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Alkenes

Alkenes. .1 Introduction .2 Nomenclature of Alkenes .3 Physical Properties of Alkenes .4 Preparation of Alkenes .5 Reactions of Alkenes. Dr Manal F. AbouTaleb. Structure Of Alkenes.

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Alkenes

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  1. Alkenes .1Introduction .2Nomenclature of Alkenes .3Physical Properties of Alkenes .4Preparation of Alkenes .5Reactions of Alkenes Dr Manal F. AbouTaleb

  2. Structure Of Alkenes • They are unsaturated hydrocarbons – made up of C and H atoms and contain one or more C=Cdouble bond somewhere in their structures. • Their general formula is CnH2n - for non-cyclic alkenes • Their general formula is CnH2n-2 - for cyclic alkenes 2

  3. SP2Hibridization: Angle of SP2 bond =120o

  4. The C = C double bond in ethene Introduction Functional group of alkenes: • Each carbon is trigonal and planar. • Bonding: sp2 hybridization for 3 s-bonds to the three atoms bonded to each carbon • pz orbital for π-bond • Typical C=C bond distance (i.e., 1.34 Å) shorter than C-C bond distance (i.e., 1.54 Å) • slightly shorter C-H distance than alkanes

  5. Nomenclature of Alkenes Nomenclature of Alkenes 1. Determine the stem name by selecting the longest possible straight chain containing the C = C double bond and use the ending ‘-ene’ • Number the parent chain so as to include both carbon atoms of the double bond, and begin numbering with the end of the chain nearer the C = C double bond • 3. Designate the position of the C = C double bond by using the number of the first atom of the double bond

  6. 4. Designate the positions of the substituent's by using the numbers obtained by application of rule 2

  7. Nomenclature of Alkenes Examples:

  8. CH2 CH2 CH2 CH CH2 CH C CH CH2 C H2 C H3 C H2 C C C H2 H2 H3 C • 5. Indicated number of double bonds by prefixes (ene, diene, triene, tetraene, etc.) Examples 1 2 3 4 5 2- ethyl -1-pentene 1 2 3 4 5 1,4-pentadiene CH3 1 2 3 4 2- methyl-1,3-butadiene

  9. Nomenclature of Alkenes 6. If two identical groups are present on the same side of the C = C double bond, the compound is designated as cis; if they are on opposite sides, the compound is designated as trans. e.g.

  10. Nomenclature of Alkenes Example Give the IUPAC names for the following alkenes: (a) (b) Answer Solution: (a)trans-3,4-dichlorohept-3-ene (b)cis-3,4-dimethyloct-3-ene

  11. (a) (b) (c) Nomenclature of Alkenes Check Point Draw the structural formula for each of the following alkenes: (a) cis-hex-3-ene (b) trans-2,3-dihydroxybut-2-ene (c) cis-1,2-dichloroethene Answer

  12. Alkenes are non polar compounds. Insoluble in water. Soluble in non polar organic solvents. They are less dense than water. Range of physical states: ≤ 4 C's are gases 5 - 17 C's are liquids ≥ 18 C's are solids The alkenes has a boiling point which is a small number of degreeslower than the corresponding alkanes. Physical Properties of Alkenes

  13. Physical Properties of Alkenes

  14. Preparation of Alkenes Elimination Reactions Dehydrohalogenation • Dehydrohalogenationis the elimination of a hydrogen halide molecule from a haloalkane in presence of KOH in alcohol

  15. Preparation of Alkenes Examples:

  16. Preparation of Alkenes Dehydrohalogenation of 2° and 3° haloalkanes can take place in more than one wayand a mixture of alkenes is formed alc. KOHCH3CH2CHClCH3 CH3CH = CHCH3 + CH3CH2CH = CH2heat 2-chlorobutane But-2-ene But-1-ene (80%) (20%) Note: the more highly substituted alkeneis formed as major product

  17. Saytzeff s Rule: In every instance in which more than one alkene can be formed, the major product is the alkene with the most alkyl substituents attached on the double bonded carbon. Br 3 -methyl cyclohexene CH3 CH3 CH3 EtOH + KOH + ∆ H 1-methyl cyclohexene Major Minor

