Chapter 3 Alkenes and Alkynes Chemistry 20

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

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. C n H 2n Containing triple bonds are alkynes. C n H 2n-2 Alkenes and Alkynes

Thermal cracking CH 3 -CH °C CH 2 =CH 2 + H 2 120° 180°

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

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. STEP 2 Number the carbon atoms starting from the end nearest a double or triple bond.

CH 2 = CH ─ CH 2 ─ CH 3 1-Butene CH 3 ─ CH=CH─ CH 3 2-Butene CH 3 | CH 3 ─ CH=C─CH 3 2-Methyl-2-butene CH 3 ─ C  C ─ CH 3 2-Butyne Naming Alkenes & Alkynes

CH 3 ─ CH 2 ─ C  C ─ CH 3 2-Pentyne CH 3 CH 3 ─ CH 2 ─ C=CH ─ CH 3 3-Methyl-2-pentene CH 2 – CH 3 CH 3 ─ CH 2 ─ C=CH ─ CH 3 3-Ethyl-2-pentene Naming Alkenes & Alkynes

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

Cis & Trans Stereoisomers C = C H H CH 3 H3CH3C C = C H H CH 3 H3CH3C cis-2-Butenetrans-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 C4H8C4H8 C4H8C4H8

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

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

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)

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

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 + H 2 H– C – C– H │ │ H H Ethene Ethane Chemical properties 1. Hydrogenation (Reduction): Pt More reactive than Alkanes

Chemical properties 1. Hydrogenation (Reduction):

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 │ │ H Cl Ethene Chloroethane Chemical properties 2. Hydrohalogenation: More reactive than Alkanes

Chemical properties 2. Hydrohalogenation: - reaction is regioselective. - 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!

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

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

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

Polymerization Propene Monomer Polymerization Polypropene (Polypropylene)

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. Cl n Polymerization Vinyl chloridePoly(Vinyl chloride) PVC

Polymers in our life

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. Polyethylene Inexpensive

Recycling