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Organic Chemistry - Morrison and Boyd

Oct 28, 2014

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Bonding in Organic Compounds

Chapter 1

1Bonding in Organic Compounds

CHAPTER SUMMARYOrganic chemistry is the study of compounds of carbon. This is a separate branch of chemistry because of the large numbers of organic compounds and their occurrences and applications.

1.1 Elements and Compounds Atoms and Molecules Elements are the fundamental building units of substances. They are composed of tiny particles called atoms; atoms are the smallest particles of an element that retains the properties of that element. Atoms are composed of a positively charged nucleus that consists of protons (charge = +1, mass = 1) and neutrons (charge = 0, mass = 1). The nucleus is surrounded by negatively charged electrons that have negligible mass. Elements combine to form compounds. A molecule is the smallest particle of a compound that retains the properties of the compound; atoms bond to one another to form a molecule.

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Chapter 1 1.2 Electron Configuration

Bonding in Organic Compounds

A. Atomic Number and Atomic Mass The atomic number of an atom is the number of protons in the nucleus; this is equal to the number of electrons surrounding the nucleus in a neutral atom. The mass number is the number of protons plus neutrons in the nucleus. Isotopes are atoms with the same number of electrons and protons but different numbers of neutrons; they have the same atomic number but different mass numbers. The atomic mass of an element is the weighted average of the naturally occurring isotopes. B. Atomic Orbitals The space electrons occupy around an atomic nucleus is described by atomic orbitals. The most common orbitals in organic chemistry are sorbitals, spherical orbitals with the atomic nucleus located in the center, and dumbbell shaped p-orbitals in which the nucleus is between the lobes. C. Filling Atomic Orbitals Orbitals exist in energy levels or shells (numbered 1-7). An atomic orbital can be occupied by 0, 1, or 2 electrons. Atomic orbitals are filled according to the Aufbau principle beginning with the lowest energy orbitals and proceeding to higher energy ones. The electron configuration of an atom is described by the orbitals occupied in each shell and the number of electrons in each orbital. D. Electron Configuration and the Periodic Table The periodic table of elements is organized according to electron configuration. Elements are placed in periods that are related to the outermost shell of electrons and in groups that are related to the number of electrons in the outer shell. All elements in a group have the same number of outer shell electrons (the same as the group number) and the same electron configuration except for the shell number (for example in Group IV, C is 2s22p2 and Si is 3s23p2; both outer shells have four electrons). E. Stable Octets The elements in Group VIII are especially stable; their outer shell configuration is known as a stable octet.

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Bonding in Organic Compounds 1.3 Ionic Bonding and the Periodic Table

Chapter 1

A. Ionic Bonding, Electronegativity, Electron Configuration, and the Periodic Table Ionic bonding involves the complete transfer of electrons between two atoms of widely different electronegativities; charged ions are formed (one positive from the loss of electrons and one negative from the gain of electrons), both of which usually have a stable octet outer shell. The ionic bond results from the attraction between the positive cation and negative anion. Electronegativity is defined as the attraction of an atom for its outer shell electrons. Electronegative elements have a strong attraction for electrons and form anions in chemical reactions; electropositive elements have relatively weak attractions for electrons and form cations. B. Electron Dot Representation of Ions The electrons in the outer shell of an anion are represented by dots surrounding the elements symbol. Anions have usually gained sufficient electrons to complete their outer shells. Cations have usually lost their outer shells, the next shell in becomes the new outer shell, a stable octet, and is not shown. 1.4 Covalent Bonding A. Covalent Bonding, Electron Configuration, and the Periodic Table Covalent bonds involve the sharing of electron pairs between atoms of similar electronegativites; in most cases one or both atoms obtain a stable octet outer shell of electrons. The most common valences in Groups I-IV of the periodic table result from the pairing of all outer shell electrons with outer shell electrons of other atoms; a stable octet results in Group IV, but Groups I-III have incomplete outer shells. The common valences of Groups V-VII result from the pairing of outer shell electrons with those of other atoms to form an octet. The predicted valences of Groups I-VII are 1,2,3,4,3,2,1 respectively. Electron dot formulas depict the outer shell of atoms in molecules showing bonding and non-bonding electron pairs. B. Covalent Bonding in Organic Compounds

