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What did you do this morning?
You probably got up, showered and put on some clothes, perhaps made from cotton or acrylic. You then might have sipped at a coffee whilst eating a slice of toast spread thickly with butter and jam. After that, you might have travelled to work or school, perhaps by car or bus, both fuelled by petrol or diesel. At some point, you sat down, pulled out your phone or computer and started reading this article.
What do these activities have in common? They all involve organic compounds. From the material of your clothes and the food you eat to the fuel for your car and the retina in your eyes, organic compounds are everywhere.
Organic compounds are molecules that are made up of carbon covalently bonded to other atoms, most commonly hydrogen, oxygen, and nitrogen.
There are hundreds of different organic compounds. In fact, thousands - perhaps even millions. They are all based on carbon atoms, covalently bonded to other elements. These are two fundamental ideas behind organic compounds. let's look at them in more detail now.
Organic molecules are all based on the element carbon. Making up the backbone of all the organic compounds in the world is a big task, but carbon successfully rises to the occasion. But what makes it so versatile?
Well, carbon has two properties in particular that make it so good at forming molecules and compounds: its tetravalency, and its small size.
Take a look at carbon's electron configuration, shown below.
Carbon's electron configuration. Anna Brewer, StudySmarter Original
You can see that it has six electrons. Two are found in an inner shell, and four are found in an outer shell. This makes it a tetravalent atom. Atoms tend to want to have full outer shells of electrons; in carbon's case, this would mean having eight electrons in its outer shell. To do this, it likes forming four covalent bonds. It's not fussy about who it bonds with - it is just as happy bonding with oxygen as it is with nitrogen. We'll look at organic compounds featuring both elements later.
You know that there are other atoms that have four electrons in their outer shell, such as silicon. Why aren't they as versatile and prevalent as carbon?
It's because carbon is a small atom. Its diminutive size means multiple carbon atoms can fit together easily in complicated structures. We say that it is good at catenation - when atoms of the same element join up in long chains.
The combination of small size and tetravalency means the possible arrangements of carbon atoms, covalently bonded both to each other and to other elements, are practically infinite. This is why we have so many different organic compounds.
To tell the truth, there is no fixed definition of an organic compound, and some carbon-based molecules are in fact not organic compounds. These include carbonates, cyanides, and carbon dioxide. The reasons behind their exclusion are mostly historic, instead of being based on any defining feature. Structures such as graphite and diamond are also excluded from the group. Because they are made from just one element, they don't count as compounds.
Organic compounds are joined together using covalent bonds.
A covalent bond is a bond formed by a shared pair of electrons.
Covalent bonds are formed when two atoms each offer up an electron to form a shared pair. The atoms are held together by the electrostatic attraction between their positive nuclei and these negative electrons. This is why most of the elements found in organic compounds are non-metals - they're the ones that can form covalent bonds.
There are a couple of exceptions to this rule - you can find some metals in organic compounds.
Firstly, transition metals can bond to organic compounds using ligand reactions. The two bond together with a dative covalent bond, using a lone pair of electrons from the organic compound. You can read more about this in Transition Metals.
Secondly, beryllium, a group 2 metal, can also form covalent bonds. You'll find out why in the article Group 2.
In this next section, we're going to look at different types of organic compounds and ways of classifying them. We can do this in different ways.
First, we'll take a look at functional groups.
A species' functional group is the particular group of atoms responsible for its chemical reactions.
The easiest way to distinguish organic compounds is by their functional group. This is the atom or combination of atoms that makes it react in a certain way. Carboxylic acids contain the carboxyl functional group, often written as COOH, whereas amines contain - you guessed it - the amine functional group, or -NH2.
You'll come across the following functional groups when looking at organic compounds.
Family name | Functional group | Prefix/suffix |
Alkane | C-C | -ane |
Alkene | C=C | -ene |
Alkyne | C≡C | -yne |
Alcohol | R-OH | -ol or hydroxy- |
Halogenoalkane | R-X | Varying suffix-ane |
Aldehyde | R-CHO | -al |
Ketone | R-CO-R | -one |
Carboxylic acid | R-COOH | -oic acid |
Ester | R-COO-R | -oate |
Amine | -NH2 | -amine or amino- |
Wondering what the prefixes and suffixes are for? We use them to name organic compounds, as you'll find out later.
Molecules with the same functional group react in very similar ways. Because of that, we tend to group them together in a homologous series.
A homologous series is a group of organic molecules with the same functional group, but different carbon chain lengths.
A homologous series has some fixed properties.
Organic molecules can also be classified as aliphatic, aromatic, or alicyclic.
