Organic Chemistry: Principles and Mechanisms (Second Edition)
Organic Chemistry: Principles and Mechanisms (Second Edition)
2nd Edition
ISBN: 9780393663556
Author: Joel Karty
Publisher: W. W. Norton & Company
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Chapter D, Problem D.1P
Interpretation Introduction

Interpretation:

The orbital interaction that represents hyperconjugation in CF3CH2+ is to be drawn. The hyperconjugation lead to greater stabilization in this CF3CH2+ or in CH3CH2+ would expect is to be explained.

Concept introduction:

Relative stabilities of carbocations are explained by hyperconjugation. In the case of primary carbocations like RCH2+, ρ AO is aligned with one of the C-H bonds in the adjacent R group, which represents a pair of electrons occupying a C-Hσ bonding orbital. The empty ρ AO and the σ bonding orbital therefore interact. In this case, constructive interference results in a new orbital that is more stable than the σ bonding orbital. The two electrons from the σ bonding MO end up in the lower-energy of the two new orbitals and are thus stabilized. The mixing of these orbitals effectively transfers some electron density from the alkyl group into the empty ρ AO.

Expert Solution & Answer
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Answer to Problem D.1P

The orbital interaction that represents hyperconjugation in CF3CH2+ is,

Organic Chemistry: Principles and Mechanisms (Second Edition), Chapter D, Problem D.1P , additional homework tip  1

The hyperconjugation leads to greater stabilization for the CH3CH2+ ion.

Explanation of Solution

In CF3CH2+ ion, the filled σ bonding orbital is of the adjacent C-F bond of CF3 which can interact with the empty ρ AO of CH2+ as the empty ρ AO overlaps with σ bonding orbital of the C-F bond. It is shown below:

Organic Chemistry: Principles and Mechanisms (Second Edition), Chapter D, Problem D.1P , additional homework tip  2

Also, in CH3CH2+, the empty ρ AO overlaps with σ bonding orbital of CH3 bond of CH3, so the two orbitals can interact. The two electrons from the σ bonding MO end up in the lower-energy of the two new orbitals and are thus stabilized.

The orbital interactions for both CF3CH2+ and CH3CH2+ are shown below,

Organic Chemistry: Principles and Mechanisms (Second Edition), Chapter D, Problem D.1P , additional homework tip  3

The F atom is more electronegative than H, therefore the C-Fσ bonding orbital is in lower energy than that of the CH3 bond of CH3CH2+. Therefore, the ΔE for the overlap of the empty ρ AO with C-Fσ bonding orbital is smaller than that for overlap of the empty ρ AO and CH3σ bonding orbital. Thus, hyperconjugation leads to greater stabilization for the CH3CH2+ ion.

Conclusion

The cation in which hyperconjugation would leads to greater stabilization is determined on the basis of orbital interactions shown above.

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