In this explainer, we will learn how to describe various reactions of alcohols and predict what products are formed.
Alcohols are a class of organic compounds that contain an (hydroxy) group:
Alcohols may be classified as primary , secondary , or tertiary depending on the position of the hydroxy group. The position of the hydroxy group will affect the reactivity of the molecule:
Alcohols are very important compounds because they can be easily made and easily converted into many other functional groups. For this reason, alcohols are often intermediates in large organic syntheses.
When an alkene is reacted with water in the presence of sulfuric acid, an alcohol can be formed. A hydrogen atom from the sulfuric acid can add to the carbon of the double bond that has the most hydrogen atoms. Then, a hydroxy group from the water will add to the other carbon of the double bond as predicted by Markovnikov’s rule. This is the hydration reaction of an alkene:
Definition: Hydration Reaction
A hydration reaction is a chemical reaction where water is reacted with a compound.
Reaction: Acid-Catalyzed Hydration of an Alkene
Hydration of an alkene is in equilibrium with dehydration of secondary and tertiary alcohols.
Definition: Dehydration Reaction
A dehydration reaction is a chemical reaction that involves the loss of water from a compound.
Reaction: Dehydration of an Alcohol
The direction of the equilibrium can be controlled by closely monitoring the temperature:
Low concentration of acid and excess water will shift the reaction to the right. If dehydration is preferred, water can be distilled as it is produced to shift the reaction to the left.
Primary alcohols can be dehydrated with sulfuric acid, but primary alcohols cannot be produced by hydration of an alkene, with the exception of ethanol. This means that dehydration of a primary alcohol is not in equilibrium with hydration of an alkene. Even so, the temperature of the reaction should be closely monitored. At temperatures around , dehydration occurs as expected:
However, at temperatures around , two primary alcohol molecules can react with one another via substitution to form a symmetrical ether:
Reaction: Acid-Catalyzed Dehydration of Primary Alcohol, to Ether
Example 1: Reaction of Ethanol with Concentrated Sulfuric Acid
Excess ethanol was reacted with concentrated sulfuric acid at to produce a product with the molecular formula . The product was found not to be an alkene or an alcohol. What class of molecule might the product be?
Ethanol has the chemical formula and the following displayed formula:
The position of the hydroxy group in a molecule of ethanol is primary . Primary alcohols can produce two different products when dehydrated with concentrated sulfuric acid, depending on the temperature of the reaction.
At temperatures around , ethanol can be dehydrated to produce ethene and water:
At temperatures around , two ethanol molecules will react with one another to produce diethyl ether and water:
The temperature of the reaction in the question was and the product was not an alkene. Therefore, it is likely that the second dehydration reaction occurred. Furthermore, the product has the chemical formula . This is the chemical formula of diethyl ether. Thus, the class of molecule of the product is an ether.
All alcohols can react with hydrohalic acids (, , and ) via a substitution reaction. A substitution reaction is a reaction where one functional group is replaced by another.
Definition: Substitution Reaction
A substitution reaction is a type of reaction where a part of a molecule is removed and replaced with something else.
In this reaction, the hydroxy group of the alcohol is replaced by the halogen of the hydrohalic acid producing a haloalkane and water:
Zinc chloride () is frequently added as a catalyst when hydrochloric acid is used for this reaction.
Reaction: Substitution of an Alcohol with a Halogen
Example 2: Reaction of a Tertiary Alcohol with Hydrochloric Acid
Consider the following reaction:
What product is formed from this reaction?
The organic reactant shown is 1-methylcyclohexanol, a tertiary alcohol. Tertiary alcohols, like primary and secondary alcohols, react with hydrohalic acids via substitution. In a substitution reaction, one functional group is replaced by another. For the reaction of an alcohol with a hydrohalic acid, the hydroxyl functional group is replaced by a halogen.
The reaction shown uses in the presence of , a catalyst. The halogen, chlorine, will replace the hydroxy group. No other changes will occur in the structure:
The product formed from this reaction is 1-chloro-1-methylcyclohexane, answer choice C.
Primary and secondary alcohols can be oxidized to compounds containing carbonyl groups () via an oxidation reaction.