Benzene rings are represented by a hexagon with a circle in the middle. Want to find out more about the wonders of benzene? Head over to Aromatic Chemistry, where all will be explained!
A third way of labelling organic compounds is using the terms saturated and unsaturated.
You might remember from earlier that a C=C double bond is the functional group found in alkenes. This makes all alkenes unsaturated compounds. The C≡C triple bond, however, is the functional group found in alkynes. Once again, this makes all alkynes unsaturated.
In biology, you'll probably come across four main groups of organic compounds that are fundamental to life. These are carbohydrates, lipids, proteins, and nucleic acids. We won't go into them here - they're much too important for that! However, you can find out more in the articles dedicated to these molecules: Carbohydrates, Lipids, Proteins, and Nucleic Acids.
Now that we know more about the different types of organic compounds, we can have a look at naming them. The practice of naming organic compounds is known as nomenclature. The official nomenclature system was created by the International Union of Pure and Applied Chemistry (IUPAC), which is the system you need to know for your exams.
To name a molecule, you use the following:
Let's look at these three ideas in more detail.
The root name of an organic compound tells you the number of carbon atoms in the molecule's longest carbon chain. This can sometimes be a little tricky to spot. Have a go at finding the longest carbon chain in the molecule below:
This example uses a type of formula that is a cross between structural and displayed formulae. We'll come on to these later.
At first glance, it might look like the carbon chain is only three carbon atoms long. But notice how the chain snakes up and to the right. In actual fact, the longest carbon chain is four atoms long.
Finding the longest carbon chain. Anna Brewer, StudySmarter Original
This next handy table tells you the root name for molecules with chain lengths ranging from just one carbon atom, up to eight carbon atoms long.
Length of chain | Root name |
1 | -meth- |
2 | -eth- |
3 | -prop- |
4 | -but- |
5 | -pent- |
6 | -hex- |
7 | -hept- |
8 | -oct- |
Our molecule above, with a chain length of four carbon atoms, therefore has the root name -but-.
Do you remember those prefixes and suffixes from the functional group table? This is where they come in. Prefixes and suffixes are used to show a compound's functional group and any additional side chains.
We looked at functional groups and their prefixes earlier on in the article, but we'll revisit them again now.
Family name | Functional group | Prefix/suffix |
Alkane | C-C | -ane |
Alkene | C=C | -ene |
Alcohol | R-OH | -ol or hydroxy- |
Halogenoalkane | R-X | Varying prefix-ane |
Aldehyde | R-CHO | -al |
Ketone | R-CO-R | -one |
Carboxylic acid | R-COOH | -oic acid |
Ester | R-COO-R | -oate |
Amine | -NH2 | -amine or amino- |
The prefix used to identify a halogenoalkane varies, depending on the halogen atom found in the molecule. For example, organic compounds containing fluorine use the prefix fluoro-, and those containing chlorine use the prefix chloro-.
You'll notice that some functional groups can use either a prefix or a suffix. In general, we use the suffix, but sometimes it can be easier to use the prefix. This happens if the molecule features multiple functional groups. In this case, you look at the priorities of the different functional groups. IUPAC has assigned each type of functional group a priority; you always use the suffix of the highest priority group. The other functional groups take prefixes instead.
In the molecule we looked at above when working out root names, you can see that there is a -CH3 group branching off from the main carbon chain. This is a side chain, and we also show it using prefixes. These prefixes are similar to root names, varying according to the length of the side chain, but end in -yl. Take a look at the first four:
Length of side chain | Prefix |
1 | methyl- |
2 | ethyl- |
3 | propyl- |
4 | butyl- |
Looking at that molecule again, we can see that it has a side chain that is one carbon atom long. It, therefore, takes the prefix methyl-.
Main and side carbon chains. Anna Brewer, StudySmarter Original
If a molecule contains two or more of the same functional group or side chain, we use quantifiers to show the amount. These go before the prefix or suffix. Here are the first three:
Number present | Quantifier |
2 | di- |
3 | tri- |
4 | tetra- |
For example, an alkene with two C=C double bonds would end with the suffix -diene.
Sometimes, a molecule needs multiple prefixes. This happens if it has two or more different side chains or functional groups. We arrange the prefixes in alphabetic order, ignoring any quantifiers such as di- or tri-.
The last part of a molecule's name is its numbering. We use numbers, sometimes called locants, to show where functional groups or side chains are attached to the main carbon chain - in other words, to show their position in the molecule. These numbers go before the prefix or suffix, separated by a hyphen. For example, if a functional group is found on the third carbon in the chain, you'd use the number 3. You can number the carbon chain from left to right or right to left. However, there is one thing to bear in mind: if you add up all the locants used to represent the molecule's different side chains and functional groups, you want the total to be the lowest possible.
