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Alkenes and Alkynes Geometric Isomers of Alkenes Addition Reactions

Alkenes and Alkynes Geometric Isomers of Alkenes Addition Reactions. ALKENES and ALKYNES. Alkenes area class of HYDROCARBONS which contain only carbon and hydrogen.

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Alkenes and Alkynes Geometric Isomers of Alkenes Addition Reactions

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  1. Alkenes and AlkynesGeometric Isomers of AlkenesAddition Reactions LecturePLUS Timberlake

  2. ALKENES and ALKYNES • Alkenes area class of HYDROCARBONS which contain only carbon and hydrogen. • UNSATURATED hydrocarbons contain either double or triple bonds. Since the compound is unsaturated with respect to hydrogen atoms, the extra electrons are shared between 2 carbon atoms forming either double bonds for alkenes or triple bonds for alkynes.

  3. Saturated and Unsaturated Compounds • 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 • Unsaturated compounds contain double or triple bonds LecturePLUS Timberlake

  4. Typical Alkene • Etheneis the number one organic chemical synthesized in the U. S. and the world. The small quantities of ethane, propane, and butane found in natural gas are converted into ethene. It can be produced by thermal cracking of ethane to produce ethene and a hydrogen molecule.

  5. Alkenes • Carbon-carbon double bonds • Names end in -ene H2C=CH2 ethene (ethylene) H2C=CH-CH3 propene (propylene) cyclohexene LecturePLUS Timberlake

  6. Typical Alkyne • Physical and chemical properties are similar to those of alkenes.

  7. Alkynes • Carbon-carbon triple bonds • Names end in -yne HCCH ethyne(acetylene) HCC-CH3 propyne LecturePLUS Timberlake

  8. ALKENE and ALKYNE NAMES Root names give the number of carbons in the longest continuous chain. The names are formed by dropping the "ane" and if the chain is 3C or less replacing it with "ene“. If the chain is 4C or more the lowest “carbon number” of the two carbons involved with the double bond plus a hyphenated ene is added.

  9. Naming Alkenes and Alkynes When the carbon chain has 4 or more C atoms, number the chain to give the lowest number to the double or triple bond. 1 2 3 4 CH2=CHCH2CH3 but-1-ene CH3CH=CHCH3 but-2-ene CH3CHCHCH3 but-2-yne LecturePLUS Timberlake

  10. Cis-Trans Isomerism • The double bond of alkenes form a rigid structure around which the atoms cannot rotate as in alkanes. This leads to a form of isomerism referred to as cis- trans isomers. This isomerism is most readily seen in the two forms of but-2-ene

  11. Cis-but-2-ene • The hydrogens occur on the same side (plane ). • Trans-but-2-ene • The hydrogens occur diagonally across from each other.

  12. Properties  Physical Properties Chemical Reactivity Alkenes are quite reactive because of the presence of the double bond. Many small compounds react by addition i.e. molecules add to the alkene to form one product. All compounds: Combustion Reaction • Boiling points depend on chain length, slightly less than alkanes. • Non polar • Insoluble in water • Less dense than water

  13. Learning Check Write the IUPAC name for each of the following unsaturated compounds: A. CH3CH2CCCH3 CH3 B. CH3C=CHCH3 C. LecturePLUS Timberlake

  14. Solutions Write the IUPAC name for each of the following unsaturated compounds: A. CH3CH2CH=CHCH3 2-pentyne CH3 B. CH3C=CHCH3 C. 2-methyl-2-butene 3-methylcyclopentene LecturePLUS Timberlake

  15. Alkene/Alkyne Addition Reactions • The double and triple bonds present in alkenes and alkynes, respectively, make them much more reactive than alkanes. • Alkenes and alkynes undergoaddition reactions • a reaction in which parts of a reactant are added to the two atoms that make up a double or triple bond

  16. Alkene/Alkyne Addition Reactions • Some of the common reactants that can add to alkenes and alkynes include: • Br2 • I2 • Cl2 • H2 (requires a transition metal catalyst) • H2O (requires a strong acid catalyst) • HBr • HCl • HI

  17. Hydrogenation • Adds a hydrogen atom to each carbon atom of a double bond H H H H Ni H–C=C–H + H2H–C–C–H H H ethene ethane LecturePLUS Timberlake

  18. Adding Halogens (Halogenation) Halogens also add to the double bond of an alkene. H2CCH2 + Cl2 LecturePLUS Timberlake

  19. Adding Hydrogen Halides Hydrogen halides also add to the double bond of an alkene H Cl H–C=C–H + HClH–C–C–H H H H H LecturePLUS Timberlake

  20. Adding Water Water also can be added to the double bond of an alkene H OH H–C=C–H + H2OH–C–C–H H HHH LecturePLUS Timberlake

  21. Alkene/Alkyne Addition Reactions • Markovnikov’s Rule:The major product obtained from the addition of an unsymmetrical reagent such as H-Br, H-Cl, or H-OH to an alkene or alkyne is the one obtained when the H atom of the reagent is added to the C atom of the multiple bond that already has the greater number of H atoms. “The rich get richer”

  22. Example Water also can be added to the double bond of an alkene H HOH H H–C = C– C–H + H2OH–C – C– C–H H HHHHH LecturePLUS Timberlake

  23. Alkene/Alkyne Addition Reactions Example: Write the structure of the product that forms when propynereacts with the following reagents (individually): Br2 (excess) H2 (excess) with Ni catalyst HBr (excess)

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