l08b - CH 203 O R G A N I C C H E M I S T R Y I...

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Conformational analysis II © Bruno I. Rubio 1 CH 203 O R G A N I C C H E M I S T R Y I Conformational analysis II Molecular strain Molecules are destabilized by many different kinds of strain: (1) bond-length strain; (2) bond-angle strain; (3) steric strain; (4) eclipsing strain (also called torsional strain). Bond-length strain When two atoms that are about to form a bond approach each other, the nega- tively charged electrons of one atom are attracted to the positively charged nucleus of the other atom. The energy of the system decreases until the atoms reach an optimum separation called the ideal bond length. If the atoms con- tinue to approach each other, however, the nucleus of one atom begins to re- pel the nucleus of the other atom and the energy of the system sharply in- creases. Bond-length strain is caused by a deviation from the ideal bond length. In organic molecules, bond-length strain is nearly always due to lengthening rather than compression. The figure below illustrates how the energy of the carbon–carbon bond in eth- ane (CH 3 CH 3 ) varies with changes in the bond length. Ethane has what is considered a Csp 3 –Csp 3 ! bond of “normal” length (154 " 10 –12 m) and “normal” strength (377 kJ/mol). In contrast, the C2–C3 ! bond of
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Conformational analysis II © Bruno I. Rubio 2 2,2,3,3-tetramethylbutane (C(CH 3 ) 3 C(CH 3 ) 3 ) is a bit longer (157 " 10 –12 m) but very much weaker (289 kJ/mol). The C2–C3 ! bond of 2,2,3,3-tetramethylbutane is lengthened and weakened as the molecule attempts to increase the distance between the bulky methyl groups at C2 and C3 in an effort to relieve unfavor- able steric crowding: H 3 C CH 3 CH 3 H 3 C CH 3 CH 3 C2–C3 bond lengthens to relieve unfavorable steric interaction among methyl groups Bond-angle strain Each hybridization state is associated with an ideal set of bond angles: 109.5° for sp 3 , 120° for sp 2 , and 180° for sp. Bond-angle strain is caused by a deviation from the ideal bond angle. Examples of molecules that have bond- angle strain are cyclopropane, cyclopropene and benzyne: cyclopropane cyclopropene benzyne The carbon–carbon bonds in cyclopropane are forced into a 60° angle (i.e., they deviate by 49.5° from ideality); cyclopropene has two alkene carbons (ideal bond angle 120°) likewise forced to 60°. Benzyne forces a triple bond (ideal bond angle 180°) into a 120° angle. Bonds that are deformed from their ideal angles are destabilized because the orbitals that make up those bonds do not overlap efficiently. The best over- lap is achieved when interacting orbitals overlap in a direct head-to-head or side-to-side manner. When bond angle strain is in play, orbitals are forced to align in an off-center orientation that weakens the resulting bond. The drawing below compares the unfavorable, off-center alignment of Csp 3 hybrid AOs to form the Csp 3 –Csp 3 ! bonds in cyclopropane versus the more favorable, direct head-to-head alignment of orbitals in bond-angle-strain-free propane.
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l08b - CH 203 O R G A N I C C H E M I S T R Y I...

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