How often have you read a food label and wondered what saturated fats are? Fats differ according to whether they contain single or double bonds in their structure! Compounds in organic chemistry are also divided into saturated and unsaturated compounds according to their bonds!
So, without further ado, let's dive into unsaturated and saturated bonds!
- First, we will talk about saturated and unsaturated carbon compounds.
- Next, we will learn about the structure of saturated bonds.
- After, we will look at the definition of unsaturated bonds and their reactivity.
- Then, we will talk about the catalytic hydrogenation of alkenes and alkynes into alkanes.
- Lastly, we will look at the basics of degree of unsaturation (IHD) and how to use it to find out how many hydrogens are missing in a compound.
Saturated and Unsaturated Carbon Compounds
Before diving into saturated and unsaturated bonds, let's review what carbon compounds are. Carbon compounds are compounds that contain carbon and at least one other element, such as hydrogen, oxygen, nitrogen, and sulfur.
Figure 1 shows some examples of carbon compounds.
Now, If you look carefully at the carbon compounds above, what do you notice about them? If you said that some of them have single bonds while others have double bonds, then you are on the right track! A carbon (C) atom is able to form single (C–C), double (C=C), and even triple bonds (C≡C)!
When an organic compound possesses the maximum number of single bonds to carbon, and no double or triple bonds, we call it a saturated bond.
A saturated bond is basically a single bond; a carbon-to-hydrogen bond. Carbon compounds that are considered saturated are those that have the maximum number of hydrogen atoms possible in their chemical structure.
Saturated Bond Structure
As an example, let's look at the saturated bond structure of methane, CH4. Here, notice that the carbon atom is surrounded by four single covalent bonds to hydrogen, making it a saturated hydrocarbon. Carbon compounds containing only carbon and hydrogen atoms (such as CH4) are called hydrocarbons, and they are the simplest form of carbon compounds!
Hydrocarbons with saturated bonds are called alkanes, and they have the general formula CnH2n+2. For example, if we had an alkane with 3 carbon atoms, its general formula would be C3H8. The saturated bond structure of propane (C3H8) is shown below. The condensed formula for propane is CH3CH2CH3.
Unsaturated Bond Definition
Next, we have unsaturated bonds. Unsaturated hydrocarbons can be alkenes and have the general formula CnH2n, or alkynes, with the general formula CnH2n-2.
Hydrocarbons with unsaturated bond contain one or more double bonds, or even a triple bond.
Ethene (C2H4) and ethyne (C2H2) are both examples of carbon compounds containing unsaturated bonds (Figure 4). Ethene is the simplest alkene in organic chemistry, and it has a double bond between the two carbon atoms. On the other hand, ethyne is the simplest alkyne, and it has a triple bond between the two carbon atoms.
Let's look at an example!
What type of hydrocarbon is CH3CHCHCH3?
To solve this problem, the first thing we need to do is count the number of carbon and hydrogen atoms in the carbon compound. In this case, there are four carbon atoms and eight hydrogen atoms.
So, since n is equal to 4, then the only way to get eight hydrogen atoms would be to use the general formula for alkenes: CnH2n. Therefore, CH3CHCHCH3 is an unsaturated hydrocarbon with a double bond.
Another organic compound that is considered unsaturated is the aromatic compound. Aromatic compounds include carbon compounds that have a benzene ring (six-membered ring) containing double bonds.
Unsaturated Bond
Let's take a moment to talk about orbitals in chemical bonds.
Sigma (σ) Bond - this is an orbital that forms bonds through direct overlap and looks like this for saturated hydrocarbons:
Pi (π) Bond - this is an orbital that forms bonds through sideways overlap and looks like this for double-bonded hydrocarbons:
Now that we know what saturated, and unsaturated bonds are, let's talk about their reactivity. But first, remember that single bonds are only made up of sigma (σ) bonds, whereas double and triple bonds contain both sigma (σ) and pi (π) bonds. Water (H2O), for example, has only single, sigma bonds!