Definition: Oxidation Reaction
An oxidation reaction is a type of reaction where the central carbon of a functional group has an increase in oxidation number. This often occurs in conjunction with an increase in the number of bonds between a carbon atom and oxygen atom.
These reactions require an oxidizing agent such as acidified potassium dichromate () or potassium permanganate (). Acidified potassium dichromate is also known as Jones reagent and is prepared by dissolving potassium dichromate in aqueous sulfuric acid to produce chromic acid ().
During oxidation, the hydroxy group and the carbon atom attached to the hydroxy group will each lose a hydrogen atom. To account for the loss of bonds, the oxygen of the hydroxy group will become double bonded to the carbon.
The functional group formed via oxidation depends on the type of alcohol and the reaction conditions used.
When acidified potassium dichromate is added to a secondary alcohol, the chromic acid in solution oxidizes the alcohol to a ketone and water is produced. In this process, the chromium (VI) in the chromic acid is reduced to chromium (IV):
The chromium (IV) undergoes further reactions in solution to reduce to chromium (III). The reaction progress can be observed as the orange chromium (VI) is reduced to green chromium (III).
Potassium permanganate has a violet color. When it is used in the oxidizing agent, the manganese is reduced, and the violet color disappears.
In the generic oxidation reaction equation, we can use the symbol to represent an oxygen from an oxidizing agent.
Reaction: Oxidation of a Secondary Alcohol, to a Ketone
Primary alcohols can also be oxidized using acidified potassium dichromate to form an aldehyde:
However, aldehydes are also easily oxidized. The remaining chromic acid in solution can oxidize the newly formed aldehyde into a carboxylic acid:
We can attempt to control the product of primary alcohol oxidation by adjusting the reaction conditions. When the reaction is performed with excess acidified potassium dichromate and is heated under reflux, the primary alcohol will be fully oxidized to a carboxylic acid.
Reaction: Oxidation of a Primary Alcohol, to a Carboxylic Acid
To prevent full oxidation and obtain the aldehyde, excess alcohol should be used, and the aldehyde should be distilled as it is produced. It should be noted that there are also alternatives to Jones reagent in practical situations, such as pyridinium chlorochromate (PCC), which cannot fully oxidize the alcohol forming only the aldehyde.
Reaction: Oxidation of a Primary Alcohol, to an Aldehyde
Potassium permanganate rapidly oxidizes aldehydes. If potassium permanganate is added to a primary alcohol, the alcohol will be fully oxidized to a carboxylic acid and the aldehyde will not be obtained.
Tertiary alcohols cannot be oxidized via these methods without breaking a carbon–carbon bond. This is because oxidation proceeds by removing a hydrogen atom from the carbon atom that the hydroxy group is attached to. In a tertiary alcohol, the hydroxy group is attached to a carbon atom that is not attached to any hydrogen atoms.
Example 3: Determining the Structure of an Alcohol from the Product of Oxidation
The following product results from the oxidation of an alcohol:
- Was the alcohol primary, secondary, or tertiary?
- Which of the following alcohols could the reactant be?
The product shown is 2-pentanone, a ketone:
Ketones can be formed via the oxidation of secondary alcohols. The correct answer is answer choice B.
In part 1, we identified the alcohol used in the oxidation to be secondary. Answer choice A is a primary alcohol and choice B is a tertiary alcohol. Choices C and D are secondary alcohols.
When a secondary alcohol oxidizes to a ketone, the position of the oxygen and carbon atoms remains unchanged. Therefore, if the product has a chain of five carbon atoms with an oxygen atom attached to the second carbon atom of the chain, the reactant must have the same basic structure:
Both choices C and D are molecules that have five carbon atoms in a chain. Answer choice C has an oxygen atom attached to the third carbon atom of the chain, while answer choice D has an oxygen atom attached to the second carbon atom of the chain:
Answer choice D has the same carbon and oxygen atom positions as the product. The correct answer is answer choice D.
Alcohols are both weak acids and weak bases. When an alcohol donates a proton, the conjugate base, an alkoxide, also called an alkoxide ion, is formed:
Alkoxides are important reagents in many organic syntheses. They can be created by reacting an alcohol with a strong base, such as sodium amide ():
Alkoxides can also be formed by adding sodium metal to an alcohol. An oxidation–reduction reaction occurs and hydrogen gas () is liberated:
This reaction can also be performed with lithium or potassium.
