In this explainer, we will learn how to identify and name alcohols and describe their physical properties.
Alcohols are organic compounds that contain at least one hydroxy functional group. The simplest alcohol is methanol, and it contains just one carbon atom that is bonded to three hydrogen atoms and a single hydroxy group. Ethanol is the second simplest alcohol molecule. It contains one ethyl moiety that is bonded to a single hydroxy group. The methanol and ethanol molecules are displayed in the following figure.
Methanol and ethanol are relatively small and low molecular mass molecules. Methanol has the rather simple chemical formula, and it has a molecular mass of just 32.04 g/mol. Ethanol has the slightly more complex chemical formula, but its molecular mass is only 46.07 g/mol.
Alcohol molecules are organic compounds that contain a hydroxy group covalently bonded to a carbon atom.
Example 1: Identifying What Functional Group Can Be Found in Every Single Different Type of Alcohol Molecule
What functional group does an alcohol contain?
Alcohols are organic compounds that contain at least one hydroxy functional group. Option D shows the hydroxy group with the term. We can use these statements to determine that option D is the correct answer for this question.
Alcohol molecules have unusual chemical and physical properties that differ from the properties of similarly sized alkanes. Alcohols are more soluble in water and they tend to have much higher boiling points and lower volatility values. The unusual properties of alcohol molecules can be understood if we remember that oxygen atoms withdraw a significant amount of electron density from covalent bonds.
Oxygen atoms are highly electronegative elements, and they withdraw a large amount of electron density from covalent bonds. The electron density becomes concentrated around the electronegative oxygen atoms and depleted around the covalently bonded hydrogen atoms. The hydroxy group becomes highly polar and it can induce the formation of hydrogen bonds when it interacts with other polar molecules. The following figure shows how a hydrogen bond can form between one polar ethanol and one polar water molecule. The hydrogen bond is depicted as a dashed red line.
Small alcohol molecules are soluble in water because of the attractive intermolecular interactions between neighboring alcohol and water molecules. Small alcohol molecules readily dissolve in liquid water because they can form strong hydrogen bonds with any water molecules that surround them. It is energetically favorable for small alcohol molecules to dissolve in liquid water because of the attractive electrostatic interactions between the polar alcohol and water molecules.
Longer alcohol molecules tend to be less soluble in water because they contain a longer nonpolar hydrocarbon chain domain. The small hydroxy groups can form hydrogen bonds with surrounding water molecules, but the larger nonpolar hydrocarbon chain domain cannot form hydrogen bonds with any type of encompassing water molecules whatsoever. It is energetically favorable for the small hydroxy groups to dissolve in water, but it is more energetically favorable for the large carbon chains to not dissolve in water.
The following table shows how the solubility of an alcohol molecule in water can be determined by the length of its hydrocarbon chain domain. It is important to appreciate here that alcohol molecules are always more soluble in water than similarly sized nonpolar alkane molecules. The nonpolar alkane molecules cannot make any hydrogen bonds with water molecules because they do not contain any hydrogen bond-forming groups such as an group.
|Formula||Name||Water Solubility (g/100 g of Water)|
|Decan-1-ol||Insoluble in water|
The strength of the hydrogen bond can also be used to explain why alcohol substances have relatively high boiling points and why they are not very volatile. Adjacent alcohol molecules are linked together with strong hydrogen bonds, and it takes a lot of thermal energy to break them apart. The following image shows how a strong hydrogen bond can form between two adjacent ethanol molecules.
The next table shows that alcohol substances tend to have higher boiling points when they are made up of molecules that have a greater number of hydroxy groups. The table indicates that ethylene glycol has a higher boiling point than ethanol , because ethylene glycol molecules have two hydroxy groups and ethanol molecules have one hydroxy group. The table also indicates that the glycerol substance has a very high boiling point primarily because it is made up of molecules that have three hydroxy groups.
|Alcohol Name||Chemical Formula||Boiling Point ()|
Many alcohol substances can be classed as weak acids because they contain polar hydroxy groups that can break apart in liquid water and produce hydrogen ions . The alcohol substances are only weakly acidic, and they have a neutral effect on litmus paper, but they are sufficiently acidic to react with strong active metals such as sodium or potassium. The following equation shows how potassium metal can be reacted with an alcohol to produce a potassium alkoxide product:
The term is used here to represent an alkyl group. The next equation shows how a representative ethanol substance can be reacted with sodium metal to produce sodium ethoxide and hydrogen gas products:
Many alcohols can be classed as either primary (), secondary (), or tertiary () alcohols. Alcohols are classed as primary alcohols if the hydroxy group is bonded to a carbon atom that is, in turn, attached to two other hydrogen atoms and one alkyl group. Primary alcohols can be described with the general formula, where is an alkyl group. Methanol cannot be described with the general formula, but it is still categorized as a primary alcohol by almost all chemistry textbooks and encyclopedias.
