Organic Chemistry Lecture Notes (PDF)

‫بسم هللا الرحمن الرحيم‬ In this Lecture 2 ◼ Alkenes & Cycloalkenes CnH2n & CnH2n-2 Structure & Nomenclature ◼3  Homework Draw all possible Isomers mention their type and IUPAC for the following MF C9H18, C8H14, C9H12 ...

‫بسم هللا الرحمن الرحيم‬ In this Lecture 2 ◼ Alkenes & Cycloalkenes CnH2n & CnH2n-2 Structure & Nomenclature ◼3  Homework Draw all possible Isomers mention their type and IUPAC for the following MF C9H18, C8H14, C9H12  Preparation: From alcohols; dehydration: + [H] H R CH 2OH heat C C + H2O CH3 CH2 CH - CH2 H+ CH3 CH2 CH = CH2 + H2O H OH OH H+ [H] heat ◼5  Preparation: From alkyl halide; by dehydrohalogenation: H H H H alc oholic KOH R C C H R C C H + HX heat H X R = H , CH 3 , C2H5 , C3H7 X = F , Cl, Br ,I 6 ◼  Preparation: From alkyl halide; by dehydrohalogenation: R-I > R-Br > R-Cl ◼ ◼ ease of dehydrohalogenation ---- R R H R C X > R C X > R C X R H H ◼ 3 alkylhalide > 2 alkylhalide > 1 alkylhalide ◼ Ease of dehydrohalogenation ---- 7 ◼  Preparation: From vicinal dihalides; by Dehalogenation: CH3 CH CH2 + Zn CH3 CH CH2 + Zn Br2 Br Br propene 8 ◼ Mechanisms of Elimination 9 ◼  The E2 Mechanism The rate of reaction depends on the conc. of substrate and nucleophile. (one-step) bimolecular process single transition state ◦ C—H bond breaks ◦ p component of double bond forms ◦ C—X bond breaks ◼10 The E2 Mechanism 11 ◼  The E2 Mechanism..– R O:.. H C C : X:.. Reactants 12 ◼  The E2 Mechanism..– R O:.. H C C : X:.. Reactants 13 ◼ The E2 Mechanism d–.. R O H.. Transition state C C d– : X:.. 14 ◼ The E2 Mechanism.. R O H.. C C.. – : X:.. Products 15 ◼  The E1 Mechanism 1. Alkyl halides can undergo elimination in absence of base. 2. Carbocation is intermediate 3. The rate of reaction depends on the conc. of substrate only. Rate-determining step is unimolecular ionization of alkyl halide. 16 ◼  CH3 Step 1 CH3 C CH2CH3 : Br:.. slow, unimolecular CH3 C CH3 + CH2CH3.. – : Br:.. 17 ◼  CH3 Step 2 C CH3 + CH2CH3 – H+ CH2 CH3 C + C CH2CH3 CH3 CHCH3 CH3 Which alkene is more stable and why? ◼ 18  Saytzeff rule: The main product is the most highly substituted alkene. CH 2 CH CH 2 CH 3 H H H H 20% H C C C C H H Br H H CH 3 CH CH CH 3 80% 19 ◼  Physical Properties of alkenes C2 – C4 alkenes are gases, C5 – C18 are liquids, while those above C18 are solids at room temperatures. All are colourless and odourless except ethene which has a rather pleasant odour. Alkenes are only slightly soluble in water but dissolve freely in organic solvents, unlike alkanes they dissolve in conc. H2SO4. Their boiling points, melting points and specific gravities, in general rise with increase of molecular weight. 20 ◼  Reactions Alkenes are more reactive than alkanes this because: 1. the p electrons of a double bond are located much farther from the carbon nuclei and are thus less firmly bound to them. 2. The overlap of atomic orbitals in forming a p-bond is not as effective as that of a  bonds..thus a p-bond is weaker than a -bond and more easily broken. N.B The p-electrons in C=C bond shield the alkene molecule from attack by nucleophilic reagents. ◼21  22 22 ◼ The electrophilic addition reaction proceeds according to one of the following mechanisms: A) Ionic mechanism. B Free radical mechanism C) Cyclic concerted mechanism. ◼ 23 A) Ionic mechanism. 24 ◼ Addition of Halogens to Alkenes Bromine and chlorine add to alkenes to give 1,2-dihaldes, an industrially important process ◦ F2 is too reactive and I2 too unreactive Cl2 reacts as “Cl+ Cl-”; Br2 is similar 25 25 ◼  Mechanism of Bromine Addition Br+ adds to an alkene producing a cyclic cation: a bromonium ion, in which bromine shares charge with carbon 26 26 ◼ Since the Br blocks one face, one must get anti (trans) addition 27 ◼27 Halohydrin Formation This is formally the addition of HO-X to an alkene (with “-OH” as the nucleophile) to give a 1,2-halo alcohol, called a halohydrin The actual reagent is the dihalogen (Br2 or Cl2 with water in an organic solvent) 28 ◼28 Mechanism: 29 29 ◼  Addition of Br2 to Cyclopentene Addition is exclusively trans (stereospecific) 30 30 ◼ Addition of Br2 to Cyclopentene 31 31 ◼  32 32 ◼ Effect of substituents on the rate of addition Electron-releasing substituents such as alkyl, aryl, - CC-, RO.