2443-012811.pdf

Quiz 1 30 point quiz. Count as a 25 point quiz Average = 18.1 / 25 Stnd. Deviation = 5.86 Chapter 3. Bonding 2 ? The most fundamental concept in Organic chemistry is the nature of the bond betwen two carbon atoms or betwen carbon and another atom. ? For the most part, these are covalent bonds to carbon, although ionic bonds wil be seen. ? Most common organic molecules are characterized by the presence of covalent bonds. ? The place to begin a study of Organic chemistry is with chemical bonds of carbon-to-carbon, carbon to hydrogen, or carbon to other atoms. To begin, you should know: 3 ? The electronic configuration of elements in the first two rows. ? The difference betwen s and p and d orbitals. ? The shape of s and p atomic orbitals and how they relate to electronic configuration. ? The difference betwen and an ionic bond and a covalent bond. ? A sense of difference in the size of the elements and their respective ions. ? Bonds are made of electrons. ? The concept of electronegativity. When completed, you should know: 4 ? s-Orbitals are sphericaly symetrical and p-orbitals are "dumbel" shaped. ? Electrons are found in orbitals at discrete distances from the nucleus in an atom. ? Electrons in the bond of a molecule are located betwen two nuclei and are at diferent energy levels than in an unbonded atom. ? Ionic bonds are formed by elctrostatic atraction of two atoms or groups that have opposite charges. ? Covalent bonds are made of two elctrons that are mutaly shared betwen two atoms: ? Mixing atomic orbitals forms hybrid molecular orbitals, and the number of s and p orbitals used to form the hybrid determines the hybridization (sp 3 for example). ? Organic molecules generaly have a backbone of carbon-carbon covalent bonds. ? Electronegativity for an atom increase to the right and up the Periodic Table. ? Polarized bonds are formed when two atoms are bonded together but one is more electronegative. ? Polarized covalent bonds are generaly weaker than non-polarized covalent bonds. ? Reactions are driven by making and breaking bonds, which relase or require energy. ? The VSEPR model is used to predict the thre-dimensional shape around an individual ato. Electrons are Important 5 ? Atoms are discreet entities that difer from one another by the number of protons, neutrons and electrons that make up each atom. ? Protons and neutrons are found in the nucleus, of course, but reactions involving organic molecules do not involve transfer, gain or los of protons or neutrons. ? Chemical reactions involve the transfer of electrons, which are the important non-nuclear constituents of an atom. ? To determine chemical reactivity, a method has ben developed to ascertain the position of electrons relative to the nucleus. ? Electrons are said to reside in orbitals. Electrons, Waves and Orbitals 6 ? The motion of elctrons has some characteristics of wave motion. ? The motion of an electron is expresed by a wave equation, which has a series of solutions and each solution is called a wavefunction. Each electron may be a described by a wavefunction whose magnitude varies from point to point in space. ? A particular solution to the Schrödinger wave equation, for a given type of elctron, is determined by the equation. Hy = Ey ? H is a mathematical operator caled the Hamiltonian operator ? E is the numerical value for the energy ? y is a particular wavefunction. ? The relationship betwen orbitals and the Schrödinger equation is apparent when its solutions are represented as the waves shown in the Figure for various values of y that corespond to diferent energies. ? The amplitude of the wave is the wave function (y) and it has a maximum (represented by +) and a minimu (represented by ?), and each point in space can be represented by spatial coordinates (x,y,z). ? The point at which the wave changes Its phase is refered to as a node. (a) (b) node node s orbital (0 nodes) p orbital (1 node) d orbital (2 nodes) Orbitals 7 (a) (b) node node s orbital (0 nodes) p orbital (1 node) d orbital (2 nodes) ? The Heisenberg uncertainty principle states that the positon and moentum of an elctron cannot be simultaneously specified. ? It is only posible to determine the probability that an electron wil be found at a particular point relative to the nucleus. ? Since the exact position of the elctron is unknown (there is uncertainty as to its position), the probability of finding the electron in a unit volume of three-dimensional space is given by /y(x,y,z)/ 2 . The position is expressed as ?y/ 2 dt, which is the probability