Alcohols are classed as secondary alcohols when the hydroxy group is bonded to a carbon atom that is, in turn, attached to one hydrogen atom and two alkyl groups. Secondary alcohols can be described with the general formula, where is an alkyl group.
Alcohols are classed as tertiary alcohols when the hydroxy group is bonded to a carbon atom that is, in turn, attached to zero hydrogen atoms and three alkyl groups. Tertiary alcohols can be described with the general formula where is an alkyl group.
Alcohol molecules can also be classed into different categories depending on the number of hydroxy groups they contain. Alcohol molecules can be classed as monohydric molecules if they contain a single hydroxy group and dihydric molecules if they contain two hydroxy groups. Methanol and ethanol can both be classed as monohydric molecules and ethylene glycol can be classed as a dihydric molecule.
Ethylene glycol is used to make hydraulic brake fluids and to produce antifreeze formulations. Ethylene glycol molecules can also be used to make printing ink and to prepare the polyethylene glycol (PEG) polymer that is used to manufacture photographic films and cassette tapes.
Alcohol molecules can be classed as trihydric molecules if they contain three hydroxy groups, and the term polyhydric is used to describe any alcohol molecule that contains multiple hydroxy groups. Some chemistry textbooks state that polyhydric alcohols have at least four hydroxy groups and others state that polyhydric alcohols have at least two or three hydroxy groups. There are many different definitions for the polyhydric term and students should be fully aware of this fact.
Glycerol can be classed as a trihydric molecule because it contains three hydroxy groups. The sorbitol sweetener and the glucose and fructose sugars can all be classed as polyhydric molecules because they contain at least five hydroxy groups.
The glucose and fructose sugars are regularly added to food products to make them more sweet and easier to eat. Sorbitol molecules can also be added to food items to make them more sweet and palatable. Glycerol is used to make creams and cosmetics. Glycerol can also be used to make different types of textiles and to prepare explosive substances such as nitroglycerine. The following equation shows how glycerol molecules can be reacted with nitric acid to make the nitroglycerine substance:
The IUPAC developed a systematic naming system for classifying all of the different types of alcohol molecules. The IUPAC alcohol name always has a stem term, and it can also include infix numbers and prefix and suffix terms. The following figure shows how the IUPAC name of one alcohol can be made up of the butane stem term and other infix numbers and prefix and suffix terms. The 2-methyl prefix term is blue in the following figure and the butane stem term is black. The -2,3-infix numbers are green and the -diol suffix is red.
The stem term is always determined from the size and molecular bonding of the longest carbon chain. The stem is the full or abbreviated name of an alkane molecule such as butane or butan. The suffix is determined from the number of hydroxy groups that are covalently bonded to the longest carbon chain. The suffix is -ol if the molecule has one hydroxy group and it is -diol or -triol if the molecule has two or three hydroxy groups. The prefix is determined from the type and number of substituents that have replaced hydrogen atoms on the longest carbon chain. The prefix is usually just a simple combination of numbers and alkyl group names such as 2-methyl or 2-ethyl-4-methyl. The infix numbers are determined from the position of the hydroxy groups on the longest carbon chain. The infix can be a single number or it can be a combination of different numbers. We will determine the IUPAC name of the following organic compound.
The longest and only carbon chain contains five carbon atoms, and we can use this information to determine that the stem is pentane. There is one hydroxy group, so the suffix must be -ol. The infix number can be determined by numbering the carbon atoms from the right- and left-hand sides of the molecule. The red numbers show how the carbon atoms could be numbered from the left-hand side of the molecule. The blue numbers show how the carbon atoms could be numbered from the right-hand side of the molecule. The red and blue number sequences show that hydroxy substituent is either at the second (2) or fourth (4) main-chain carbon atom and the infix is therefore either -2- or -4-. We will have to choose just one of these two infix options. The red color numbers are the preferred choice here because we always want the infix numbers to be as low as possible. We do not need to determine a prefix for the name of this molecule because it does not have any alkyl substituents.
