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1 Introduction to Organic Chemistry Organic chemistry is the study of the chemistry of carbon compounds. Over 16 million carbon containing compounds are known and 90% of the new compounds discovered or synthesized are also organic or carbon containing. All of the biomolecules essential to life such as nucleic acids, amino acids, carbohydrates, lipids and fats are organic compounds. In the last 60 years the increased life expectancy is directly linked to life saving drugs that minimize the dangers of microbial infections. These antibiotics were introduced from 1940s onwards and radically changed the course of treatment of common diseases. Many of the most effective modern vaccines depend on organic chemistry. What are the unique properties of carbon that leads to such a huge diversity of compounds? Stability of carbon-carbon bonds, The existence of carbon-carbon multiple bonds Carbon-oxygen, carbon-nitrogen, carbon-halogen bonds add to the huge variety of compounds Carbon-carbon bonds lead to linear and branched chain molecules as well as Compounds with rings REVISION OF CONCEPTS STUDIED IN CHEM101 Carbon is tetravalent Methane H H C H H H H C H H 2 H H C C H ethane H H H H C C C H H propane H H H H H H H H H C C C C H H butane H H C O N H tetravalent divalent trivalent monovalent H H C C N H amine H H H H H H H H C C H H O H H alcohol H H C C H H H F halide Multiple Carbon-Carbon Bonds H H C C H ethane H H H H H C C H H ethane/ethylene H C C H ethyne/acetylene C C C C C C single double triple Chemical Bonding Atomic orbitals Electrons populate different energy levels, or electron shells, and the way in which electrons fill the energy levels creates the atoms of the periodic table (rows of periodic table) 3 Orbitals 3-dimensional volume in which find electron (electron density) Shape of orbitals are important s orbitals are spherical p orbitals are like dumbbell Orbitals increase in energy in ranking 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p....... For Organic chemists 1s, 2s, 2p are most important!! The inert gas configuration of electrons, a full outer energy level is especially stable. (Octet rule) The ground state electronic configuration is the lowest energy state of an atom and can be written in either of two ways: Sodium would be written as 1s2 2s2 2p6 3s1 or [Ne]3s1 When sodium loses an electron from its outer shell it achieves the electronic configuration of neon; Na+ 1s2 2s2 2p6 Ionic Bonds When atoms lose or gain electrons, they develop charges and are called ions. Positively charged ions are cations, and negatively charged ions are called anions. Li + . lithium cation Ffluoride anion 4 Electronegativity Electronegativity measures the tendency of an atom to attract electrons. Electronegativity increases in going left to right across a row in the Periodic Table. In some groups, it decreases in going down a column. Li Row 2 Be B C N O F increasing electronegativity Ionic bonds are formed when atoms of very different electronegativities interact. Group 7 F decreasing Cl electronegativity Br I Atoms at extreme left or right of periodic table lose or gain electrons to achieve inert gas configuration (very stable). Octet rule!!! Example Na+ ClBoth atoms achieve the stable inert gas configuration by loosing and gaining an electron. Resultant compound is ionic most electronegative Linus Pauling (1901-1994) Nobel Prize in Chemistry 1954 Nobel Peace Prize 1962 5 Electronegativity and Types of Chemical Bonds The electronegativity difference between interacting atoms determines the type of bond that is formed. F2 compound electronegativity difference type of bond 4.0 - 4.0 = 0 HF 4.0 - 2.1 = 1.9 polar covalent nonpolar covalent LiF 4.0 - 1.0 = 3.0 ionic Lithium Fluoride Lithium, a metal, has a very low electronegativity. Fluorine, a nonmetal, has a very high electronegativity. When they react, lithium gives up its single outermost (valence-level) electron, while fluorine takes on a single electron to fill the second energy level (valence-level). Li + F Li + F elect ron configurat ion of neon elect ron configurat ion of helium In the solid state, lithum fluoride forms a very stable ionic crystal lattice structure where each Li+ is surrounded by six F- , and each Fis surrounded by six Li+. Strong electrostatic attractive forces among the ions of opposite charge stabilize the solid state structure. 6 Carbon and Covalent Bonds Carbon has 4 outer shell electrons 1s2 2s2 2p2 Loosing 4 electrons to become C4+ or C4- would require far too much energy. Carbon shares electrons with the atom to which it is bonded and achieves an octet of electrons Example methane: each hydrogen atom has a share of two electrons and the carbon a share of 8. Covalent Bonding Covalent Bonding When two atoms of the same or similar electronegativities react, they achieve a noble gas electron configuration by sharing electrons in covalent bonds. A two electron bond is shown by a dash or line. Examples H2 H. . . C . + H. . H :H or H-H H H or : H :C H HCH H H : : CH 4 H. + Note: Carbon, Group 4, needs 4 electrons to reach the electron configuration of neon. Hydrogen achieves the electron configuration of helium by forming a single two electron bond. Sharing of electrons results in the covalent bond Simple examples H2, O2 (Lewis) 7 Covalent Bonding When two atoms of the same or similar electronegativities react, they achieve a noble gas electron configuration by sharing electrons in covalent bonds. A two electron bond is shown by a dash or line. Examples H2 H. . . C . + H. . H :H or H-H H H or H :C :H HCH H H : : CH 4 H. + Note: Carbon, Group 4, needs 4 electrons to reach the electron configuration of neon. Hydrogen achieves the electron configuration of helium by forming a single two electron bond. 8 The Octet Rule By sharing 6 electrons, each nitrogen achieves the electron configuration of neon with an octet of electrons in the valence level. This tendency to reach 8 electrons in the valence level is called the octet rule. Examples of the Octet Rule N N nitrogen : : : : H H:C :H H methane H:O:H H :F : water hydrogen fluoride The above structures are called Lewis structures in honor of G.N. Lewis. In a Lewis structure, all the valence-level (outermost) electrons are shown as dots. The Lewis structure of an atom is the chemical symbol with the valence-level electrons shown as dots. 9 Example CH3F methyl fluoride The Lewis structures of the atoms are: : : . . C . .F H . H . H . . or H HC F H : : H H H :C : F : or H C F : H H 14 valence electrons : : : : : The remaining 6 valence electrons are nonbonding electrons around the fluorine atom. The octet rule applies to both the carbon and fluorine atoms. H H :C : F H : : The four covalent bonds to the central carbon atom account for 8 of the valence electrons: There are 4 + 7 + 3 = 14 valence electrons available for chemical bonding and as nonbonding electron pairs. 10 Theory of Chemical Bonding 3 Key ideas of the 19th Century 1 4 bonds to Carbon - Tetravalent (Kekulé and Couper) 2 Tetrahedral Shape of Substitution at Carbon Spatial direction of 4 bonds from Carbon is Tetrahedral (van’t Hoff and Le Bel) 3 Covalent bonding occurs by sharing electrons Covalent Bonds and Lewis Structures Covalent bonding occurs by sharing electrons Fluorine Oxygen Water H2O Ammonia NH3