of an elctron being in a smal elment of the volume dt. ? This smal volume can be viewed as a charge cloud if it contains an electron, and the charge cloud represents the region of space where we are most likely to find the electron in terms of the (x,y,z) cordinates. ? Ploting y versus distance from the nucleus in the Figure leads to the familiar s, p and d orbitals. These charge clouds are known as orbitals. Atomic Orbitals 8 ? The energy levels represent the region in space where elctrons are found relative to the nucleus. ? There are several quantum levels in atoms, particularly in high atomic mas elments. ? This means that there are diferent energy levels asociated with each type of elctron shel, so there are different types of s orbitals: 1s, 2s, 3s, 4s, etc., similar in shape but differing in energy (their relative distance from the nucleus). ? There are 2p, 3p, 4p orbitals, and 3d, 4d, 5d, and, 4f, 5f, 6f orbitals. Based on the Periodic Table: ? The first row elments (H, He) have only the spherical s-orbitals. ? The second row (Li, Be, B, C, N, O, F, He) has the 1s orbital and the 2s- and 2p- orbitals are in the outermost shell. ? The third row introduces 3s-, and 3p-orbitals. ? d-Orbitals apear in the fourth row. Each shel wil have one s, thre p, five d and seven f orbitals (1, 2, 3, 4). * Note that d- and f-orbitals acept more electrons or give up more electrons in chemical reactions than a p-orbital because more orbitals are involved. s p d (4 types) d z 2 s, p, and d orbitals Electronic Configuration - 2nd Row 9 ? The description of the way elctrons are distributed in the atomic orbitals of an element is known as its electronic configuration. The electronic configuration of each atom wil show the type, quantum level, and number of electrons. ? The electronic configuration of each element is described by the order in which the various orbitals fil as the number of electrons increases for each element of the Periodic table. ? Each individual orbital can hold no more than two electrons. ? Electrons have the property of spin, which is asociated with a magnetic dipole. The spin quantum number was introduced in General chemistry, and it is one of four quantum numbers where self-rotation of the electron gives rise to an angular momentum vector. ? Each electron wil have spin and the symbol ? is used to indicate an electron with a certain spin quantum number. ? A single orbital containig two elctrons, also known as a filed orbital, is represented by two opposed arrows (??). 1 Li 2 Be 13 B 14 C 15 N 16 O 17 F 18 Ne 1s 2s 2p ? The symbol ?? indicates that when an orbital contains two elctrons, those two elctrons are spin paired. ? Note that if two elctrons ocupy one orbital, spin pairing is lower in energy than if two elctrons of the same spin are forced to occupy the same orbital. ? The Pauli exclusion principle states that if there are several orbitals of equal energy (such as the thre 2p-orbitals), each orbital wil fil with one electron before any orbitals contain two. ? Atempting to fil one orbital with two spin paired elctrons before filing the next orbital would require more energy. Electrons have like charges, and two electrons wil repel if they are in the same orbital. 1s 1s H He Electronic Configuration and the Aufbau Principle 10 ? Electrons have like charges, and two elctrons wil repel if they are in the same orbital. ? Ading two elctrons to two 2p orbitals is lower in energy than ading two elctrons to a single 2p orbital, so electrons "fil" orbitals to conserve energy. ? The concept of filing orbitals with elctrons in ascending order of orbital energy until al available electrons have been used is known as the aufbau procedure. ? The order in which elctrons fil is generalized by the mnemonic 1s ? 2s ? 2p ? 3s ? 3p ? 4s ? 3d ? 4p ? 5s ? 4d ? 5p ? 6s ? 4f ? 5d ? 6p ? Hydrogen has an elctronic configuration 1s 1 and helium is 1s 2 . ? The "1" represents the row of the Periodic Table, the "s" represents the orbital, and the superscript "1" represents the number of electrons in that orbital. ? Lithium begins the second row and the 2s orbitals begin to fil. The elctron configuration of lithium is 1s 2 2s 1 . ? Berylium is 1s 2 2s 2 , which fils the 2s orbital. ? The next elment begins to fil the 2p orbitals: boron has the configuration 1s 2 2s 2 2p 1 ? Carbon is 1s 2 2s 2 2p 2 , and continues to the Noble gas neon with a configuration 1s 2 2s 2 2p 6 . ? In the third row, sodium begins to fil the 3s orbital (1s 2 2s 2 2p 6 3s 1 ) and continues to argon, with a configuration of 1s 2 2s 2 2p 6 3s 2 