Organic Compounds

Organic compounds definition

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.

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.

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.

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 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.

We explore all of these groups in more detail in the article Functional Groups.

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 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.

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.

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

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

Let's start with general formulae.

Organic compound general formulae

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.

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.

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.

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

Organic compound skeletal formulae

The final type of formula we'll look at is the skeletal formula.

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.

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

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!

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:

• E-Z isomers differ in their arrangement of atoms or groups around a \( C=C\) double bond. You'll find E-Z isomerism in alkenes such as but-2-ene.
• Optical isomers differ in their arrangement of 4 different atoms or groups around a central carbon atom. They form non-superimposable, mirror-image molecules of each other.