Samuel Redstone University of Utah Hyperconjugation Samuel Redstone University of Utah

Hyperconjugation: Definitions Hyperconjugation is the donation of a sigma bond into an adjacent empty or partially filled p orbital, which results in an increased stability of the molecule. Hyperconjugation contributes to the resonance stabilization of this tertiary carbocation, where electrons from the C-H sigma bonding orbital are donated to the empty p orbital of the cation. Hyperconjugation was first described by Baker and Nathan in 1935 to describe the “abnormal behavior” of alkyl-substituted compounds.1 1. J. Chem. Soc., 1935, 1844-1847.

Trends in Hyperconjugation Less electronegative atoms make for better sigma bond donors. This is because more electronegative atoms have lower lying HOMOs, which have poor overlap with the LUMO of the electron acceptor. > In other words, less electronegative atoms are more willing to share there electrons with the carbocation. Therefore, the 2-methyl butane is more stabilized through hyperconjugation than the 3-fluoro 2-methyl butane.

Trends in Hyperconjugation 2. Lower lying LUMOs make for better electron acceptors, as there is better orbital overlap with the HOMO of the electron donor. > > > Atomic orbital sp π* sp2 π* sp3 π* 3. The greater the number of substituents on a carbocation, the more stabilized it will be through hyperconjugation. > > Therefore, tertiary carbocations are more stable than secondary or primary.

Other types of Hyperconjugation Occasionally, electron density will be donated from a filled 𝜋 or p orbital into an adjacent σ* orbital. This is known as negative hyperconjugation. Other times, electron density will be donated from a p orbital into an adjacent 𝜋* orbital; the result is an overall neutral charge on the molecule. This is known as neutral hyperconjugation.

Consequences of Hyperconjugation 1. Conformational Preferences of Esters Esters exist in two conformers, E and Z. (E) Conformer (Z) Conformer     Because the Z conformation allows for the overlap of the lone pair with the C—O σ* it is favored over the E conformation, where the lone pair is aligned with the C—R σ*.2 (Z) Conformer ΔG=+4.8 kcal/mol (E) Conformer No resonance, poor orbital overlap! 2. Chem. Phys. Lett., 1981, 84, 267.

Consequences of Hyperconjugation 2. The Secondary Kinetic Isotope Effect The kinetic isotope effect (KIE) is the change in rate of a reaction when an atom is replaced with a heavier isotope. A secondary KIE is when the isotope substitution occurs at an atom where a bond is not being broken or formed. Because heavier atoms form stronger bonds, they are less willing to contribute to hyperconjugation. Therefore, reactions that require the elimination of a leaving group proceed more slowly. For example: Due to the weaker C—H bond, Reaction 1 proceeds faster than Reaction 2. This is because the C—H σ bond is more willing to push electrons into the empty orbital of the carbocation and contribute to hyperconjugation. 1. KH/KD is the ratio of the rates of the individual reactions. A KH/KD >1 suggests that the reaction occurs more slowly with the isotopic substitution. 2. KH/KD = 1.15, as found by Streitwieser, et al.3 *Note: TsO, or tosylate, is a leaving group. J. Chem. Phys., 1951, 19, 342.

Problems 1. Rank the ability of the following bonds to contribute to hyperconjugation: C—F, C—O, C—N, C—H. C—F > C—O > C—N > C—H C—H > C—N > C—O > C—F C—H > C—O > C—N > C—F C—F > C—H > C—N > C—O 2. The following molecule will not experience hyperconjugation. Why is that? The hybridization of the molecule does not permit proper orbital overlap. The deuterium does not contribute hyperconjugation very well. There is poor overlap between the LUMO of the N—Me bond and HOMO of the N—D bond. Hyperconjugation does not occur on heteroatoms such as nitrogen. 3. Which of the following most readily ionizes? B. D. A. C. 4. Identify whether the following molecules have positive, negative, or neutral hyperconjugation. i. ii. iii. Positive Negative Neutral Positive Negative Neutral Positive Negative Neutral

Solutions B C A i. B ii. C iii. A

Contributed by: Samuel Redstone (Undergraduate) University of Utah, 2016