Let's look at an example.
How many sigma (σ) and pi (π) bonds does the molecular structure of propene has?
If you were to draw the structure of of the alkene propene (C3H6), you would find that it has 7 single bonds and one double bond. Now, since double bonds are made up of one sigma plus one pi bond, the structure of propene has eight sigma bonds and one pi bond in total!
The presence of these pi (π) bonds in alkenes and alkynes make them more chemically reactive than alkanes because π bonds are easier to break, allowing each carbon atom to become an active site that can form a new covalent bond with another atom (one pi bond turns into 2 sigma bonds).
Double Saturated Bond
When talking about unsaturated and saturated bonds, there is a technique called catalytic hydrogenation, which can be used to saturate a double bond! The process of catalytic hydrogenation of ethene (C2H4) into ethane (C2H6) can be seen in Figure 9, below.
First, ethene attaches to a metal catalyst surface covered with hydrogen atoms. Then, double bond between the two carbon atoms gets broken and the hydrogen atoms covalently bond to the carbon atoms. This addition of two hydrogen atoms produces a saturated alkane (ethane).
Hydrogenation can also be used to convert an alkyne into an alkane. For example, the addition of hydrogen gas to ethyne turns it into ethane!
Saturated vs. Unsaturated Bond
Let's continue learning about unsaturated and saturated bonds by looking at the degree of unsaturation (IHD) of hydrocarbons. The degree of unsaturation (IHD) rules helps us quickly determine which molecule is more saturated/less saturated with hydrogen.
Having 1 IHD is the equivalent of that compound missing 2 hydrogens.
Rings or double bonds count as 1 IHD
Triple bonds count as 2 IHD
Let's look at an example.
What would be the degree of unsaturation of the following compound?
Figure 11. Drawing of an organic compound.
The first thing we need to do is count the number of double bonds/triple bonds and rings the structure has. In this case, the structure has 4 double bonds and 1 ring. Since double bonds or rings count as 1 IHD, this structure will then have an IHD of 5.
Now, If 1 IHD is the equivalent of missing two electrons, then having 5 IDH means that this unsaturated molecule is missing 10 hydrogen atoms!
Figure 12. Finding the degree of unsaturation of an organic compound.
Saturated and Unsaturated Bonds Difference
To finish off, let's make a table to review the difference between saturated and unsaturated bonds.
| Saturated Compounds | Unsaturated Compounds |
Saturated compounds are those that only have single bonds between carbon atoms. | Unsaturated compounds are carbon compounds that have double or triple bonds. |
Alkanes (ex. propane) are examples of saturated carbon compounds. | Examples of compounds with unsaturated bonds include alkenes and alkynes. |
Now, I hope that you were able to gain a better understanding of what unsaturated and saturated bonds are!
Unsaturated and Saturated Bonds - Key takeaways
- Carbon compounds are compounds that contain carbon and at least one other element such as hydrogen, oxygen, nitrogen, and sulfur.
- A saturated bond is basically a single bond. Carbon compounds that are considered saturated are those that have the maximum number of hydrogen atoms possible in its chemical structure.
- Hydrocarbons with unsaturated bond contain one or more double bonds, or even a triple bond between C–C atoms.
- To convert an unsaturated alkyne or an alkene into a saturated alkane, chemists can use the process of catalytic hydrogenation.
References
- Theodore Lawrence Brown, Eugene, H., Bursten, B. E., Murphy, C. J., Woodward, P. M., Stoltzfus, M. W., & Lufaso, M. W. (2018). Chemistry : the central science (14th ed.). Pearson.
- Moore, J. T., & Langley, R. (2021b). McGraw Hill : AP chemistry, 2022. Mcgraw-Hill Education.
- Master Organic Chemistry - An Online Organic Chemistry Resource. (2019b). Master Organic Chemistry. https://www.masterorganicchemistry.com/
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