Advanced Organic Chemistry (Chapter 1) sh.Javanshir 1-2-انرژی پیوند- قطبیت- قطبش پذیری انرژی پیوند: MoleculeBond Energy (kcal) Length (pm) Ethane C (sp 3 ) -H Ethylene C (sp 2 ) -H Acetylene C (sp) -H

Advanced Organic Chemistry (Chapter 1) sh.Javanshir Comparison of C-C bonds: a) Bond lengths are strongly depend on hybridization but little influenced by other factors.

Advanced Organic Chemistry (Chapter 1) sh.Javanshir Bond Lengths between sp 3 Carbons in Some Compounds similar bonds have fairly constant lengths from one molecule to the next, though exceptions are known. The variation is generally less than 1%.

Advanced Organic Chemistry (Chapter 1) sh.Javanshir b) Bond energies are not independent of the remainder molecule. BondBond Energy (kcal/mol) CH 3 CH 2 -H98 H 2 C=CH-CH 2 -H85 PhCH 2 -H85 Homolytic Bond Dissociation Energies

Advanced Organic Chemistry (Chapter 1) sh.Javanshir پایداری رادیکال های آلیل وبنزیل از طریق رزونانس باعث می شود پیوند C-H ضعیف تر شود Stable Allyl and Benzyl Radicals:

Advanced Organic Chemistry (Chapter 1) sh.Javanshir 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

Advanced Organic Chemistry (Chapter 1) sh.Javanshir انرژی تفکیک هترولیتیک در فاز گازی a) Very High in Gas Phase b) Low in Solution

Advanced Organic Chemistry (Chapter 1) sh.Javanshir الکترونگاتیوی و قطبیت Bondpolarity: Unequal distribution of electron density in covalent bond (  =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 IP s and IP p are the ionization potentials of the s and p electrons and a and b are the number of s and p electrons, respectively.

Advanced Organic Chemistry (Chapter 1) sh.Javanshir

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

Advanced Organic Chemistry (Chapter 1) sh.Javanshir Effect of Polarity on Acidity:

Advanced Organic Chemistry (Chapter 1) sh.Javanshir In Solution: Acetic Acid > Propionic Acid In Gas Phase: Acetic Acid < Propionic Acid Reason: Solvation Parameter in Solution and Charge Accommodation in Gas Phase

Advanced Organic Chemistry (Chapter 1) sh.Javanshir تعیین کنفورماسیون دی کلرواستیلبن چگونه میتوان تعیین کرد کدام فرم برتر است؟

Advanced Organic Chemistry (Chapter 1) sh.Javanshir 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 polarization effects transmitted through σ-bonds Positive (+I) (electron donating)/ negative (-I) (electron withdrawing).

Advanced Organic Chemistry (Chapter 1) sh.Javanshir 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.

Advanced Organic Chemistry (Chapter 1) sh.Javanshir 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.

Advanced Organic Chemistry (Chapter 1) sh.Javanshir اثر میدانی

Advanced Organic Chemistry (Chapter 1) sh.Javanshir The field-effect order of alkyl groups attached to unsaturated systems is tertiary>secondary>primary> CH 3, but this order is not always maintained when the groups are attached to saturated systems.

Advanced Organic Chemistry (Chapter 1) sh.Javanshir قطبش پذیری- سختی ونرمی

Advanced Organic Chemistry (Chapter 1) sh.Javanshir Polarizibility: A Fundamental Atomic Property Which is Related to Electronegativity. Softness: Ease of Distortion Hardness: Difficulty of Distortion Factors That cause increasing the Hardness: a) Increasing The Electronegativity b) Decreasing The size of the Atom c) Increasing the oxidation Number in Metal Cations Lewis Acids: Hard Acids Prefer Hard Bases and Soft Acids Prefer Soft Bases

Advanced Organic Chemistry (Chapter 1) sh.Javanshir 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.

Advanced Organic Chemistry (Chapter 1) sh.Javanshir مقیاس سختی  = (E I – A)/2 E I انرژی یونیزاسیون = A الکترون خواهی = نرمی 

Advanced Organic Chemistry (Chapter 1) sh.Javanshir these considerations are important because they allow us to understand the Lewis acid-base interactions and reactivity (= hard acids (species) react preferentially with hard bases(species) and soft with soft). In fact the reactivity of the hard centers is dominated by electrostatic attractions, while the reactivity of the soft centers is dominated by mutual electron cloud polarization.

Advanced Organic Chemistry (Chapter 1) sh.Javanshir 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. اصل سختی بیشینه PRINCIPLE OF MAXIMUM HARDNESS R P TS

Advanced Organic Chemistry (Chapter 1) sh.Javanshir degree of electron transfer between the reactants How it works: One can consider the degree of electron transfer between the reactants. This can be expressed numerically as: اصل سختی بیشینه PRINCIPLE OF MAXIMUM HARDNESS Example: Consider radical halogenation of an alkane (in our case methane). Methyl halide (product) is formed as a result of combination of a halogen and methyl radicals according to the following scheme: کاهش سختی وقطبیت افزایش واکنش پذیری (MeI>Br>Cl>F)  x = absolute electronegativity of species x(y)  x = hardness of species x (y) افزایش درجه انتقال الکترون=> افزایش خصلت یونی

Advanced Organic Chemistry (Chapter 1) sh.Javanshir 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 stable product). Outcome = Bond between two harder reactants is more stable than bond between two softer reactants.