What did you do this morning?

You woke up, got dressed in clothes made of cotton or acrylic, and ate toast with butter and jam while sipping coffee. Then, you headed off to school or work, traveling by car or bus powered by petrol or diesel. Now, you're reading this article. All of these activities involve organic compounds. Organic compounds are in your clothes, food, fuel, and even your eyes.

This article is all about organic compounds in chemistry. First, we'll define what organic compounds are and then explore the different types, like saturated and alicyclic compounds. Next, we'll talk about organic compound nomenclature and how to represent them with formulae. Finally, we'll dive into isomerism. If you're interested in learning more about the science behind the things you use and consume every day, keep reading to discover the world of organic compounds.

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.

Take a look at carbon's electron configuration, shown below.

‍

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.

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 made up of covalent bonds. A covalent bond happens when two atoms share a pair of electrons. The atoms are held together because their positive nuclei are attracted to the negative electrons. Non-metals are usually the elements that form covalent bonds in organic compounds.

However, there are some exceptions. Transition metals can bond with organic compounds using a dative covalent bond, which happens when the organic compound shares a lone pair of electrons. You can find more information about this in our article on Transition Metals. Additionally, beryllium, a group 2 metal, can also form covalent bonds. Learn more in our article on 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

The functional group of a species is the group of atoms responsible for its chemical reactions. The easiest way to differentiate between organic compounds is by their functional group. For example, carboxylic acids have the COOH functional group while amines have the -NH2 functional group.

There are several functional groups found in organic compounds, which we explore in-depth in our article on Functional Groups. Prefixes and suffixes are used to name organic compounds.

Molecules with the same functional group have similar reactions, so they are grouped together in a homologous series. A homologous series is a group of organic molecules with the same functional group but different carbon chain lengths. In a homologous series, all members have the same functional group, and they differ only in the number and arrangement of -CH2- groups in their carbon chain. They have the same chemical properties and undergo the same reactions, but they may have different physical properties. A general formula expresses the basic ratio of different atoms in a molecule and represents all members of a homologous series.

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 -CH2- 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

Organic compounds can also be classified as saturated or unsaturated. Saturated compounds only contain single C-C bonds while unsaturated compounds have one or more double C=C bonds or triple C≡C bonds.

Alkenes have a C=C double bond, making them all unsaturated compounds. On the other hand, alkynes have a C≡C triple bond, making them unsaturated as well.

In biology, there are four main organic compounds that are essential to life: carbohydrates, lipids, proteins, and nucleic acids. These molecules are crucial to the functioning of biological systems. You can find more information about each of these organic compounds in our articles on 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.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 is 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.

‍

Nomenclature is a complicated topic, and so we've created a whole article specially dedicated to solving its mysteries. Head over to Organic 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 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.

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 x 4) + 2 = 10 hydrogen atoms. Its molecular formula is therefore C4H10.

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 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?

‍

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 CH3CH2OH and the following displayed formula:

‍

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.

‍

Organic compound skeletal formulae

Skeletal formulae are a shorthand representation of a molecule's structure and bonding, omitting certain atoms and bonds to simplify the diagram. They're useful because they save time when drawing structures repeatedly.

In skeletal formulae, carbon atoms are left out, and they're represented by the vertices of the lines. Any unlabelled vertex, junction, or end of a line is assumed to have a carbon atom. Carbon-hydrogen bonds are also omitted, and it's assumed that each carbon atom forms exactly four covalent bonds, with any unshown bonds being carbon-hydrogen bonds.

For instance, ethanol's displayed formula, CH3CH2OH, can be translated into a skeletal formula.

‍

‍

Isomerism in organic compounds

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 isomerismStereoisomerism

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: 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: 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, C3H8O, 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.

‍

Stereoisomerism

Stereoisomerism is another type of isomerism where molecules have the same molecular and structural formula, but differ in the spatial arrangement of atoms. To identify stereoisomers, you need to examine the molecule's displayed formula, which shows the arrangement of atoms and bonds. There are two subtypes of stereoisomerism:

E-Z isomers, which differ in the arrangement of atoms or groups around a C=C double bond. They are found in alkenes like but-2-ene. Optical isomers, which differ in the arrangement of four different atoms or groups around a central carbon atom. They are mirror images of each other and cannot be superimposed. Understanding stereoisomerism is crucial in fields such as pharmaceuticals, where two stereoisomers of a drug can have vastly different effects on the body. Mastery of isomerism is essential for any student of organic chemistry.

To summarize, organic compounds are molecules made up of carbon covalently bonded to other atoms, and they have different functional groups that determine their chemical properties. They can be classified in different ways, including by their structure and whether they are saturated or unsaturated. Organic compounds are named using IUPAC nomenclature, which includes a root name, prefixes and suffixes to indicate functional groups, and locants to show their position. Formulae are used to represent organic compounds, with different types of formulae showing different levels of detail. Isomerism is also common in organic compounds, with structural isomers and stereoisomers being the two main types. Understanding these concepts is essential for studying organic chemistry and its applications in fields such as medicine, agriculture, and materials science.

Organic Compounds

What are organic compounds?

Organic compounds are molecules that are made up of carbon covalently bonded to other atoms, most commonly hydrogen, oxygen, and nitrogen.

What are volatile organic compounds?

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.

How do you determine solubility in water of organic compounds ?

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.

What are organic compounds used for?

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.

What are the four main organic compounds? 

In biology, the four main types of organic compound are carbohydrates, lipids, proteins, and nucleic acids.

‍