Carbon and its Compounds · Lesson 3 of 13
Saturated And Unsaturated Carbon Compounds
“Single, double or triple: carbon changes the bond and changes the family.”
• Construct structures of ethane, ethene and ethyne from their molecular formulae. • Distinguish saturated from unsaturated carbon compounds. • Explain how carbon's valency determines the number of bonds in a structure. • Compare single, double and triple carbon-carbon bonds. • Explain why unsaturated carbon compounds are generally more reactive.
The molecular formula tells us how many atoms are present, but a structure shows how those atoms are connected. Carbon structures are built by satisfying carbon's valency of four and hydrogen's valency of one.
Building The Structure Of Ethane
Ethane has formula C₂H₆. Start by joining the two carbon atoms with one single bond. A single C–C bond uses one valency of each carbon, leaving three valencies on each carbon. Attach three hydrogen atoms to each carbon. Every carbon now has four bonds and every hydrogen has one.
Problem
Build the structure of C₂H₆ step by step.
- 1.Place two carbon atoms next to each other.
- 2.Join them with a single C–C bond.
- 3.Each carbon has used one of its four valencies, so three valencies remain on each carbon.
- 4.Attach three H atoms to the first carbon and three H atoms to the second.
- 5.The completed structure is CH₃—CH₃.
A Saturated Carbon Compound contains only single bonds between its carbon atoms.
Ethane and propane are saturated because every carbon-carbon link is a single bond. Saturated compounds are generally less reactive than corresponding unsaturated compounds.
Building The Structure Of Ethene
Ethene has formula C₂H₄. Begin with a single bond between the two carbon atoms. If two hydrogens are attached to each carbon, one valency remains unsatisfied on each carbon. The two carbon atoms therefore share one additional electron pair, forming a double bond.
Problem
Build the structure of C₂H₄.
- 1.Join two carbon atoms initially.
- 2.Attach two hydrogen atoms to each carbon.
- 3.Each carbon still needs one more bond to reach valency four.
- 4.Form a second bond between the two carbon atoms.
- 5.The result is H₂C=CH₂, containing a C=C double bond.
Building The Structure Of Ethyne
Ethyne has formula C₂H₂. Each carbon is attached to one hydrogen, leaving three valencies on each carbon to be satisfied by bonding with the other carbon. The two carbons therefore form a triple bond.
Problem
Build the structure of C₂H₂.
- 1.Place two carbon atoms together.
- 2.Attach one hydrogen to each carbon.
- 3.Each carbon has used one valency with hydrogen and still needs three more bonds.
- 4.Create three shared pairs between the carbon atoms.
- 5.The result is HC≡CH.
An Unsaturated Carbon Compound contains one or more carbon-carbon double bonds or triple bonds.
Ethene and ethyne are unsaturated compounds. Their multiple bonds make them more reactive than saturated compounds, which is why unsaturated hydrocarbons readily undergo addition reactions later in the chapter.
| Compound | Formula | Carbon-Carbon Bond | Classification |
|---|---|---|---|
| Ethane | C₂H₆ | Single | Saturated |
| Ethene | C₂H₄ | Double | Unsaturated |
| Ethyne | C₂H₂ | Triple | Unsaturated |
A Reliable Structure-Building Method
For simple carbon compounds, first connect the carbon atoms. Next, add enough hydrogen atoms to satisfy the remaining valencies. If carbon still has unsatisfied valencies after the correct number of hydrogens has been used, increase the carbon-carbon bond order from single to double or triple.
Never accept a structure until every carbon has four bonds and every hydrogen has one bond.
Quiz
Which compound is saturated?
Why does ethene contain a double bond?
Which structure represents ethyne?
What defines an unsaturated carbon compound?
Which pair contains only unsaturated compounds?
Practice Problems
- Construct the structure of ethane from C₂H₆ and explain each bonding step.
- Construct the structure of ethene from C₂H₄ and show exactly why a double bond is required.
- Construct the structure of ethyne from C₂H₂ and explain why a triple bond is required.
- Classify CH₃—CH₃, CH₂=CH₂ and HC≡CH as saturated or unsaturated and justify each answer.
- Explain a general method for checking whether a drawn carbon structure satisfies all valencies.
- Compare the number of hydrogen atoms in ethane, ethene and ethyne and connect the difference to bond order.
Key Takeaways
• Structures are built by satisfying carbon's valency of four and hydrogen's valency of one. • Ethane contains a single carbon-carbon bond and is saturated. • Ethene contains a double bond and ethyne contains a triple bond, so both are unsaturated. • Unsaturated compounds contain one or more C=C or C≡C bonds. • Checking every atom's valency is the safest way to verify a structural formula.