The alcohols they are organic compounds that are characterized by having a hydroxyl group (-OH) linked to a saturated carbon; that is, a carbon that is linked to four atoms by single bonds (without double or triple bonds).
The generic formula for this vast and versatile family of compounds is ROH. To be considered an alcohol in a strictly chemical sense, the OH group must be the most reactive in the molecular structure. This is important to be able to affirm, among several molecules with OH groups, which of them is an alcohol..
One of the quintessential alcohols and the best known in popular culture is ethyl alcohol or ethanol, CH3CHtwoOH. Depending on its natural origin, and therefore on its chemical environment, its mixtures can originate an unlimited spectrum of flavors; some, that even show positive changes to the palate with the fly of the years.
They are the mixtures of organic and inorganic compounds with ethyl alcohol, which gives rise to its consumption in social and religious events since times before Christ; such as with grape wine, or with glasses served for a celebration, in addition to punches, candies, panettones, etc..
The enjoyment of these drinks, in moderation, is the result of the synergy between ethyl alcohol and its enveloping chemical matrix; without it, as a pure substance, it becomes extremely dangerous and triggers a series of negative health consequences.
It is for this reason that the consumption of aqueous mixtures of CH3CHtwoOH, like those bought in pharmacies for antiseptic purposes, represents a great risk to the body.
Other alcohols that are also very popular are menthol and glycerol. The latter, as well as erythrole, is found as an additive in many foods to sweeten and preserve them during storage. There are government entities that dictate which alcohols can be used or consumed without collateral effects..
Leaving behind the daily use of alcohols, chemically they are very versatile substances, because starting from them, other organic compounds can be synthesized; to the point that some authors think that with a dozen of them all the compounds necessary to live on a desert island can be created.
Article index
Alcohols have a general formula of ROH. The OH group is linked to the alkyl group R, whose structure varies from one alcohol to another. The union between R and OH is through a single covalent bond, R-OH.
The following image shows three generic structures for alcohols, keeping in mind that the carbon atom is saturated; that is, it forms four simple links.
As can be seen, R can be any carbon skeleton as long as it does not have substituents more reactive than the OH group..
For the case of primary alcohol, 1st, the OH group is linked to a primary carbon. This is easily verified by observing that the atom in the center of the left tetrahedron is bonded to one R and two H.
The secondary alcohol, 2º, is verified with the carbon of the tetrahedron in the center now linked to two R groups and one H.
And finally, there is the tertiary alcohol, 3º, with the carbon linked to three R groups.
Depending on the type of carbon linked to the OH, there is the classification of primary, secondary and tertiary alcohols. In the tetrahedra, the structural differences between them have already been detailed. But, all alcohols, regardless of their structure, share something in common: the amphiphilic character..
You don't need to tackle a structure to notice it, just its ROH chemical formula. The alkyl group is made up almost entirely of carbon atoms, "assembling" a hydrophobic backbone; that is, it interacts very weakly with water.
On the other hand, the OH group can form hydrogen bonds with water molecules, thus being hydrophilic; that is, loves or has an affinity for water. So, alcohols have a hydrophobic backbone, attached to a hydrophilic group. They are apolar and polar at the same time, which is the same as saying that they are amphiphilic substances.
R-OH
(Hydrophobic) - (Hydrophilic)
As will be explained in the next section, the amphiphilic character of alcohols defines some of their chemical properties..
The alkyl group R can have any structure, and yet it is important as it allows alcohols to be cataloged.
For example, R can be an open chain, as is the case with ethanol or propanol; branched, such as t-butyl alcohol, (CH3)twoCHCHtwoOH; it can be cyclical, as in the case of cyclohexanol; or it may have an aromatic ring, as in benzyl alcohol, (C6H5) CHtwoOH, or in 3-Phenylpropanol, (C6H5) CHtwoCHtwoCHtwoOh.
The R chain can even have substituents such as halogens or double bonds, such as for the alcohols 2-chloroethanol and 2-Buten-1-ol (CH3CHtwo= CHCHtwoOH).
Considering then the structure of R, the classification of alcohols becomes complex. For this reason, the classification based on their structure (1st, 2nd and 3rd alcohols) is simpler but less specific, although it is enough to explain the reactivity of alcohols..
One of the main properties of alcohols is that they associate through hydrogen bonds.
In the image above, you can see how two ROH molecules form hydrogen bonds with each other. Thanks to this, alcohols are usually liquids with high boiling points..
For example, ethyl alcohol has a boiling point of 78.5 ° C. This value increases as the alcohol becomes heavier; that is, the R group has a greater mass or number of atoms. Thus, n-butyl alcohol, CH3CHtwoCHtwoCHtwoOH, has a boiling point of 97ºC, slightly lower than that of water.
Glycerol is one of the alcohols with the highest boiling point: 290ºC.
Why? Because not only the mass or structure of R influences, but also the number of OH groups. Glycerol has three OH's in its structure: (HO) CHtwoCH (OH) CHtwo(OH). This makes it capable of forming many hydrogen bonds and holding its molecules together more tightly..
