Chapter 2 Polar Covalent Bond Covalent bond in which the electron pairs are not shared equally.

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Nelson Simmons
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1 hapter 2 Polar ovalent Bond ovalent bond in which the electron pairs are not shared equally. Pure ovalent Bond (non-polar) increasing bond polarity Ionic Bond X X X Y X + Y - Electronegativity, c ability of an atom in a molecule to attract electron density to itself. Structure and Bonding 1

Pure ovalent Bond (non-polar) increasing bond polarity Ionic Bond X X X

2 Dc = 1.9 -l = 0.9 -Br = 0.7 -I = 0.4 Decreasing polarity Increasing ionic character Dc Li 3.0 Be B N ionic covalent Polar covalent bonds electron density is not shared equally. Electrons spend more time with the more electronegative atom. Structure and Bonding 2

3 So what? Polar bond dipoles are additive and may result in molecular dipole giving a polar molecule. Affects physical properties (i.e., melting point, boiling point, viscosity, vapor pressure). δ δ δ+ δ δ B δ+ δ 3 δ δ+ non-polar non-polar polar 3 Regions of high or low electron density provide sites for chemical reactions to occur. 3 δ δ+ 3 δ + N R δ+ δ+ δ N 2 R NR Rules for writing Lewis Structures onsider l 2 (phosgene). 1. alculate the total number of valence electrons in the structure. (Don t forget the charge!). l.... l 2(7 e - ) = 14 e - 4 e - 6 e - total 24 e - Structure and Bonding 3

3 δ δ+ 3 δ + N R δ+ δ+ δ+ 3 3 3 N 2 R 3 2 + NR Rules for writing Lewis Structures onsider l 2 (phosgene). 1.

4 2. Write the symbols for the atoms present in the structure with the correct configuration. Generally, the atom with the smallest electronegativity will be the central atom. is always a terminal atom (because it can form only one bond) and and the halides (, l, Br, I) are usually terminal. l l 3. Use lines to indicate electron pair bonds between pairs of symbols. Use remaining electrons to make lone pairs so that each atom has an octet (duet for hydrogen). l l 18 e - l l l l Structure and Bonding 4

is always a terminal atom (because it can form only one bond) and and the halides (, l, Br, I) are usually terminal. l l 3.

5 4.Indicate the formal charge of the atoms. the sum of the formal charges is equal to the charge of the species... = (# val. e - s) - (# bonds) - (# unshared e - s)..(l) = = 0..() = = 0.. () = = 0 l l onsider , acetate 24 e - sp 3 sp 2 resonance hybrids Structure and Bonding 5

.. = (# val. e - s) - (# bonds) - (# unshared e - s)..(l) = 7-1 - 6 = 0.

6 Evaluating Resonance orms Resonance forms must have the same structure. Thus N -, N -, and N - would not be considered as they have different connectivities. N N N The position of the atoms must be the same in all resonance hybrids. Resonance forms in which atoms bonded to one another have the same charge are unfavorable. Thus, for the resonance hybrids of N 2 shown below, hybrid c is not as important of a contributor as a and b. + N N N a b c The most important resonance forms of a given resonance hybrid have the smallest number of formal charges and the lowest values for these charges. The best forms have no formal charge at all. Thus, for the cyanate ion, N -, c is not an important contributor to the resonance hybrid. The distribution of positive and negative formal charges should be in agreement with the electronegativities of the atoms. onsider N Structure and Bonding 6

Thus, for the resonance hybrids of N 2 shown below, hybrid c is not as important of a contributor as a and b.

7 Exceptions to the ctet Rule 1. If an element has less than four valence electrons, it may have less than an octet of electrons in a molecule. B l Be l l Al l l 2. Elements from the 3rd period or beyond may have more than an octet of electrons in a molecule. P 6 - (48 e - s) S 4 (34 e - s) P - S The octet rule 1 s orbital and 3 p orbitals 8 electrons in valence shell. rom the 3rd period on, elements can use d orbitals greater capacity for electrons. Structure and Bonding 7

B l Be l l Al l l 2. Elements from the 3rd period or beyond may have more than an octet of electrons in a molecule.

8 Pl 3 (32 e-'s) l P l l P l l +1 P -1 l l l l l +1 P -1 l l P l l l Acids and Bases Brønsted-Lowry Acid--proton donor Base--proton acceptor acid + + base conjugate base conjugate acid N N 4 + Structure and Bonding 8

9 Acid strength acid dissociation equilibrium in water A A - K a pk + [3 ][A = [A] a = log K a ] Relative acid strengths Acid Name pk a onjugate Name base 3 2 ethanol ethoxide 2 water hydroxide N hydrocyanic 9.31 N - cyanide acid 3 2 acetic acid acetate hydrofluoric fluoride acid N 3 nitric acid N 3 nitrate l hydrochloric acid -7.0 l - chloride Structure and Bonding 9

74 - hydroxide N hydrocyanic 9.31 N - cyanide acid 3 2 acetic acid 4.76-3 2 acetate hydrofluoric 3.

10 Acids and Bases Lewis Acid--electron pair acceptor Base--electron pair donor Lewis bases have one or more lone pairs of electrons that can be readily donated l + N l N 4 Lewis base Electron deficient compounds are good Lewis Acids (need an acceptor orbital) B - + [ ]- B B N + B - N + l l Al l + [ l ]- l Al l l l - Structure and Bonding 10

+ N 4 Lewis base Electron deficient compounds are good Lewis Acids (need an acceptor

11 Representing molecules complete (Kekulé) structure all bonds and atoms are shown condensedstructure only - bonds necessary for clarity are shown ( 3 ) 2 skeletal structure atoms are not shown Represented by intersection of lines (bonds) or ends of terminal lines atoms bonded to are not shown assume valency is 4 and so we know how many are present Atoms other than and are shown Structure and Bonding 11

shown Represented by intersection of lines (bonds) or ends of terminal lines atoms bonded to are not

12 Examples, ( 3 ) Problems 1. alculate the formal charges on the indicated atoms in each compound below l P l l 2. Draw the Lewis structure of 2 and chose the best structure based on formal charge. lower energy Lewis structure Structure and Bonding 12

13 3. Use δ-/ δ+ convention and the crossed arrow ( ) to show the direction of the expected polarity of the indicated bonds in the following compounds. The - bond in fluorobenzene The -Sibond in tetramethyl silane 3 3 Si 3 3 Structure and Bonding 13

Self Assessment_Ochem I

Self Assessment_Ochem I UTID: 2013 Objective Test Section Identify the choice that best completes the statement or answers the question. There is only one correct answer; please carefully bubble your choice on the scantron sheet.