3p 6 . An Atom in the Second Row 11 x y z 2p z 2p y 2p x 2s orbital 1s orbital ? To ?construct? an atom, ther is a 1s orbital, a 2s orbital, and three 2p orbitals in the second row of equal energy ? The thre p-orbitals have the same shape and energy. ? The space volume for each electron is described in a Cartesian (thre-cordinate) system, x,y,z. ? The Figure shows the 1s, 2s and al thre 2p orbitals superimposed onto the tri-coordinate system, to illustrate the spatial relationships of the orbitals in an elemental atom, one to the other. ? The 1s orbital is represented by the spherical "gren dot" at the center. ? The 2s orbital is represented by the "black circle" (meant to represent a sphere) that is larger than the 1s sphere, showing its greater distance from the nucleus. ? The thre 2p orbitals are labeld as red for the 2p z , blue for the 2p x , and yelow for the 2p y orbitals. ? The purpose of this simplistic picture is to give a mental image that a 2p electron is more easily removed than the 1s or 2s electrons because it is further from the nucleus. The nucleus is the convergence point of the tri-coordinate system. The orbitals are closely associated with the atomic nucleus (atomic orbitals), so the electrons are closely associated with the atom. Chemical Bonds 12 ? A chemical reaction between atoms or groups of atoms wil usualy produce new combinations of atoms held together by what is called a chemical bond. ? When one element reacts with another it does so via its electrons, not by the protons and neutrons in the nucleus, and the resulting bond between the atoms is composed of two electrons. Two major types of bonds wil be considered. ? A covalent bond is formed by the mutual sharing of valence electrons betwen two atoms. In other words, sharing electron density holds the atoms together. ?An ionic bond is formed transfer of electrons from one atom to another, resulting in ions (+ and ?) that are held together by electrostatic atraction. Ionic Bonds 13 ? LiF has an ionic bond, where the positively charged Li is electrostaticaly bound to the negatively charged F. ? Is Li is more likely to donate electrons or acept electrons? A similar question can be asked for F. ? The "octet rule" is a useful tol, and it states that in the second row, a maximum of eight elctrons can occupy the valence shell (the Ne configuration). ? In the first row, a maximum of two elctrons can ocupy the valence shel (the He configuration) and the second row can accommodate a maximum of eight elctrons to give the Ne configuration (a total of 10 electrons). ? Assume there is an energetic preference for transfering electrons to atain the Noble gas configurations. ? If one 2s valence elctron is lost from lithium during elctron transfer proceses, the result is Li + with a 1s 2 configuration (the He configuration). This is energetically more favorable than lithium gaining an extra electron to give Li ? , with a 1s 2 2s 2 configuration. ? Los of an electron from Li does not lead to helium, but to a positively charged lithium ion (Li + ) with a 1s 2 configuration. ? If electron transfer removes an electron from F to give F + , the electronic configuration is 1s 2 2s 2 2p 4 . ? If electron transfer ads one electron to a fluorine atom, however, the result is F ? which has the 1s 2 2s 2 2p 6 configuration (the Ne configuration). ? Adition of an electron to fluorine does not give Ne, but rather the fluoride ion F ? with the configuration 1s 2 2s 2 2p 6 . F ? is more stable than F + . Ionization Potential and Electron Affinity 14 ? An atom with a valence electronic configuration such as Li (Group 1) wil lose an electron during an electron transfer process, and an atom with a configuration such as fluorine (Group 17) wil gain one electron. ? The energy required for the loss of one electron from an atom is caled its ionization potential. ? The energy required for the gain of one electron into an atom is caled its electron affinity. ? This means that the energy gained or lost for an atom is a measurable quantity. ? Electron transfer to form ions is the basis for the known ionic bonding in many molecules composed of alkali metals (Groups 1 and 2) and halogens (in Group 17): LiF, NaCl, KBr, NaI, etc. C H H H H Li F Ionic Group 1,2 with Group 17 C in ?midle? Shares electrons Covalent bond Electrons donated to one Atom or the other - charged Ionization potential - E to donate electron Electron afinity - E to accept electron neutral Ionic versus Covalent Mp, 870°C Bp, 1676°C Temp to dissociate: >350 K 3227°C Temp to dissociate: >60 K 327°C (begins