Bond Energies and Polarity in Chemistry

1. 1-2-انرژی پیوند- قطبیت- قطبش پذیری 1-2-1-انرژی پیوند:

2. Comparison of C-C bonds: a) Bond lengths are strongly depend on hybridization but little influenced by other factors.

3. similar bonds have fairly constant lengths from one molecule to the next, though exceptions are known. The variation is generally less than 1%. Bond Lengths between sp3 Carbons in Some Compounds

4. b) Bond energies are not independent of the remainder molecule. Homolytic Bond Dissociation Energies

5. Stable Allyl and Benzyl Radicals: پایداری رادیکال های آلیل وبنزیل از طریق رزونانس باعث می شود پیوندC-H ضعیف تر شود

6. Homolytic Bond Dissociation Energies ضعیف شدن پیوند ها با افزایش استخلاف ها ناشی از افزایش پایداری رادیکال های استخلافی نسبت به رادیکال های نوع اول است

7. انرژی تفکیک هترولیتیک در فاز گازی a) Very High in Gas Phase b) Low in Solution

8. 1-2-2-الکترونگاتیوی و قطبیت Bondpolarity: Unequal distribution of electron density in covalent bond (m=e.d) Electronegativity: The tendency of an atom to attract electrons can be defined for an atom or group. Allred and Rochow defined electronegativity in terms of the electrostatic attraction by the effective nuclear charge Zeff. A more recent formulation of electronegativity where IPs and IPp are the ionization potentials of the s and p electrons and a and b are the number of s and p electrons, respectively. 8

11. Bond strength is increased by electronegativity differences. Some Group Electronegativites Relative to H = 2.176

12. Effect of Polarity on Acidity:

13. In Solution: Acetic Acid > Propionic Acid • In Gas Phase: Acetic Acid < Propionic Acid • Reason: Solvation Parameter in Solution and Charge Accommodation in Gas Phase

14. تعیین کنفورماسیون چگونه میتوان تعیین کرد کدام فرم برتر است؟ دی کلرواستیلبن

15. INDUCTIVE EFFECT (IE)اثر القایی The polarization of one bond caused by the polarization of an adjacent bond is called the inductive effect. polarization effects transmitted through σ-bonds Positive (+I) (electron donating)/ negative (-I) (electron withdrawing).

16. The moment of p-cresol (1.57 D) is quite far from the predicted value of 1.11 D. In some cases, molecules may have substantial individual bond moments but no total moments at all because the individual moments are canceled out by the overall symmetry of the molecule.

17. اثر میدانی • The other effect operates not through bonds, but directly through space or solvent molecules, and is called the field effect. • The field effect depends on the geometry of the molecule but the inductive effect depends only on the nature of the bonds. اثر القایی اتم های کلر یکسان ولی اثر میدانی انها متفاوت است، به دلیل اینکه اتم های کلر در سمت چپ به گروه کربوکسیل نزدیک تر است .

18. اثر میدانی

19. The field-effect order of alkyl groups attached to unsaturated systems is tertiary>secondary>primary> CH3, but this order is not always maintained when the groups are attached to saturated systems.

20. 1-2-3-قطبش پذیری- سختی ونرمی

21. Polarizibility: A Fundamental Atomic Property Which is Related to • Electronegativity. • Softness: Ease of Distortion • Hardness: Difficulty of Distortion • Factors That cause increasing the Hardness: • Increasing The Electronegativity • Decreasing The size of the Atom • Increasing the oxidation Number in Metal Cations • Lewis Acids: Hard Acids Prefer Hard Bases and Soft Acids Prefer Soft Bases

22. Soft Bases: The donor atoms are of low electronegativity and high polarizability and are easy to oxidize. They hold their valence electrons loosely. • Hard Bases: The donor atoms are of high electronegativity and low polarizability and are hard to oxidize. They hold their valence electrons tightly. • Soft Acids: The acceptor atoms are large, have low positive charge, and contain unshared pairs of electrons (p or d) in their valence shells. They have high polarizability and low electronegativity. • Hard Acids: The acceptor atoms are small, have high positive charge, and do not contain unshared pairs in their valence shells. They have low polarizability and high electronegativity.

23. مقیاس سختی h = (EI – A)/2 EIانرژی یونیزاسیون = A الکترون خواهی= نرمی s = 1/h

25. these considerations are important because they allow us to understand the Lewisacid-base interactions and reactivity (=hard acids (species) react preferentially with hardbases(species) and soft with soft). In fact the reactivity of the hard centers is dominated by electrostatic attractions, while thereactivity of the soft centers is dominated by mutual electron cloud polarization.

26. اصل سختی بیشینهPRINCIPLE OF MAXIMUM HARDNESS • Pearson(1987) suggested that a chemical system tends to arrange itself so as to achieve maximum hardness and the most stable state of the system is that in which η is maximized. P R TS

27. اصل سختی بیشینه PRINCIPLE OF MAXIMUM HARDNESS How it works: One can consider the degree of electron transfer between the reactants. Thiscan be expressed numerically as: cx= absolute electronegativity of species x(y) hx = hardness of species x (y) Example: Consider radical halogenation of an alkane (in our case methane). Methyl halide (product) isformed as a result of combination of a halogen and methyl radicals according to the followingscheme: کاهش سختی وقطبیت افزایش واکنش پذیری افزایش درجه انتقال الکترون=> افزایش خصلت یونی (MeI>Br>Cl>F)

28. This is also in agreement with the known reactivity of methyl halides (MeI>Br>Cl>F). Because when similar (= comparable) bonds are considered, the reaction partners of highest hardness display higher net charge transfer. This in turn means higher exothermicity of the product-bond formation (= more thermodynamically stableproduct). Outcome = Bond between two harder reactants is more stable than bond between two softer reactants.