Reaction: Reaction of Alcohol with Active metal to Form Metal Alkoxide
Example 4: Determining the Products of the Reaction of Ethanol and Sodium Metal
Which of the following is true upon the reaction of ethanol with sodium metal?
- Formation of sodium ethoxide with the evolution of hydrogen gas
- Formation of sodium ethoxide with the evolution of carbon dioxide gas
- Formation of sodium acetate with the evolution of hydrogen gas
- Formation of sodium acetate with the evolution of carbon dioxide gas
- Formation of sodium ethoxide with the evolution of carbon monoxide gas
Ethanol, a primary alcohol, has the chemical formula and the following displayed formula:
When an alcohol is reacted with sodium metal, hydrogen is removed from the hydroxy group, the metal is oxidized, and an alkoxide, the conjugate base of an alcohol, is formed. When this process occurs to two alcohol molecules, the removed hydrogen atoms can combine to form hydrogen gas () that bubbles off of the reaction:
The metal alkoxide formed is named by replacing the word metal with the metal used in the reaction and the prefix alk- with the prefix for the number of carbon atoms attached to the group. Thus, the metal alkoxide formed when sodium reacts with ethanol will be sodium ethoxide:
The statement that is true is answer choice A.
The reaction map below summarizes the alcohol reactions we have learned.
Example 5: Reactions of Propan-2-ol
The reaction scheme below shows various reactions a molecule of propan-2-ol can undergo:
- Which product would be a ketone?
- What name would product C have?
- How many different positional isomers would result from the reaction to form product A?
- 1 isomer
- 2 isomers
- 4 isomers
- 3 isomers
Propan-2-ol is a secondary alcohol. When secondary alcohols react with hydrochloric acid in the presence of zinc chloride, a substitution reaction occurs, and chloroalkane is produced. When secondary alcohols react with concentrated sulfuric acid at temperatures greater than , a dehydration reaction occurs, and an alkene is produced. When secondary alcohols react with acidified potassium dichromate in reflux, an oxidation reaction occurs, and a ketone is produced. Product B will be a ketone.
In part 1, we established that secondary alcohols react with hydrochloric acid in the presence of zinc chloride via a substitution reaction to produce a chloroalkane. In this substitution reaction, the hydroxy functional group of the alcohol is replaced by chlorine from the hydrochloric acid:
The name of the product from this reaction is 2-chloropropane, answer choice B.
When secondary alcohols are reacted with concentrated sulfuric acid, a dehydration reaction occurs that produces an alkene and water. This reaction is in equilibrium, and at temperatures above , the equilibrium can be shifted toward the alkene:
When a molecule containing an alcohol is dehydrated, the molecule will lose the hydroxy group and an atom of hydrogen attached to a carbon atom two positions away from the hydroxy group. A carbon–carbon double bond will be formed in between the carbon that lost the hydrogen atom and the carbon that lost the hydroxy group.
In a molecule of propan-2-ol, there are six hydrogen atoms that could be removed during dehydration. These hydrogen atoms are circled in the figure below:
If any one of the hydrogen atoms on the carbon labeled with 1 is lost during dehydration, then the following product will be produced:
This compound is the alkene, propene.
If any one of the hydrogen atoms on the carbon labeled 3 is lost during dehydration, then the following product will be produced:
This compound is also propene. Therefore, the product of the dehydration of propan-2-ol will be propene regardless of which of the six possible hydrogen atoms are removed during the reaction. Thus, the correct answer is answer choice A: only one positional isomer is formed from this reaction.
- Primary, secondary, and tertiary alcohols can be dehydrated with sulfuric acid to produce alkenes:
- In the presence of sulfuric acid, primary alcohols may react to form ethers:
- All alcohols can undergo substitution with hydrohalic acids to produce haloalkanes:
- Secondary alcohols react with oxidizing agents to produce ketones:
- Primary alcohols react with oxidizing agents to produce aldehydes or carboxylic acids, depending on the reaction conditions:
- Alcohols are deprotonated into alkoxides when reacted with sodium metal: