The discussion of organic chemistry to this point has described only compounds of carbon and hydrogen. Although all organic compounds contain carbon, and almost all have hydrogen, most of them contain other elements as well. The most common other elements in organic compounds are oxygen, nitrogen, sulfur, and the halogens.
The halogens resemble hydrogen because they need to form a single covalent bond to achieve electronic stability. Consequently, a halogen atom may replace any hydrogen atom in a hydrocarbon. Figure 1 shows how fluorine or bromine atoms proxy for hydrogen in methane.
Figure 1. Methane and two derivatives.
Halogens can replace any or all of the four hydrogens of methane. If the halogen is fluorine, the series of replacement compounds is
CH 4 CH 3F CH 2F 2 CHF 3 CF 4
Such halogenated compounds are called organic halides or alkyl halides. The substituted atoms may be fluorine, chlorine, bromine, iodine, or any combination of these elements.
The previously mentioned ethylene molecule is planar; that is, all six atoms lie in a single plane because the double bond is rigid. In Figure 2, the stiff double bond prevents the molecule from being “twisted” around the axis between the carbon atoms.
Figure 2. Ethylene.
If a reaction substitutes a different atom such as a bromine atom for one or more hydrogen atoms, the resulting compound can exist in either of two different structural configurations. The configuration with the bromines adjacent is called cis (from the Latin derivative for “on this side”), whereas the configuration with bromines opposite is called trans (which means “on the other side”). The two configurations are different substances with unique chemical and physical properties. They are described as being geometric isomers. See Figure 3.
Figure 3. Geometric isomers.
Figure 4 lists some common classes of organic compounds containing oxygen or nitrogen. The main carbon‐bearing part of the compound attaches to the bond extending leftward in the second column. The examples use the ethyl C 2H 5– unit as the carbon chain attached to the functional group, but the immense number of organic compounds arises from the fact that virtually any carbon chain can be attached at that site.
Figure 4. Common functional groups.
If you compare the carbon‐oxygen bonding, you will observe that oxygens may be bonded to carbon by either single or double bonds.
Both alcohols and carboxylic acids have a single hydrogen bonded to an oxygen in the functional group. In aqueous solution, such hydrogens can become detached, producing slightly acidic solutions.
The amines contain nitrogen bonded to one, two, or three carbon chains. These compounds are derivatives of ammonia, hence the name of the class, as shown in Figure 5.
Figure 5. Ammonia.
Consider three possible amines created by replacing hydrogen with the –CH 3 methyl group. See Figure 6.
Figure 6. Methyl derivatives of ammonia.
Of course, more complex carbon groups can be attached at any of the three bonds to nitrogen. Notice that the nitrogen atom is truly the core atom in an amine, in contrast to the functional groups in alcohols, aldehydes, and carboxylic acids, in each of which the functional group must be at the end of the molecule.
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.
- This article is about organic compounds in chemistry.
- We'll start by defining organic compounds before looking at the different types of organic compounds.
- You'll learn terms such as saturated and alicyclic.
- After that, we'll explore organic compound nomenclature and ways of representing these molecules using formulae.
- Finally, we'll look at isomerism.
Organic compounds definition
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 the two fundamental ideas behind 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.
Carbon in organic compounds
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.
- Its small size.
Tetravalency
Take a look at carbon's electron configuration, shown below.
Fig. 1 - Carbon's electron configuration
You can see that carbon has six electrons. Two are found in an inner shell, whilst four are found in its outer shell (also known as its valence shell). These four outer shell electrons make carbon a tetravalent atom. Atoms tend to want to have full outer shells of electrons, and in carbon's case, this means having eight valence electrons. To achieve a full outer shell, the atom needs to form 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. This means that carbon forms compounds with a range of different elements, and we'll look at examples of organic molecules featuring both oxygen and nitrogen later.
Size
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.
Bonding in organic 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.
Types of organic compounds
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.
- The easiest way to group organic molecules is by their functional group.
- We can also distinguish between aliphatic, aromatic, and alicyclic compounds.
- Another useful label is saturated or unsaturated.
First, we'll take a look at functional groups.
Functional groups in organic compounds
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 \(-NH_2\)
Types of functional groups
You'll come across the following functional groups when looking at organic compounds.
Family nameFunctional groupPrefix/suffixAlkane\(C-C\)-aneAlkene\(C=C\)-eneAlkyne\( C\equiv N\)-yneAlcohol\(R-OH\)-ol or hydroxy-Halogenoalkane\(R-X\)Varying suffix-aneAldehyde\(R-CHO\)-alKetone\(R-CO-R\)-oneCarboxylic acid\(R-COOH\)-oic acidEster\(R-COO-R\)-oateAmine\( -NH_2 \)-amine or amino-We explore all of these groups in more detail in the article Functional Groups.
