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Mechanisms of organic reactions. How Organic Reactions Occur Homolytic bond breaking (radical): A-B  A  + B  radicals are formed Heterolytic bond breaking.

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Presentation on theme: "Mechanisms of organic reactions. How Organic Reactions Occur Homolytic bond breaking (radical): A-B  A  + B  radicals are formed Heterolytic bond breaking."— Presentation transcript:

1 Mechanisms of organic reactions

2 How Organic Reactions Occur Homolytic bond breaking (radical): A-B  A  + B  radicals are formed Heterolytic bond breaking (polar): A-B  A + + :B - ions are formed

3 Heterolytic Reactions Nucleophile has an electron-rich atom (e.g Cl -, CN -, NH 3 ) and can form a bond by donating a pair of electrons to electron-poor atom Electrophile has an electron-poor atom (e.g H +, CH 3 + ) and can form a bond by accepting a pair of electrons from a nucleophile A + + :B -  A:B

4 Kinds of Organic Reaction I. Addition reactions Addition reactions - two reactants add together to form a single new product with no atoms „left over“ Elimination reactions Elimination reactions - single reactant splits into two products

5 Kinds of Organic Reaction II. Substitution reactions Substitution reactions - two reactants exchange parts to give new products Rearrangement reactions Rearrangement reactions - single reactant undergoes a reorganization of bonds and atoms

6 Nuccleophilic Substitution reactions SN1 & SN2 SN1 & SN2

7 Uni-/Bi- molecular reactions S N 1 & S N 2 Unimolecular reaction SN1 Unimolecular reaction SN1 only one of the reactant molecules is present in the transition state R - X + Y -  R + + X - + Y - R + + Y -  R - Y Bimolecular reaction SN2 Bimolecular reaction SN2 both of reactants must be present together in transition state Y - + R - X  [Y…R…X]  R - Y + X -

8 Electrophilic Substitution  - an electrophile (E  ) reacts with an aromatic ring and substitutes for one of the hydrogens MECHANISM Resonance forms of  complex

9 Electrophiles Substance that is „electron-loving“ Has an electron-poor atom Lewis acids Lewis acids (AlCl 3, FeCl 3 ) catalyze formation of electrophilic molecules Lewis acid – accepts electron pair Lewis base – donates electron pair

10 Aromatic Electrophilic Substitutions I Halogenation MECHANISM X = Cl

11 Aromatic Electrophilic Substitutions II. Friedel-Crafts alkylation reactionFriedel-Crafts alkylation reaction Friedel-Crafts acylation reactionFriedel-Crafts acylation reaction AlCl 3 benzene + HCl tert- butylchloride tert- butylbenzene

12 Aromatic Electrophilic Substitutions III. Nitration

13 Substituent Effects in Substituted Aromatic Rings 1. Ortho- and para-directing substituents increase the electron density inside the ring Y = - NH 2, -OH, -F, -Cl, alkyl 2. Meta-directing substituents decrease the electron density inside the ring Y = - NO 2, -CN, -COOH, -SO 3 H

14 2. Nucleophilic Substitution Reactions of Alkyl Halides Reactions of Alkyl Halides Nu : - + CH 3 -Br  CH 3 -Nu + Br - Nucleophile = HS - > CN - > I - > HO - > Cl -

15 3. Radical Substitution Initiation Initiation Cl 2  2Cl  Propagation Propagation Cl  + H 3 C-CH 3  H 3 C -CH 2  + HCl CH 3 -CH 2  + Cl 2  H 3 C -CH 2 Cl + Cl  Termination TerminationH 3 C -CH 2  + Cl   H 3 C -CH 2 Cl UV

16 Electrophilic addition Markovnikov´s Rule: Markovnikov´s Rule: In the addition of HX to an alkene, the H attaches to the carbon with more H. cis addition – both groups attach to the same side of the double bond trans addition –groups attach to the opposite side of the double bond

17 Elimination Reactions Dehydration Dehydrohalogenation

18 Organic halide SN1 Remember the reactivity order Benzylic > allylic > 3° RX> 2° RX> 1° RX

19 SN2 Remember the reactivity order 1° RX > 2° RX > 3° RX > allylic >Benzylic

20 SN2 example

21 E1 Remember the reactivity order Benzylic > allylic > 3° RX> 2° RX> 1° RX

22 E1 & SN1

23 E2 Remember the reactivity order 1° RX > 2° RX > 3° RX > allylic >Benzylic

24 ALCOHOLS & Phenols Forming the phenoxide ion Remember that the negative charge here is localized, therefore this ion more stable than the alkoxide ion (from alcohol)

25 Mechanism of the Kolbe-Schmitt Reaction

26 Ethers

27 Williamson synthesis

28 Aldehydes & Ketones IDOFORM REACTION OF THE METHYL KETONE

29 Aldol condensation

30 Amines HOFMANN rearrangement


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