We can combine the pentane stem with the -2- infix and the -ol suffix terms to determine that the molecule has the pentan-2-ol IUPAC name. You should notice here that the pentane stem was abbreviated to the pentan- term because the -ol suffix starts with a vowel. The terminal “e” letter of any alkane stem term should always be removed if the suffix starts with a vowel.
We will now try to determine the IUPAC name for a dihydric molecule that has a single methyl group attached to one of the main-chain carbon atoms.
The longest carbon chain contains four carbon atoms, and this implies that the stem is butane. There are two hydroxy groups and one methyl group substituents, so the suffix must be -diol and the prefix should include the methyl- term.
The infix and prefix numbers are difficult to determine for this molecule because we have to think about the position of both hydroxy and methyl groups. The infix and prefix numbers can be determined by first considering the position of the hydroxy groups and then considering the position of the less important methyl substituent. The hydroxy groups always take precedence over other less important alkyl substituents.
The red numbers show how the carbon atoms can be counted from the left-hand side of the molecule. The blue numbers show how the carbon atoms could be counted from the right-hand side of the molecule. The red numbers put the leftmost hydroxy group at the second main-chain carbon atom (2) and the rightmost hydroxy group at the third main-chain carbon atom (3). The blue numbers put the leftmost hydroxy group at the third main-chain carbon atom (3) and the rightmost hydroxy group at the second main-chain carbon atom (2). The red and blue numbers both indicate that the infix should be -2,3- and that the molecule should therefore include the butane-2,3-diol term.
We can, however, determine that we should be using the red numbers here because they put the methyl substituent at the second main-chain carbon atom (2), whereas the blue numbers put it at the third main-chain carbon atom (3). The prefix is 2-methyl if we use the red numbers, and it is 3-methyl if we use the blue numbers. The red numbers are the preferred choice here because they make the prefix number term have a lower value. We can combine the 2-methyl prefix with the butane-2,3-diol term to determine that the molecule has the 2-methylbutane-2,3-diol IUPAC name. You should notice here that we did not abbreviate the butane stem and change it to be the butan- term because the -diol suffix does not start with a vowel.
The following image shows the IUPAC name for some other types of alcohol molecules.
You should be able to realize from these representative examples that alkyl substituents should be listed in alphabetical order and that cyclic alcohol molecules can be named according to the same conventions as straight-chain molecules.
The IUPAC name does not usually match the common name of an alcohol. Lots of alcohol molecules have the alkyl alcohol common name but the alkanol IUPAC name. Methanol is regularly called methyl alcohol and ethanol is frequently called ethyl alcohol. It is also quite normal to use the name propyl alcohol for the propan-1-ol molecule and the name isopropyl alcohol for the propan-2-ol molecule. Many people also use the name isobutyl alcohol for what should officially be called 2-methylpropan-1-ol and just as many people use the name tertiary butyl alcohol, or the tert-butyl alcohol, for what should be called 2-methylpropan-2-ol. This information is summarized in the following table.
|Condensed Structural Formula||Class of Alcohol||Common Name||IUPAC Name|
Example 2: Determining the Correct IUPAC Name for a Six-Membered Carbon Chain That Has a Single Hydroxy Substituent
Using the IUPAC nomenclature, what name does the following alcohol have?
The longest chain contains six carbon atoms and there is only one hydroxy group. The stem must be hexane and the suffix must be -ol. The longest and only carbon chain contains six carbon atoms, and this means that we can either sequentially number the carbon atoms from the left-hand side to the right-hand side or from the right-hand side to the left-hand side. The following figure uses red numbers to show how we could order the numbers from the left-hand side to the right-hand side and blue numbers to show how we could order the numbers from the right-hand side to the left-hand side.
The IUPAC naming system specifies that we should also use the lowest possible sequential numbering system if more than one numbering system is available. The lowest possible sequential numbering system would start from the far-left carbon atom on the hexane chain and this would mean the hydroxy group is bonded to the second carbon atom (2). This discussion can be used to determine that the correct IUPAC classification for this compound is hexan-2-ol, or answer E.
