cem_850_2004_thermochem - CEM 850 Fall 2004 Some notes on...

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CEM 850, Fall 2004 Some notes on Thermochemistry, Bond Strengths, and Strain energies Ned Jackson
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Alkane H f values (kcal/mol) 1 -17.9 2 -20.04 3 -25.02 4 -30.37 -32.07 5 -35.08 -36.73 -40.14 6 -39.96 ... -44.35 Carbon number n Branching Octanes: 7 -44.89 ... -48.96 8 -49.82 ... -53.99 Octane 2-methylheptane 3-methylheptane 4-methylheptane 3-ethylhexane 2,5-dimethylhexane 2,4-dimethylhexane 2,3-dimethylhexane 3,4-dimethylhexane 3-ethyl-2-methylpentane -49.82 -51.50 -50.82 -50.69 -50.40 -53.21 -52.44 -51.13 -50.91 -50.48 2,2-dimethylhexane 3,3-dimethylhexane 3-ethyl-3-methylpentane 2,3,4-trimethylpentane 2,2,3-trimethylpentane 2,2,4-trimethylpentane 2,3,3-trimethylpentane 2,2,3,3-tetramethylbutane -53.71 -52.61 -51.38 -51.97 -52.61 -53.57 -51.73 -53.99
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Alkane H f values show Systematic Patterns • Can we estimate H f by summing energy equivalents for transferable molecular “building blocks?” Bond Equivalents Group Equivalents Fragment transferability in comparisons between compounds implies deep similarity
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Bond Equivalents • Estimate H f values from C-H, C-C bonds: – Ethane H f = -20.04; 6 C-H, 1 C-C – Propane H f = -25.02; 8 C-H, 2 C-C C-H = -3.765; C-C = +2.55 – Predict H f (C 5 H 12 ) = 4C-C + 12C-H = -34.98 N-pentane -35.08 but isopentane = -40.14 Bond equivalents fail for branching.
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Group Equivalents All alkanes can be expressed in terms of four building blocks: (CH 3 ) i (CH 2 ) j (CH) k (C) l (nonspecified bonds implicitly to C) Enthalpy Equivalents: – CH 3 -10.08 – CH 2 -4.95 CH -1.90 C +0.50
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Group Equivalents (cont’d) Analogous equivalents for alkenes and aromatics can be similarly derived, with value for CH 3 held at -10.08 kcal/mol no matter what it’s attached to. This method defines a “strainless ideal” for hydrocarbons of arbitrary formula, and allows the definition of “strain.”
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Strain Energies Cycloalkanes (CH 2 ) n 2 12.54 2x(-4.95) = -9.9 22.44 3 12.74 3x(-4.95) = -14.85 27.59 4 6.78 4x(-4.95) = -19.8 26.58 5 -18.26 5x(-4.95) = -24.75 6.49 6 -29.43 6x(-4.95) = -29.7 0.27 Ring Size n ² H f ² H f Hypothetical Strainless (CH 2 ) n Strain E.
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Thermochemistry--why care? Besides simple reaction H and G values, detailed energetics define reaction direction Combined with bond strengths and kinetics of the reactions of interest, even imperfect energetic ideas put limits on mechanistic possibilities Lead in to tools for comparing reactions!
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Bond Strengths An X-Y bond, as defined by its atoms X and Y, is not a uniform (thus transferable) molecule building block The bond equivalent approach did not lead to a reliable method for H f estimation A group equivalent approach was required Some bond strengths allow development of group equivalent ideas for reactions
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