Alcohol is a word that is familiar to both organic chemists and the average person. In this article, we will focus on the organic chemistry usage of the word. You will learn the structure of alcohol, its classification, its nomenclature, and some of its reactions. You will also get to explore several examples of alcohols and their real-world uses!
Topics Covered in Other Articles
- Functional Groups in Organic Chemistry
- Nucleophile – What is it?
- Sn1 Reaction
- Sn2 Reaction
- E1 Reaction
- E2 Reaction
Alcohols are compounds with a hydroxyl group (OH group) bonded to an sp3 hybridized carbon atom. That is, the carbon with the OH bonds to three other things that are either hydrogen atoms, alkyl groups, or both. This makes alcohol different from carboxylic acid, another common hydroxyl-containing functional group because, in carboxylic acids, the carbon with the OH double bonds to another oxygen atom.
Classification of Alcohols
The classification of alcohols depends on how many alkyl groups are attached to the carbon atom with the OH group. With that in mind, an alcohol can be classified as a primary, secondary, or tertiary alcohol.
In a primary alcohol, the carbon with the OH group has 1 alkyl group attached to it. In a secondary alcohol, the number of attached alkyl groups is 2. And, you guessed it, that number is 3 in tertiary alcohols.
By the way, chemists count methanol as a primary alcohol despite it having no alkyl groups attaching to the carbon with the OH.
Phenol is a special kind of alcohol. The name “phenol” refers both to the family of molecules with an OH group attached to an aromatic ring and the simplest member of that family, C6H5OH. Phenols have a number of characteristics that are very different from normal alcohols. For example, thanks to their ability to form resonance structures, phenols are way more acidic than normal alcohols. Thus, the reactivity of phenols is different from normal alcohols, allowing them to participate in reactions that normal alcohols typically don’t. It is for this reason that we will not spend too much focus on phenols in this article.
Quick Facts on Alcohols
- Structure: A carbon single bonded to an oxygen that’s single bonded to a hydrogen
- General formula: R-OH or ROH
- pH: Neutral (around 7). Alcohols are generally weak acids whose most acidic proton is the H of the OH.
- pKa: Generally from 15 to 20
- Methanol (CH3OH): 15.3
- Ethanol (C2H5OH): 15.9
- Phenol (C6H5OH): 9.99
- Solubility: The solubility of an alcohol in polar solvents decreases as the size of the hydrophobic region (typically the hydrocarbon chain) of that alcohol increases. For example, methanol and ethanol are miscible in water, while nonanol is insoluble in water.
- IR Spectroscopy: A concentrated alcohol solution produces a broad signal around 3200 – 3600 cm-1
To name an alcohol using IUPAC nomenclature, follow these steps:
- First, identify the parent, which is the longest chain that contains the OH group.
- Next, assign a number to the OH group and other substituents (if there are any) to indicate their position within the molecule. The OH group should get the smallest number possible.
- Finally, change the suffix “-e” of the parent alkane into “-ol”.
If a molecule has multiple OH groups, insert prefixes like “di” or “tri” before “-ol”. In such cases, keep the “-e” of the parent alkane.
If the alcohol part (more specifically, the hydroxyl group) is just a substituent of a molecule (i.e., there exist higher priority functional groups), then we put the prefix “hydroxy-” in front of the parent name of that molecule.
General Reaction Trends
The reactivity hotspot in alcohols is the OH group. You will see many reactions that involve removing the whole OH group or just the H. The high electronegativity of oxygen makes the C and H bonded to it electrophilic and thus reactive to electron-rich molecules.
But before jumping into the reactions of alcohol, let’s see how one can synthesize alcohol first.
Preparation of Alcohols
Alkenes can undergo hydration to produce alcohols. You can read more about these hydration reactions in our article on alkenes. One such example of an addition that produces an alcohol is the hydroboration oxidation.
We can reduce aldehydes or ketones into alcohols with a reducing agent like sodium borohydride (NaBH4) or lithium aluminum hydride (LAH). Moreover, the reactivity of LAH also allows it to reduce carboxylic acids and esters into alcohols. These reducing agents act as proton sources, giving their protons to the reactant. Contrarily, ketones, aldehydes, and carboxylic are produced by oxidizing alcohols.
Grignard reagents are alkyl halides that are treated with magnesium. A Grignard reagent has a nucleophilic carbon that can perform attacks on various electrophiles and thus is useful in building carbon skeletons. In our case, aldehydes, ketones, and esters can all be attacked by the Grignard reagent and undergo subsequent protonation to produce alcohols.
Reactions of Alcohols
Alcohols can undergo substitution reactions to form alkyl halides. Tertiary alcohols only undergo SN1 reactions; secondary alcohols can undergo SN1 reactions but with a slow rate and thus generally prefer to undergo SN2 reactions; primary alcohols only undergo SN2 reactions.
It’s important to note that since OH is a bad leaving group, the conversion of OH into a better leaving group is a required step in the mechanism of every alcohol substitution reaction.
Secondary and tertiary alcohols can undergo an E1 reaction to form alkenes under acidic conditions. Since this reaction also removes a water molecule, chemists also call it a “dehydration reaction”. Primary alcohols can also undergo a dehydration reaction via an E2 mechanism, but with a much slower rate than secondary and tertiary alcohols.
Primary alcohols can undergo oxidation to produce aldehydes or carboxylic acids. You can control the product by carefully choosing the oxidizing agent. A strong oxidizer like chromic acid (H2CrO4) will oxidize the primary alcohol straight into a carboxylic acid, while a milder and more selective oxidizer like pyridinium chlorochromate (abbreviated as PCC) will only oxidize the primary alcohol into an aldehyde.
Secondary alcohols will only produce ketones upon oxidization, and tertiary alcohols will not undergo oxidation at all. (To understand why, recall that the removal of a hydrogen atom is the best way to increase a molecule’s oxidation state. Unfortunately, in tertiary alcohols, the carbon with the OH group isn’t attached to any removable hydrogen atom.)
Alcohols can react with carboxylic acids in the presence of a catalytic acid and heat to produce esters. Chemists call this reaction the Fischer esterification. You can learn more about it in this article: Ester Functional Group and Esterification.
Alcohols can also produce esters when reacting with acid chlorides (also called acyl chlorides) in the presence of neutralizing pyridine.
Examples of Alcohols
- Methanol (or methyl alcohol or wood alcohol) (CH3OH) is most commonly used to make fuels for cars and ships.
- Ethanol (or ethyl alcohol) (C2H5OH) is the alcohol found in alcoholic drinks. Different types of drinks have different ethanol concentrations. For example, whiskey has a higher concentration of ethanol than champagne, and champagne has a higher concentration of ethanol than standard beer.
- 2-propanol (or isopropyl alcohol) ((CH3)2CHOH) is used in cleaning and disinfecting products like detergents and hand sanitizers.
- Ethane-1,2-diol (or ethylene glycol) ((CH2OH)2) is the chief ingredient of antifreeze.
- Geosmin (C12H22O) is a bicyclic alcohol that gives the rain its distinct earthy smell.
- Menthol (C10H20O) is responsible for the minty, refreshing sensation that products like toothpaste, mouthwash, and chewing gum give.
- Cholesterol (C27H46O) is a type of lipid that helps the body make cell membranes, several hormones, bile acid, and vitamin D.