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Saturated and Unsaturated Organic Compounds Chemistry Tutorial

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Saturated Hydrocarbons

A hydrocarbon is a compound composed of only carbon atoms (C) and hydrogen atoms (H).

The bonds between carbon atoms, and between carbon and hydrogen atoms, are all covalent bonds.

Each carbon atom can make 4 covalent bonds. These bonds can be to hydrogen atoms or to other carbon atoms.

If all the covalent bonds between the carbon atoms in the hydrocarbon molecule are single bonds, C−C, then the hydrocarbon molecule is said to be saturated.

The alkane homologous series (CnH2n+2) provides many examples of saturated hydrocarbons:

Examples of Saturated Hydrocarbon Compounds

(only single bonds between carbon atoms)

Molecular structure Molecular formula Name
  H
|		  
H− C −H
  |
H		  
CH4 methane
  H
|		   H
|		  
H− C − C −H
  |
H		   |
H		  
C2H6 ethane
  H
|		   H
|		   H
|		  
H− C − C − C −H
  |
H		   |
H		   |
H		  
C3H8 propane
  H
|		   H
|		   H
|		   H
|		  
H− C − C − C − C −H
  |
H		   |
H		   |
H		   |
H		  
C4H10 butane
  H
|		   H
|		   H
|		   H
|		   H
|		  
H− C − C − C − C − C −H
  |
H		   |
H		   |
H		   |
H		   |
H		  
C5H12 pentane

Even branched-chain alkanes are saturated because all the covalent bonds between carbon atoms are single bonds:

Examples of Branched-chain Saturated Hydrocarbons

(only single bonds between carbon atoms)

Molecular structure Molecular formula Name
      H
|		      
  H H-C-H H  
  |   |   |  
H− C − C − C −H
  |
H		   |
H		   |
H		  
C4H10 2-methylpropane
      H
|		          
  H H-C-H H   H  
  |   |   |   |  
H− C − C − C − C −H
  |
H		   |
H		   |
H		   |
H		  
C5H12 2-methylbutane

Even if the carbon atoms form a "ring" instead of a "chain", the molecules will still be saturated as long as all the covalent bonds between the carbon atoms are single bonds. When the carbon atoms all join up to form a ring, we refer to these as cyclic compounds. We name these cyclic alkanes by placing the prefix "cyclo" in front of the name of the alkane.

Examples of Saturated Cyclic Hydrocarbon Compounds

(only single bonds between carbon atoms)

Molecular structure Molecular formula Name
C3H6 cyclopropane
C4H8 cyclobutane
C5H10 cyclopentane
C6H12 cyclohexane

A saturated hydrocarbon is an alkane and can be a:

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Unsaturated Hydrocarbons

An unsaturated hydrocarbon is a molecule made up of only carbon and hydrogen atoms, but 1 or more of the covalent bonds between the carbon atoms is not a single bond, it is either a double bond (C=C) or a triple bond (C≡C). (1)

If the hydrocarbon molecule contains a double bond it is an alkene.

If the hydrocarbon molecule contains a triple bond it is an alkyne.

A molecule belonging to either the alkene or alkyne homologous series is an unsaturated hydrocarbon.

The carbon atoms can be arranged in straight chains, branched chains, or in rings (cyclic compounds), as long as there is at least 1 double bond (C=C) and/or 1 triple bond (C≡C) the molecule will be unsaturated.(2)

Examples of Unsaturated Organic Compounds

(a double bond or triple between carbon atoms)

Homologous Series Molecular structure Molecular formula Name
Alkene
H       H
  \   /  
    C=C    
  /   \  
H       H
C2H4 ethylene
(ethene)
H       H   H  
  \     |   |  
    C = C − C −H
  /         |  
H           H  
C3H6 propene
(propylene)
H       H   H   H  
  \     |   |   |  
    C = C − C − C −H
  /         |   |  
H           H   H  
C4H8 but-1-ene
(1-butene)
        H
|		      
H     H-C-H H  
  \     |   |  
    C = C − C −H
  /         |  
H           H  
C4H8 2-methylpropene
(isobutylene)
Alkyne
H−C≡C−H
 C2H2 acetylene
(ethyne)
      H
| 		 
H−C≡ C − C −H
      |
H 		 
C3H4 propyne
(prop-1-yne)
      H
| 		  H
| 		 
H−C≡ C − C − C −H
      |
H 		  |
H 		 
C4H6 but-1-yne
(1-butyne)
Cyclic alkene C6H10 cyclohexene

An unsaturated hydrocarbon is a compound composed of only carbon and hydrogen atoms, and contains at least one double and/or triple bond between between two of the carbon atoms.

Unsaturated hydrocarbons can be :

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Tests for Saturated and Unsaturated Hydrocarbons

Saturated hydrocarbons can be distinguished from unsaturated hydrocarbons in the laboratory because saturated hydrocarbons are less chemically active (reactive) than unsaturated hydrocarbons. This means alkenes and alkynes more likely to readily react with a chemical reagent than an alkane.