Multiple locants are separated by commas.
Sounds confusing? Let's put all this information together and practice naming some organic compounds.
When naming organic compounds, follow these steps.
Here's a molecule for you to work on.
Its longest carbon chain is four carbon atoms long. This gives it the root name -but-. Here's the chain, shown in pink.
The root name of an unknown molecule. Anna Brewer, StudySmarter Original
This molecule is a halogenoalkane. Specifically, it contains two chlorine atoms. It therefore needs the suffix -ane and the prefix chloro-. Because there are two chlorine atoms, the prefix will be preceded by the quantifier di-.
Finally, we need to look at numbering. Remember that this shows the position of functional groups on the main carbon chain. Where are the chlorine atoms located?
If we number the carbon chain from left to right, we can see that the chlorine atoms are found on carbons 2 and 4. If we number it from right to left, we can see that they are found on carbons 1 and 3. Remember that if we add up these numbers, we want to make the lowest total possible. We'd therefore number the carbon chain from right to left.
Numbering an unknown molecule. Anna Brewer, StudySmarter Original
If we put the root name, prefix, quantifier, and locants together, we arrive at this molecule's name: 1,3-dichlorobutane.
Here's another example.
An unknown molecule for you to name. Anna Brewer, StudySmarter Original
First, look at its longest carbon chain. Here it is three carbon atoms long, giving it the root name propyl-.
The root name of an unknown molecule. Anna Brewer, StudySmarter Original
Next, look at its functional groups. It contains two functional groups: a R-COOH group (also known as a carboxyl group), making it a carboxylic acid, and an R-OH group (also known as a hydroxyl group), making it an alcohol. To make life a little simpler, we use one prefix and one suffix. If you read the deep dive earlier on in the article, you'll know that we use the suffix of the highest priority functional group. In this case, the carboxyl group takes priority. Our molecule therefore ends in -oic acid. To show the hydroxyl group, we use the prefix hydroxy-.
This molecule also has a methyl group. We can show this using the prefix methyl-. Remember that we arrange prefixes in alphabetical order, so hydroxy- will come before methyl-.
The functional groups and side chains found on an unknown molecule. Anna Brewer, StudySmarter Original
Finally, let's look at numbering. In this case, we deviate from the rules a little bit. The carboxyl group is always found at the end of a carbon chain and always takes first position. The carbon atom within the carboxyl group is known as carbon 1, and we number the rest of the functional groups and side chains accordingly. Here, the methyl group is bonded to carbon 2, and the hydroxyl group is bonded to carbon 3.
The final naming of an unknown molecule. Anna Brewer, StudySmarter Original
Putting that all together, what do we get? 3-hydroxy-2-methylpropanoic acid.
Let's focus our attention on ways of representing organic compounds. We do this using chemical formulae. There are a few different types you need to know about. These include:
One formula, two formulae - formula is the singular, and formulae is the plural. Don't get them mixed up!
Let's start with general formulae.
A general formula is a formula that shows the basic ratio of atoms in a compound or molecule. It can be applied to a whole homologous series.
If you want to represent a whole family of compounds with the same functional group, you can use a general formula. They're useful because they can be applied to all the members of a homologous series.
General formulae express the numbers of atoms of each element in a compound in terms of n. For example, all alkanes have the general formula CnH2n+2. The formula tells us that if an alkane has n carbon atoms, it will have 2n+2 hydrogen atoms. This means that once we know the number of carbon atoms in an alkane, we can always find out its number of hydrogen atoms - you double the carbon number and add 2. Of course, we can go backwards as well - subtracting 2 from the number of hydrogens and then halving the result gives you the number of carbons. The general formula works for all of the alkanes in the alkane homologous series, from the very small to the very large.
General formulae are good at representing a whole family of compounds, but they aren't good at specifying an individual compound. We can do this in several ways. The first way of representing a specific compound is by using its molecular formula.
A molecular formula is a formula that shows the actual number of atoms of each element in a compound.
Let's say that we have an alkane with four carbon atoms. From the general formula, we know that it has hydrogen atoms. Its molecular formula is therefore C4H10.
There's a problem when we only rely on molecular formulae to represent molecules: different molecules can have the same molecular formula. You'll see more of this when we look at isomerism later on. A different type of formula we can use is a structural formula.
A structural formula is a shorthand representation of the structure and arrangement of atoms in a molecule, without showing every bond.
When writing structural formulae, we move along the molecule from one end to the other, writing out each carbon and the groups attached to it separately.
