Alkenes.1Introduction.2Nomenclature of Alkenes.3Physical Properties of Alkenes.4Preparation of Alkenes.5Reactions of Alkenes

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

Angle of SP 2 bond =120 o SP 2 Hibridization:

Introduction Functional group of alkenes: The C = C double bond in ethene  Each carbon is trigonal and planar.  Bonding: sp 2 hybridization for 3 s-bonds to the three atoms bonded to each carbon  p z 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

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’ 2.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 Nomenclature of Alkenes

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

Nomenclature of Alkenes Examples:

CCC C CH2H2 H3H3 CH2H2 CH3H3 H2H2 H2H2 2- ethyl -1-pentene CH 2 CH CH 2 CH 3 C CH CH 2 1,4-pentadiene 2- methyl-1,3-butadiene  5. Indicated number of double bonds by prefixes (ene, diene, triene, tetraene, etc.) Examples

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.

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

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 Nomenclature of Alkenes (a) (b) (c)

Physical Properties of Alkenes 13 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 C's are liquids ≥ 18 C's are solids The alkenes has a boiling point which is a small number of degrees lower than the corresponding alkanes.

Physical Properties of Alkenes NameFormulaBoiling point (°C) Melting point (°C) Density at 20 °C (g cm -3 ) EtheneCH 2 = CH — PropeneCH 3 CH = CH But-1-eneCH 3 CH 2 CH = CH Pent-1-eneCH 3 (CH 2 ) 2 CH = CH Hex-1-eneCH 3 (CH 2 ) 2 CH = CH cis-But-2-eneCH 3 CH = CHCH 3 (cis) trans-But-2-eneCH 3 CH = CHCH 3 (trans) Methylbut-1- ene CH 3 CH 3 C(CH 3 ) = CH

Preparation of Alkenes Dehydrohalogenation is the elimination of a hydrogen halide molecule from a haloalkane in presence of KOH in alcohol Elimination Reactions Dehydrohalogenation

Preparation of Alkenes Examples:

Preparation of Alkenes Dehydrohalogenation of 2° and 3° haloalkanes can take place in more than one way and a mixture of alkenes is formed alc. KOH CH 3 CH 2 CHClCH 3  CH 3 CH = CHCH 3 + CH 3 CH 2 CH = CH 2 heat 2-chlorobutaneBut-2-eneBut-1-ene (80%)(20%) Note: the more highly substituted alkene is formed as major product

Br + KOH CH 3 H EtOH ∆ + 1-methyl cyclohexene MajorMinor CH 3 3 -methyl cyclohexene 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.

Preparation of Alkenes Dehydration is the removal of a water molecule from a reactant molecule, in the presence of Mineral acids (H 2 SO 4, H 3 PO 4 ) Dehydration of Alcohols The presence of H 2 SO 4 to prevent the reversible reaction

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

Preparation of Alkenes hydrogenation of alkynes using Lindlar’s catalyst produces alkenes prevent further hydrogenation of the alkenes formed to alkanes Addition Reactions Hydrogenation

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

Addition of hydrogen bromide to C = C double bond yields a bromoalkane Electrophilic Addition Reactions Addition of Hydrogen Bromide Reactions of Alkenes

Examples: Reactions of Alkenes

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

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

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

Alkenes react rapidly with Br 2 in 1,1,1-trichloroethane at room temperature and in the absence of light Addition of Bromine Reactions of Alkenes e.g.

The behaviour of alkenes towards Br 2 in CH 3 CCl 3 is a useful test for the presence of carbon-carbon multiple bonds Reactions of Alkenes Add Br 2 in CH 3 CCl 3 to excess alkene The reddish brown colour of Br 2 is decolourized

In an aqueous solution of Br 2, the following equilibrium exists Addition of Bromine Water Reactions of Alkenes Br 2 + H 2 OHBr + HOBr Bromic(I) acid The bromine atom bears a partial positive charge while the oxygen atom bears a partial negative charge ∵ oxygen is more electronegative than bromine

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

Alkenes react with cold and concentrated H 2 SO 4 to form alkyl hydrogensulphates Addition of SulphuricAcid Reactions of Alkenes e.g.

The large bulky –OSO 3 H group makes the alkyl hydrogensulphate very unstable. Two possible further reactions take place: 1.Regeneration of alkenes Reactions of Alkenes 2.Production of alcohols

In the presence of metal catalysts (e.g. Pt, Pd or Ni), H 2 is added to each atom of C = C double bond to form an alkane Catalytic Hydrogenation Reactions of Alkenes e.g.

Addition of H 2 O: Hydration 35 Only one product is possible from the addition of H 2 O in presence of acids as catalysts to symmetrical alkenes such as ethene and cyclohexene.  Only one product is possible from the addition of H 2 O 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.  However, addition reactions to unsymmetrical alkenes will result in the formation of Markovonikov’s product preferentially.

36 Addition of HCN

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

Check Point (c)Give the reaction products for the following reactions: Ni (i)CH 3 CH = CH 2 + H 2  conc. H 2 SO 4 (ii)CH 3 CH = CHCH 3  (iii)CH 3 CH = CHCH 3 + Br 2  Answer Reaction of Alkenes (c)(i)CH 3 CH 2 CH 3 (ii) (iii)

Ozonolysis is a widely used method for locating the double bond of an alkene Ozonolysis Reactions of Alkenes (unstable) The unstable ozonide is reduced directly by treatment with Zn and H 2 O

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 Polymerization Reactions of Alkenes

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