Alcohols

Alcohol definition

Here's ethanol. It is by far the most common and best-known alcohol.

Ethanol. StudySmarter Originals

Notice the oxygen and hydrogen atoms on the right? They form a hydroxyl group, and we represent it with the letters -OH. All molecules with a hydroxyl group are alcohols.

The hydroxyl group

Let's take a closer look at the hydroxyl group, -OH. There are a couple of things to note.

• Firstly, the group is polar. This is because oxygen is more electronegative than hydrogen and attracts the bonded pair of electrons towards itself. This makes oxygen partially negatively charged, and hydrogen partially positively charged.
• Secondly, oxygen has two lone pairs of electrons. These squash the C-O and O-H bonds together. It results in a bond angle of 104.5°, making alcohols v-shaped molecules.

The polarity and bond angle of the hydroxyl group.StudySmarter Originals

Alcohol general formula

Because alcohols share a functional group, they form their own homologous series - a family of compounds with similar chemical properties that can be represented by a general formula.

Alcohols with just one hydroxyl group all have the general formula C_nH_2n+1OH. This is handy, because it means that once we know the number of carbon atoms in an alcohol, we can work out its number of hydrogen atoms. An alcohol with n carbon atoms has 2n+1 hydrogen atoms, plus an extra -OH group. For example, an alcohol with 3 carbon atoms has $2\left(3\right)+1=7$ hydrogen atoms, plus an extra one from the -OH group. In total, it has 3 carbon atoms, 1 oxygen atom, and 8 hydrogen atoms.

Types of alcohol

Alcohols can be classified into three types in chemistry: primary, secondary, or tertiary. An alcohol's classification all to do with the molecule's alpha carbon.

To be more precise, classification involves how many R groups the alpha carbon is attached to, where an R group is a shorthand representation for any other hydrocarbon chain.

Here's how primary, secondary, and tertiary alcohols differ.

• In primary alcohols, the alpha carbon is bonded to zero or one R groups. This means that the alpha carbon, and therefore the hydroxyl group, is always found at the end of the molecule. We show that alcohols are primary alcohols with the symbol 1°.
• In secondary alcohols, the alpha carbon is bonded to two R groups. These can be exactly the same as each other or completely different - it doesn't matter. We show them with the symbol 2°.
• In tertiary alcohols, the alpha carbon is bonded to three R groups. Once again, these can be alike or totally different. We show tertiary alcohols with the symbol 3°.

Primary, secondary, and tertiary alcohols. Note the number of R groups attached to the alpha carbon atom in the different alcohol classifications.StudySmarter Originals

Wondering how we name alcohols? We'll look at that next.

Nomenclature of alcohols

Naming alcohols is pretty simple. We follow all of the usual IUPAC nomenclature rules. Note the following:

• We use a root name to show the length of the molecule's parent chain.
• We use a suffix to show the molecule's highest priority functional group, known as its parent functional group. If the highest priority functional group is the hydroxyl group, we use the suffix -ol. However, if there are higher priority functional groups present, we instead show that the molecule is an alcohol using the prefix hydroxy-.
• We use numbers, called locants, to indicate the position of the hydroxyl group on the carbon chain. Locants also show the position of any substituents or other functional groups present. You can number the carbon chain from either direction, but remember that you firstly want the parent function group to have the lowest possible locant. If both numbering possibilities give the same number locant, you then want to locants of any other substituents or prefixes to add up to the lowest total possible.
• If there are multiple of the same functional group or substituent present, we use quantifiers such as -di- or -tri-.

Let's look at an example. Have a go at naming the alcohol below.

Properties of alcohols

The properties of alcohols are greatly influenced by the polar hydroxyl group. We touched on this earlier, but let's go over it again now.

The hydroxyl group. StudySmarter Originals

As we discovered, the hydroxyl group is polar. This is because oxygen is a lot more electronegative than hydrogen. The oxygen atom pulls the bonded pair of electrons it shares with hydrogen over towards itself, leaving hydrogen with a partial positive charge. Because hydrogen is such a small atom, it has a high charge density. Hydrogen's charge density is so high, in fact, that it is attracted to the lone pairs of electrons on the oxygen atom of an adjacent alcohol molecule. We call this hydrogen bonding. It is a type of intermolecular force that is much stronger than other intermolecular forces (such as van der Waals forces and permanent dipole-dipole forces).

Hydrogen bonding between adjacent hydroxyl groups. StudySmarter Originals

Now we'll explore how hydrogen bonding affects the properties of alcohols.

Melting and boiling points

Alcohols have high melting and boiling points compared to similar alkanes. This is because the hydrogen bonding holding adjacent alcohol molecules together is strong and requires a lot of energy to overcome. In contrast, alkanes are only held together by van der Waals forces. These are a lot weaker than hydrogen bonds and much easier to overcome. This gives alkanes low melting and boiling points.

As with all organic molecules, alcohols follow trends in melting and boiling points:

• As chain length increases, melting and boiling points increase. This is because the molecules experience stronger van der Waals forces.
• As branching increases, melting and boiling points decrease. This is because the molecules can't fit as closely together and so experience weaker van der Waals forces.

