Overview
This lecture covers the chemistry of hydrocarbons, focusing on alkanes, alkenes, and alkynes, their structural differences, naming rules, isomerism, and how these relate to fats and nutrition.
Hydrocarbons: Structure and Types
- Alkanes are saturated hydrocarbons with only single (sigma) bonds; each carbon forms four bonds.
- Unsaturated hydrocarbons have double or triple bonds, forming alkenes (double) and alkynes (triple).
- Alkenes have at least one carbon-carbon double bond; alkynes have at least one carbon-carbon triple bond.
Naming Conventions
- Alkanes end in -ane, alkenes in -ene, and alkynes in -yne.
- The carbon chain is numbered to assign the lowest possible number to double or triple bonds.
- Prefixes (meth-, eth-, prop-, etc.) indicate the number of carbons and remain the same across alkane, alkene, alkyne.
Bonding and Isomerism
- Single (sigma) bonds allow free rotation; double and triple bonds restrict rotation.
- Double bonds consist of one sigma and one pi bond; triple bonds have one sigma and two pi bonds.
- Restricted rotation in alkenes causes cis-trans (geometric) isomerism: "cis" (same side), "trans" (across).
- Alkynes do not form cis-trans isomers due to their bonding structure.
Chemical Reactions of Alkenes and Alkynes
- Pi bonds in alkenes and alkynes are less stable and undergo addition reactions.
- Hydrogenation adds H₂ across double/triple bonds, often needing a catalyst.
- Halogenation, hydration, and oxygenation are other addition reactions that break double/triple bonds.
- Polymerization can link multiple hydrocarbon units via opened double bonds.
Fats and Their Chemistry
- Fats (triglycerides) consist of three fatty acids bonded to glycerol.
- Saturated fats have only single bonds (alkanes); stable, solid at room temperature, but harder for the body to process.
- Unsaturated fats contain double bonds; more reactive, usually liquid, and healthier.
- Hydrogenation converts unsaturated fats to saturated or partially saturated (mono-unsaturated) fats.
- Omega-3 fats have a double bond three carbons from the end of the fatty acid chain; essential for health.
- Cis fats are natural; trans fats (from partial hydrogenation) act like saturated fats and are unhealthy.
Key Terms & Definitions
- Alkane — saturated hydrocarbon with only single bonds.
- Alkene — unsaturated hydrocarbon with at least one double bond.
- Alkyne — unsaturated hydrocarbon with at least one triple bond.
- Sigma bond — single covalent bond allowing free rotation.
- Pi bond — additional bond in double/triple bonds that restricts rotation.
- Cis-trans isomerism — isomerism due to restricted rotation around double bonds.
- Hydrogenation — addition of hydrogen to a double or triple bond.
- Triglyceride — molecule of fat formed by three fatty acids bonded to glycerol.
- Saturated fat — fat with no double bonds (all alkanes).
- Unsaturated fat — fat with one or more double bonds (alkenes/alkynes).
- Trans fat — unsaturated fat with trans isomerism, unhealthy for humans.
Action Items / Next Steps
- Review structural formulas for alkanes, alkenes, and alkynes.
- Practice naming and identifying cis/trans isomers.
- Read more about fatty acid structure and function (next lecture).