محاضرة-2 عضوية-1 د. حجاج حسن PDF

‫بسم هللا الرحمن الرحيم‬ ‫جامعة اسيوط االهلية‬ ‫جامعة اسيوط‬ ‫كلية الصيدلة‬ ‫كلية الصيدلة‬ PHARMACEUTICAL ORGANIC CHEMISTRY-1 Pharm D Student (1st level, 1st semester) 1. Introduction to organic chemistry...

‫بسم هللا الرحمن الرحيم‬ ‫جامعة اسيوط االهلية‬ ‫جامعة اسيوط‬ ‫كلية الصيدلة‬ ‫كلية الصيدلة‬ PHARMACEUTICAL ORGANIC CHEMISTRY-1 Pharm D Student (1st level, 1st semester) 1. Introduction to organic chemistry 2. Chemistry of aliphatic hydrocarbons (2) ‫ حجاج حسن‬/‫دكتور‬  Electronegativity is a measure of the ability of an atom in a particular molecule to attract bonding electrons. Electronegativity and bond polarity The difference in electronegativity between two atoms detect the extent of bond polarity Compound F2 HF LiF Electronegativity 4.0 - 4.0 = 0 4.0 - 2.1 = 1.9 4.0 - 1.0 = 3.0 Difference Type of Bond Non polar Polar covalent Ionic (non- covalent covalent)  bonds connect atoms of.. the.. same electronegativity are nonpolar: H—H : F F:.... The greater the difference in electronegativity between two bonded atoms; the more polar the bond. Polar Covalent Bonds: Difference in electronegativity of atoms < 2 +.. − H..F: Ionic Bonds: Difference in electronegativity > 2 (NaCl, LiF) centers of positive and negative charge do not coincide, otherwise cancelling of charge occur. 4 Polarity of Bonds and Molecules Bond Dipole Moments 5 Why C is so unique? Bonding Properties of Carbon Carbon forms covalent bonds in all its elemental forms and compounds. – The ground state electron configuration of C is [He]2s22p2; the formation of carbon ions is therefore energetically unfavorable. – Carbon has an electronegativity of 2.5, which is midway between that of most metals and nonmetals. Carbon prefers to share electrons. Carbon exhibits catenation, the ability to bond to itself and form stable chain, ring, and branched compounds. – The relatively small size of the C atom allows it to form short, strong bonds. – The tetrahedral shape of the C atom allows catenation. Bond C-C N-N O-O Energy 347 163 157  Comparison of Carbon and Silicon As atomic size increases down the group, bonds between identical atoms become longer and weaker. So, C–C bond is stronger than a Si–Si bond. The bond energies of a C–C bond, a C–O bond, and a C– Cl bond are very similar, while Si–Si bond is weaker than Si–C and Si–O. Si has low energy d orbitals available for reaction, allowing Si compounds to be more reactive than C compounds. C compounds can undergo a variety of reactions and remain stable, while Si compounds cannot.  Diversity and Reactivity of Organic Molecules ✓ Many organic compounds contain heteroatoms, atoms other than C and H. The most common of these are O, N, and the halogens. ✓ Most reactions involve the interaction of electron rich area in one molecule with an electron poor site in another. – C–C bonds and C–H bonds tend to be unreactive. – Bonds between C and a heteroatom are usually polar, creating an imbalance in electron density and providing a site for reactions to occur. 8  Drawing Chemical Structures Empirical Formulas and Molecular Formulas Empirical Formula CH2O Molecular Formula C2H4O2 Structural Formulas Lewis Structure Condensed Structural Formula CH3COOH Line–Angle (Dot-Line) Formulas 9  Drawing the chemical structure H H H H C Br CH3 CH2 CH2 Br C C CH3CH2CH2Br Br H H H Skeletal structure or Condensed formula Dot –line formula Lewis structure 10  Resonance a molecule or ion can be represented by two or more Lewis structures that differ only in the positions of the electrons and are called resonance structures or resonance forms. The actual molecule is said to be a resonance hybrid of its resonance forms. resonance hybrid resonance structures or resonance forms resonance structures or resonance forms resonance hybrid Notes: - Resonance structures exist only on paper. - We are only allowed to move electrons - All of the structures must be proper Lewis structures. - The energy of the resonance hybrid is lower than the energy of any 11 contributing structure.  Resonance and Inductive Effects Resonance is the process whereby -electrons can be delocalised by exchanging double bonds and single bonds. - Resonance can be used to delocalise both lone pairs of electrons and charges which are adjacent to double bonds. - Delocalisation of positive and negative charges lead to relatively stable cations and anions, respectively. Resonance of Charges and Lone Pairs - Inductive effects are caused by differences in electronegativity between bonded atoms, which leads to a polarisation of the bond. - The inductive effects are short range 12 (unlike resonance effects).  