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Chapter 1

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A single bond has one bonding pair of electrons; there are two bonding pairs (four electrons) in a double bond and three bonding pairs in a triple bond. The number of bonds formed by elements commonly found in organic compounds is: C - 4; N - 3; O, S - 2; H - 1; F, Cl, Br, I - 1. A carbon can have four single bonds, two double bonds, a double and two single bonds, or a triple and a single bond; all total four bonds. These bonds can be represented by electron dot or line bond formulas. C. Drawing Electron Dot Formulas In drawing electron dot formulas, one must use every atom in the molecular formula and satisfy the valence (the number of bonds formed) for each. A good procedure involves bonding together atoms with valences greater than one with single bonds, inserting double and triple bonds until all valences can be satisfied with the available monovalent atoms, and finally attaching the monovalent atoms. D. The Structural Nature of Compounds Ionic compounds are composed of positive and negative ions in a ratio that will provide an electrically neutral compound. The atoms of a covalent compound are attached to one another to form molecules. Dissolution of an ionic compound in water produces solvated ions whereas covalent compounds have solvated molecules. E. Polyatomic Ions and Formal Charge Polyatomic ions are charged species in which several atoms are connected by covalent bonds. The magnitude and location of the ion's charge is called the formal charge. The formal charge on an atom is equal to the group number of the atom on the periodic table minus the non-bonding electrons and half of the bonding electrons. F. Polar Covalent Bonds A polar covalent bond is composed of atoms with similar but not equal electronegativities. The more electronegative atom is partially negative and the other is partially positive. 1.5 An Orbital Approach to Covalent Bonding A. Sigma and Pi Covalent Bonds

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Bonding in Organic Compounds

Chapter 1

A covalent bond is formed by the overlap of two atomic orbitals each with one electron. There are two types: sigma and pi. A sigma bond involves the overlap of two atomic orbitals head-to-head in one position (such as two s-orbitals, an s and a p-orbital, or two p-orbitals). A pi-bond involves the overlap of parallel p-orbitals at both lobes. B. Electron Configuration of Carbon Bonding in carbon involves the promotion of a 2s electron to an empty 2p orbital thus creating four unpaired electrons, one in the 2s and one in each of the three 2p orbitals. This allows carbon to be tetravalent. C. Shapes of Organic Molecules The shapes of organic molecules are predicted using the following principle: atoms and non-bonding electron pairs attached to a common central atom are arranged as far apart in space as possible. If there are four surrounding groups, the shape is tetrahedral; with three, the groups protrude to the corners of a triangle (trigonal); and with two, the region is linear. D. Carbon with Four Bonded Atoms A carbon with four bonded atoms is sp 3-hybridized, tetrahedral, and has approximately 109o bond angles. The four atomic orbitals on carbon (an s and three p's) combine, through a process called hybridization, to form new orbitals with different geometric orientations. The four new sp3orbitals are raindrop shaped and are oriented to the corners of a tetrahedron. All bonds to the carbon are sigma bonds. E. Carbon Bonded to Three Atoms A carbon with three bonded atoms is sp2-hybridized, trigonal, and has approximately 120o bond angles. There are three new sp2-hybrid orbitals directed to the corners of a triangle; these form sigma bonds with other atoms. The remaining p-orbital overlaps with a parallel p-orbital of an adjacent, sigma bonded atom to form a pi-bond and complete the double bond. F. Carbon Bonded to Two Atoms A carbon with two bonded atoms is sp-hybridized, linear, and has 180o bond angles. There are two new sp hybrid orbitals that are directed opposite to one another on a straight line; these form sigma bonds. The two remaining p-orbitals overlap with p-orbitals on a similarly hybridized atom to 5

Chapter 1

Bonding in Organic Compounds

form two pi-bonds and complete the triple bond. Alternatively, the two porbitals can overlap with counterparts on two adjacently bonded sp2hybridized atoms forming two double bonds. G. Bonding in Organic Compounds A Summary A carbon with four bonded groups is tetrahedral, sp3-hybridized, and has 109.5O bond angles. A carbon with three is trigonal, sp2-hybridized, and has 120O bond angles. A carbon with two bonded groups is linear, sp-hybridized, and has 180O bond angles. A single bond is a sigma bond; a double bond is composed of one sigma bond and one pi-bond; a triple bond is one sigma and two pi-bonds. Triple bonds are stronger than double bonds and double bonds are stronge