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: The VSEPR model predicts bond angles of 120° about each carbon of a double bond.

Alkenes CIS TRANS 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 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.

Example 12-3, p. 336

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’s rule: H adds to the less substituted carbon and X to the more substituted carbon.

Hydration-addition of water

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.

Table 12-2, p. 349

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)