Video Transcript
In this video, we will learn how to identify and name alcohols and describe their physical properties. We will examine a few chemical reactions that produce alcohols and list a few important uses.
Alcohols are compounds that contain a hydroxy group, an oxygen atom bonded to a hydrogen atom. The reactivity and behavior of an alcohol depends largely on the number of hydroxy groups in the molecule and their general position. Alcohols may be monohydric, containing one hydroxy group; dihydric, containing two hydroxy groups; trihydric, containing three hydroxy groups; or polyhydric, containing four or more hydroxy groups. As poly- means many, some sources describe any alcohol with two or more hydroxy groups as polyhydric. Monohydric alcohols may be further classified as primary, secondary, or tertiary depending on the position of the hydroxy group.
The hydroxy group of a primary alcohol is bonded to a carbon atom that has one alkyl substituent. The hydroxy group of a secondary alcohol is bonded to a carbon atom that has two alkyl substituents. And in a tertiary alcohol, the hydroxy group is bonded to a carbon atom that has three alkyl substituents. Now that we can recognize the structure of alcohols, let’s learn how to name alcohol molecules to help us distinguish between them.
Here is a sample alcohol molecule that we would like to give a name. To name an alcohol following the rules established by the International Union of Pure and Applied Chemistry, we begin by naming the longest continuous chain of carbon atoms that contains the hydroxy group. This name consists of a prefix indicating the length of the carbon chain and a suffix that indicates the type of carbon-carbon bonds present.
In the example molecule, the hydroxy group is bonded to a carbon atom that is part of a five-carbon continuous chain. We name this chain pentane: pent- for the five carbon atoms of the chain and -ane for alkane, meaning that the carbon atoms are single bonded to one another. Next, we number the carbon atoms of the chain so that the hydroxy group has the lowest possible numbered position.
Looking at the molecule, we could number the carbon atoms from left to right or from right to left. Numbering the carbon atoms from left to right places the hydroxy group on the fourth carbon atom of the chain. But if we number from right to left, the hydroxy group is placed on the second carbon atom of the chain. This means that for this molecule, we want to number the carbon chain from right to left rather than left to right. This is because numbering the carbon chain from right to left places the hydroxy group on the carbon atom with the lowest possible position number.
We then add the position number to the name followed by the ending -ol, which indicates that the molecule is an alcohol. We separate the name, position number, and alcohol ending with dashes. When there is only one hydroxy group in the molecule, the e in the base chain name is often removed. Finally, we would want to name any additional substituents that appear in the molecule. This molecule does not have any additional substituents, so the final name is pentan-2-ol.
There are a few additional considerations when naming alcohols. A two-carbon saturated alcohol only has one unique hydroxy group position. As such, the position number of the hydroxy group for this molecule does not appear in the name. Thus, this alcohol is given the name ethanol, not ethan-1-ol. Some alcohols have common names that are frequently used in addition to their IUPAC name. Ethanol is commonly known as ethyl alcohol. Propane-2-ol is commonly called isopropyl alcohol. And 2-methylpropan-2-ol goes by the names tert-butanol, tert-butyl-alcohol, or t-butyl-alcohol.
We also know that alcohols may be dihydric or trihydric. These alcohols are named following the same basic rules. But as these molecules contain more than one hydroxy group, we need to include all of the hydroxy position numbers in the name. In addition, the ending -ol is changed to indicate how many hydroxy groups are in the molecule. If there are two hydroxy groups in the molecule, the suffix -diol is used. And if the molecule contains three hydroxy groups, the suffix -triol is used. Thus, the dihydric alcohol shown here is given the name ethane-1,2-diol and is commonly called ethylene glycol. The trihydric alcohol shown is given the name propane-1,2,3-triol and is commonly called glycerol. Notice that when naming diols and triols, the e at the end of the base chain name is not dropped.
Now that we can recognize and name alcohols, let’s take a look at how alcohols are produced. Alkyl halides, also called haloalkanes, undergo a substitution reaction when heated in the presence of a strong base to produce an alcohol and a salt. A substitution reaction is a chemical reaction where a part of a molecule is removed and is replaced by something else. Over the course of this reaction, the halide is removed, and it is replaced by a hydroxy group provided by the strong base. Here is the generic equation for the substitution reaction involving an alkyl halide and a strong base. The halogen represented by an x may be chlorine, bromine, or iodine. And the metal ion of the strong base represented by an M is typically lithium, sodium, or potassium.
When comparing iodoalkanes, bromoalkanes, and chloroalkanes, iodoalkanes tend to undergo substitution more rapidly than comparable bromo- and chloroalkanes. This is because the carbon iodine bond is weaker than a carbon-bromine bond or carbon-chlorine bond. And the iodide anion produced via the substitution reaction is more stable than a bromide or chloride ion. Alcohols can also be produced by reacting an alkene to double-bonded carbon atoms with water in the presence of sulfuric acid. This is an example of a hydration reaction, a chemical reaction in which a substance combines with water.
The alcohol produced via the hydration of an alkene can be predicted using Markownikoff’s rule. Markownikoff’s rule is best understood by using reaction mechanisms and carbon cations but can be simplified to state that the acidic hydrogen atom will add to the carbon of the double bond that has the greatest number of hydrogen substituents. In this reaction, the acidic hydrogen atom is provided by the sulfuric acid. It adds to the carbon of the double bond that has the greatest number of hydrogen substituents. This then allows for the hydroxy group from the water molecule to add to the carbon of the double bond that has the least number of hydrogen substituents. So the hydration of propene will produce propan-2-ol. The hydration of an alkene is used in industry to produce ethanol from ethene, a compound obtained from the thermal cracking of petroleum distillates.
