1 Electrophilic Reactions

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3 Electrophilic Reactions Addition to unsaturated carbon Alkenes + Alkynes Unsaturated hydrocarbon Unsaturated hydrocarbon: contains one or more carbon- carbon double or triple bonds - alkene - alkene: contains a carbon-carbon double bond and has the general formula C n H 2n - alkyne - alkyne: contains a carbon-carbon triple bond and has the general formula C n H 2n-2

4 Electrophilic Reactions Addition to unsaturated carbon Alkenes IUPAC Nomenclature en -use the infix -en- to show the presence of a carbon-carbon double bond - number the parent chain to give the 1st carbon of the double bond the lower number - follow IUPAC rules for numbering and naming substituents - for a cycloalkene, the double bond must be numbered 1,2

5 Electrophilic Reactions Addition to unsaturated carbon Alkenes Some alkenes, particularly low-molecular-weight ones, are known almost exclusively by their common names

6 Electrophilic Reactions Addition to unsaturated carbon Alkenes The cis,trans system: The cis,trans system: configuration is determined by the orientation of atoms of the main chain

7 Electrophilic Reactions Addition to unsaturated carbon Alkenes Configuration: E,Z Assign a priority to the substituents on each carbon of the double bond Z if the groups of higher priority are on the same side of the double bond, the configuration is Z (German: zusammen, together) E if the groups of higher priority are on opposite sides of the double bond, the configuration is E (German: entgegen, opposite)

8 Electrophilic Reactions Addition to unsaturated carbon Alkenes – E,Z configuration Priority rules 1.Priority is based on atomic number; the higher the atomic number, the higher the priority 2. If priority cannot be assigned on the basis of the atoms bonded directly to the double bond, look to the next set of atoms; priority is assigned at the first point of difference 3. Atoms participating in a double or triple bond are considered to be bonded to an equivalent number of similar atoms by single bonds

9 Electrophilic Reactions Addition to unsaturated carbon Alkenes –Addition of hydrogen halides Alkenes –Addition of water

10 Electrophilic Reactions Addition to unsaturated carbon Alkenes –Addition of hydrogen halides to double bonds with substituted carbons Carbocations are planar -> attack from both sides possible!!

11 Electrophilic Reactions Addition to unsaturated carbon Alkenes –Addition of hydrogen halides to double bonds with substituted carbons regioselective reaction: -> Addition is regioselective (regioselective reaction: a reaction in which one direction of bond-forming or bond-breaking occurs in preference to all other directions) Markovnikov’s rule: Markovnikov’s rule: in additions of HX, H adds to the carbon with the greater number of hydrogens

12 Electrophilic Reactions Addition to unsaturated carbon Alkenes –Addition of hydrogen halides to double bonds with substituted carbons -> Carbocation rearrangements

13 Electrophilic Reactions Addition to unsaturated carbon Alkenes –Addition of halogens

14 Electrophilic Reactions Addition to unsaturated carbon Alkenes –Addition of halogens -> Stereochemical consequences of electrophil addition -> gives 2,3 dibromobutane as a pair of enantiomers, R,R and S,S -> a result of anti addition

15 Electrophilic Reactions Addition to unsaturated carbon Alkenes –Addition of halogens -> Stereochemical consequences of electrophil addition -> gives 2,3 dibromobutane as a meso R,S isomer

16 Electrophilic Reactions Addition to unsaturated carbon Alkenes –Addition of halogens -> Stereochemical consequences of electrophil addition -> gives 1,2 dibromocyclohexan as a pair of enantiomers, R,R and S,S -> a result of anti addition

17 Electrophilic Reactions Addition to unsaturated carbon Alkenes –Addition of halogens -> Stereochemical consequences of electrophil addition

18 Electrophilic Reactions Addition to unsaturated carbon Alkenes –Addition of halogens in water -> Halo alcohol (halohydrin) Halo alcohols can be used to make -> epoxides

19 Electrophilic Reactions Addition to unsaturated carbon Alkenes –Addition of halogens in water under basic conditions -> Epoxides

20 Electrophilic Reactions Addition to unsaturated carbon Alkynes IUPAC nomenclature yn use the infix -yn- to show the presence of a carbon-carbon triple bond number the parent chain to give the 1st carbon of the triple bond the lower number follow IUPAC rules for numbering and naming substituents

21 Electrophilic Reactions Addition to unsaturated carbon Alkynes –Addition of halides

22 Electrophilic Reactions Addition to unsaturated carbon Alkynes –Addition of halides

23 Electrophilic Reactions Addition to unsaturated carbon Alkynes – Hydration gives a keton

24 Electrophilic Reactions Addition to conjugated systems Alkenes – conjugated systems Dienes, trienes, and polyenes n 2n for an alkene with n carbon-carbon double bonds, each of which can show cis-trans isomerism, 2n cis-trans isomers are possible consider 2,4-heptadiene; it has four cis-trans isomers, two of which are drawn here

25 Electrophilic Reactions Addition to conjugated systems Alkenes – conjugated systems - vitamin A has five double bonds - vitamin A is the all-trans isomer

26 Electrophilic Reactions Addition to conjugated systems Alkenes – addition of halides

27 Electrophilic Reactions Addition to conjugated systems Alkenes – addition of halides Conjugate addition

28 Electrophilic Reactions Addition to conjugated systems Alkenes – addition of halogens

29 Electrophilic Reactions Carbocations as electrophiles Cationic polymerization Polystyrene

30 Electrophilic Reactions Carbocations as electrophiles Cationic polymerization – intramolecular electrophilic addition

31 Electrophilic Reactions Carbocations as electrophiles Cationic polymerization – intramolecular electrophilic addition Terpenoids and Steroids Huge group of natural products -> flavouring, aromatherapy oils, vitamins (A, K), steroid hormons (lanosterol, cholesterol) Terpene: Terpene: a compound whose carbon skeleton can be divided into two or more units identical with the carbon skeleton of isoprene

32 Electrophilic Reactions Carbocations as electrophiles Cationic polymerization – intramolecular electrophilic addition Terpenoids and Steroids –myrcene, C10H16, a component of bayberry wax and oils of bay and verbena –menthol, from peppermint –Vitamin A

33 Electrophilic Reactions Carbocations as electrophiles Cationic polymerization – intramolecular electrophilic addition Terpenoids and Steroids Lemon oilPine oilEucalyptus oil

34 Catalytic Hydration Addition of hydrogen to alkenes and alkynes Catalytic hydration -> NOT a radical reaction (Chapter 9: Radical reactions) Involvement of atomic hydrogen -> mechanism has more in common with radical reaction than with ionic reaction

35 Catalytic Hydration Addition of hydrogen to alkenes and alkynes