IGCSE Hydrocarbon Classification, Names & General Formulas

When you start learning about organic chemistry for the very first time, the very first thing you will learn are the concepts of hydrocarbons. Like, what a hydrocarbon is, their classification, general formula and so on. Hydrocarbons and their derivatives are the core of organic chemistry. 

So firstly, lets define a Hydrocarbon:
A Hydrocarbon is a compound which is made of hydrogen and carbon only.

Let’s get one thing clear, a hydrocarbon is a compound made out of only Hydrogen and Carbon atoms. So if there are any other atom(s) of any other elements present in the compound, then it is not a hydrocarbon. Please have a look at the Simple Example below:

Compound 1

Compound 2

  • Is a Hydrocarbon: This compound is made of Hydrogen and Carbon ONLY.
alcohol hydrocarbon
  • Not a Hydrocarbon: Because apart from carbon and hydrogen it also contains an oxygen atom.

In this blog, I am going to discuss some of the important concepts of Hydrocarbons, their classification, their general formula, homologous series, how to name them. Also covering some important concepts about one of the most important derivative of Hydrocarbon, which is Alcohol.

What will this blog cover and what it will not

Before you start reading this blog, let me make it clear what this blog will cover and what it will not. So please have a look at the table below, which shows the topics this blog will cover vs what it will not.

THIS BLOG WILL COVERTHIS BLOG WILL NOT COVER
Definition of Hydrocarbon Structural isomerism
Differentiating between Hydrocarbon and non-hydrocarbon Covering indepth about derivatives of Hydrocarbons
Classification of Hydrocarbons Reactions of derivatives of hydrocarbons
General Formulas of Homologus series Polymerization
Homologous Series Extraction of crude oil, cracking
Naming Hydrocarbons Polymerization

Touching up on a derivative of hydrocarbon

In the initial part of this blog, I have already discussed what a hydrocarbon is, I have also showed how to differentiate between a hydrocarbon and a non-hydrocarbon. If you’ve missed this part, then please CLICK HERE, to review that part again.

A Closer Look at Carbon and Hydrogen atoms

Hydrocarbons are covalent compounds. The carbon and hydrogen atoms covalently bond with each other to form a hydrocarbon. If your concepts about covalent bonding is not clear, then you can have a look at my blog “What is Covalent Bonding and How it is formed“. 

Carbon Atom: The carbon atom has four unpaired electrons in it’s outer shell. This means that it can form up to four covalent bonds. The diagram in the right with four lines shows that carbon is available to form four covalent bonds. Four atoms could attach themselves to carbon atom by doing covalent bond with it.

Hydrogen Atom: compared to a carbon atom, hydrogen atom has only one electron in it’s outer shell. Hence it can form only one covalent bond. So a hydrogen atom can attach itself to carbon by doing a simple covalent compound. The single line beside the hydrogen atom represents that it can form only one covalent bond with the carbon compound.

Example of the Simplest Hydrocarbon

The most simple hydrocarbon consists of only one carbon atom. As the carbon atom can form 4 covalent bonds, so it bonds covalently with 4 other hydrogen atoms. And this the product below which is formed. It’s name is methane. Do not worry about the naming too much, I will cover that part in great detail later on this blog.

Methane

Now lets have a look at an example showing a hydrocarbon with two carbon atoms. The name of this hydrocarbon is ethane. 

ethane
Ethane

Classification of Hydrocarbons

There are many types of hydrocarbons, but in IGCSE chemistry we will be dealing with only 2 of them, Alkane and Alkene. Plus I will cover some concepts about a derivative of hydrocarbons called Alcohols.

Alkane is the most simplest type of hydrocarbon, in this type of hydrocarbon the carbon atoms simply joins up with as many hydrogen atoms possible. Each carbon atom forms four covalent bonds. Please have look at two Alkanes below, methane and ethane.

Methane
ethane
Ethane

An Alkene is also a hydrocarbon, but it is has one carbon to carbon double bond (C=C) in it’s structure. Which means it doesn’t use up all it’s unpaired electrons to bond with hydrogen. There is no alkene with only one carbon atom, because the alkene needs to have at least two carbon atoms for a carbon to carbon bond to exist. Below are the examples of two alkenes, ethene(2 carbon atoms), propene(3 carbon atoms).

Ethene
Propene

Derivatives of Hydrocarbons

There are many derivatives of hydrocarbons, hydrocarbon derivative it means that the compound is formed from a hydrocarbon but is not a hydrocarbon anymore as it contains atoms from other elements other than Carbon or Hydrogen. Some hydrocarbon derivatives are natural and some are synthetic. But in this blog, I am going to touch up one of the most important derivative of hydrocarbons, that is Alcohol.

The structure of Alcohol is very similar to that of alkanes, no carbon to carbon double bond is present in its structure. The only difference is that one hydrogen atom is replaced by an -OH group(will discuss about Functional group later on this blog), please have a look at a simple 2 carbon alcohol structure below, it is called ethanol. An Alcohol is not a hydrocarbon as it contains an oxygen atom apart from the usual carbon and hydrogen atoms. You can call simply call it a derivative of hydrocarbon. To learn how alcohol is obtained from sugarcane, you can have a look my blog “Production of Ethanol From Sugarcane“.

