1559Tch0101-1710/22/051:48Page2 EQA 2.chapter 1 STRUCTURE AND BONDNG IN ORGANIC MOLECULES Keys to the Chapter 1-2 and 1-3.Coulomb Forces;Bonds d to each othe r (as de d in this chapter).but difference betw en the atoms.the bondins electrons tend to be closer to the more electr onegative atom,thereby creating a partial charge separation.In general,for A less electronegative than B.we have A:B See the in the vicinity of more partial negative charge it themore in its vicinity.the more partial posi arge it be to compare colors fro t maps.how e co anic chem oms with opposite charges or polarities are attra h other.Then. lectrons mo up of electrons.it's necessary to keep track of how many electrons are involved.and where they are located. Lewis structres are of paramount importance in this bookkeeping process. 1-4.Lewis Structures Whether you've ever done Le wis structures before or not.follow the rules in Section 1-4 very closely.Becom aroun mon a or the that you will quickly and confidently be able to picture a Lewis structure for any y of the tyres of species vo u gain confidence through practice.you will be able to use shorthand notations nic compounds and other anic or ing anic species Thes kkeeping system to help us keep track of electrons in reactions 1-5. Resonance Forms arrows are in kind of n is the se of oe-eade play in organic chemistry.As shown in this section,many species have structures that cannot be represented hich can only b descr inte▣ nnature betweent wo or more co a molecule by drawing the resonance forms separated by double-beaded arrows and enclosed in brackets.The eis called the resonance hybrid.The only difference between the reson ce forms is a differen one Lewis structure.In cases like this you need to be only and if t The second convention in Section 1-5 is the use of show the movement of electron pairs In this section the only gplicationisinsho g ho w the elec se arrows you leam and understand or on movement The guidelines in the textbook for determining e degree to which each resonance form contributes to the actual structure are the ones you will use the most when dealing with the most common atoms in organiKeys to the Chapter 1-2 and 1-3. Coulomb Forces; Bonds “Unlike charges attract” and “like charges repel.” These consequences of elementary physics dealing with elec￾trostatics and Coulomb’s law are central to a basic understanding of chemistry. Not only do they determine whether, and how strongly, atoms will bond to each other (as described in this chapter), but they also influence an even more complicated process: whether two molecules are likely to react with each other. Time and time again we will return to simple electrostatics, in the context of the properties of the individual elements, to ex￾plain the reactions of organic chemistry. Most organic molecules contain polarized covalent bonds. In bonds of this type, one or more pairs of electrons are shared between two atoms, but because of an electronegativity difference between the atoms, the bonding electrons tend to be closer to the more electronegative atom, thereby creating a partial charge separation. In general, for A less electronegative than B, we have A

: B
. See the specific examples in Section 1-3. The computer-generated electrostatic potential maps in Section 1-3 are vi￾sual representations of relative charge distribution and, therefore, bond polarity: In any such map, the more red in the vicinity of an atom, the more partial negative charge it contains, and the more blue in its vicinity, the more partial positive charge it bears. (Be careful not to compare colors from different maps, however. The col￾ors have been scaled for each map individually to bring out even small polarity differences as clearly as pos￾sible.) As you will see later on, most of the reactions in organic chemistry follow a general pattern. First, two nonbonded atoms with opposite charges or polarities are attracted to each other. Then, electrons move from the “electron-rich” to the “electron-poor” atom to form a new covalent bond between them. Because bonds are made up of electrons, it’s necessary to keep track of how many electrons are involved, and where they are located. Lewis structures are of paramount importance in this bookkeeping process. 1-4. Lewis Structures Whether you’ve ever done Lewis structures before or not, follow the rules in Section 1-4 very closely. Become familiar with the number of electrons around common atoms and the common arrangements of these electrons in the bonds of molecules. This familiarity, brought about by doing lots of examples, is the best way to ensure that you will quickly and confidently be able to picture a Lewis structure for any of the types of species you will encounter later on. As you gain confidence through practice, you will be able to use shorthand notations, such as lines instead of dots for bonding electron pairs. Organic chemistry involves reactions between organic compounds and other organic or inorganic species. These reactions can involve both bond-breaking and bond-forming processes, and the key to both is the movement of electrons. Lewis structures provide the bookkeeping system to help us keep track of electrons in reactions. 1-5. Resonance Forms Two important conventions involving arrows are introduced in Section 1-5. The first is the use of double-headed arrows between resonance forms. This is a special kind of notation because of the special role resonance forms play in organic chemistry. As shown in this section, many species have structures that cannot be represented by a single Lewis structure. They can only be described as intermediate in nature between two or more con￾tributing forms, each of which by itself is an incomplete picture of the molecule’s structure. We represent such a molecule by drawing the resonance forms separated by double-headed arrows and enclosed in brackets. The true structure is called the resonance hybrid. The only difference between the resonance forms is a different location for the electrons from one to the next. The same geometrical arrangement of the atoms is maintained in all the resonance forms. Molecules that actually exist as resonance hybrids are often represented by only one Lewis structure. In cases like this you need to be aware of the fact that this is a shortcut used for conve￾nience purposes only and that the real structure is still the resonance hybrid—the other resonance forms are implied even if they aren’t written down. The second convention in Section 1-5 is the use of curved arrows to show the movement of electron pairs. In this section the only application is in showing how the electron pairs shift in going from one Lewis struc￾ture of a resonance hybrid to another. Pictorial descriptions of electron movement using these arrows will be very useful tools to help you learn and understand organic chemistry. The guidelines in the textbook for determining the relative degree to which each resonance form contributes to the actual structure are the ones you will use the most when dealing with the most common atoms in organic 2 • Chapter 1 STRUCTURE AND BONDING IN ORGANIC MOLECULES 1559T_ch01_01-17 10/22/05 1:48 Page 2