Solubility

Short-chain alcohols are soluble in water, whilst long-chain alcohols are insoluble. This is because the alcohol's polar hydroxyl group can also hydrogen bond with water molecules, dissolving the alcohol. However, in long-chain alcohols, the nonpolar hydrocarbon chain gets in the way of the hydrogen bonding and prevents the alcohol from dissolving.

Acidity

Alcohols are slightly acidic. According to the Brønsted-Lowry theory of acids and bases, all molecules which donate protons (H⁺) are acids. The hydroxyl group of alcohols tends to release H⁺ because of its polarity. Due to oxygen being more electronegative than hydrogen, the shared pair of electrons shifts towards oxygen, making the -OH bond weaker and easier to break. The release of H⁺ makes alcohol acidic.

Producing alcohols

We mentioned that alcohols are important stepping stones in synthesis pathways. You can use them to make lots of other organic compounds. But how do we make alcohols themselves?

There are a few different ways. You might already be familiar with some of them, while some will be new.

Synthetic methods of alcohol production

In organic chemistry, it is often helpful to draw big synthesis mind maps that link different organic compounds, showing how you might get from one to the other and what conditions or catalysts you need. We'd highly recommend you make one if you haven't already, and gradually add to it as you learn more and more organic reactions. With such a map, it is easy to see all the different ways of producing a type of organic molecule; alcohols are no exception. Here are some examples of reactions that produce alcohols.

• Hydrating alkenes using steam and a phosphoric acid catalyst. For example, hydrating ethene gives ethanol.
• Reacting halogenoalkanes with the hydroxide ion in a mixture of alcohol and water. For example, mixing bromoethane with potassium hydroxide gives ethanol and potassium bromide. This is an example of a nucleophilic substitution reaction.
• Hydrolysing esters using water and a strong acid catalyst. For example, hydrolysing methyl ethanoate gives methanol and ethanoic acid.
• Reducing carboxylic acids, aldehydes, or ketones using a reducing agent such as LiAlH₄ or NaBH₄. Note that you can reduce aldehydes or ketones using both of these reducing agents, but you can only reduce carboxylic acids using LiAlH₄. For example, reducing propanal produces propanol.

Let's now summarise the reagents, products, and conditons for the chemical reactions by which alcohols can be produced:

Here's a quick version of a synthesis map, showing how you can make alcohols from other organic compounds.

A synthesis map centred on alcohols. StudySmarter Originals

Natural methods of alcohol production

The most common way of making alcohol for industrial purposes is through the hydration of ethene. But if we want to make alcohol for drinks such as wine, beer, or cider, we use a different process: fermentation.

Fermentation involves supplying tiny little yeast cells with plant carbohydrates such as sugar cane or sugar beet. The yeast breaks down the plant matter, converting it into ethanol. Fermentation takes place in anaerobic conditions at around 35°C.

Reactions of alcohols

Alcohols are fairly reactive, thanks to their polar hydroxyl group. They also make great fuels. in this next section, we're going to look at some of the other reactions involving alcohols.

• Alcohols combust in oxygen to produce carbon dioxide and water.
• They can be oxidised by an oxidising agent such as acidified potassium dichromate into aldehydes, ketones, and carboxylic acids. For example, oxidising ethanol produces ethanal, which can be further oxidised into ethanoic acid.
• They can be dehydrated using a strong acid catalyst to produce alkenes. For example, dehydrating propan-1-ol produces propene. This is a type of elimination reaction.
• They react in halogenation reactions to produce a halogenoalkane and water. These reactions take place under varying conditions, depending on the type of alcohol and the halogen used, and are examples of nucleophilic substitution reactions. For example, treating ethanol with sodium bromide and concentrated sulphuric acid produces bromoethane, water, and sodium sulphate.
• They react with carboxylic acids to form an ester in an example of a condensation reaction. For example, heating methanol and ethanoic acid with a strong acid catalyst produces methyl ethanoate and water. This is also known as esterification.

A synthesis map centred on alcohols.StudySmarter Originals

Examples of alcohols including isopropyl alcohol

Finally, let's explore a few further examples of alcohols. Here are some other common alcohols and their uses.

• Methanol, CH₃OH, is a precursor to methanal. This compound is also known as formaldehyde and is used to make paints, cosmetics, and even explosives!
• Ethanol, C₂H₅OH, is well-known for its use in alcoholic drinks but is also a great solvent. You'll find it in cosmetics and cleaning products.
• Ethanol is also used to bulk up fuels. For example, 'gasohol' is an ethanol-petrol mixture containing between 10 and 20 percent ethanol. The ethanol used always comes from fermentation, which makes gasohol a sustainable way to supplement our fuels whilst reducing carbon dioxide emissions.
• Bottles of rubbing alcohol, a powerful disinfectant used for sterilising and cleaning, contains mostly isopropyl, C₃H₇OH. This has the IUPAC name of propan-2-ol. Isopropyl is also used as a detergent, solvent, and anti-freeze.