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Compounds like tetraethyl silane or tetraethyltin can be made from their tetrachlorides using ethylating Grignards, but what about substituting carbon, would it work or steric hindrance would prevent full substitution?

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The reaction of a Grignard reagent with carbon tetrachloride is not a nucleophilic attack by the carbanion moiety on the central carbon of the carbon tetrachloride. Rather, the Grignard reagent attacks the chlorine, leading to the formation of products in which a dechlorinated fragment of the original carbon tetrachloride added to or dehydrogenated additional substrates. Davis et al.[1] give an example of ethyl and phenyl Grignard reagents reacting with carbon tetrachloride in the presence of cyclohexane, forming products:

  • 3,3'-bicyclohexyl, in which the cyclohexane molecules were dehydrogenated

  • Dichloromethylcyclohexane, in which a $\ce{CCl2}$ fragment was inserted into the cyclohexane

  • 7,7-dihalonorcaranes, in which the carbon tetrachloride carbon added to the six-carbon ring (which was again dehydrogenated) to form the norcarane core

The dechlorination is favored not only by steric crowding of the carbon. The carbon also acts as an acceptor of pi back-donated electrons, effectively transferring positive charge and electrophilic character to the chlorine.

Note also that Tetraphenylmethane was synthesized by Gomberg[2] not by Grignard addition but by a diazo coupling to triphenylbromomethane.

References

  1. M. Davis, L.W. Deady, A.J. Finch, J.F. Smith, "Some reactions of grignard reagents with chloroform and carbon tetrachloride in the presence of cyclohexene". Tetrahedron 29(2): 349-352, ISSN 0040-4020, https://doi.org/10.1016/S0040-4020(01)93300-9.

  2. Gomberg, M. (1898). "On tetraphenylmethane". J. Am. Chem. Soc. 20(10): 773–780. https://doi.org/10.1021/ja02072a009.

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  • $\begingroup$ Thank you a lot for your answer, if it is not possible to make it using carbon tet, then I think there is another aproach. First, react propionic acid methyl ester with EthMgBr, to form triethtylmethanol, then substitute -OH group formed using PBr3 with bromine, but at this point I am not sure how to substitute the bromide with ethyl, do you have any ideas? $\endgroup$ Commented Jun 13 at 11:24
  • $\begingroup$ When you get $\ce{Et3CBr}$, the Grignard coupling is more likely to work than with carbon tet. But others may have a more informed opinion. $\endgroup$ Commented Jun 13 at 11:27
  • $\begingroup$ If it is 3,3-diethylpentane you want, consider this. t-BuLi reacts with tetrahydrofuran (THF) to form the lithium anion of acetaldehyde and ethylene. Excess t-BuLi adds to ethylene to form 3-ethylpentan-3-yl)lithium. Protonation affords the hydrocarbon, 2,2-dimethylbutane. If you can prepare (3-ethylpentan-3-yl)lithium, then this organometallic should lead to the target hyrocarbon. $\endgroup$
    – user55119
    Commented Jun 14 at 0:11
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    $\begingroup$ I have discovered that probably the only reagent that could substitute the bromide with ethyl giving substantial yields is Lithium diethylcuprate - a Gilman reagent. $\endgroup$ Commented Jun 15 at 10:41

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