Organic Chemistry I
Organic Chemistry I CHEM 2010
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This 23 page Class Notes was uploaded by Columbus Kerluke on Sunday October 11, 2015. The Class Notes belongs to CHEM 2010 at East Tennessee State University taught by Yu-Lin Jiang in Fall. Since its upload, it has received 34 views. For similar materials see /class/221419/chem-2010-east-tennessee-state-university in Chemistry at East Tennessee State University.
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Date Created: 10/11/15
2 Polar Covalent Bonds i and Bases Based on McMurry s OrganC ChemSL734 6th edition Chapter 2 Acids 21 Polar Covalent Bonds Electronegativity Covalent bonds can have ionic character These are polar covalent bonds Bonding electrons attracted more strongly by one atom than by the other X X x Y X Y Symmetrical covalent Polar covalent Ionic bond bond bonds Thomson Br Bond Polarity and Electronegativity Electronegativity EN intrinsic ability of an atom to attract the shared electrons in a covalent bond Differences in EN produce bond polarity Arbitrary scale As shown in Figure 22 electronegativities are based on an arbitrary scale F is most electronegative EN 40 Cs is least EN 07 3 The Periodic Table and i Electronegativity H 21 Be L1 16 10 Na Mg 09 12 K Ca 08 10 Rb Sr 08 10 Cs Ba 07 09 Sc 13 Y 12 La 10 Ti Zr V Cr 15 16 16 Nb M0 14 16 1398 Hf Ta 13 15 17 18 Mn 1 Tc 19 Re 19 5Ru 22 Os 22 B 20 1415 Zn Ga A 16 16 1 Cd In Au 17 17 24 Hg T1 19 18 Thomson Brooks Coie Bond Polarity and Electronegativity Metals on left side of periodic table attract electrons weakly lower electronegativities Halogens and other reactive nonmetals on right side of periodic table attract electrons strongly higher electronegativities Electronegativity of C 25 Bond Polarity ancl Inductive Effect Nonpolar Covalent Bonds atoms with similar electronegativities Polar Covalent Bonds Difference in EN of atoms lt 2 Ionic Bonds Difference in electronegativities gt 2 approximately Other factors solvation lattice energy etc are important in ionic character Bond Polarity and Inductive Effect Bonding electrons are pulled toward the more electronegative atom in the bond C acquires partial positive charge 8 Electronegative atom acquires partial negative charge 8 Inductive effect shifting of electrons in a bond in response to the electronegativities of nearby atoms Electrostatic Potential Maps Electrostatic potential maps show calculated charge distributions Colors indicate electronrich red and electronpoor blue regions a 3 Ci ng I Chlorine EN 2 30 Hz H Carbon EN 25 H Difference 05 Chloromethane F I Carbon EN 25 H H Lithium EN 10 Difference 15 Methyllithium Thomson Brooks Cole 22 Polar Covalent Bonds Dipole Moments Molecules as a wholeare often olar from vectorsummatlon of ndIVIdual ond polarltles and lonepair contributions 5 K 02 electron excess C1 5 I r 02 electron de cit H H H Chloromethane M 187 D Polar Covalent Bonds Dipole Moments Dipole moment Net molecular polarity due to difference in summed charges p magnitude of charge Qat end of molecular dipole times distance r between charges u Qgtlt r in debyesD 1 D 3336 x 1030 coulomb meter 10 Dipole Moments in Water and Ammonia Large dipole momenis Electronegativities of O and N gt H Both 0 and N have lonepair electrons oriented away from all nuclei Lone pair I IT Net 39 0 47K i H pairs Water H2O u 185 D Ammonia N39H3 u 147 D 2004 Thomson BrooksCole 1 1 TABLE 21 Dipole Moments of Some Compounds Dipole moment Dipole moment Compound D Compound D NaCl 90 NHg 147 O CH O quot1 HC N 346 CCL O 0 CH3CH3 0 Nitromethane CH301 187 0 IquotIgO 185 Benzene CH30H 170 BR 0 H20 N N 150 Diazomethane 2004 Thomson BrooksCoke 12 Absence of Dipole Moments In symmetrical molecules the dipole moments of each bond has one in the H Cl oquot Ill I C a x H H 01 I Cl H H H C1 H Methane Tetrachloromethane Ethane pLOD M0D M0D Thomson Brooks Cole 13 Cis amp trans12dichIoroethylenes Problem 226 cisl2Dichloroethylene trans12Dichloroethylene 2004 Thomson BrooksCole 14 23 Formal Charges Sometimes it is necessary to have structures with formal Charges on individual atoms We compare the bonding of the atom in the molecule to the valence electron structure If the atom has one more electron in the molecule it is shown with a charge If the atom has one less electron it is shown with a charge 15 Formal Charges Number ef Number ef valence electrons valence electrene in ee atem I in bound atom Formal charge of bending electrons Number of nemhonding electrons Number of valence electrons ll 16 Nitromethane H O Formal positive charge C N F0rmm negatlve charge H I39I Nitromethane 2004 Thomson BrooksCole 17 For the nitramethane nitrogen CHENOQ HgNj H 103 Nitrogen valence electrons Nitrogen bonding electrons Nitrogen uonhonding electrons Formal charge 5 g 0 For the singly bonded nitromethane oxygen Oxygen valence electrons Oxygen bending electrons Oxygen nonbonding electrons Formal charge 6 g 6 3301 1 6 2 6 1 18 g Formal Charges TABLE 22 A Summary of Formal Charges on Atoms Atom C 0 I l CI II II II Structure 39C C C 39N l l Number 3 4 3 4 2 3 2 1 of bonds Lone 0 0 1 0 2 1 2 3 pairs Formal 1 0 1 1 D 1 1 0 1 charge 2004 Thomson BrooksCole 19 i 24 Resonance Some molecules have structures that cannot be shown with a single Lewis representation In these cases we draw Lewis structures that contribute to the final structure but which differ in the position of the 7c bonds or lone pairs Such a structure is delocalized and is represented by resonance forms 20 24 Resonance The resonance forms are connected by a doubleheaded arrow Double bond to this oxygen w H O O C N lt gt C N r III HH 0 HH 0 Nitromethane L Or to this oxygen Thomson Brooks Cole 21 i Resonance Hybrids A structure with resonance forms does not alternate between the forms Instead it is a hybrid of the two resonance forms so the structure is called a resonance hyb d For example benzene C6H6 has two resonance forms with alternating double and single bonds In the resonance hybrid the actual structure all of the CC bonds are equivalent midway between double and single bonds 22