Chemistry 20 Chapter 3 Alkenes and Alkynes Hydrocarbons Large family of organic compounds Composed of only carbon and hydrogen Saturated hydrocarbons Unsaturated hydrocarbons Alkanes H Alkenes, Alkynes & Aromatics H C-C C=C H C C C C C H CC C H 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 Thermal cracking CH3-CH3 800-900°C CH2=CH2 + H2 120° 124.7° 121.7° H H H H3 C 180° H H C C C C C CC-H H-C H H H H Ethylene Ethylene H H Prop ene e Acetylen (an alken e) (an alk yn e) Naming Alkenes & Alkynes Using the IUPAC alkane names: Alkene names change the end to -ene. Alkyne names change the end to -yne STEP 2 Number the carbon atoms starting from the end nearest a double or triple bond. STEP 3 Give the location and name of each substituent (alphabetical order) as a prefix to the name of the main chain. Give the location for double and triple bond Naming Alkenes & Alkynes CH2= CH ─ CH2─ CH3 1 3 2 4 CH3─ CH=CH─ CH3 1 2 3 2 3 2 3 2-Methyl-2-butene 1 CH3─ CC ─ CH3 1 2-Butene 4 CH3 | CH3─ CH=C─CH3 4 1-Butene 4 2-Butyne Naming Alkenes & Alkynes CH3─ CH2─ CC ─ CH3 5 4 3 2 2-Pentyne 1 CH3 CH3─ CH2─ C=CH ─ CH3 5 4 3 2 3-Methyl-2-pentene 1 CH2 – CH3 CH3─ CH2─ C=CH ─ CH3 5 4 3 2 1 3-Ethyl-2-pentene Conformation Under normal condition, no rotation is possible about the carbon-carbon double and triple bond. Flat or planar molecules Cis & Trans Stereoisomers H H H C=C H3C CH3 C=C CH3 C 4H 8 H3C H C 4H 8 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 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 6 4 3 5 4 1 2 3-Meth ylcyclop entene (not 5-methylcyclopen ten e) 1 5 2 3 4-Ethyl-1-meth ylcyclohexen e (not 5-ethyl-2-methylcyclohexene) Dienes, Trienes, and Polyenes Alkenes that contain more than one double bond. diene, triene, … Alkane name: -ne 1 2 4 CH2 =CHCH2 CH=CH2 1,4-Pentadien e 1 2 3 CH3 CH 2 =CCH=CH2 2-Meth yl-1,3-b utadiene (Isoprene) 1 2 3 1,3-Cyclopentad iene 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. –C = C – – C – C– Exothermic reactions Products are more stable (have the lower energy). Chemical properties More reactive than Alkanes 1. Hydrogenation (Reduction): • 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–C=C–H + H2 Ethene Pt Pt H H │ │ H– C – C– H │ │ H H Ethane Chemical properties 1. Hydrogenation (Reduction): Chemical properties More reactive than Alkanes 2. Hydrohalogenation: • A hydrogen halide (HCl, HBr, or HI) adds to alkene to give haloalkane. H H H H │ │ │ │ H–C=C–H + HCl Ethene H– C – C– H │ │ H Cl Chloroethane 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. CH3 CH=CH2 + HCl Prop ene Cl H CH3 CH-CH2 H Cl CH3 CH-CH2 2-Ch loroprop ane 1-Chlorop ropan e (not formed) 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). CH3 CH=CH2 Propene CH3 CH3 C=CH2 + + 2-Methylp ropen e H2 O H2 O H2 SO4 OH H CH3 CH-CH2 2-Propan ol CH3 H2 SO4 CH3 C-CH2 HO H 2-Methyl-2-prop anol Chemical properties More reactives than Alkanes 4. Halogenation: • A halogen atom adds to each carbon atom of a double bond. • Usually by using an inert solvent like CH2Cl2. H H │ │ CH3–C=C–CH3 + Cl2 2-Butene CH2Cl2 H H │ │ CH3– C – C– CH3 │ │ Cl Cl 2,3-dichlorobutane Polymerization Polymer: a long-chain molecule produced by bonding together many single parts called monomers. The most important reactions of alkenes in industry. nCH2 =CH2 initiator CH2 CH2 n (polymerization) Ethylene Polyethylene Polymerization Polypropene (Polypropylene) monomer un its show n in red Polymerization n Propene CH3 CH3 CH3 CH3 CH2 CH-CH2 CH-CH2 CH-CH2 CH CH3 CH2 CH Part of an extended polymer chain The repeating un it Monomer n 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 Vinyl chloride n Polymerization Cl Poly(Vinyl chloride) PVC Polymers in our life Polyethylene • Low-density polyethylene (LDPE): Inexpensive – 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. Recycling Cod e 1 PET Polymer Common Uses poly(eth ylen e terephthalate) soft drin k bottles, hous ehold ch emical b ottles , films, textile fib ers 2 HD PE high-den sity polyethylen e milk and w ater jugs, grocery bags, sq ueezable bottles 3 V poly(vinyl ch loride), PV C low -density polyethylen e sh amp oo bottles, pip es, show er curtains, vinyl siding, w ire in sulation, floor tiles sh rin k w rap , tras h and grocery bags, san dw ich b ags , squ eeze bottles 5 PP polypropylene 6 PS polystyrene 7 all other p lastics plas tic lids, clothin g fibers, bottle cap s, toys , diaper linings styrofoam cu ps, egg carton s, disp os able uten sils, packaging materials, app lian ces various 4 LD PE