Yeast can be used to produce ethanol and carbon dioxide from small sugars such as glucose and sucrose or from starch macromolecules that are made up of smaller sugar monomers. The following equations show how the Hawamdiya sugar company produces an ethanol product from a molasses (sucrose) reactant through a fermentation process:
Ethanol can also be produced through the catalytic hydration of ethene molecules. Industrial chemists regularly pass ethene and water molecules over sulfuric or phosphoric (V) acid catalysts to produce an ethanol product. The following equations show how these chemical reactions can occur:
The reactions are usually reversible reactions, but they can be repeated again and again until the vast majority of the reactant ethene molecules have been converted into an ethanol product.
Water can similarly be combined with longer-chain alkene molecules to make relatively large alcohol molecules. Markovnikov’s rule can be used to understand how water molecules and other hydrogen halide molecules add across the carbon–carbon double bond of longer-chain alkene molecules. Markovnikov’s rule states that the hydrogen atom of the or molecules adds to the carbon atom of the alkene molecule that has the greater number of covalently bonded hydrogen atoms. This means that 2-methylbut-2-ene molecules would produce 2-methylbutan-2-ol molecules if they were reacted with water molecules. The reaction between 2-methylbut-2-ene and water molecules is shown in the following diagram:
Example 3: Understanding How Ethanol Can Be Produced through the Fermentation of Glucose Sugars or the Catalytic Hydration of Ethene Molecules
The given reactions show two different methods for the preparation of ethanol:
- What name is given to the process shown in method 1?
- What name is given to the process shown in method 2?
- Catalytic hydration
Fermentation describes the process whereby yeasts convert small sugars such as glucose to ethanol and carbon dioxide. This statement can be used to determine that option C is the correct answer for the first part of this question.
Catalytic hydration describes the process whereby ethene molecules are combined with steam in the presence of a catalyst to produce ethanol. This statement can be used to determine that option A is the correct answer for the second part of this question.
Alcohol substances can also be produced through substitution reactions as halogenoalkane (alkyl halide) molecules are reacted with an aqueous solution of sodium or potassium hydroxide. The hydroxide ions interact with the halogenoalkane molecules and new alcohol products are produced as the carbon–halogen bond breaks through fission processes. The following equation shows how an alcohol product can be produced by reacting a halogenoalkane with an aqueous solution of potassium hydroxide: Note that is the alkyl radical and is the halide radical.
Halogenoalkanes are compounds that contain a halogen atom covalently bonded to an alkyl group.
Bromoethane (ethyl bromide) and 2-bromopropane can be heated under reflux with a solution of sodium or potassium hydroxide to produce alcohol products. The following equation shows how bromoethane molecules can be reacted with an aqueous solution of potassium hydroxide to produce an ethanol product, the ethanol product can be classed as a primary alcohol because it can be described with the general formula where is an alkyl group:
The following equation shows how 2-bromopropane molecules can be reacted with an aqueous solution of potassium hydroxide to make a propan-2-ol product; the propan-2-ol product can be classed as a secondary alcohol because it can be described with the general formula where is an alkyl group:
The next figure shows how 2-bromo-2-methylpropane can similarly be reacted with an aqueous solution of potassium hydroxide to make a 2-methylpropan-2-ol product. The 2-methylpropan-2-ol product can be classed as a tertiary alcohol because it can be described with the general formula where is an alkyl group:
The rate of the substitution reactions depends on both the polarity and strength of the carbon–halogen bond. The substitution reactions should occur most rapidly if the carbon–halogen bond is highly polar and if the carbon–halogen bond is very weak and easy to break. The carbon–halogen bond cannot, however, be both highly polar and very weak and easy to break. The carbon–halogen bonds are stronger when they are more polar. The carbon–fluorine bond is the most polar carbon–halogen bond and it is also the strongest carbon–halogen bond. The carbon–iodine bond is the least polar carbon–halogen bond and it is also the weakest carbon–halogen bond. The following figure shows why a carbon–halogen bond cannot be both highly polar and very weak and easy to break.
Chemical experiments have been used to show that the strength of the carbon–halogen bond is significantly more important for determining the rate of alkaline hydrolysis substitution reactions. The halogenoalkane molecules tend to react with hydroxide ions most rapidly if the carbon–halogen bond is relatively weak. The following table compares the strengths of four different carbon–halogen bonds. The bond strengths are shown as average bond energy values. The strongest bonds have the highest average bond energy values. The weakest bonds have the lowest average bond energy values.
|Bond||Average Bond Energy (kJ/mol)|
The carbon–halogen bond strengths are closely related to the carbon–halogen bond lengths. The carbon–halogen bonds tend to be weaker when the halogen atoms are larger and the carbon–halogen bonds are longer. The following figure shows the relationships between the size of the halogen atoms and the carbon–halogen bond length and bond strength values.
