Organic Chemistry ucture and Function CHAPTER 1 &Neil E.Schore Structure and Bonding in Organic Molecules Jiaxi Xu BUCT 1-1 The Scope of Organic Chemistry:An Overview The Scope of Organic Chemistry:An Overview Alkyres C-C triple bonds (Chapter 13) egen-ag9aloroups,omiycartonand bond strengths and reactons 2 Nitrogen cor u(Caperup Functional (Chapter 8) 1-1 The Scope of Organic Chemistry:An Overview 2couibnibForcesAsmpliedMewoiBondng aaaomakyge 0+20+2m一26+n 8taeaoane2a8oaeoteae9m3caeas (Substrates)
1 Organic Chemistry Structure and Function Fifth Edition W. H. Freeman & Company, New York K. Peter C. Vollhardt & Neil E. Schore Jiaxi Xu BUCT CHAPTER 1 Structure and Bonding in Organic Molecules The Scope of Organic Chemistry: An Overview 1-1 •Functional groups determine the reactivity of organic molecules. •Alkanes – No functional groups, only carbon and hydrogen. (Chapter 2) •Alkane Reactions – Alkane bond strengths and reactions. (Chapter 3) •Cyclic Alkanes – New properties and changes in reactivity (Chapter 4) •Stereoisomerism – Same connectivity – different relative positioning of substituents in space (Chapter 5) •Haloalkanes – Substitution Reactions and Elimination Reactions (Chapters 6 and 7) •Alkenes – C-C double bonds (Chapter 8) The Scope of Organic Chemistry: An Overview 1-1 •Alkynes – C-C triple bonds (Chapter 13) •Aldehydes and Ketones – Carbonyl Compounds – C=O double bonds. (Chapters 16 and 17) •Amines – Nitrogen containing functional group (Chapter 21) •Tools For Identification – Spectroscopy (Chapters 10, 11, 14 and 20) •Carbohydrates and Amino Acids – Multiple Functional Groups (Chapters 24 and 26) The Scope of Organic Chemistry: An Overview 1-1 •Synthesis is the making of new molecules •Wöhler’s Synthesis of Urea: •Synthesis – Construct complex organic chemicals from simpler, more readily available ones (Chapter 8). •Reactions are the vocabulary, and mechanisms are the grammar of organic chemistry •Reactants (Substrates) – Starting compounds •Products •Reaction Mechanism – Underlying details of a reaction •Reaction Intermediate – Chemical species formed and then destroyed on the pathway between reactants and products. Coulomb Forces: A Simplified View of Bonding 1-2 Bonds are made by simultaneous coulombic attraction and electron exchange. When two atoms approach, the electrons of one are attracted by the protons of the other and vice-versa. 2 (+) charge ( ) charge Attracting Force = constant distance × − × Energy is released as the two atoms approach each other. When the atoms get too close together the energy begins to rise again due to repulsions between the two nuclei and the two sets of electrons
e6eo "2ae ⊙·⊙-。。 ⊙⊙-·⊙ 1-3 lonic and Covalent Bonds:The Octet Rule The periodic table underlies the octet rule. ce electrons (Helium 2)and are 2+一NT,wN *119 kcal mol 三 8 kcal mor sare shared to achieve a lence electro eitomof the same Li Be Na.Mg :+2:,Mg:: 2aS8常n9n n 2
2 This minimum energy is called the bond strength, and the distance between the two nuclei at this point is called the bond length. 1-3 Ionic and Covalent Bonds: The Octet Rule 1. Covalent Bonds are based on the sharing of electrons. If the electrons are not shared equally, a polar covalent (partially ionic) bond is formed, otherwise a pure covalent bond is formed. 2. Ionic Bonds are based on the transfer of one or more electrons from one atom to another. The resulting cation and anion are electrostatically attracted to each other. 1-3 Ionic and Covalent Bonds: The Octet Rule The periodic table underlies the octet rule. Electrons in atoms occupy levels or shells of fixed capacity. The first has room for 2, the second 8, and the third 16. Nobel gases have 8 valence electrons (Helium 2) and are particularly stable. Other elements lack octets in their outer electron shells and tend to form molecules in such a way as to create a stable octet arrangement. In pure ionic bonds, electron octets are formed by transfer of electrons. Alkali metals react with halogens by the transfer of one electron from the alkali metal to the halogen. Both atoms achieve a noble gas configuration: the alkali metal that of the preceding inert gas, the halogen that of the following inert gas. IPNa = +119 kcal mol-1 EACl = -83 kcal mol-1 -LE = -120 kcal mol-1 ΔE = -84 kcal mol-1 Valence electrons are conveniently indicated by placing dots around the symbol for an element. The letters represent the nucleus and the core electrons, and the dots represent the valence electrons: Hydrogen can either lose an electron to form an H+ ion, or gain an electron to form a H- , or hydride, ion: In covalent bonds, electrons are shared to achieve octet configurations • Ionic bonds between identical atoms of the same element do not form. • The high ionization potential of hydrogen prevents it from forming ionic bonds with halogens and other nonmetallic elements. • Ionic bonds are also unfeasible for carbon since it would require the loss of 4 electrons to achieve the octet of the preceding inert gas, or the gain of 4 electrons to achieve the octet of the following inert gas
W are2c8segrmemegeemheseearemsand fecin pol cova oulsion co the shapes of molec Electron-Dot Model of Bonding:Lewis Structures .rawn by fol ing simple rule 2.Count the mtable (non-metal Correct Lewis Structure correct Lewis Struct H:: 4山 3