Here's an example. Take the molecular formula C3H6O. This could represent multiple different compounds - for example, propanal or propanone. Propanal has the structural formula CH3CH2CHO. This tells us that it has a -CH3 group, bonded to a -CH2- group, bonded to a -CHO group. In contrast, propanone has the structural formula CH3COCH3. This tells us that it has a -CH3 group, bonded to a -CO- group, bonded to a -CH3 group. Do you notice the slight difference?
Structural formulae. Anna Brewer, StudySmarter Original
If we want to show all of the bonds in a compound, we use its displayed formula. Displayed formulae often come in handy when drawing reaction mechanisms.
Displayed formulae show every atom and bond in a molecule.
In displayed formulae, we represent bonds using straight lines. A single straight line tells us that we have a single bond, whereas a double straight line tells us we have a double bond. Although they can be a pain to draw out, displayed formulae are useful because they give us important information about a molecule's unique structure, bonding, and arrangement of atoms.
For example, ethanol has the structural formula CH3CH2OH and the following displayed formula:
Displayed formula of ethanol. Anna Brewer, StudySmarter Original
In this example, we've drawn all the bonds as if the molecule were flat on the page. However, bonds aren't like that in real life. If we want to show a bond sticking out of the page, we use a wedged line. If we want to show a bond protruding backwards into the page, we use a dashed line. Here's an example using methane.
Drawing 3D chemical molecules. Anna Brewer, StudySmarter Original
The final type of formula we'll look at is the skeletal formula.
Skeletal formulae are another type of formula that act as a shorthand representation of a molecule, showing some aspects of its structure and bonding. It omits certain atoms and bonds in order to simplify the diagram.
Drawing displayed formulae over and over again takes a lot of time. This is where skeletal formulae come in handy. They're an easy way of showing a molecule's structure and bonding without drawing every atom and bond. As in displayed formulae, you represent bonds using straight lines. However, you leave out carbon atoms. You represent these missing carbons using the vertices of the lines, assuming that there is a carbon atom at every unlabelled vertex, junction, or end of a line. You also omit carbon-hydrogen bonds. Instead, you assume that each carbon atom forms exactly four covalent bonds, and that any bonds that aren't shown are carbon-hydrogen bonds.
Sound confusing? Let's take a look at an example. We've already seen the displayed formula of ethanol, CH3CH2OH. Here's how it translates into a skeletal formula.
Skeletal formula of ethanol. Anna Brewer, StudySmarter Original
We've learnt about types of organic compounds and the different formulae we can use to represent them. Finally, let's look at isomerism.
Isomers are molecules with the same molecular formula, but different arrangements of atoms.
Do you remember how earlier we mentioned that molecular formulae aren't that helpful, as one molecular formula can represent multiple different molecules? Well, this is why. Isomers contain exactly the same number of atoms of each element, but the atoms are arranged differently.
There are two main types of isomerism in chemistry.
Structural isomers are molecules with the same molecular formula but different structural formulae.
Let's revisit propanal and propanone. As we discovered, they both have the same molecular formula: C3H6O. However, they have different structural formulae. Propanal has the structural formula CH3CH2CHO, and propanone has the structural formula CH3COCH3. This makes them structural isomers.
Structural isomerism can be further split into three subtypes:
Position isomerism in propanol. Anna Brewer, StudySmarter
Another type of isomerism is stereoisomerism. If you thought structural isomers were similar, you better brace yourself - stereoisomers are even more alike!
Stereoisomers have both the same molecular formula and the same structural formula, but different arrangements of atoms in space.
To identify stereoisomers, you need to look at a molecule's displayed formula. Remember, this is a formula that shows every atom and bond. It also shows the arrangement of atoms and bonds; this is where stereoisomers differ.
Once again, there are a couple of subtypes of stereoisomerism:
For more examples of structural and stereoisomerism in action, take a look at Isomerism.
Organic compounds are molecules that are made up of carbon covalently bonded to other atoms, most commonly hydrogen, oxygen, and nitrogen.
Volatile organic compounds, also known as VOCs, are organic compounds that readily turn into a gas at room temperature. They're emitted by certain solids and liquids.
In general, polar organic compounds are soluble in water. These include molecules with hydroxyl, carboxyl, or amine functional groups. However, nonpolar molecules are insoluble in water. These include molecules with long hydrocarbon chains.
Organic compounds have practically infinite uses. We use them as fuels for vehicles, find them in pharmaceuticals and soaps, eat them as food, and use them as structural materials within our body.
In biology, the four main types of organic compound are carbohydrates, lipids, proteins, and nucleic acids.
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