Iodoalkane molecules tend to react with hydroxide ions more quickly than either bromoalkane or chloroalkane molecules because the carbon–iodine bond is weaker than the carbon–bromine or carbon–chlorine bond. Fluoroalkane molecules tend to react very slowly and ineffectively with hydroxide ions because the carbon–fluorine bond is very strong and it is very difficult to break.
Example 4: Identifying the Ideal Halogenoalkane (Alkyl Halide) for Producing Alcohol Molecules through Alkaline Hydrolysis Reactions
Alcohols are prepared in the laboratory by the hydrolysis of alkyl halides in a strong alkali, like potassium hydroxide . Which of the following alkyl halides is the best choice for this reaction?
- Alkyl bromide
- Alkyl iodide
- Alkyl chloride
- Alkyl fluoride
Halogenoalkane (alkyl halide) molecules can be reacted with an aqueous solution of sodium or potassium hydroxide to make an alcohol product. New alcohol products are produced as hydroxide ions interact with the halogenoalkane molecules and the carbon–halogen bond breaks through fission processes. The rate of the substitution reactions depends on the strength of the carbon–halogen bond. The halogenoalkane molecules tend to react with the hydroxide ions more rapidly if the carbon–halogen bond is relatively weak. The carbon–iodine bond is weaker than carbon–bromine , carbon–chlorine , and carbon–fluoride bonds. The alkyl iodide molecule will react with an aqueous solution of potassium hydroxide more rapidly than the other listed options because it contains the weakest carbon–halogen bond. This line of reasoning can be used to determine that option B is the correct answer for this question.
The alcohols are indeed an important group of compounds that have many commercial and industrial applications. Methanol is an important chemical feedstock and it is also used in automotive antifreeze substances, in rocket fuels, and as a polar solvent. Methanol is additionally used as a fuel for monster trucks and USAC sprint cars because it does not produce an opaque cloud of smoke when cars and vehicles crash on the racetrack. The following equation describes the chemical equation for the complete combustion of methanol:
It is important to realize, however, that methanol intoxication can cause severe visual dysfunction and death. Small amounts of ingested methanol can destroy parts of the central nervous system, and this can cause permanent neurological dysfunction and irreversible blindness.
Ethanol is used to produce alcoholic beverages and is used as a general-purpose antiseptic and disinfectant. It is also used as a chemical solvent and as an additive for automotive gasoline. Ethanol has a relatively high octane rating and it can be blended with other fuels to reach minimum octane number requirements that are set by national automotive regulatory bodies. We might see an increase in the use of ethanol fuel in the near future. Ethanol fuels have lower carbon footprints than conventional crude oil distillation products and an increase in the use of low carbon footprint fuels could help us decelerate global warming. Ethanol is also used to make special thermometers partly because it is less toxic than mercury but also because it has a freezing point of and a boiling point of .
It is important to state here that the excessive consumption of ethanol can be extremely hazardous and it can cause irreparable damage to the human body. Alcoholics are prone to developing multiple different types of cancers and their livers tend to be scarred and damaged. The excessive consumption of alcoholic beverages has also been associated with the development of hypertension, heart disease, stroke, and digestive problems.
Example 5: Identifying What Products Are Produced during the Complete Combustion of Methanol
Methanol has been suggested as an alternative fuel for internal combustion engines. What products are formed from the complete combustion of methanol in excess oxygen?
Methanol molecules produce carbon dioxide and water when they completely combust in a plentiful supply of oxygen. Carbon dioxide molecules have the chemical formula and water molecules have the chemical formula. We can use these statements to determine that option E is the correct answer because it shows the chemical formula of carbon dioxide and water molecules.
- Alcohols contain at least one hydroxy group .
- Alcohols are able to form hydrogen bonds.
- Alcohols are more soluble in water than comparable alkanes and they also have higher boiling points and are less volatile.
- The IUPAC developed nomenclature for classifying primary, secondary, and tertiary alcohols.
- Ethanol can be produced through fermentation.
- Alcohols can be produced through the catalytic hydration of alkenes.
- Alcohols can be prepared in the laboratory by reacting alkyl halides with water or hydroxide ions.
- Ethanol and methanol have many important commercial and industrial applications.