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.
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Jetzt kostenlos anmeldenAh, 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 groups, -OH.
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.
Let's take a closer look at the hydroxyl group, -OH. There are a couple of things to note.
The polarity and bond angle of the hydroxyl group.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.
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 the members of this group of compounds are all alcohols, as opposed to any other type of organic molecule.
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.
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, 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.
Naming alcohols is pretty simple. We follow all of the usual IUPAC nomenclature rules. Note the following:
Stuck with naming compounds? IUPAC Nomenclature has you covered.
Let's look at an example. Have a go at naming the alcohol below.
Name the following alcohol and give its classification.
An unknown alcohol to be named.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, meaning that the hydroxyl group is the parent functional group. As a result, we 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. The first numbering possibility means that the parent functional group has the lowest possible locant, and so in this case, we number the parent chain's carbon atoms from the left. Putting that all together, we get 4-chlorobutan-2-ol.
Our unknown alcohol, with its functional groups and parent chain labelled, is known as 4-chlorobutan-2-ol.StudySmarter Originals
The classification of our unknown alcohol. The alpha carbon is bonded to two R groups, making it a secondary alcohol.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. 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
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.
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.
Check out Brønstead-Lowry Acids and Bases for further information about acids and bases.
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.
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.
Let's now summarise the reagents, products, and conditons for the chemical reactions by which alcohols can be produced:
Reactions that Produce Alcohol | ||
---|---|---|
Reaction | Reagents and Conditions | Products |
Electrophilic addition of steam to alkene | H2O(g) and H3PO4 as catalyst | Alcohol |
Oxidation of alkenes with cold dilute acidified potassium manganate (VII) | KMnO4/H+ | Diol + MnO2 (dark brown ppt) |
Nucleophilic substitution of a halogenoalkane | NaOH and heat | Alcohol + sodium salt |
Reduction of an aldehyde or ketone | NaBH4 or LiAlH4 | Alcohol + Al3+ |
Reduction of a carboxylic acid | LiAlH4 | Alcohol + Al3+ |
Hydrolysis of an ester | dilute acid or dilute alkali and heat | Alcohol + carboxylic acid / carboxylate salt |
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
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.StudySmarter Originals
We can use an oxidation reaction to test for some alcohols. Mixing a primary or secondary alcohol with orange acidified potassium dichromate 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.
An alcohol of the formula CH3CH(OH)-R can be tested for the CH3CH(OH)- group by its reaction with alkali iodine, I2. This results in the formation of triidomethane, CHI3 (also known as iodoform), which is a yellow precipitate. The reaction goes like this:
Identification of CH3CH(OH)- group in alcohols by reaction with iodine | StudySmarter Originals
If the yellow precipitate of triiodomethane is formed, the test is positive and confirms the presence of the CH3CH(OH)- group in the initial alcohol. Besides being a yellow precipitate, triiodomethane, also has a faint "medical" smell due to its use as an antiseptic.
You'll find out more about all of the various reactions mentioned above in Reaction of Alcohols.
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.
What is an alcohol?
Alcohols are organic compounds containing one or more hydroxyl group, -OH.
What functional group do alcohols contain?
The hydroxyl group, -OH
What is the general formula of an alcohol?
CnH2n+1OH
What suffix do we use to name alcohols?
-ol
True or false? The hydroxyl group is nonpolar.
False
Compare and contrast primary, secondary, and tertiary alcohols.
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