dissociation) Carbon 16 ? Carbon has an electronic configuration of 1s 2 2s 2 2p 2 with four valence electrons. ? The bonds in molecules that contain carbon are usualy formed by sharing electrons with another atom in what is known as a covalent bond, rather than by complete transfer of electrons to form an ionic bond. ? Carbon has the electronic configuration 1s 2 2s 2 2p 2 , so there are four electrons in the outmost shel. ? To achieve the helium configuration by electron transfer (1s 2 ), four electrons must be lost (fourth ionization potential). To achieve the Neon configuration (1s 2 2s 2 2p 6 ) four electrons must be gained (fourth electron afinity). In both cases, the energy requirements are absurd. ? It is unlikely that an elemental carbon atom wil form bonds by the same type of electron transfer as found with LiF. ? For reactions in this bok, elemental carbon such as that found in diamond or graphite wil not be converted into carbon containing molecules. ? To say that carbon "transfers electrons and shares electrons" does not mean that atomic carbon wil react with other atoms or other compounds for form a bond to become covalent. Carbon in a Molecule 17 ? Electrons are found in orbitals, ? The orbitals for atomic carbon are diferent from the orbitals for covalent carbon in a molecule; carbon in the element is diferent from carbon in a molecule. ? The distinction is that the bonding in organic molecules is covalent and it involves carbon, but elemental carbon is not readily converted to such compounds. ? This means the direct conversion of elemental carbon (graphite or diamond for example) into an organic molecule such as methane is unlikely. Both graphite and diamond are known for their stability, not for their ability to react with things. ? The fundamental conclusion is that the placement of electrons in a bond to carbon in a molecule (molecular carbon) is different from that in elemental carbon. Covalent Carbon-Carbon Bond 18 ? When electrons are concentrated in orbitals on a single atom, the electrons are said to be "localized" on that atom. ? When an atom is part of a covalent bond, it shares electrons with the other atom; i.e., the electrons in the bond are not localized on one atom but rather are shared by both atoms and concentrated betwen the atoms. ? The electrons that constitute the bond are concentrated betwen both nuclei rather than localized on one or the other. nucleus-1 nucleus-2 Two atoms that re not bonde. The lectrons in each p orbital re localized on the atom As the atoms are brought within bondig distance, the orbitals of ne atom ust be directd towards the orbitl f th ther in order t share the letrons t frm cvlt bnd When the covalent bond is formed, the orbital on each atom has ben distorted -the orbital is dirctd twrds the ther at. T lectrs r nw srd twen th atoms and the greatst oncentration f lctron density is betwe the uclei (black atoms) nuclus nucleus aximu orbital overlap (mi elctrn dnsity) Molecular Orbitals 19 ? For an isolated atom of a given elment, the elctrons are in atomic orbitals, but in a molecule the electrons reside in diferent orbitals known as molecular orbitals. ? One way to visualize a molecular orbital is to "mix" atomic orbitals to indicate the directionality of the new orbital towards the atom to which the bond is formed. Mixing a spherical s-orbital and a "dumbbell-shaped" p orbital, for example, leads to a hybrid orbital. ? This new molecular orbital is more or les a hybrid of the s and p-orbitals from which it was derived, with characteristics of both. ? In the C?H bond of a molecule, the elctron density must be concentrated betwen the C and H nuclei if there is a covalent bond. In other words, in a C-H bond, the lectrons shared by carbon are directed toward the hydrogen atom. Likewise, the electrons shared by hydrogen will be directed towards the carbon atom. ? This fact is acknowledged in the hybrid orbital, with directionality, as in 1. The directionality simply reflects the fact that 1 is used to form a covalent bond, and the shared elctron density wil be directed towards the other atom in that bond. ? A covalent bond is described as the overlap of two hybrid orbitals to form a covalent bond known as a sigma bond. ? A sigma bond is defined as a bond betwen two atoms where the electron density is concentrated symmetrically on a line between the two nuclei. + combine and hybriize + + hybrid orbital C- !-bond hybrid orbital hybrid orbital http:/ww.chem.unc.edu/faculty/murphy/1251/slides/C19b/sld06.htm http:/ww.madsci.org/posts/archives/200-10/97192131.Ch.r.html Promote electrons Valence Bond Theory & Hybrid Orbitals Sigma Bond (?