3 • In these and similar cases, covalent bonding occurs. Atoms share electrons to achieve a noble gas configuration. •In certain cases, one atoms supplies both of the electrons in the bond: •Often 4 electron (double) and 6 electron (triple) bonds are formed: In most organic bonds, the electrons are not shared equally: polar covalent bonds. •Pure covalent bonds (perfect sharing of electrons) and ionic bonds (complete transfer of electrons) are two extreme types of bonding. •Most bonds lie somewhere between these extremes and are called polar covalent bonds. •Each element can be assigned an electronegativity value which represents its electron accepting ability when participating in a chemical bond. •The larger the difference in electronegativety between two atoms participating in a chemical bond, the more ionic is the bond. •Bonds between atoms of different electronegativity are said to be polar bonds. A partial negative charge is found on the atom of higher electronegativity and an equal but positive charge on the other atom. •As a rule of thumb, electronegativity differences less than 0.3 represent pure covalent bonds, from 0.3 to 2.0 polar covalent bonds, and greater than 2.0 ionic bonds. •The separation of opposite charges in polar covalent molecules results in the formation of dipoles: •In symmetrical molecules such as CO2 and CCl4, the individual dipoles will cancel and the molecule is left with a zero dipole moment. Electron repulsion controls the shapes of molecules. •The shapes of molecules can be predicted using the VSEPR method. •Bonding and non-bonding electron pairs on the same atom will arrange themselves in three-dimensions to be as far apart as possible. •In the case of 2 electron pairs, as in BeCl2, a linear arrangement results. For 3 electrons pairs, as is in BCl3, a trigonal arrangement results, and in the case of 4 electron pairs, a tetrahedral arrangement occurs: 1-4 Electron-Dot Model of Bonding: Lewis Structures Lewis structures are drawn by following simple rules. 1. Draw the molecular skeleton 2. Count the number of available valence electrons • Add one electron for each negative charge, if an anion. • Subtract one electron for each positive charge, if a cation. 3. Depict all covalent bonds by two shared electrons, giving as many atoms as possible a surrounding electron octet, except for H, which requires a duet. • Elements at the right of the periodic table (non-metals) may contain lone pairs of electrons. Correct Lewis Structure Incorrect Lewis Structures
4.Assign charges to atoms in the molecule. obl orponds Charge(valence electrons in free,neutral atom) “ 空 e。 The octet rule does not always hold. Covalent bondscan be depicted by straight lines. 1.ho: Structures of this type are called. 3atea3nwmopehe8tb2rtnlatomyc =-” 1-5 Resonance Forms at is its true structure? The carbonate ion has rrect Lewis stru of s the vereo caled over all three
4 It is often necessary to use double or triple bonds to satisfy the octet rule: 4. Assign charges to atoms in the molecule. Charge = (# valence electrons in free, neutral atom) – - (# unshared electrons on the atom) – ½(# bonding electrons surrounding the atom) In molecules such as nitric acid, charges occur on individual atoms, even though the molecule itself is neutral. The octet rule does not always hold. 1. The molecule or ion has an odd number of electrons. NO, CH3, NO2 2. The central atom has a deficiency of electrons. CH3, BeCl2, BH3 3. Past row 2 of the periodic table, the central atom may be surrounded by more than 8 electrons (expanded octet). Covalent bonds can be depicted by straight lines. Bonding pairs of electrons are most often represented as straight lines: single bonds as a single line, double bonds as two parallel lines, and triple bonds as three parallel lines. Lone pairs of electrons are either shown as dots or are omitted. Structures of this type are called Kekulé structures. 1-5 Resonance Forms The carbonate ion has several correct Lewis structures. Three equivalent structures must be drawn to accurately represent the carbonate ion. The only difference between these structures is the placement of electrons. But what is its true structure? The “true” structure can be thought of as the average of all three structures which is called a resonance hybrid. The 2 negative charges are delocalized over all three oxygen atoms
Other Structures with amaximum e8%883 c40 The el as9neaaaweg 一 c s-Ay 5
5 Other examples of resonance: Not all resonance forms are equal. 1. Structures with a maximum of octets are most important. 2. Charges should be preferentially located on atoms with compatible electronegativity. If this conflicts with rule 1, then rule 1 takes precedence. 3. Structures with less separation of opposite charges are more important resonance contributors than those with more charge separation. In some cases charge separation is necessary and guideline 1 takes precedence over guideline 2: If there are two or more charge separated resonance structures which comply with the octet rule, the most favorable one places the charges on atoms of compatible electronegativity: Atomic Orbitals: A Quantum Mechanical Description of Electrons around the Nucleus 1-6 The electron is described by wave equations. An electron within an atom can have only certain definite energies called energy states. Moving particles such as electrons exhibit a wavelength determined by the de Broglie relation: h λ= mv Where h is Plank’s constant, m is the mass of the electron in kg, and v is the velocity of the electron in m/s. The electron waves contain nodes, where the amplitude of the wave changes sign, and can interact with each other, producing either constructive or destructive interference: The wave theory of electron motion is called quantum mechanics. The quantum mechanical equations describing the motion of the electrons are called wave equations. The solutions of these equations are called wave functions and are represented by the Greek letter, ψ. The square of the wave function, evaluated at a point in space (x,y,z) represents the probability of finding the electron at that point at any given time. Each wave function corresponds to a specific discrete energy and the system is said to be quantized