Part of the syllabus for A level organic chemistry is the study of alcohols. When you come to revise this group of organic substances, it’s crucial to learn about their general structures, functional groups, chemical properties, physical properties, and reactions. Continue reading for an overview of each of these topics so that you can enter the classroom more prepared.
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What You Need to Know About Alcohols
By definition, an alcohol is an organic or carbon-based substance that has at least one hydroxyl functional group (-OH), which is chemically bonded to an alkyl substituent. Therefore, the names of alcohols are often based on the name of the alkyl substituents. For example, the simplest alcohol is called methanol because of the methane substituent.
These are the key points that you need to understand about alcohols:
- The alcohol is defined by the hydroxyl functional group that’s attached to an alkyl substituent, which is a saturated hydrocarbon derivative.
- Depending on how many alkyl groups are attached to the carbon with the hydroxyl functional group, an alcohol can be classified as either primary, secondary, or tertiary.
- Not all organic compounds that have a hydroxyl functional group can be classified as alcohols.
- The hydroxyl functional group has strong polarisation towards the oxygen atom because the electron density is concentrated near the oxygen.
- The hydroxyl functional groups are the ones responsible for increasing the boiling points of alcohols.
- The polarity of the hydroxyl group has a direct effect on the solubility of alcohol in water, despite the nonpolar nature of the alkyl substituents.
- When dissolved in water, alcohols become slightly less acidic compared to water.
General Formula and Structure of Alcohols
Alcohols are classified as a group because of the hydroxyl functional group. The specific type of alcohol is identified based on the alkyl substituent, which is also referred to as the R chain. This R chain is just a hydrocarbon chain that varies in terms of the number of carbon atoms linked together with corresponding hydrogens.
The general formula for alcohol is very similar to the general formula of alkanes or saturated hydrocarbons – but with one key difference, which is the hydroxyl group. It can be written as:
Compare that to the general formula of alkane, which is CnH2n+2 . As you can see, one hydrogen is replaced by the hydroxyl group. In terms of molecular structure, alcohols have three main forms: primary, secondary, and tertiary. The form is determined by the number of R chains attached to the carbon with the hydroxyl group, also known as the alpha carbon. See the illustration below:
As you may recall, a carbon atom has four valence electrons. This means that it can bond with four other elements or groups of atoms, such as hydrocarbon chains, to become chemically stable based on the octet rule.
- Primary alcohols: Only one R group is bonded to the alpha carbon. Some examples of this include ethanol, propanol, and butanol. Methanol is an exception because it only has one carbon for its R group, which is directly attached to the -OH functional group.
- Secondary alcohols: Two hydrogens are replaced by R groups. Examples include 2-propanol and 2-butanol.
- Tertiary alcohols: Three R groups are attached to the alpha carbon. An example of this is tert-butanol, more formally known as 2-methyl-2-propanol.
Nomenclature of Alcohols
Similar to other chemicals, many alcohols have common and formal names. For example, the formal name for propanol is propan-1-ol. The IUPAC nomenclature standard is the formal standard for naming alcohols.
The IUPAC system for naming alcohols uses the -ol suffix together with the name of the parent alkane but dropping the “e”. At the same time, a number is used to indicate the location of the -OH group. The systematic way of naming alcohols can be summarised in three steps.
- Step 1: Identify the longest carbon chain that has the hydroxyl functional group. Drop the final -e in the name of the alkane then add the suffix -ol.
- Step 2: Count the number of carbons in the longest chain, starting from the nearest carbon to the -OH group. Use the appropriate number to indicate the location of the hydroxyl group.
- Step 3: Finally, identify the substituents, and assign the appropriate number corresponding to their locations.
Physical Properties of Alcohols
Alcohols are polar despite having saturated hydrocarbon chains because of the hydroxyl group. The slightly unbalanced electron density of the functional group makes it polar. The oxygen in the functional group is more negatively charged compared to the rest of the chain while the hydrogen in the functional group is more positive.
The polarity of alcohol allows the alcohol molecules to form hydrogen bonds with each other and with many other molecules of polar compounds. Being a polar substance, alcohol is highly soluble or miscible in water.
Alcohols have hydrogen bonds that tend to make their boiling points higher than the parent alkanes. Below is a table showing the comparative boiling points of water and some alcohols. Similar to alkanes, their boiling points increase as the molecule becomes larger.
You can compare the alcohol boiling points with the boiling points of the parent hydrocarbons, as shown in the table below:
Alcohols are volatile and flammable liquids. Therefore, they can be used as fuel, not only for alcohol lamps but also for engines. Aside from the most common reaction that we know about, which is combustion, alcohols can also undergo other types of chemical reactions. Here is a summary of the different types of chemical reactions that alcohols can undergo:
- Deprotonation: Aqueous solutions of alcohols are slightly weaker acids than pure water. Therefore, they can react with strong base solutions like sodium hydride. As a result, alcohols can become proton (hydrogen ion) donors. The reactions of alcohols with strong bases produce salts known as alkoxides.
- Nucleophilic substitution: This reaction takes place after the oxygen in the hydroxyl group is first protonated. Water becomes a stable leaving group. The reactions of tertiary alcohols with hydrochloric acid produce alkyl halides. The hydroxyl group is replaced by chlorine.
- Dehydration: The unbonded lone pairs of electrons of oxygen in the hydroxyl group make alcohol a weak base in the presence of strong acids like sulphuric acid. Treating it with acid can convert alcohols into alkenes through dehydration. Strong acids act as catalysts in the dehydration reaction.
- Protonolysis: This involves proton transfer. This type of reaction typically occurs in tertiary alcohols when they react with strong acids, which produces carbocations. This reaction is related to alcohol dehydration.
- Esterification: Alcohols can react with carboxylic acids to form esters. The reaction is called Fischer esterification. It typically involves catalysts.
- Combustion: The complete combustion of alcohol produces water and carbon dioxide as byproducts, along with heat and light. This is a rapid chemical reaction and the most common type of alcohol reaction. See the example below:
- Oxidation: Alcohol oxidation involves an oxidiser, otherwise known as an electron acceptor. It commonly involves a catalyst but may not necessarily need an energy input. Combustion is a type of oxidation reaction. The oxidation of alcohol can produce ketones, aldehydes, and carboxylic acid.
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