Alcohol - the life of a party, the culprit behind our wild nights and hangovers. But it's not just a recreational drug, it's also an important compound in chemistry. Alcohols are organic compounds with many applications, from solvents to fuels. Did you know that glucose, the sugar we use for energy, is actually an alcohol?
Despite its usefulness, alcohol abuse is the biggest risk factor for poor health and death among young British people. Shockingly, almost half of adults in the UK drink weekly and a quarter exceed the Chief Medical Officer's low-risk guidelines.
In this article, we're going to dive into the chemistry of alcohols. We'll start by defining what an alcohol is and exploring the different types. You'll also get to practice naming them! We'll then look at their properties like melting, boiling points and solubility. We'll also examine how they're produced and some of their characteristic reactions. Lastly, we'll discuss units of alcohol and the impact of excessive drinking on the body. So let's get started on this fascinating and informative journey into the world of alcohols!
Alcohols are organic compounds containing one or more hydroxyl groups, -OH. Here's ethanol. It is by far the most common and best-known alcohol.
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.
Alcohols belong to a homologous series - which is a group of compounds with similar chemical properties that share a functional group and can be represented by a general formula. A general formula shows the basic ratio of atoms of each element in a compound and can be applied to the whole homologous series.
For alcohols with one hydroxyl group, the general formula is CnH2n+1OH. This 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 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.
We don't show the general formula as CnH2n+2O because separating the -OH hydroxyl group out helps to identify that these compounds are alcohols, and not any other type of organic molecule. To learn more about homologous series and other features of organic compounds, check out our Organic Compounds section.
Let's take a closer look at the hydroxyl group, -OH. Firstly, this group is polar because oxygen is more electronegative than hydrogen, attracting the bonded pair of electrons towards itself. This creates a partially negative charge on the oxygen atom and a partially positive charge on the hydrogen atom. Secondly, oxygen has two lone pairs of electrons, which compress the C-O and O-H bonds together. This results in a bond angle of 104.5°, making alcohols v-shaped molecules. To learn more about electronegativity, polarity, and how lone pairs of electrons affect the shape of molecules, check out our Electronegativity, Polarity and Shapes of Molecules section.
Alcohols can be classified into three types based on the number of R groups attached to the alpha carbon, which is the carbon atom directly bonded to the hydroxyl group. An R group is a shorthand representation for any other hydrocarbon chain.
The three types of alcohols are primary, secondary, and tertiary alcohols.
A primary alcohol has one R group attached to the alpha carbon. In other words, the alpha carbon is only attached to one other carbon atom, which can be an alkyl or an aryl group.
A secondary alcohol has two R groups attached to the alpha carbon. In other words, the alpha carbon is attached to two other carbon atoms, which can be alkyl or aryl groups.
A tertiary alcohol has three R groups attached to the alpha carbon. In other words, the alpha carbon is attached to three other carbon atoms, which can be alkyl or aryl groups. By identifying the number of R groups attached to the alpha carbon, we can classify an alcohol as primary, secondary, or tertiary.
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.
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.
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.
Wondering how we name these alcohols? We'll look at that next.
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-. Stuck with nomenclature? Organic Compounds has you covered.
Let's look at some examples. Have a go at naming the alcohol below.
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.
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.
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.
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.
You can read more about hydrogen bonding in Intermolecular Forces.
Now we'll explore how hydrogen bonding affects the properties of alcohols.
Alcohols have higher melting and boiling points compared to similar alkanes due to the strong hydrogen bonding holding adjacent alcohol molecules together. This requires a lot of energy to overcome. In contrast, alkanes are only held together by weak van der Waals forces, making them easier to overcome and giving them low melting and boiling points.
Like all organic molecules, alcohols follow trends in melting and boiling points. As chain length increases, so do melting and boiling points because the molecules experience stronger van der Waals forces. As branching increases, melting and boiling points decrease because the molecules can't fit as closely together, leading to weaker van der Waals forces.
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.
Alcohols are slightly acidic. According to Bronsted-Lowry theory, all molecules which donate protons (H+) are acids. The hydroxyl group of alcohols tend to release H+ because of its polarity. Due to O being more electronegative, the shared pair of electrons shifts towards O, weakening the O-H bond. The release of H+ makes alcohol acidic.
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.
Hydrating alkenes using steam and a phosphoric acid catalyst → produces alcohols
Reacting halogenoalkanes with the hydroxide ion in a mixture of alcohol and water → produces alcohols through nucleophilic substitution reaction
Hydrolysing esters using water and a strong acid catalyst → produces alcohols
Reducing carboxylic acids, aldehydes, or ketones using a reducing agent such as LiAlH4 or NaBH4 → produces alcohols
By knowing these chemical reactions and the required reagents and conditions, it is possible to make alcohols from a variety of organic compounds. Building a synthesis map can help to visualize the different pathways and connections between various organic compounds, making it easier to plan and execute complex organic synthesis.