-Bond) 21 nucleus nucleus maximu orbital overlap (ai elctrn density) ? When two identical atoms share electrons in a covalent bond, most (but not al) of the electron density is equaly distributed betwen the two nuclei (in the "space" betwen the two atoms). ? The electron density that constitutes the bond is located in orbitals betwen the nuclei. This means that the charge density of the shared electrons is greatest betwen the two nuclei. ? This orbital picture describes a sigma bond (? bond. ? The orbitals used to form covalent bonds in a molecule are caled molecular orbitals (they are diferent from atomic orbitals). This means that the electrons in the covalent bond are found in molecular orbitals. Bond Length 22 nucleus nucleus maximu orbital overlap (i elctrn dnsity) ? The strength of a sigma bond is related to the amount of electron density concentrated betwen the nuclei. ? In the C-C ?-bond there is a symetrical distribution of electron density) betwen the two nuclei, commonly caled a single bond betwen the two atoms, as in a carbon- carbon single bond. ? Each ?-bond is composed of two shared electrons. ? Bond length is the measured distance betwen the nuclei of each atom participating in the bond (the internuclear distance) and it has ben measured in angstroms (Å), where 1 Å = 1 x 10 -10 meters. Modern measurements are reported in picometers (pm, or 1x10 ?12 meters, or 0.01 Å). Therefore, 1 Å = 10 pm. ? A longer bond has less electron density per unit length than if the bond is short, meaning it is easier to disrupt the bond (easier to break); it is weaker. Conversely, a shorter bond wil have more electron density per unit length, and it is stronger. ? Bond length is, in part, a function of the size the atoms. ? The internuclear distance betwen two atoms of larger atomic radius wil be greater than that betwen two atoms of smaler atomic radius. ? The size of each nucleus (number of protons and neutrons), the number of valence electrons, and the nature of the orbitals in which they reside determine the bond length. Bond Length 23 nucleus nucleus maxium orbital overlap (i elctrn dnsity) ? Al covalent bonds are not the same. ? Bonds betwen diferent nuclei wil have diferent strengths and the distance betwen the nuclei wil be diferent. The bond length and bond energy of a C-C bond is diferent from an O?O bond, or a C?O bond. ? All carbon-carbon bonds are not the same. A C?C bond such as R 3 C?CH 3 (the R represents any carbon group) wil be diferent from the C?C unit in R 2 CHCH 2 C?CH 2 R, for example. Bond Molecule Type Bond Length (Å) C?I ?Br C?l ? O C?N ?O C?F ?H N? O?H ? alkyl halide alkyl halide alkyl halide alkane hydrogn peroxide (HOH) amine ether alkyl halide alkane amine alcohl water 2.139 1.938 1.76 1.541 .8 1.472 1.426 1.379 1.094 1.012 0.97 0.958 Bond Length (pm) 213.9 193.8 176.7 154.1 8 147.2 142.6 137.9 109.4 10.2 97 95.8 Linear Combination of Atomic Orbital (LCAO) model 24 ? Using the hydrogen atom and its atomic orbitals, predictions may be made about the orbitals in a molecule. ? In one model, the atomic orbitals of the atom are mixed and combinig two such atoms may lead to a molecule. This idea of mixing atomic orbitals to form a molecular orbital is caled the Linear Combination of Atomic Orbital (LCAO) model, and in some cases it helps predict the relative energy of these molecular orbitals. ? Begin with two individual hydrogen atoms, each has one electron in a 1s orbital. ? If the 1s orbital from each hydrogen atom (there are two) are mathematicaly "mixed", two new orbitals are formed. ? The new orbitals are associated with the dihydrogen molecule (two molecular orbitals for H?H), but they must be of a diferent energy than the orbitals from the atoms. ? This means that the electrons shared betwen the two atoms are no longer associated with an individual atom, so they must be at a diferent energy relative to each hydrogen nucleus. This diference is represented by two diferent orbitals at diferent energy levels. ? Mixing two atomic orbitals generates two molecular orbitals, so orbitals are not gained or lost by this process. ? Each orbital represents an energy level, and since the orbitals must be of a diferent energy, one orbital must be lower in energy and one is higher when compared to the atomic orbitals. ENRGY required to "hold" elctrons H atomic rbital H 2 molecular orbitals H atomic rbital H atomic rbital H atomic rbital H 2 molecular orbitals bondig antibondig Michael Smith 2443-012811.ppt