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Chapter 3 Alkenes and Alkynes. Unsaturated Hydrocarbons Contain carbon - carbon multiple bonds. Alkenes C=C double bonds Alkynes C ≡C triple bonds Aromatics benzene rings . Alkenes. Structure:
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Chapter 3 Alkenes and Alkynes
Unsaturated Hydrocarbons Containcarbon-carbon multiple bonds. Alkenes C=C double bonds Alkynes C≡C triple bonds Aromatics benzene rings
Alkenes • Structure: • The VSEPR model predicts bond angles of 120° about each carbon of a double bond.
Alkenes • Cis-trans isomerism • because of restricted rotation about a carbon-carbon double bond, an alkene with two different groups on each carbon of the double bond shows cis-trans isomerism. CIS TRANS
(d) (c) Problem 43, p. 355
Naming Alkenes Step 1: Name the longest chain that contains the C=C bond. Use the IUPAC root and the –ene ending. Step 2: Number the longest chain so the C=C bond gets the lowest number possible. Step 3: Designate the C=C bond in the name with the lowest-numbered carbon.
Examples: 1 2 3 4 CH3-CH=CH-CH3 2-butene 6 5 4 3 2 1 CH3-CH2-CH2-CH=CH-CH3 2-hexene
Naming Alkenes, cont. Step 4: Locate and name attached groups. Step 5: Combine all the names as you did with alkanes.
Alkynes - IUPAC Names • follow the same rules as for alkenes, but use the ending -yne to show the presence of the triple bond.
(d) Example 12-1, p. 333
Cycloalkenes • To name a cycloalkene: • number the carbon atoms of the ring double bond 1 and 2 in the direction that gives the lower number to the substituent encountered first. • number and list substituents in alphabetical order.
Physical Properties • Alkenes and alkynes are nonpolar compounds. • The only attractive forces between their molecules are London dispersion forces. • Their physical properties are similar to those of alkanes with the same carbon skeletons. • Alkenes and alkynes are insoluble in water but soluble in one another and in nonpolar organic liquids.
Alkene Reactions Alkenes are quite chemically reactive Many reactions are addition reactions:
Reactions of Alkenes • The most common reaction is addition hydrohalogenation halogenation
Alkene Reactions, cont. Hydrogenation (addition) reactions can occur in the presence of a catalyst (Pt, Pd, or Ni). The hydrogenation of vegetable oils is an important commercial process.
Addition of H2 • Virtually all alkenes add H2 in the presence of a transition metal catalyst, commonly Pd, Pt, or Ni.
Addition of a Halogen (Halogenation) • Addition takes place readily at room temp. • reaction is generally carried out using pure reagents, or mixing them in a nonreactive organic solvent • addition of Br2 is a useful qualitative test for the presence of a carbon-carbon double bond
Addition of HX (Hydrohalogenation) • Addition of HX (HCl, HBr, or HI) to an alkene • H adds to one carbon of the C=C and X to the other. • Markovnikov’srule: H adds to the less substituted carbon and X to the more substituted carbon.
Addition of H2O • Hydration follows Markovnikov’s rule; H adds to the less substituted carbon and OH adds to the more substituted carbon.
1. 4. 2. 5. 3.
Polymerization Polymers – long chain products made up of repeating units. Monomer – the starting material that becomes the repeating units of a polymer.
Polymerization • Show the structure of a polymer by placing parentheses around the repeating monomer unit. • Place a subscript, n, outside the parentheses to indicate that this unit repeats n times. • The structure of a polymer chain can be reproduced by repeating the enclosed structure in both directions. • following a section of polypropene (polypropylene)
