Alkanes, alkenes, and alkynes are three major families of hydrocarbons, which are compounds made only of carbon and hydrogen. They are important because they form the basis of fuels, plastics, solvents, and many starting materials in organic chemistry. The family a molecule belongs to depends on whether its carbon chain contains only single bonds, at least one double bond, or at least one triple bond.
These bonding differences control the molecule's formula, shape, and typical reactions.
Alkanes are saturated hydrocarbons because each carbon has the maximum possible number of hydrogen atoms, while alkenes and alkynes are unsaturated because multiple bonds reduce the number of hydrogens. A carbon-carbon single bond allows rotation and usually has tetrahedral geometry around carbon, while a double bond is rigid and planar, and a triple bond is linear. Multiple bonds contain pi bonds, which are more exposed and reactive than sigma bonds.
Because of this, alkenes and alkynes often undergo addition reactions, while alkanes are less reactive and commonly undergo combustion or substitution.
Key Facts
- Alkanes contain only carbon-carbon single bonds and have the general formula CnH2n+2 for acyclic chains.
- Alkenes contain at least one carbon-carbon double bond and have the general formula CnH2n for acyclic chains with one double bond.
- Alkynes contain at least one carbon-carbon triple bond and have the general formula CnH2n-2 for acyclic chains with one triple bond.
- A single bond is one sigma bond, a double bond is one sigma bond plus one pi bond, and a triple bond is one sigma bond plus two pi bonds.
- Typical bond angles are about 109.5° for alkane carbons, about 120° for alkene carbons, and 180° for alkyne carbons.
- Complete combustion of a hydrocarbon follows hydrocarbon + O2 -> CO2 + H2O and releases energy.
Vocabulary
- Hydrocarbon
- A hydrocarbon is an organic compound made only of carbon and hydrogen atoms.
- Saturated hydrocarbon
- A saturated hydrocarbon has only single bonds between carbon atoms and contains the maximum number of hydrogen atoms.
- Unsaturated hydrocarbon
- An unsaturated hydrocarbon has at least one carbon-carbon double or triple bond and therefore fewer hydrogen atoms than the matching alkane.
- Pi bond
- A pi bond is a covalent bond formed by sideways overlap of orbitals and is found in double and triple bonds.
- Addition reaction
- An addition reaction adds atoms across a double or triple bond, converting part or all of the multiple bond into single bonds.
Common Mistakes to Avoid
- Using the wrong general formula, such as applying CnH2n+2 to an alkene. This is wrong because each double bond removes two hydrogen atoms compared with the matching alkane.
- Thinking double and triple bonds rotate freely like single bonds. This is wrong because pi bonds lock the bonded carbons in place unless the pi bond is broken.
- Calling all hydrocarbons nonreactive. This is wrong because alkenes and alkynes are relatively reactive due to exposed pi bonds, especially in addition reactions.
- Forgetting that shape changes with bond type. This is wrong because single-bonded carbons are usually tetrahedral, double-bonded carbons are trigonal planar, and triple-bonded carbons are linear.
Practice Questions
- 1 Write the molecular formulas for the straight-chain alkane, alkene, and alkyne that each contain 5 carbon atoms.
- 2 A hydrocarbon has the formula C6H10 and contains one carbon-carbon triple bond. Does it fit the alkyne general formula for an acyclic molecule, and why?
- 3 Explain why ethene reacts more readily with bromine than ethane, using the difference between sigma bonds and pi bonds.