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 RCHtwoOH, since there is only one alkyl group R.
The R group of the formula RCHtwoOH can be any: a chain, a ring, or heteroatoms. When it comes to a chain, short or long, we are dealing with the most representative primary alcohols; among them are methanol and ethanol, two of the most synthesized at industrial levels.
Physically they are like other alcohols, and their boiling or melting points depend on the degree of their branching; but chemically, they are the most reactive. In addition, 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.
Article index
The primary alcohols derived from linear alkanes are the most representative. However, in reality any structure, whether linear or branched, can be classified within this type of alcohols as long as the OH group is linked to a CHtwo.
So, structurally they all have in common the presence of the group -CHtwoOH, called methylol. A characteristic and consequence of this fact is that the OH group is less hindered; that is, it can interact with the medium without spatial interference from other atoms.
Likewise, a less hindered OH means that the carbon atom that carries it, that of the CHtwo, may undergo substitution reactions through an SN mechanismtwo (bimolecular, without the formation of a carbocation).
On the other hand, an OH with greater freedom to interact with the medium, is translated as 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..
The primary alcohols are of all the most acidic. For an alcohol to behave like a Bronsted acid, you must donate an H ion+ to the medium, to say water, to become an alkoxide anion:
ROH + HtwoOR <=> RO- + H3OR+
The negative charge of RO-, specifically from the RCHtwoOR-, is less repulsed by the electrons in the two C-H bonds than by the electrons in the C-R bond.
The alkyl group then exerts the greatest repulsion, destabilizing RCHtwoOR-; 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 through the electronegativity difference, creating the dipole moment: HtwoCδ+-ORδ-H. Oxygen attracts electron density from both CHtwo as from H; the positive partial charge of carbon repels a little that of hydrogen.
The R group transfers some of its electron density to the CHtwo, which helps to decrease its positive partial charge and with it its repulsion for the hydrogen charge. The more R groups there are, the lower the repulsion, and therefore the tendency of H to be released as H+.
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 bind to H+ to transform into the hydronium ion, H3OR+; 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.
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 + HtwoOR
RX is a primary alkyl halide (CH3Cl, CH3CHtwoBr, 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 (SOCltwo) requires the presence of pyridine to react.
CH3(CHtwo)3CHtwoOH + SOCltwo => CH3(CHtwo)3CHtwoCl + SOtwo + HCl
This reaction corresponds to the halogenation of 1-pentanol to become 1-chloropentane in the presence of pyridine.
Alcohols can be oxidized to aldehydes and carboxylic acids, depending on the reagent. Pyridinium chlorochromate (PCC) oxidizes primary alcohol to aldehyde, using dichloromethane (CHtwoCltwo):
CH3(CHtwo)5CHtwoOH => CH3(CHtwo)5COH
This is the oxidation of 1-heptanol to 1-heptanal.
Meanwhile, potassium permanganate (KMnO4) first oxidizes the alcohol to aldehyde, and then oxidizes the aldehyde to carboxylic acid. When potassium permanganate is used to oxidize alcohols, the breaking of the bond between carbons 3 and 4 must be avoided..
CH3(CHtwo)4CHtwoOH => CH3(CHtwo)4COOH
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.
Primary alcohols can be converted to ethers when heated in the presence of a catalyst, usually sulfuric acid:
2 RCHtwoOH => RCHtwoOCHtwoR + HtwoOR
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 '+ HtwoOR
A well-known reaction is that of ethanol with acetic acid, to give ethyl acetate:
CH3CHtwoOH + CH3COOH <=> CH3COOHCHtwoCH3 + HtwoOR
Primary alcohol is the most susceptible to Fischer esterification reactions.
At high temperatures and in an acid medium, generally sulfuric acid, alcohols dehydrate to give rise to alkenes with the loss of a water molecule.
CH3CHtwoOH => HtwoC = CHtwo + HtwoOR
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:
RCHtwoOH => R = CHtwo (which is also equal to RC = CHtwo)
The rules for naming a primary alcohol are the same as for the other alcohols; with the exception that sometimes it is not necessary to list the OH-bearing carbon.
In the upper image there is a main chain of seven carbons. The carbon bound to the OH is assigned the number 1, and then it begins to be counted from left to right. Its IUPAC name is therefore: 3,3-diethylheptanol.
Note that this is an example of a highly branched primary alcohol.
Finally, some primary alcohols are mentioned based on their traditional and systematic nomenclature:
-Methyl, CH3Oh
-Ethyl, CH3CHtwoOh
-n-propyl, CH3CHtwoCHtwoOh
-n-hexyl, CH3CHtwoCHtwoCHtwoCHtwoCHtwoOh
These are derivatives of linear alkanes. Other examples are:
-2-phenylethanol, C6H5CHtwoCHtwoOH (C6H5 = benzene ring)
-2-propen-1-ol (allyl alcohol), CHtwo= CHCHtwoOh
-1,2-ethanediol, CHtwoOHCHtwoOh
-2-chloroethanol (ethylenechlorohydrin), ClCHtwoCHtwoOh
-2-buten-1-ol (crotyl alcohol), CH3CH = CHCHtwoOh
Yet No Comments