-and R2N-) increase the rate of addition by raising the electron density in the double bond Electron-withdrawing substituents such as halogens, -CO , -COOH , -CN , -NO2 and – SO3H groups decrease the rate of electrophilic addition reaction by lowering the electron density in the double bond 33 ◼  Addition of HX acids to alkenes “hydrohalogenation” An example of addition reactions following the ionic electrophilic mechanism is the hydrohalogenation reaction of C=C or CC bonds. Hydrogen halides HX add rapidly to the double bond of alkene to yield an alkyl halide. R H H H C C + H X H C C H H H X H unsymetrical alkene H H H H C C + HBr H3C C C H CH 3 H Br H 34 ◼  ◼ Markovinkove’s Rule: When an unsymmetrical reagent (e.g HBr, H-OH, R-OH, H2SO4, HCN, …) adds to unsymmetrical alkene (e.g CH3 CH=CH2) the positive part of the reagent becomes attached to the double bonded-carbon atom having the greatest number of hydrogen atoms. I.e “the rich gets richer”. CH3 CH CH2 + HBr CH3 CH CH3 Br The order of reactivity of HX acids is HF > HCl > HBr > HI ◼Modern statement of Markovinkov’s rule: “In the addition of unsymmetrical reagent to a double bond, the positive portion of the adding reagent attaches itself to the carbon of a double bond so as to yield the more stable carbocations. 35 ◼ Anti –Markovinkov’s Rule: The presence of electron withdrawing substituent e.g– F,. NO2, etc in the alkene molecule would result in the formation of less stable carbocation and addition would appear to go anti-Markovinkov’s, i.e, Hydrogen adds to the carbon of the double bond with fewer, number of hydrogen atom. for example. F3 C-CH CH2 + HBr F3C CH2 CH2 Br 36 ◼  Free Radical Mechanism: “Peroxide effect or” Karasch effect” In presence of organic peroxides (ROOR) alkenes react with HBr via an anti-Markovinkov’s rule following a free radical addition mechanism. Hydrogen adds to the carbon atom poor in hydrogen while the halogen (Br) adds to the carbon rich in hydrogen. R-O-O-R CH3 CH CH2 + HBr CH3 CH2 CH2 Br ◼37 ◼ Step (1):. R-O-O-R 2 R-O ◼ Step (2): R-O. + H : Br R-OH + Br. Step (3): Br (a). Br. + CH3-CH CH2 CH3 CH CH2 Br O (b) Br. + CH3-CH CH2 CH3 CH2 CH2 The order of stability of free radicals is 3>2>1 Step (4):.. CH3 - CH - CH2 Br + H : Br CH3 CH2 CH2 Br + Br 38 ◼ Oxidation 39 ◼  i) Mild oxidation alkenes give glycols when oxidation is carried out by cold KMnO4 or Osmium oxide OsO4 KMnO4 CH2 CH2 + cold CH2 CH2 OH OH OSO4 CH3 CH CH2 CH3 CH CH2 OH OH O O OH OsO4 NaHSO3 Os H2 O O O OH A cyclic osmate cis-1,2-Cyclopentan ediol (a cis glycol) ◼40 41 ◼ ii) Strong Oxidation: Strong oxidation results in cleavage of the alkene. The oxidative cleavage of alkenes has frequently been used to prove the location of the double bond in an alkene chain or ring. # of H atoms Type of Product 2 CO2 1 -COOH 0 Ketone 42 ◼  Oxidation with O3 Treatment of an alkene with ozone followed by a Either oxidation or reduction of the ozonide to the final products. cleaves the C=C and forms two carbonyl groups in its place ◼43 44 ◼ Reduction of Alkenes (Hydrogenation) Requires Pt or Pd as powders on carbon and H2 Hydrogen is first adsorbed on catalyst Reaction is heterogeneous (process is not in solution) 45 ◼45 Syn Addition 46 46 ◼ Mechanism of Catalytic Hydrogenation 47 47 ◼  Dienes and Polyenes Conjugated dienes Non conjugated dienes Allenes Cycloalkenes Preparation Via Diels-Alder Reaction: CH2 CH2 CH CH2 O CH CH2 200 C + CH CH2 CH CH2 CH2 CH2 (Diene) (Dienophile) (Adduct) ◼48  r your at fo te ks nt n ion Tha ! Th s an k ks an T h ◼ 49