Chemistry GCSE Revision: Organic Chemistry

by Lucy Bell-Young

Organic chemistry is one of the most difficult science subjects – one that even science majors find challenging. If you’re in secondary school and preparing for your GCSE in science, you probably know what we’re talking about. 

Not only does organic chemistry involve understanding complex concepts and memorising molecular structures, you’ll also need to learn the standard nomenclature system and how it’s related to the chemical formulas and molecular structures of the organic compounds. This article will run you through the basics of organic chemistry so that you have a good foundation of knowledge when revising this subject.

What is Organic Chemistry All About?

Organic chemistry is primarily about the study of carbon-based compounds, including their chemical properties, physical properties, structures, and reactions. An organic compound is identified as having carbon atoms that are held together through covalent bonding.

Organic compounds are typically synthesised by living organisms and are found in things like animal and plant tissues, fruits and vegetables, nails, milk, and horns. But organic compounds aren’t only in the cells of living things. They’re found throughout the world, land or sea, and often exist in wood, paper, petroleum, and even gasoline. The main examples of organic compounds in this respect are the hydrocarbon mixtures in crude oil or fossil fuel.

Organic chemistry often overlaps with biochemistry, where the main concern is how chemicals function inside a living organism. Organic chemistry, on the other hand, mainly deals with the chemical behaviour and synthesis of carbon-based chemicals outside living organisms. That means it’s very useful in developing pharmaceutical products, manufacturing polymers like plastics, and processing fossil fuels.

Organic chemistry covers a wide range of organic compounds, including hydrocarbons, organometallic compounds, and other carbon-based compounds, especially those that contain oxygen, nitrogen, sulfur, phosphorus, and halogens. 

Crude Oil, Hydrocarbons and Alkanes

Your GCSE revision for organic chemistry should be systematic. You need to first understand the underlying principles of the subject, and then apply these principles to specific examples. It’s also important to know the different categories of organic compounds, such as hydrocarbons. Hydrocarbons are organic compounds that include crude oil mixtures, most of which are alkanes.

Classifications of hydrocarbons

What Are the Different Types of Hydrocarbons?

There are several classes of organic molecules. Hydrocarbons are the simplest because they’re only composed of carbon and hydrogen. The most complex organic compounds, on the other hand, are biomolecules, which include proteins, lipids, carbohydrates, and nucleic acids found in living organisms. While these are organic compounds, they’re not the focus of organic chemistry.

Hydrocarbons are particularly important in organic chemistry because they’re the main source of energy that powers industries and transportation. Hydrocarbon derivatives, particularly plastics, are also crucial materials for manufacturing a wide range of commercial products. Based on the standard IUPAC nomenclature, hydrocarbons are classified into three categories:

  1. Saturated hydrocarbons

These are the simplest type of hydrocarbons, and only have single bonds. As such, they’re saturated with hydrogen because all the other non-carbon-to-carbon bonds are occupied by hydrogen atoms. For example, methane has one carbon atom but four hydrogen atoms. 

Acyclic saturated hydrocarbons are called alkanes, and have the generalised formula of CnH2n+. This means that for every carbon, the number of hydrogen is double that number plus two. 

Straight chain saturated hydrocarbons are the easiest to name and to deduce the formula of. For example, a hydrocarbon of this type with seven carbons is called heptane, just as a seven sided shape is called a heptagon. Using the rule above, we can then figure out that the chemical formula would be C7H16: we know that there are seven carbon atoms, so we double this to get 14, and add two when determining the number of hydrogen atoms.

The generalised formula for saturated hydrocarbons with rings is CnH2n+2(1-r). The “r” represents the number of rings. Alkanes that have exactly one ring are called cycloalkanes. 

Alkanes are the foundation of refined petroleum fuels and related products. They’re either linear or branched in form. Petroleum fuels are mixtures of alkanes and other types of hydrocarbons. For example, gasoline is typically composed of the following by volume:

  • 4-8% alkanes
  • 2-5% alkenes
  • 25-40% isoalkanes
  • 3-7% cycloalkanes
  • 1-4% cycloalkenes
  • 0-50% total aromatics (0.5-2.5% benzene)

Saturated hydrocarbons are very useful for creating other products, particular polymers, because other chemicals or elements can easily react through substitutions. For example, chloroform can be synthesised from methane through a chlorination reaction.

  1. Unsaturated hydrocarbons

These hydrocarbons have one or more double or triple bonds between their carbon atoms. Hydrocarbons with double bonds are called alkenes, and have the generalised formula CnH2n. This generalised formula is applicable to those without cyclic structures. Hydrocarbons that have triple bonds are called alkynes with the generalised formula CnH2n−2.

  1. Aromatic hydrocarbons

Aromatic hydrocarbons are cyclic in their molecular structures, or at least have one aromatic ring per molecule. They are called arenes and the most common example is benzene. Approximately 10% of non-methane emissions from gasoline-powered vehicles are aromatic hydrocarbons.

Fact sheet of benzene

More Organic Chemistry

Organic chemistry also devotes a significant portion of its study to functional groups. This is not only a mere convention in classifying organic chemicals but more importantly about identifying the structures and chemical properties of various types of organic chemicals.

The main categories of organic chemicals based on the functional groups are:

  • Hydrocarbons: As previously mentioned, these are compounds that contain only carbon and hydrogen atoms. Compounds are either linear or cyclic. They can also have several isomers or molecular variations.
  • Groups containing halogen: These are haloalkane compounds that contain halogens such as fluorine, chlorine and bromine. The functional groups are named after the halogen. For example, a haloalkane that contains iodine is called iodoalkane. The generalised formula is R-X, where R represents the alkane chain while the X represents the halogen.
  • Groups containing oxygen: These groups have wide variations in terms of molecular structure. They include alcohol (hydroxyl), ketone (carbonyl), ester (carboalkoxy), and carboxylic acids (carboxyl). There are other more complex forms that are defined by the functional group and structure of oxygen bonding.
  • Groups containing nitrogen: These groups are very important in living organisms because they form protein molecules and other biomolecules. Some examples are amines, azides, and pyridine derivatives.
  • Groups containing sulphur: Organic chemicals that contain sulphur have unique chemical properties and reactivity because of their capacity to form more bonds than oxygen. Examples include sulphides, sulphoxides, and sulphonate esters. These groups are important because many antibiotics are based on sulphur organic compounds.
  • Groups containing phosphorus: The most important molecule that contains phosphorus is DNA, which carries the genetic code of all eukaryotic organisms. Other examples are phosphines, phosphates, and phosphoric acid.
  • Groups containing boron: Examples of these groups are either acids or esters. They include boronic acids and boronic esters.

GCSE Organic Chemistry Sample Questions

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