Concept of acidity and basicity Different definitions of acids and bases: In 1923, Brønsted and Lowry defined acids and bases based on the transfer of protons: Brønsted–Lowry acid is any species that can donate a proton Brønsted–Lowry base is any species that can accept a proton. 13 Acid Strength Because they span such a wide range, acid-dissociation constants are often expressed on a logarithmic scale. 14 Some of the factors that affect the acid strength (stability of conjugate bases) are electronegativity, size, resonance and 15 hybridization. 1. Electronegativity 2. Size 16 3- Resonance Stabilization 4- Inductive Effects Electron-withdrawing atoms and groups can also stabilize a conjugate base through the sigma bonds of the molecule. 17  5- Hybridization effects consider the relative acidities of differently hybridized compounds containing C—H bonds. The higher the ratio of s-character of the hybrid orbital, the closer the lone pair is held to the nucleus, and the more stable the conjugate base, then the acid is stronger. 18  2- Lewis Acids and Bases Lewis bases are species that can donate electrons Lewis acids are species that can accept electron 19  Lewis Bases Lewis bases can accept protons as well as Lewis acids, therefore the definition encompasses that for Brønsted bases Most oxygen- and nitrogen-containing organic compounds are Lewis bases because they have lone pairs of electrons Some compounds can act as both acids and bases, depending on the reaction 20  Intermolecular forces (van der Waals forces) Mainly affect physical properties; boiling points and melting points, solubility properties of organic compounds (like dissolve like). 1. the dipole–dipole forces of polar molecules 2. the London dispersion forces (induced dipole): London force arises from temporary dipole moments that are induced in a molecule by other nearby molecules. 21 3. the “hydrogen bonds” that link molecules having OH or NH groups ❑ A hydrogen atom can participate in hydrogen bonding if it is bonded to oxygen, nitrogen. ❑ Although hydrogen bonding is a strong form of intermolecular attraction, it is much weaker than a normal or covalent bond. ❑ Hydrogen bonding has a large effect on the physical properties of organic compounds, as shown by the boiling points of ethanol (ethyl alcohol) and dimethyl ether, two isomers of molecular formula: Rank the following compounds in order of increasing boiling points: 22 Different ways of writing organic reactions 23  Kinds of Organic Reactions ❑ Addition reactions – two molecules combine ❑ Elimination reactions – one molecule splits into two ❑ Substitution – parts from two molecules exchange 24  How Organic Reactions Occur: Mechanisms The mechanism is a detailed description of how bonds are broken and formed as starting material is converted into product. Steps in Mechanisms A step involves either the formation or breaking of a covalent bond Steps can occur in individually or in combination with other steps When several steps occur at the same time they are said to be concerted Types of Steps in Reaction Mechanisms Bond formation or breakage can be symmetrical or unsymmetrical Symmetrical- homolytic 25 Unsymmetrical- heterolytic  Bond Breaking and Bond Making There are only two ways to break (cleave) a bond: the electrons in the bond can be divided equally or unequally between the two atoms of the bond. Radical Reactions Bond breaking Bond forming 26 Bond breaking polar reactions Bond forming An electrophile, an electron-poor species, combines with a nucleophile, an electron-rich species 27 Polar reactions: Reagents in organic Chemistry: Nucleophiles & Electrophiles Nucleophiles Electrophiles 28 Neutral molecules with vacant orbitals e.g. AlCl3, FeCl3, SO3. 29 30 Classification of Organic Compounds: - Hydrocarbons - Hydrocarbon Derivatives According to the nature of chemical properties into: 1- Aliphatic compounds. They are acyclic or cyclic compounds, which has no aromatic rings (no benzene ring). 2-Aromatic compounds. They are cyclic compounds which have closed system of delocalized π-electron like benzene. Pharm. Org. chemistry-1 Hybridization of atoms, Nature of chemical bonds, electronegativity of atoms, Polarity, Dipole moment, Resonance, acidic and basicity, types of Reactions of organic compounds and reaction mechanisms. The structure, properties, synthesis and reactions of Hydrocarbon compounds: ALKANES, ALKENES and ALKYNES. Stereochemistry. Concept of aromaticity, aromatic electrophilic substitutions reactions, Aromatic Classes and polynuclear aromatic compounds 31  Organic compounds are further classified by the presence of characteristic functional groups. What is a functional group? A functional group is defined as an atom or a group of atoms that effectively determines the chemical and physical properties of an organic compound. 32 33