alkene and alkyne

In organic chemistry, an alkyne is an unsaturated hydrocarbon containing at least one carbon—carbon triple bond. The simplest acyclic alkynes with only one triple bond and no other functional groups form a homologous series with the general chemical formula CₙH₂ₙ−2

1. Alkenes and Alkynes Md. Saiful Islam BPharm, MSc North South University Fb Group: Pharmacy Universe

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

3. 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

4. Un-branched alkenes containing 2-4 carbons are gaseous, 5-18 are liquids and over 18 are solids. Insoluble in water but soluble in organic solvents. Lighter than water with limiting density less than 0.8 Alkenes have higher MP than that of same length of alkanes which is attributed by the presence of extra pi bond that makes the double bond polar and also forms induced dipole moments within the molecules Physical properties

5. Thermal cracking CH 3 -CH 3 800-900 °C CH 2 =CH 2 + H 2 120 ° 180 °

6. Preparation of Alkynes: Elimination Reactions of Dihalides Treatment of a 1,2-dihalidoalkane with KOH or NaOH produces a two-fold elimination of HX (double dehydrohalogenation ) Vicinal dihalides are available from addition of bromine or chlorine to an alkene Intermediate is a vinyl halide ( vinyl substituent = one attached to C=C)

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

8. 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.

9. 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 1 2 3 4 1 2 3 4 4 3 2 1 1 2 3 4

10. 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 5 4 3 2 1 5 4 3 2 1 5 4 3 2 1

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

12. Cis & Trans Stereoisomers C = C H H CH 3 H 3 C C = C H H CH 3 H 3 C 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 C 4 H 8 C 4 H 8

13. 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

14. Dienes, Trienes, and Polyenes Alkenes that contain more than one double bond. Alkane name : -ne diene, triene, … 1 2 4 1 2 3 1 2 3

15. 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)

16. 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).

17. 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

18. Chemical properties 1. Hydrogenation (Reduction):

19. 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

20. 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!

21. 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).

22. 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

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

24. Polymerization Propene Monomer Polymerization Polypropene (Polypropylene)

25. 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 Cl n Polymerization Vinyl chloride Poly(Vinyl chloride) PVC

26. Polymers in our life

27. 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

28. Recycling

29. The END