Introduction to Organic Reactions

1. Chapter 28 Introduction to Organic Reactions 28.1Types of Reactive Species in Organic Chemistry 28.2Ways of Breaking Covalent Bonds 28.3Inductive and Resonance Effects 28.4Types of Organic Reactions

2. 28.1 Types of Reactive Species in Organic Chemistry (SB p.82) Free Radicals • Electrically neutral atoms or groups of atoms possessing an unpaired electron • Highly reactive because of the unstable electronic configuration • e.g.

3. 28.1 Types of Reactive Species in Organic Chemistry (SB p.82) Electrophiles and Nucleophiles • Electrophiles • electron-deficient species that tend to accept electron(s) • possess an empty orbital to receive the electron pair • cations or free radicals seeking electron-rich centres • Nucleophiles • electron-rich species that tend to seek an electron-deficient site for reaction • possess lone pairs of electrons • anions or molecules with lone pairs of electrons

4. 28.1 Types of Reactive Species in Organic Chemistry (SB p.83)

5. 28.1 Types of Reactive Species in Organic Chemistry (SB p.83) (a) Nucleophile (b) Electrophile (c) Nucleophile (d) Electrophile and nucleophile (e) Electrophile (f) Electrophile and nucleophile Check Point 28-1 Identify the following chemical species as electrophiles, nucleophiles, or one that could act as both an electrophile and a nucleophile. (a) Cl– (d) CH3Cl (b) C2H5+ (e) NO2+ (c) NH3 (d) CH3OH Answer

6. 28.2 Ways of Breaking Covalent Bond (SB p.83) A curly arrow with half an arrow head ‘ ’ is used to indicate the movement of a single electron. Homolysis

7. 28.2 Ways of Breaking Covalent Bond (SB p.84) Energy must be supplied in, either in form of heat or irradiation with light. e.g. chlorine undergoes homolysis readily when heated, or when irradiated with light of a wavelength that can be absorbed by the molecule to form two chlorine radicals

8. 28.2 Ways of Breaking Covalent Bond (SB p.84) General equation of homolysis of a bond to carbon: e.g. methane undergoes homolysis to form a methyl radical and a hydrogen radical

9. 28.2 Ways of Breaking Covalent Bond (SB p.84) • a curly arrow with a full arrow head ‘ ’ is used to indicate the movement of an electron pair. • 2 charged fragments or ions formed Heterolysis

10. 28.2 Ways of Breaking Covalent Bond (SB p.85) Heterolysis of a bond requires the bond to be polarized Thegreater the difference in electronegativity between the atoms, thegreater is the polarization of the bonds.

11. 28.2 Ways of Breaking Covalent Bond (SB p.85) The product of heterolysis of a bond to carbon depends on the electronegativity of the atom that is bonded to the carbon atom.

12. 28.2 Ways of Breaking Covalent Bonds (SB p.85) (a) Homolysis (b) Heterolysis (c) Homolysis Check Point 28-2 Which type of bond fission, homolysis or heterolysis, is most likely to occur in: (a) a bond between identical atoms? (b) a bond between atoms having widely different electronegativities? (c) a bond between atoms having similar electronegativities? Answer

13. 28.3 Inductive and Resonance Effects (SB p.86) Inductive Effect Due to the difference in electronegativity between two atoms linked up by  bonds, the bonding electrons will displace towards the more electronegative atom. The atom exhibits a partial negative charge. The electronic effect of a group that is transmitted by the polarization of electrons in  bonds is called an inductive effect.

14. 28.3 Inductive and Resonance Effects (SB p.86) Electron-withdrawing group (X) exerts a negative inductive effect. Electron-donating group (Y) exerts a positive inductive effect. Inductive effect is represented by an arrow head in the middle of the covalent bond pointing in the direction of the displacement of electrons.

15. 28.3 Inductive and Resonance Effects (SB p.87) 1. Groups which exert negative inductive effects (i.e. electron-withdrawing groups): e.g. –NO2 > –F > –COOH > –Cl > –Br > –I 2. Groups which exert positive inductive effects (i.e. electron-releasing groups): e.g. alkyl groups like –CH3, –C2H5, –C3H7

16. 28.3 Inductive and Resonance Effects (SB p.87) tert-butyl carbocation is the most stable because electron-donating groups exert positive inductive effects to reduce the positive charge on the carbon atom. The greater the number of alkyl groups attached to the central carbon atom, the more stable is the carbocation.

17. 28.3 Inductive and Resonance Effects (SB p.87) Resonance structures Resonance Effect Resonance effect is an electronic effect involving  bond electrons or electrons present in unhybridized p orbitals. The ion become more stable when the charge of the ion can be reduced or dispersed.

18. 28.3 Inductive and Resonance Effects (SB p.88) The actual structure of carboxylate ion is the resonance hybrid of the resonance structures. • The negative charge of the anion is dispersed • This resonance stabilization is responsible for the high acidity of carboxylic acids

19. 28.3 Inductive and Resonance Effects (SB p.88) Another example: Carbocation with the positively charged carbon atom directly bonded to a benzene ring Its actual structure is represented by four resonance structures shown below:

20. 28.3 Inductive and Resonance Effects (SB p.88) Solution: (a) Conjugate base 2 is more stable. The anion is stabilized by resonance effect and the negative charge of the anion is dispersed over two oxygen atoms. The two resonance structures of the anion are shown below: Example 28-1 The following equations represent the ionizations of two organic acids: (a) Which conjugate base is more stable? Explain your answer. Answer

21. 28.3 Inductive and Resonance Effects (SB p.88) Solution: (b) Conjugate base 1 is less stable because there is no resonance effect stabilizing the anion. Moreover, the positive inductive effect of the electron-releasing CH3CH2– group further destabilizes the anion. (c) Acid 2 is a stronger acid than acid 1. Example 28-1 (cont’d) (b) Which conjugate base is less stable? Explain your answer. (c) Which is a stronger acid? Answer

22. 28.3 Inductive and Resonance Effects (SB p.89) (a) (b) (i) Negative inductive effect (ii) Negative inductive effect (iii) Negative inductive effect (iv) Positive inductive effect Check Point 28-3 (a) Draw the two resonance structures for propanoate ion (CH3CH2COO–). (b) State whether the following species exhibit positive or negative inductive effects. (i) –I (ii) –NO2 (iii) –COOH (iv) –C2H5 Answer

23. 28.4 Types of Organic Reactions (SB p.90) Substitution Reactions • An atom or a group of atoms of the reactant molecule is replaced by another atom or group of atoms • Characteristic reactions of saturated compounds • e.g. H2O CH3– Cl + NaOH  CH3– OH + NaCl

24. 28.4 Types of Organic Reactions (SB p.90) Addition Reactions • Two molecules react to give a single product • Characteristic reactions of compounds with multiple bonds • e.g.

25. 28.4 Types of Organic Reactions (SB p.90) Elimination Reactions • Atoms or groups of atoms are removed from two adjacent atoms of the reactant molecule • Method for preparing compounds with multiple bonds • e.g.

26. 28.4 Types of Organic Reactions (SB p.91) Condensation Reactions • Two or more molecules join together, with a small molecule being removed • e.g.

27. 28.4 Types of Organic Reactions (SB p.91)  Rearrangement Reactions • A molecule undergoes reorganization of its constituent atoms or groups of atoms • e.g.

28. The END