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I am struggling with the above question. It will be the conjugate base of trichloromethane that is involved. The same is true of the carboxylate. However, I have no idea about the reaction mechanisms. Please help.

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You are correct that the first step with both reagents involves the formation of the trichloromethyl anion ($\ce{CCl3^{-}}$). This anion can eject $\ce{Cl^{-}}$ and irreversibly form dichlorocarbene ($\ce{:CCl2}$). Carbenes are very reactive intermediates, reactive enough to add to a double bond and form a strained 3-membered ring (a cyclopropane).

The reaction you've shown is said to be stereospecific. That is, you started with cis-2-butene and wound up with a cis-dimethyl-dichlorocyclopropane; the cis olefin geometry is preserved in your cyclopropane product. This indicates that the reaction involved singlet dichlorocarbene. Carbenes can exist as singlets or triplets. As the following figure indicates, singlet carbenes add stereospecifically (your case), triplets do not. The "slow spin flip" step in the triplet carbene reaction is slow enough that rotation can occur about what was the olefinic bond and this rotation gives rise to cis and trans cyclopropane isomers.

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I think that both reactions involve carbene as common intermediate which then is the species involved in the subsequent cyclopropanation reaction. These are the two equations which lead to the formation of the carbene: $$\ce{KOH + CHCl3 -> :CCl2 + KCl + H2O}$$ $$\ce{Cl3CCO2Na -> :CCl2 + NaCl + CO2}$$ Subsequently, the alkene undergoes cyclopropanation by dichlorocarbene. In the second case, heat is required in order to promote decarboxylation, which is entropically favored.

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It will be the conjugate base of trichloromethane that is involved.

Only as the initial intermediate! $¸\ce{CHCl3 + OH- ->Cl3C^- + H2O}$

The carbanion will lose chloride and yield dichlorocarbene $\ce{Cl3C- -> