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Chapter 3 Alkenes and Alkynes

Chemistry 20. Chapter 3 Alkenes and Alkynes. Hydrocarbons. Large family of organic compounds Composed of only carbon and hydrogen. Saturated hydrocarbons Alkanes. Unsaturated hydrocarbons Alkenes, Alkynes & Aromatics. C = C. C C. C - C. Alkenes and Alkynes.

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Chapter 3 Alkenes and Alkynes

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  1. Chemistry 20 Chapter 3 Alkenes and Alkynes

  2. Hydrocarbons Large family of organic compounds Composed of only carbon and hydrogen Saturated hydrocarbons Alkanes Unsaturated hydrocarbons Alkenes, Alkynes & Aromatics C = C C C C - C

  3. Alkenes and Alkynes • Saturated compounds (alkanes): • Have the maximum number of hydrogen • atoms attached to each carbon atom. • Unsaturated compounds: • Have fewer hydrogen atoms attached to • the carbon chain than alkanes. • Containing double bond are alkenes. • CnH2n • Containing triple bonds are alkynes. • CnH2n-2

  4. 800-900°C CH3-CH3 CH2=CH2 + H2 120° Thermal cracking 180°

  5. Naming Alkenes & Alkynes Using the IUPAC alkane names: Alkene names change the end to -ene. Alkyne names change the end to -yne

  6. STEP 2 Number the carbon atoms starting from the end nearest a double or triple bond. Give the location for double and triple bond STEP 3 Give the location and name of each substituent (alphabetical order) as a prefix to the name of the main chain.

  7. Naming Alkenes & Alkynes CH2= CH ─ CH2─ CH31-Butene CH3─ CH=CH─ CH32-Butene CH3 | CH3─ CH=C─CH32-Methyl-2-butene CH3─ CC ─ CH32-Butyne 1 3 2 4 2 1 3 4 4 2 1 3 1 3 2 4

  8. Naming Alkenes & Alkynes CH3─ CH2─ CC ─ CH3 2-Pentyne CH3 CH3─ CH2─ C=CH ─ CH3 3-Methyl-2-pentene CH2 – CH3 CH3─ CH2─ C=CH ─ CH3 3-Ethyl-2-pentene 1 3 5 2 4 1 3 5 2 4 3 1 5 2 4

  9. Conformation Under normal condition, no rotation is possible about the carbon-carbon double and triple bond. Flat or planar molecules

  10. Cis & Trans Stereoisomers H H H CH3 C = C C = C H3C H3C H CH3 C4H8 C4H8 cis-2-Butene trans-2-Butene The same molecular formula and the same connectivity of their atoms but a different arrangement of their atoms in space. mp & bp of cis < mp & bp of trans

  11. Naming of Cycloalkenes Number the carbon atoms of the ring double bond 1 and 2 in the direction that gives the substituent lower number. List substituents in alphabetical order. No location for C = C

  12. 2 4 2 3 1 1 1 2 3 Dienes, Trienes, and Polyenes Alkenes that contain more than one double bond. Alkane name: -ne diene, triene, …

  13. Chemical properties of Alkenes & Alkynes More reactive than Alkanes Addition of Hydrogen (Hydrogenation-Reduction) Addition of Hydrogen Halides (Hydrohalogenation) Addition of water (Hydration) Addition of Bromine & Chlorine (Halogenation)

  14. –C = C – – C – C– Chemical properties of Alkenes & Alkynes Addition reactions Double bond is broken and two new single bonds are formed. Exothermic reactions Products are more stable (have the lower energy).

  15. Chemical properties • A hydrogen atom adds to each carbon atom of a double bond. • A catalyst such as platinum or palladium is used (Transition metals). • H H H H • │ │ Pt │ │ • H–C=C–H + H2 H– C – C– H • │ │ • H H • Ethene Ethane More reactive than Alkanes 1. Hydrogenation (Reduction): Pt

  16. Chemical properties 1. Hydrogenation (Reduction):

  17. Chemical properties • A hydrogen halide (HCl, HBr, or HI) adds to alkene to • give haloalkane. • H H H H • │ │ │ │ • H–C=C–H + HCl H– C – C– H • │ │ • HCl • Ethene Chloroethane More reactive than Alkanes 2. Hydrohalogenation:

  18. Chemical properties 2. Hydrohalogenation: • reaction isregioselective. • Markovnikov’s rule: H adds to double bonded carbon that • has the greater number of H and halogen adds to the other carbon. The rich get richer!

  19. Chemical properties • Water adds to C=C to give an alcohol. • Acid catalyst (concentrated sulfuric acid). • A regioselective reaction (Markovnikov’s rule). 3. Hydration (addition of water):

  20. Chemical properties • A halogen atom adds to each carbon atom of a double bond. • Usually by using an inert solvent like CH2Cl2. • H H H H • │ │ │ │ • CH3–C=C–CH3 + Cl2 CH3– C – C– CH3 • │ │ • Cl Cl • 2-Butene 2,3-dichlorobutane More reactives than Alkanes 4. Halogenation: CH2Cl2

  21. Polymerization Polymer: a long-chain molecule produced by bonding together many single parts called monomers. The most important reactions of alkenes in industry.

  22. Polymerization Polypropene (Polypropylene) Polymerization Propene Monomer

  23. Naming of polymers Prefix “ploy-” + name of the monomer Propene Polypropene If the name of monomer consists of two words: Its name is enclosed in parentheses. n Polymerization Cl Cl Vinyl chloride Poly(Vinyl chloride) PVC

  24. Polymers in our life

  25. Polyethylene • Low-density polyethylene (LDPE): • A highly branched polymer; polymer chains do not pack well and the London dispersion forces between them are weak. • Softens and melts above 115°C. • Approximately 65% is used for the production of films (also used for packaging and trash bags). • High-density polyethylene (HDPE): • Only minimal chain branching; chains pack well and the London dispersion forces between them are strong. • It has a higher melting point than LDPE and it is stronger. • It can be blow molded to squeezable jugs and bottles. Inexpensive

  26. Recycling

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