General Formulas

What is General Formula? With the help of the general formula we can obtain the molecular formula of a specific hydrocarbon, given that we know the number of carbon atom(s) present in the hydrocarbon. Now you might ask that what is a molecular formula? The molecular formula simply tells us the number of atoms present from each element. For example, C2H6 is a molecular formula, it tells us that the compound has 2 carbon atoms and 6 hydrogen atoms in it. Of course you can draw a hydrocarbon and count the number of carbon and hydrogen atoms present, but that would be a massive waste of time right? Imagine if you were told to write down the molecular formula of a hydrocarbon with 10 carbon atoms!

In the table below, are listed down the general formulas of Alkane, Alkene and Alcohol

Hydrocarbon/Derivative of HydrocarbonGeneral Formula
AlkaneCnH2n+2
AlkeneCnH2n
AlcoholCnH2n+2O

Examples:

Now, let me show you how you can use the General Formula to find out the molecular formula of each type hydrocarbon.

Example 1: Finding the molecular formula of an Alkane with 3 carbon atoms
The General Formula of Alkane is CnH2n+2, the number of carbon atoms is 3 so n is 3, so the number of hydrogen atoms are 2(3)+2, which is 8. Hence the molecular formula for this alkane is: C3H8.

Example 2: Finding the molecular formula of an Alkene with 4 carbon atoms
The General Formula of Alkene is CnH2n, the number carbon atoms is 4 so n is 4, hence according to the formula, the number of hydrogen atoms are 2(4). Therefore the molecular formula for this alkene is C4H8.

Example 3: Finding the molecular formula of an Alcohol with 6 carbon atoms
The General Formula of Alcohol is CnH2n+2O, the number of carbon atoms is 6 so n is 6, hence according to the General Formula, the number of hydrogen atoms are 2(6)+2= 14. Hence the molecular formula of an alcohol with 6 carbon atoms is C6H14O

Functional Group

The Functional Group in an hydrocarbon or derivative of hydrocarbon is an atom or group of atoms which gives the compound unique chemical and physical properties. For example, the functional group in Alkene is the carbon to carbon double bond, the function group in Alcohol is the -OH group, while Alkane does not have any functional group.

Homologous series

The Homologous series is a family or group of hydrocarbons or derivatives having the same functional group, same general formula and similar chemical properties. They also have a similar trend in physical properties. For example, Alkane is a homologous series, Alkene is a homologous series, Alcohol is also a homologous series. There are also a bunch of other homologous series such as carboxylic acid, aldehydes and so on, but you do not need to focus on these ones. In this blog we are only going to focus on Alkane, Alkene and Alcohol.

Naming a Hydrocarbon

Naming a hydrocarbon and understanding what the name means is one of the most important concepts of organic chemistry. The name of a hydrocarbon mainly depends on the number of carbon atoms and in which homologous series the hydrocarbon is in. There are also other factors in the naming such as structure, isomerism etc. but for now I am going to ignore these factors to simplify things, as this blog is mainly for beginners who are starting out with organic chemistry. But if you want a more in-depth information about naming organic compounds along with different structures, then please have a look at my blog “Best IGCSE Organic Chemistry Notes“.

The name of a hydrocarbon is made out of two parts, the PREFIX and SUFFIX. The prefix part comes first and then followed by the suffix.

Name of Hydrocarbon = PREFIX + SUFFIX

The prefix part represents the number of carbon atoms the hydrocarbon has, while the suffix part represent that to which homologous series the hydrocarbon belongs to. Before we go on and start naming some hydrocarbons, lets have a look at the table of prefix and suffix. Only in the case of alcohol there is an “an” in between the prefix and suffix, it will be clarified to you in the examples below.

PREFIX lists representing the number of carbon atoms

No. of Carbon Atoms PREFIX
1Meth-
2Eth-
3Prop-
4But-
5Pent-
6Hex-
7Hep-
8Oct-
9Non-
10Dec-

SUFFIX representing Homologous series

Here I am providing only the suffix of alkane, alkene and alcohol as we are dealing with only three homologous series in this blog.

Homologous series SUFFIX Name
Alkane -ane
Alkene-ene
Alcohol-ol

Examples

Now lets have a look at some examples so it will be clear to you of how to use the prefix and suffix to name a hydrocarbon.

Example 1: Naming an Alkane with 6 carbon atoms. Please refer to the prefix and suffix table above.

Here the number of carbon atoms are 6, so the Prefix is “Hex”
The homologous series is Alkane, so the suffix is “ane”
Name of hydrocarbon = Prefix + Suffix = Hex + ane = Hexane

Example 2: Naming an Alkene with 5 carbon atoms. Please refer to the prefix and suffix table above.

Here the number of carbon atoms are 5, so the Prefix is “Pent”
The homologous series is Alkene, so the suffix is “ene”
Name of hydrocarbon = Prefix + Suffix = Pent + ene = Pentene

Example 3: Naming an Alcohol with 8 carbon atoms. Please refer to the prefix and suffix table above.
There is an “an” in between the prefix and suffix when naming a alcohol

Here the number of carbon atoms are 8, so the Prefix is “Oct”
The homologous series is Alcohol, so the suffix is “ol”
Name of hydrocarbon = Prefix +”an”+ Suffix = Oct +”an”+ol = Octanol

Conclusion

So in this blog we covered the basic to advanced concepts about hydrocarbons. Now we know the different classification of hydrocarbons, Alkane and Alkene. We also learned a few things about a derivative of hydrocarbon called Alcohol. We learned in great detail about general formula, homologous series and naming different kinds of hydrocarbons.

If this blog helped you out, then please feel to share this blog with your friends who might need help with Organic Chemistry. Also please have look at other blogs in this website, you might find something useful.

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