  18. Preparation of Alkenes Dehydration of Alcohols Dehydration is the removal of a water moleculefrom a reactant molecule, in the presence of Mineral acids (H2SO4, H3PO4) The presence of H2SO4 to prevent the reversible reaction

  19. Preparation of Alkenes The experimental conditions of dehydration depend on the structures of alcohols e.g.

  20. Preparation of Alkenes Addition Reactions Hydrogenation • hydrogenation of alkynesusing Lindlar’s catalyst produces alkenes • prevent further hydrogenation of the alkenes formed to alkanes

  21. Reactions of Alkenes Alkenes are more reactive than alkanes Reason: presence of the C = C double bond Energetically favourable!!  alkenes undergo addition reactionsand the reactions are exothermic π-bonds weaker than σ-bonds

  22. Reactions of Alkenes Electrophilic Addition Reactions Addition of Hydrogen Bromide Addition of hydrogen bromideto C = C double bondyields a bromoalkane

  23. Reactions of Alkenes Examples:

  24. Reactions of Alkenes Propene reacts with HBr to give 2-bromopropane(major product) and 1-bromopropane (minor product) The formation of two possible products can be explained by the reaction mechanism.

  25. Reactions of Alkenes Markownikoff’s rule states that in the addition of HX to an unsymmetrical alkene, the hydrogen atomadds to the carbon atomof the carbon-carbon double bond that already hasthe greater number of hydrogen atoms. Example:

  26. Reactions of Alkenes 2-bromopropane is the major product because the more stable secondary carbocation is formed in the first step

  27. Reactions of Alkenes Addition of Bromine Alkenes react rapidly with Br2in 1,1,1-trichloroethane at room temperature and in the absence of light e.g.

  28. The reddish brown colour of Br2 is decolourized Add Br2 in CH3CCl3 to excess alkene Reactions of Alkenes The behaviour of alkenes towards Br2 in CH3CCl3is a useful test for the presence of carbon-carbon multiple bonds

  29. Br2 + H2O HBr + HOBr Bromic(I) acid Reactions of Alkenes Addition of Bromine Water In an aqueous solution of Br2, the following equilibrium exists The bromine atom bears a partial positive chargewhile the oxygen atom bears a partial negative charge∵ oxygen is more electronegative than bromine

  30. Reactions of Alkenes When bromic(I) acid reacts with alkenes, bromohydrin is formed e.g.

  31. Reactions of Alkenes Addition of SulphuricAcid Alkenes react with cold and concentrated H2SO4 to form alkyl hydrogensulphates e.g.

  32. Reactions of Alkenes The large bulky –OSO3H groupmakes the alkyl hydrogensulphate very unstable. Two possible further reactions take place: 1.Regeneration of alkenes 2.Production of alcohols

  33. Reactions of Alkenes Catalytic Hydrogenation In the presence of metal catalysts(e.g. Pt, Pd or Ni), H2 is added to each atom of C = C double bond to form an alkane e.g.

  34. Addition of H2O: Hydration • Only one product is possible from the addition of H2O in presence of acids as catalysts to symmetrical alkenes such as ethene and cyclohexene. • However, addition reactions to unsymmetrical alkenes will result in the formation of Markovonikov’s product preferentially. 35

  35. Addition of HCN

  36. (b) (i) (ii) Reaction of Alkenes Check Point 30-3 (b) What is the major product of each of the following reactions? (i) (ii) Answer

  37. (c) (i) CH3CH2CH3 (ii) (iii) Reaction of Alkenes Check Point (c) Give the reaction products for the following reactions: Ni (i) CH3CH = CH2 + H2  conc.H2SO4 (ii) CH3CH = CHCH3  (iii) CH3CH = CHCH3 + Br2  Answer

  38. (unstable) Reactions of Alkenes Ozonolysis Ozonolysis is a widely used method for locating the double bond of an alkene The unstable ozonide is reduced directly by treatment with Zn and H2O

  39. Reactions of Alkenes Polymerization Polymers: Compounds that consist of very large molecules made up of many repeating units Monomer: Each repeating unit Polymerization: The reaction by which monomers are joined together Addition polymerization: alkene monomers are joined together without the elimination of small molecules Addition polymer: The polymer produced by addition polymerization

  40. Reactions of Alkenes Poly(ethene) Monomer: ethene Depending on the conditions, two kinds of poly(ethene) are formed

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