4
$\begingroup$

I'm trying to compare the relative stability of two carbocations:

$$\ce{CH3-\overset{+}{C}H-Cl}\quad\text{vs}\quad\ce{CH3-\overset{+}{C}H2}$$

Does the $\ce{Cl}$ exert a +M effect to stabilise the carbocation, or does it exert a −I effect to destabilise it?

$\endgroup$
10
$\begingroup$

As Oscar Lanzi suggested, both $+M$ and $-I$ applies here, but $\ce{Cl}$ stabilizes carbocation, meaning $+M$ is more effective than $-I$. This fact was confirmed by this peer-reviewed paper (Ref.1):

The lowering of $\ce{C_\beta–H}$ stretching frequencies in carbocations 1a–d and 2a–c induced by hyperconjugation was tested as a possible probe for estimating the electron donating ability of $\alpha$-substituents. Conclusions are based on the results of high level quantum chemical calculations confirmed with experimental FT-IR spectra. Because the decrease in the $\ce{C_\beta–H}$ stretching frequency is comparable in 1b and in 1c, and in 2b and 2c respectively, it follows that $\alpha$-substitution $\color{red}{\text{by a methyl group}}$ or $\color{red}{\text{by chlorine}}$ stabilizes a carbocation with $\color{red}{\text{almost the same effectiveness}}$.

The effect of the chlorine $n$-electron back donation is evident by a partially double bond character of the $\ce{C+–Cl}$ bond in $\alpha$-chlorocarbocations observed experimentally in IR spectra (Ref.2). In this experiment, FT-IR spectrum of $\ce{Cl3C+}$ cation has displayed the $\ce{C–Cl}$ stretching frequency at $\pu{1040 cm–1}$, which is $\pu{250 cm–1}$ higher than in neutral alkyl chlorides (e.g., characteristic $\ce{C–Cl}$ stretching frequency of $\ce{CCl4}$ is $\pu{785 cm-1}$). This is indicative of partial double bond character of $\ce{C+–Cl}$ bond suggested by the resonance structures:

$$\ce{Cl2C+-Cl <-> CH2C=Cl+}$$

The pioneering work of Olah and coworkers has also predicted this phenomena by $\ce{^{13}C}$-NMR studies of trihalocarbocations (Ref.3):

$$ \begin{array}{c|c|c|c} \ce{HCX3} & \delta\ce{^{13}C}\text{ of }\ce{CHX3} & \ce{^+CX3} & \delta\ce{^{13}C}\text{ of }\ce{^+CX3} & \Delta \delta\ce{^{13}C} \\ \hline \ce{HCCl3} & 77.7 & \ce{^+CCl3} & 236.3 & 158.6 \\ \ce{HCBr3} & 12.3 & \ce{^+CBr3} & 207 & 194.7 \\ \ce{HCI3} & -139.7 & \ce{^+CI3} & 95 & 234.7\\ \hline \end{array} $$

Olah has suggested that the decreasing trend of $\Delta \delta\ce{^{13}C}$ by going $\ce{I}$ to $\ce{Br}$ to $\ce{Cl}$ is in agreement with the positive charge stabilization by back-bonding in order of $\ce{Cl > Br > I}$.

References:

  1. Milan Mesić, Igor Novak, Dionis E. Sunko, Hrvoj Vančik, "$\ce{C–H}$ Hyperconjugation in $\alpha$-chlorocarbocations," J. Chem. Soc., Perkin Trans. 2 1998, (11), 2371-2374 (https://doi.org/10.1039/A805772I).
  2. Hrvoj Vančik, Ksenija Percač, Dionis E. Sunko, "Chloromethyl cations in cryogenic antimony pentafluoride matrixes and the generation of carbocations from hydrocarbons," J. Am. Chem. Soc. 1990, 112(20), 7418–7419 (https://doi.org/10.1021/ja00176a065).
  3. George A. Olah, Ludger Heiliger, G. K. Surya Prakash, "Stable carbocations. Part 276. Trihalomethyl cations," J. Am. Chem. Soc. 1989, 111(20), 8020–8021 (https://doi.org/10.1021/ja00202a056).
$\endgroup$
3
$\begingroup$

You get both, but the +M effect wins. See the discussion of the effect of atoms with lone pairs over here.

$\endgroup$
1
$\begingroup$

I would say that many books suggest that +m effect overshines the -I effect but I feel it depend upon the reaction where thee intermediates are formed and type of reaction is happening and stability of intermediate involves the thermodynamics for a general exam like situation you can mark A is more stable than B.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.