Fermentation is the most common way of making alcohol for drinks like wine, beer, or cider. It involves supplying yeast cells with plant carbohydrates such as sugar cane or sugar beet, which are then broken down into ethanol. Fermentation takes place in anaerobic conditions at around 35°C.
While fermentation is slower than the hydration of ethene, it is a more sustainable option as it uses renewable plant matter and is carbon neutral. Ethanol produced through fermentation can also be converted into other organic compounds using reactions like dehydration into ethene, which can then be polymerised into sustainable polymers like poly(ethene). For more information on making alcohols through hydration reactions, fermentation, or nucleophilic substitution reactions, check out our resources on Reactions of Alkenes, Production of Ethanol, Nucleophilic Substitution Reactions, and Reactions of Esters and Aldehydes and Ketones.
Alcohols have a polar hydroxyl group, which makes them reactive and great fuels. They can combust in oxygen to produce carbon dioxide and water, and can be oxidised by an oxidising agent such as acidified potassium dichromate to produce aldehydes, ketones, and carboxylic acids. Alcohols can also be dehydrated using a strong acid catalyst to produce alkenes, and react with hydrogen halides to produce a halogenoalkane and water through nucleophilic substitution reactions. Additionally, they can react with carboxylic acids to form an ester in a condensation reaction, such as heating methanol and ethanoic acid with a strong acid catalyst to produce methyl ethanoate and water.
Understanding these reactions is important for various applications, such as the production of aldehydes, ketones, and carboxylic acids for use in the pharmaceutical and chemical industries, or the production of esters for use in fragrances and flavors. Additionally, the combustion of alcohols is a key process in energy production, as it releases energy in the form of heat and can be used as a fuel source.
Testing for alcohols can be done through oxidation reactions or using solid phosphorous pentachloride (PCl5). However, it's important to note that each test has limitations and may also give positive results with other compounds like aldehydes, carboxylic acids, or water. To get a more accurate result, a combination of tests may be used.
One specific test for the alcohol group CH3CH(OH)- is the reaction with alkali iodine (I2), which produces triiodomethane (CHI3), also known as iodoform. This reaction involves adding iodine to the alcohol, along with a little NaOH solution. NaOH reacts with I2 to form NaOI, which oxidises the alcohol to form an ester (or an aldehyde if the "R" group is hydrogen). NaOH also removes any color of iodine, which prevents a false positive test. In the next step, the hydrogens in the CH3- group are substituted by reaction with I2 in the presence of OH- ions. In the final step, the breaking of C-C bonds takes place in the presence of OH- ions. CHI3 is produced along with an ion RCO2-. The formation of CHI3 is a positive test for the presence of the CH3CH(OH)- group.
Overall, testing for alcohols is important in various applications, including identifying unknown substances in chemistry labs or testing alcoholic beverages for compliance with regulations.
Triiodomethane is a yellow precipitate that is formed when an alcohol is oxidized. This is a positive test that confirms the presence of the CH3CH(OH)- group in the initial alcohol. Triiodomethane is also used as an antiseptic and has a faint "medical" smell.
Oxidation of alcohols is a chemical reaction in which the alcohol is converted into an aldehyde or a ketone. Dehydration is a reaction in which an alcohol is converted into an alkene. Esterification is a reaction in which an alcohol is converted into an ester.
Examples of alcohols include ethanol, methanol, isopropyl alcohol, and butanol.
Alcohol units are a way to measure the alcohol content of drinks. One unit is equal to 8g of pure alcohol, which is approximately the amount of alcohol that an adult human can get rid of in one hour. Current UK guidelines recommend keeping your alcohol intake to below 14 units a week, and to spread your drinking out over several days. Alcohol is partially toxic to humans and can impair your ability to think straight and interfere with hormone production.
Great summary! Here are some key takeaways about alcohols:
What is alcohol?
Alcohols are organic compounds containing one or more hydroxyl group, -OH.
What are examples of alcohols?
Examples of alcohol include methanol, ethanol and isopropyl, correctly named propan-2-ol.
What are the types of alcohol?
Alcohols can be split into three different types: primary, secondary, and tertiary. Their classification depends on the number of R groups bonded to the alpha carbon. Primary alcohols have zero or one R groups bonded to the alpha carbon, whilst secondary have two, and tertiary have three.
What are the uses of alcohols?
We find most alcohol in our everyday lives in the form of ethanol in alcoholic beverages. But alcohol is also used as a solvent, in fuels, and as a disinfectant.
How do you make alcohols?
Industrially, alcohols are made by hydrating ethene or fermenting biomass using yeast.
