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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.
Alcohols are organic compounds containing one or more hydroxyl group, -OH.
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.
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.
A general formula is a formula showing the basic ratio of atoms of each element in a compound, that can be applied to a whole homologous series.
Check out Organic Compounds, where you'll learn about the other features of a homologous series.
Alcohols with just one hydroxyl group all have the general formula CnH2n+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.
Why don't we show the general formula as CnH2n+2O? Well, separating the -OH hydroxyl group out helps show that this group of compounds are alcohols, as opposed to any other type of organic molecule.
Let's take a closer look at the hydroxyl group, -OH. There are a couple of things to note.
The hydroxyl group. Anna Brewer, StudySmarter Originals
For more on electronegativity, polarity, and how lone pairs of electrons affect the shape of molecules, check out Electronegativity, Polarity and Shapes of Molecules.
Alcohols can be classified into three types in chemistry.
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.
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
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
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 is pretty simple. We follow all of the usual IUPAC nomenclature rules. Note the following:
Stuck with nomenclature? Organic Compounds has you covered.
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
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
You can read more about hydrogen bonding in Intermolecular Forces.
Now we'll explore how hydrogen bonding affects the properties of alcohols.
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:
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.
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.
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.
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
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.
Fermentation is a much slower process than the hydration of ethene, but it is a much more sustainable option. Ethene comes from crude oil, a non-renewable resource, and the processing and burning of crude oil releases carbon dioxide into the atmosphere. On the other hand, fermentation uses renewable plant matter. Overall, it is carbon neutral - any carbon dioxide released is offset by carbon taken in when the plants are growing.
Because of this, scientists are increasingly looking towards fermentation as a source of organic molecules for industrial processes. Once we get ethanol, we can then convert it into other organic compounds, using reactions like the ones you've plotted out on your synthesis map. For example, ethanol can be dehydrated into ethene, which can then be polymerised into polymers such as poly(ethene). Sustainable plastic, anyone?
Feel like you need more information? In Reactions of Alkenes, we go over making alcohols in hydration reactions, and in Production of Ethanol, you can directly compare ethene hydration with fermentation. In Nucleophilic Substitution Reactions, you'll find the mechanism behind producing alcohols from halogenoalkanes, while in Reactions of Esters and Reactions of Aldehydes and Ketones, you'll see ways of making alcohols from aldehydes, ketones, and esters.
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.
A synthesis map centred on alcohols. Anna Brewer, StudySmarter Originals
We can use an oxidation reaction to test for some alcohols. Mixing a primary or secondary alcohol with orange potassium dichromate and an acid causes the potassium dichromate to turn green. But watch out - this test doesn't work for tertiary alcohols, and it will also give a positive result with aldehydes.
You could instead test for alcohols using solid phosphorous pentachloride, PCl5. If an alcohol is present, the reaction will produce white steamy fumes of hydrogen chloride which turn damp litmus paper red. However, this test also gives positive results with water and carboxylic acids. Your best bet is to use a combination of the two tests.
You'll find out more about oxidising alcohols in Oxidation of Alcohols and explore dehydration in Alcohol Elimination Reaction. Esterification is covered in the article Reactions of Esters.
Finally, let's explore a few further examples of alcohols. Here are some other common alcohols and their uses.
To measure the alcohol content of drinks, we use the term 'units'. One unit is equal to 8g of pure alcohol. This is approximately the amount of alcohol that an adult human can get rid of in one hour and is theoretically a way to track your drinking. 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. It acts as a suppressor of the central nervous system, slowing down your reaction time and impairing your ability to think straight. It also interferes with hormone production, leading to increased levels of feel-good hormones such as dopamine. This is why alcohol is loved by so many, and plays such a major role in our lives.
Alcohols are organic compounds containing one or more hydroxyl group, -OH.
Examples of alcohol include methanol, ethanol and isopropyl, correctly named propan-2-ol.
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.
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.
Industrially, alcohols are made by hydrating ethene or fermenting biomass using yeast.
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