Skip to 0 minutes and 6 seconds Esters are a class of organic compounds that all contain the formula RCO2R. Low molecular weight esters typically have pleasant fruity smells, including apple, banana and strawberries, and consequently, they are often used in flavourings and perfumes. One example of a fruity ester is propyl ethanoate, which is a colourless liquid that smells of pears. Here is the structure of propyl ethanoate, CH3CO2CH2CH2CH3 - notice that there is a propyl group (CH3CH2CH2-) bonded to oxygen and that the ethanoate group is CH3CO2-. It can be easily be made in the lab by reacting the alcohol, propan-1-ol, with the carboxylic acid, ethanoic acid, in an esterification reaction - notice that water is formed as a byproduct.
Skip to 0 minutes and 54 seconds You will see that a small amount of an acid, such as sulfuric acid, is used in the reaction. The acid increases the rate of formation of the ester and only a small amount is required, as it is not changed chemically during the reaction. So it acts as a catalyst. So, let’s consider the structure of propyl ethanoate - the carbon chain has a zig-zag shape and the bond angles around the planar carbonyl group are 120 degrees One of the best ways of learning about the structure and shape of organic compounds is through molecular models, and here is a molecular model of propyl ethanoate.
Skip to 1 minute and 32 seconds Having a physical molecular model that you can build and rotate in your hands will help you to imagine, in 3-dimensions, what organic compounds, like esters, look like. So, your challenge in this activity is to build a realistic molecular model of propyl ethanoate using readily available household materials. You may like to consider using sweets, cocktail sticks, pipe cleaners, plasticine, buttons, magnets, straws, pencils, or even Christmas decorations, but the most important thing is to use your imagination and be creative. Post pictures of your creation on our site for the world to see!
Making a molecular model
During this exercise we are going to use readily available materials – that we can all find around the house or buy easily at the supermarket – to make a molecular model of a fragrance molecule.
We would like you to have a go at making propyl ethanoate, and we want you to be as creative as you can with the materials you use! But remember that we still want to be able to see a true representation of the molecule (i.e for it to have the correct 3D shape with bond angles that look about accurate).
The following ingredients will be required:
- Sweets, Play-Doh balls or other round objects to be used as atoms, in three different colours (ideally red, white and black)
- Cocktail sticks, straws, pipe-cleaners etc. (something to be used as bonds).
Ensure that you read through all of the instructions before beginning the experiment to ensure that you understand exactly what will happen at each step.
Step one: pick your fragrance molecule
We have made an attempt at recreating the ester we would like you to make (propyl ethanoate) - the skeletal structure is shown below.
Step two: pick your materials
For our molecules we have chosen to use red gummy sweets for oxygen, black gummy sweets for carbon, white mini marshmallow faces for hydrogen and cocktail sticks for the bonds. But this is where you get to be really creative!
You could use fruit, marzipan, Play-Doh… anything that you have around the house that could work as atoms and molecules - we’ll leave that side of things up to you.
Step three: have a go
Here's a picture of our gummy sweet creation!
It can be quite hard to get models to stand upright, but the main thing is that you can use your model to help you visualise fragrance molecules in 3D.
If you get stuck trying to make your molecular model, have a look at our help sheet (below) which could be handy when trying to get the right bond angles around your atoms.
When you are done we would love to see your creative masterpieces! Why not upload a picture of your model to our our open Padlet or use the Twitter hashtag #FLchemistry, to reveal how you got on. Also, let us know how you got on with the task using the comments section below.
If you need any guidance on using the Padlet, then further information is available - we would very much like for everyone to feel part of the learning community, so look forward to seeing your contributions.
For your safety it is advised not to eat any part of the molecular model you have made at any stage of the experiment. Dispose of the samples into a general waste bin. Take appropriate precautions when using cocktail sticks - or other sharp objects - to ensure you do not injure yourself.
© University of York