene. A more suitable general formula for this type of reaction, specifically for a primary alcohol, would be:
RCH 2 OH => R = CH 2 (which is also equal to RC = CH 2)
Nomenclature
Example of primary alcohol. Source: Gabriel Bolívar.
The rules for naming a primary alcohol are the same as for the other alcohols; with the exception that it is sometimes not necessary to list the OH-bearing carbon.
In the upper image there is a seven-carbon main chain. The carbon bound to the OH is assigned the number 1, and then it begins to count from left to right. Its IUPAC name therefore is: 3,3-diethylheptanol.
Note that this is an example of a highly branched primary alcohol.
Examples
Finally, some primary alcohols are mentioned based on their traditional and systematic nomenclature:
-Methyl, CH 3 OH
-Ethyl, CH 3 CH 2 OH
-n-propyl, CH 3 CH 2 CH 2 OH
-n-hexyl, CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 OH
These are derivatives of linear alkanes. Other examples are:
-2-phenylethanol, C 6 H 5 CH 2 CH 2 OH (C 6 H 5 = benzene ring)
-2-propen-1-ol (allyl alcohol), CH 2 = CHCH 2 OH
-1,2-ethanediol, CH 2 OHCH 2 OH
-2-chloroethanol (ethylenechlorohydrin), ClCH 2 CH 2 OH
-2-buten-1-ol (crotyl alcohol), CH 3 CH = CHCH 2 OH
References
- Morrison, RT and Boyd, RN (1987). Organic Chemistry. (5 ta Edition). Addison-Wesley Iberoamericana
- Carey, FA (2008). Organic Chemistry. (6 ta Edition). McGraw-Hill, Interamerica, Editores SA
- Mel Science. (2019). How the oxidation of primary alcohols takes place. Recovered from: melscience.com
- Royal Society of Chemistry. (2019). Definition: primary alcohols. Recovered from: rsc.org
- Chriss E. McDonald. (2000). The Oxidation of Primary Alcohols to Esters: Three Related Investigative Experiments. J. Chem. Educ., 2000, 77 (6), p 750. DOI: 10.1021 / ed077p750