AT Lecture 4

AT Lecture 4 - BISC 330L Biochemistry Spring Semester 2012...

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BISC 330L: Biochemistry Spring Semester 2012 USC Lecture 4: Wednesday, 18 January 2012 1. Pauling and Electronegativity 2. Begin Chapter 2 material…
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"The nature of the chemical bond. IV. The energy of single bonds and the relative electronegativity of atoms." May 18, 1932. J. Am. Chem. Soc. 54 (September 1932): 3570- http://osulibrary.oregonstate.edu/sp ecialcollections/coll/pauling/bond/in dex.html
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In his classic, Nature of the Chemical Bond , Linus Pauling showed for chemical reaction, Reactant(R) Product(P), H Δ o is related to bond dissociation energies (D A-B ) by H Δ o = Σ (React. bond D A-B values) - Σ (Prod. bond D A-B values) Comparing D A-B with D A-A and D B-B , he found D A-B = 0.5(D A-A + D B-B ) + Δ AB = d A + d B + Δ AB , where Δ AB is additional bonding energy arising from the electronegativity difference (X A -X B ) , which contributes to bond polarity and stability by increasing attraction between atoms A and B in A-B bond. In a balanced reaction (same atoms on both sides), only the Δ AB component contributes to H Δ o . He then showed Δ AB (in kcal/mole) = 23|X A -X B | 2 (approx.), so that when equation is balanced (same atoms on both sides) Summary of Pauling Method for Estimating H o
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Major covalent bonds in biochemistry and their dissociation energies (D A-B , kcal mol -1 )* *[ L. Pauling. Nature of the Chemical Bond , 3rd Ed., 1960, p.85] H-H 104.2 H-C 98.8 H-N 93.4 H-O 110.6 C-C 83.1 C-H 98.8 C-N 69.7 C-O 84.0 N-N 38.4 N-H 93.4 N-C 69.7 O-O 33.2 O-H 110.6 O-C 84.0 S-S 50.9 S-H 81.1 S-C 62.0 (P-P 51.3) (P-H 76.4) P-O (84.2) Electronegativity values: H 2.1 C 2.5 N 3.0 O 3.5 P 2.1 S 2.5 “The power of an atom in a molecule to attract electrons to itself.”
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Example: H Δ o of H 2 O H 2 + 0.5 O 2 H 2 O 2H 2 + O 2 2H 2 O 2 (H-H) + 1 (O=O)
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This note was uploaded on 03/15/2012 for the course BISC 330L taught by Professor Petruska,tower during the Spring '07 term at USC.

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AT Lecture 4 - BISC 330L Biochemistry Spring Semester 2012...

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