Wondering what the prefixes and suffixes are for? We use them to name organic compounds, as you'll find out in IUPAC Nomenclature.
Homologous series
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.
- All members can be represented by a general formula. This is a formula that expresses the basic ratio of different atoms in a molecule. We'll explore it in more depth in just a second.
- Members all have the same functional group, as we mentioned above.
- Members differ only by the number and arrangement of \(-CH_2\) groups in their carbon chain.
- All members have the same chemical properties and undergo the same reactions. However, they might have different physical properties.
Aliphatic, aromatic, and alicyclic compounds
Organic molecules can also be classified as aliphatic, aromatic, or alicyclic.
- Aliphatic compounds are based on carbon chains full of \(-CH_2\) groups. They don't feature any benzene rings, and can have long straight chains or form cyclic rings. Aliphatic compounds with cyclic rings are called alicyclic compounds.
- In contrast, aromatic compounds contain benzene rings with delocalised pi electrons. We represent these rings using 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!
Saturated and unsaturated compounds
A third way of labelling organic compounds is using the terms saturated and unsaturated.
- Saturated compounds contain only single \(C-C\) bonds.
- Unsaturated compounds contain one or more double \(C=C\) bonds or triple \( C\equiv C\) bonds.
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 \( C\equiv C\) triple bond, however, is the functional group found in alkynes. Once again, this makes all alkynes unsaturated.
Biological organic compounds
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.
Naming organic compounds
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:
- A root name, to show the length of the molecule's longest carbon chain.
- Prefixes and suffixes, to show any functional groups and side chains (known as substituents).
- Numbers, known as locants, to show the position of functional groups and side chains.
For example, take the molecule 2-bromopropane. The root name -prop- tells us that this molecule is based on a propane chain, which is three carbon atoms long. The suffix -ane indicates that it is an alkane, whilst the prefix bromo- lets us know that this molecule has an additional bromine atom, and so is in fact a halogenoalkane. How about the number 2? That shows that the bromine atom is attached to the second carbon atom in the chain.
Fig. 2 - 2-bromopropane
Nomenclature is a complicated topic, and so we've created a whole article specially dedicated to solving its mysteries. Head over to IUPAC Nomenclature for more.
Organic compound formulae
Let's now 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:
- General formula
- Molecular formula
- Structural formula
- Displayed formula
- Skeletal formula
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.
Organic compound 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 \(C_nH_{2n+2}\) . 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.
Organic compound molecular formulae
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 \( (2\times 4) + 2 = 10\) hydrogen atoms. Its molecular formula is therefore \(C_4H_{10}\)
Organic compound structural formulae
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 \( C_3H_6O\). This could represent multiple different compounds - for example, propanal or propanone.
Propanal has the structural formula \( CH_3CH_2CHO\). This tells us that it has a \( -CH_3\) group, bonded to a \( -CH_2-\) group, bonded to a \( -CHO\) group. In contrast, propanone has the structural formula \( CH_3COCH_3\) .This tells us that it has a \( -CH_3\) group, bonded to a\( -CO-\) group, bonded to a\( -CH_3\) group. Do you notice the slight difference?
Fig. 3 - Structural formulae
Organic compound displayed formulae
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 \( CH_3CH_2OH\) and the following displayed formula:
Fig. 4 - Displayed formula of ethanol
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.
Fig. 5 - Drawing 3D chemical molecules
Organic compound skeletal formulae
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, \( CH_3CH_2OH\) . Here's how it translates into a skeletal formula.
Fig. 6 - Skeletal formula of ethanol
Isomerism in organic compounds
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 isomerism
- Stereoisomerism
Structural isomerism
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: \( C_3H_6O\) . However, they have different structural formulae. Propanal has the structural formula \( CH_3CH_2CHO\) , and propanone has the structural formula \( CH_3COCH_3\) . This makes them structural isomers.
Structural isomerism can be further split into three subtypes:
- Chain isomers differ in the arrangement of their carbon chain. For example, one isomer might be straight, whilst the other might be branched.
- Functional group isomers have different functional groups. Propanal and propanone are great examples of this - the first is an aldehyde, the second is a ketone.
- Position isomers differ in their placement of the functional group on their carbon chain. For example, propan-1-ol and propan-2-ol are both isomers with the same molecular formula, \(C_3H_8O\) and the same functional group, an \(-OH\) group. But whilst in propan-1-ol the functional group is found on carbon 1, in propan-2-ol, the functional group is found on carbon 2.
Fig. 7 - Position isomerism in propanol
Stereoisomerism
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.