On the other hand, some alcohols are solid at room temperature; as the same glycerol at a temperature below 18ºC. Therefore, the statement that all alcohols are liquid substances is incorrect..
In homes it is very common to resort to the use of isopropyl alcohol to remove a stain that is difficult to remove on a surface. This solvent capacity, very useful for chemical syntheses, is due to its amphiphilic character, previously explained.
Fats are characterized by being hydrophobic: that is why it is difficult to remove them with water. However, unlike water, alcohols have a hydrophobic part in their structure.
Thus, its alkyl group R interacts with fats, while the OH group forms hydrogen bonds with water, helping to displace them..
Alcohols can react as acids and bases; that is, they are amphoteric substances. This is represented by the following two chemical equations:
ROH + H+ => ROHtwo+
ROH + OH- => RO-
RO- is the general formula of what is known as an alkoxide.
There are two ways to name alcohols, the complexity of which will depend on their structure.
Alcohols can be called by their common names. Which are? For this, the name of the R group must be known, to which the ending -ico is added, and is preceded by the word 'alcohol'. For example, the CH3CHtwoCHtwoOH is propyl alcohol.
Other examples are:
-CH3OH: methyl alcohol
-(CH3)twoCHCHtwoOH: isobutyl alcohol
-(CH3)3COH: tert-butyl alcohol
As for common names, you must start by identifying R. The advantage of this system is that it is much more precise than the other..
R, being a carbon skeleton, may have branches or several chains; the longest chain, that is, with more carbon atoms, is the one that will be given the name of alcohol.
To the name of the alkane of the longest chain, the ending 'l' is added. That is why at CH3CHtwoOH is called ethanol (CH3CHtwo- + OH).
Generally, the OH should have the lowest possible enumeration. For example, the BrCHtwoCHtwoCHtwo(OH) CH3 is called 4-Bromo-2-butanol, and not 1-Bromo-3-butanol.
The oil cracking process produces a mixture of alkenes of four or five carbon atoms, which can be easily separated.
These alkenes can be converted into alcohols by the direct addition of water or by the reaction of the alkene with sulfuric acid, followed by the addition of the water that cleaves the acid, originating the alcohol.
In the presence of a suitable catalyst, alkenes react with carbon monoxide and hydrogen to generate aldehydes. Aldehydes can be easily reduced to alcohols by a catalytic hydrogenation reaction.
Often there is such a synchronization of the oxo process that the reduction of the aldehydes is almost simultaneous with their formation..
The most used catalyst is dicobalt octocarbonyl, obtained by the reaction between cobalt and carbon monoxide.
The fermentation of carbohydrates by yeast is still of great importance in the production of ethanol and other alcohols. Sugars come from sugar cane or starch obtained from different grains. For this reason, ethanol is also called "grain alcohol"
Although it is not the main function of alcohols, the presence of ethanol in some of the beverages is one of the most popular knowledge. Thus, ethanol, a product of the fermentation of sugar cane, grapes, apples, etc., is present in numerous drinks for social consumption.
-Methanol is used in the production of formaldehyde, through its catalytic oxidation. Formaldehyde is used in the manufacture of plastics, paints, textiles, explosives, etc..
-Butanol is used in the production of butane ethanoate, an ester used as a flavoring in the food and confectionery industries..
-Allyl alcohol is used in the production of esters, including diallyl phthalate and diallyl isophthalate, which serve as monomers.
-Phenol is used in the production of resins, nylon manufacturing, deodorants, cosmetics, etc..
-Alcohols with a straight chain of 11-16 carbon atoms are used as intermediates to obtain plasticizers; for example, polyvinyl chloride.
-The so-called fatty alcohols are used as intermediates in the synthesis of detergents.
-Methanol is used as a paint thinner, as are 1-butanol and isobutyl alcohol.
-Ethyl alcohol is used as a solvent for many compounds insoluble in water, being used as a solvent in paints, cosmetics, etc..
-Fatty alcohols are used as solvents in the textile industry, in colorants, in detergents and in paints. Isobutanol is used as a solvent in coating materials, paints and adhesives.
-Methanol is used as a fuel in internal combustion engines and a gasoline additive to improve combustion.
-Ethyl alcohol is used in combination with fossil fuels in motor vehicles. For this purpose, extensive regions of Brazil are destined to the cultivation of sugar cane for the production of ethyl alcohol. This alcohol has the advantage of producing only carbon dioxide in its combustion..
When ethyl alcohol is burned, it produces a clean, smokeless flame, which is why it is used as fuel in field kitchens..
-Gelled alcohol is produced by combining methanol or ethanol with calcium acetate. This alcohol is used as a heat source in field stoves, and since it spills it is safer than liquid alcohols.
-The so-called biobutanol is used as a fuel in transportation, as well as isopropyl alcohol that can be used as a fuel; although its use is not recommended.
Isopropyl alcohol at 70% concentration is used as an external antiseptic to eliminate germs and retard their growth. Likewise, ethyl alcohol is used for this purpose..
Cyclohexanol and methylcyclohexanol are used in textile finishing, furniture processing and stain removers.
Yet No Comments