Another industrial method for producing ethanol is alcoholic fermentation, a biological process which converts sugars into cellular energy and produces ethanol and carbon dioxide. In this process, yeast is added to glucose and fructose extracted from sugar cane, beets, or corn. The yeast then converts the sugar molecules into ethanol and carbon dioxide. We’ve looked at how to produce alcohols via substitution of an alkyl halide, hydration of an alkene, and alcoholic fermentation. Now let’s examine some properties of alcohols.
Alcohols tend to have higher boiling points than comparable alkanes. To understand why this tends to be true, let’s examine methane and methanol. The carbon and hydrogen atoms of methane have very similar electronegativity values. This means that the electrons in each bond are equally shared and the molecule is nonpolar. Despite being nonpolar and having no permanent regions of partial positive and negative charge, methane molecules do experience a weak electrostatic attraction to one another. In a molecule of methanol, the oxygen atom is much more electronegative than the hydrogen atom and can attract the shared electrons of the bond more strongly. The greater electron density around the oxygen atom results in the oxygen atom having a partial negative charge, while the hydrogen atom has a partial positive charge.
The separation of charge across a distance means that methanol is polar. The electronegativity difference between an oxygen atom and a hydrogen atom is so large that the partial positively charged hydrogen atom of one methanol molecule can experience a strong electrostatic attraction to the lone pair electrons from an oxygen atom of another methanol molecule. This strong electrostatic attraction is called hydrogen bonding. Methanol will have a higher boiling point than methane because more energy is required to overcome the electrostatic attraction between methanol molecules than the electrostatic attraction between methane molecules.
Alcohols also tend to be more soluble in water than alkanes. Water molecules, like alcohol molecules, have an oxygen atom bonded to a hydrogen atom. Due to the large electronegativity difference between the atoms, the oxygen atom has a large partial negative charge, and the hydrogen atoms have a large partial positive charge. As such, two water molecules exhibit strong hydrogen bonding with one another. When water is added to an alkane like hexane, the two liquids do not mix to form a solution. We know that nonpolar alkane molecules exhibit weak electrostatic attractions with other alkane molecules and that water molecules exhibit strong hydrogen bonding with other water molecules.
Hexane molecules and water molecules do exhibit electrostatic attractions with one another. This electrostatic attraction is stronger than the attraction between two hexane molecules but is weaker than the attraction between two water molecules. As water molecules have a stronger electrostatic attraction with one another than with the hexane molecules, the two substances will not mix. When water is added to an alcohol like methanol, the two liquids do mix to form a solution. This is because the methanol molecules and water molecules can form hydrogen bonds with one another, similar to the intermolecular attractions that each had on its own. It’s worth noting that not all alcohols are soluble in water.
Let’s consider ethanol and hexanol. Both molecules have hydroxy groups that are polar and can hydrogen bond with other alcohol molecules. Both molecules also have a hydrocarbon portion that is nonpolar. Ethanol is soluble in water, but hexanol is considerably less soluble in water and is often described as insoluble in water. This means that the large hydrocarbon portion of hexanol must affect the solubility more than the polar hydroxy group. Thus, small alcohols like methanol and ethanol are soluble in water. But as the length of the carbon chain grows, the alcohol tends to exhibit more nonpolar characteristics, and the solubility of the alcohol in water decreases.
Before we summarize what we’ve learned, let’s take a look at some of the many uses of alcohols. Methanol and ethanol are important organic solvents and reagents for many syntheses. They can also be used as fuel. Methanol is commonly used as a fuel for monster trucks and sprint cars. And ethanol is commonly added to gasoline to increase the octane rating. Ethanol may be found in hand sanitizer as it is a general-purpose antiseptic agent and disinfectant. It is also used to produce alcoholic beverages and is used in thermometers as a safer alternative to mercury. Ethane-1,2-diol or ethylene glycol is used in vehicles in colder countries as an engine coolant, as a mixture of ethylene glycol in water has a lower freezing point than water alone.
Propane-1,2,3-triol, also called glycerol or glycerin, is used as a treatment for burns and wounds and a moisturizer added to skincare products and soaps. Glycerin is also an important reagent for the production of nitroglycerin, a compound used to treat heart problems and manufacture explosives.
In this video, we’ve learned how to identify, name, and prepare alcohols. We’ve also looked at some properties and important uses of alcohols. Let’s summarize these topics with the key points. Alcohols are molecules that contain at least one hydroxy group, an oxygen atom bonded to a hydrogen atom. Alcohols may be classified as primary, secondary, or tertiary. IUPAC rules are used to name alcohols. The name of an alcohol will end in -ol. Alcohols can be produced by reacting a haloalkane with a strong base in a substitution reaction.
In industry, ethanol is produced via hydration of ethene or via alcoholic fermentation of sugars. Alcohols tend to have higher boiling points and be more soluble in water than alkanes due to their ability to form hydrogen bonds. And finally, there are many commercial and industrial uses of alcohols, including use as a solvent, disinfectant, and moisturizer.