For a qualitative test(3) for unsaturation, a test for the presence or absence or unsaturation, we want to choose a chemical reaction that produces an effect that is easy to observe, a colour change for example.

Two common reagents used to detect whether a hydrocarbon is saturated or unsaturated are:

Bromine Water Test

The preferred test for unsaturation uses bromine water, Br2(aq)(4). Bromine water is an aqueous solution of bromine that is a brown colour if the solution is concentrated, but gradually changes to a more orange colour then to yellow with increasing dilution.

Low-molecular mass hydrocarbons are colourless gases. Gases can be bubbled through the bromine water, if a chemical reaction occurs the colour of the bromine water will "fade", we say the bromine water decolourises.
The decolourisation of the bromine water indicates that the hydrocarbon is unsaturated.
If the bromine water does not decolourise this indicates the presence of a saturated hydrocarbon.(5)

Some hydrocarbons are colourless liquids at room temperture and pressure. The density of these hydrocarbons is less than the density of water so these liquid hydrocarbons float in a layer above the bromine water.
If a few drops of bromine water is added to a saturated liquid hydrocarbon such as cyclohexane and shaken, decolourisation of the bromine water is NOT observed under standard laboratory conditions. The mixture will separate into 2 distinct layers, a colourless layer of cylcohexane floating on top of the yellow-brown bromine water layer at the bottom.

If a few drops of bromine water is added to an unsaturated liquid hydrocarbon such as cyclohexene and shaken, the bromine water readily decolourises (becomes increasingly less coloured until it is colourless).
If there is an excess of the unsaturated hydrocarbon, then the mixture will settle into 2 distinct but colourless layers with the colourless organic layer floating above the colourless aqueous layer.

Potassium Permanganate Test

Concentrated aqueous solutions of potassium permanganate are a purple colour which fades through to a pink colour as the solution is increasingly diluted.

Low molecular mass gaseous hydrocarbons can be bubbled through potassium permanganate solution.
If the hydrocarbon is saturated there will be no visible colour change (the potassium permanganate solution stays pink-purple).
If the hydrocarbon is unsaturated the potassium permanganate solution will decolourise (pink colour fades to colourless).

For colourless liquid hydrocarbons, add a few drops of potassium permanganate solution and shake.
If the hydrocarbon is saturated there will be no visible colour change (the potassium permanganate solution stays pink-purple).
If the hydrocarbon is unsaturated the potassium permanganate solution will decolourise (pink colour fades to colourless).

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Other Saturated and Unsaturated Organic Compounds

The term "saturated" can be applied to any organic molecule that contains only single bonds between the carbon atoms (C−C), even if another functional group is present in the molecule.

So, for example, we can have

The term "unsaturated" can be applied to any organic molecule that contains a double bond (C=C) or a triple bond (C≡C), even if another functional group is present in the molecule.

So, for example, we can have

Qualitative tests for saturation become a bit trickier when a functional group, other than a double or triple bond, is present.(6)

Footnotes:

(1) You can have a double bond and a triple bond in the same molecule, these are often referred to as "enynes", or, if there is more than one double and/or triple bond they are called "polyenynes".

(2) This works well for aliphatic hydrocarbons, but ofcourse aromatic compounds are a bit trickier ... refer to the tutorial on benzene for an introduction to this.

(3) A qualitative test for saturation will tell us whether the compound is saturated or unsaturated, but if it is unsaturated then a qualitative test will not tell us how many double and/or triple bonds are present.
A quantitative analysis of an unsaturated compound will tell us how many double and/or triple bonds are present in the compound.

(4) You can also use bromine dissolved in an organic solvent such as tetrachloromethane (carbon tetrachloride) instead of water. Note that both bromine and hydrocarbons are soluble in tetrachloromethane so distinct layers will not form.
When exposed to strong light, alkanes will undergo a substitution reaction with the bromine which will produce hydrogen bromide, HBr, which is only slightly soluble in tetrachloromethane, so you can test the vapour built up above the solution in a test tube with blue litmus paper which will turn red due to the presence of HBr. The reaction between bromine and an unsaturated hydrocarbon is an addition reaction so no hydrogen bromide (HBr) is produced, so if you test the gas above the solution in the test tube with blue litmus paper then the litmus paper will not change colour.
This bromine solution is not usually used in schools because tetrachloromethane is a known carcinogen.

(5) This is true as long as you don't expose your reaction mixture to strong sunlight or even place it close to bright light bulb! Exposure to strong light will produce a chemical reaction (a substitution reaction) with saturated hydrocarbons.

(6) Even the presence of conjugation within a molecule alters its stability (and hence reactivity).
A discussion of the possible types of reactions is beyond the scope of this very introductory tutorial.
Be aware that the terms "saturated" and "unsaturated" can be applied to organic compounds other than just hydrocarbons for now.