Organic Compounds

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.

• This article is about organic compounds in chemistry.
• We'll start by defining organic compounds before looking at the different types.
• After that, we'll explore their nomenclature and ways of representing them.
• Finally, we'll look at isomerism.

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 two fundamental ideas behind organic compounds. let's look at them in more detail now.

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, and its small size.

Tetravalency

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.

Carbon's 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 -NH₂.

Types of functional groups

You'll come across the following functional groups when looking at organic compounds.

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 -CH₂- 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 -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.

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≡C bonds.

Compound side chains

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:

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

Quantifiers

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:

For example, an alkene with two C=C double bonds would end with the suffix -diene.

Order of prefixes

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-.

Organic compound numbering

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.

Sounds confusing? Let's put all this information together and practice naming some organic compounds.

Examples of naming organic compounds

When naming organic compounds, follow these steps.

1. Find the number of carbons in the longest carbon chain. This gives you your root name.
2. Identify any functional groups or side chains. These give you prefixes and suffixes.
3. Add quantifiers to show the number of each functional group or side chain.
4. Add locants to show the position of each functional group or side chain.

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.

Organic compound formulae

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:

• 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 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.

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

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 CH₃CH₂OH and the following displayed formula:

Displayed formula of ethanol. Anna Brewer, StudySmarter Original

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, CH₃CH₂OH. Here's how it translates into a skeletal formula.

Skeletal formula of ethanol. Anna Brewer, StudySmarter Original

Isomerism in organic compounds

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: 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.

Position isomerism in propanol. Anna Brewer, StudySmarter

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.