Alcohols

Ah, alcohol. Found at almost every party and gathering, responsible for feelings of euphoria and elation, it is both loved for its role as a social stimulant and cursed for its harmful effect on the body. In fact, alcohol abuse is the biggest risk factor for death and poor health amongst British people aged 15-49. Despite its dangers, it is our most widely used recreational drug. Statistics show that almost half of adults in the UK drink at least once a week, whilst a quarter drink over the Chief Medical Officer's low-risk guidelines.

However, alcohol is more than just a guilty pleasure you might enjoy on a night out. Alcohols are actually important organic compounds. They're useful stepping stones in synthesis pathways and have many applications, from solvents to fuels. Glucose, the sugar used in respiration and the body's primary source of energy, is an alcohol. In this article, we're going to take a chemistry-based dive into the wonderful world of these organic compounds, alcohols.

• We will be delving into the chemistry of alcohols.
• We'll start by defining alcohol before looking at the different types.
• Next, we'll explore their nomenclature. You will be able to practice naming different alcohols.
• We'll then consider their properties, such as melting and boiling points and solubility.
• After that, we'll look at both producing alcohols and some of their characteristic reactions.
• Finally, we'll take a deep dive into units of alcohol, and the effect of alcohol on the body.

What are alcohols?

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

Ethanol. Anna Brewer, 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.

General formula of alcohols

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 hydrogen atoms, plus an extra one from the -OH group. In total, it has 3 carbon atoms, 1 oxygen atom, and 8 hydrogen atoms.

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.

Types of alcohol

Alcohols can be classified into three types in chemistry.

• Primary
• Secondary
• Tertiary

How do you know which type an alcohol is? It's all to do with the molecule's alpha carbon. The alpha carbon is the carbon atom directly bonded to the hydroxyl group. 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.

Primary alcohols

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°.

Ethanol is a good example of a primary alcohol. It is a primary alcohol because its alpha carbon is only bonded to one R group - in this case, a methyl group.

Methanol is another example. Notice how it contains just one carbon atom, which must therefore be the alpha carbon. Its carbon atom isn't bonded to any R groups. This makes methanol a primary alcohol too.

Primary alcohols. Anna Brewer, StudySmarter Originals

Secondary alcohols

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°.

For example, propan-2-ol is a secondary alcohol. Its alpha carbon is bonded to two R groups. In this case, they are both methyl groups.

Secondary alcohols. Anna Brewer, StudySmarter Originals

Tertiary alcohols

As you can probably guess, in tertiary alcohols the alpha carbon is bonded to three R groups. Once again, these can be alike or totally different.

For example, look at 2-methylpropan-2-ol. Its alpha carbon is bonded to three R groups. As before, they are all methyl groups.

Tertiary alcohols. Anna Brewer, StudySmarter Originals

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

Naming 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 longest carbon chain.
• We use the suffix -ol to show that the molecule is an alcohol. However, if there are other functional groups present, we instead use 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 side chains or other functional groups present. You can number the carbon chain from either direction, but remember that you want the locants to add up to the lowest total possible.
• If there are multiple of the same functional group present, we use quantifiers such as -di- or -tri-.

Let's look at some examples. Have a go at naming the alcohol below.

An unknown alcohol to be named. Anna Brewer, StudySmarter Originals

Its longest carbon chain is four carbon atoms long, giving it the root name -but-. It has a hydroxyl group and a chlorine atom, so we'll need the suffix -ol and the prefix chloro-. Numbering the carbons from the left, the hydroxyl group is found on carbon 2 and the chlorine group is found on carbon 4. Numbering from the right, the hydroxyl group is found on carbon 3 and the chlorine group is found on carbon 1. Numbering from the right gives us a lower total than numbering from the left, so in this case we number the carbons from the right. Putting that all together, we get 1-chlorobutan-3-ol.

Naming an unknown alcohol. Anna Brewer, StudySmarter Originals

How about this next alcohol?

A second unknown alcohol. Anna Brewer, StudySmarter Originals

Its longest carbon chain is three atoms long, giving it the root name -prop-. It contains two hydroxyl groups and one methyl group, giving it the suffix -ol and the prefix methyl-. But because it contains two hydroxyl groups, we need to use the quantifier -di- before the suffix. It doesn't matter which end of the molecule we number the carbons from - in both cases, the hydroxyl groups are found on carbons 1 and 3 and the methyl group is found on carbon 2. This molecule is therefore 2-methylpropan-1,3-diol.

Naming a second unknown alcohol. Anna Brewer, StudySmarter Originals

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. Anna Brewer, 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. Anna Brewer, 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.

Producing alcohols

We mentioned that alcohols are important stepping stones in synthesis pathways. You can use them to make lots of other different 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. With such a map, it is easy to see all the different ways of producing a type of organic molecule; alcohols are no exception.

• 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.

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. Anna Brewer, 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 with hydrogen halides to produce a halogenoalkane and water. These reactions take place under varying conditions, depending on the type of alcohol and the halogen used. For example, treating ethanol with sodium bromide and concentrated sulphuric acid produces bromoethane, water, and sodium sulphate. This is an example of a nucleophilic substitution reaction.
• 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. Anna Brewer, 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.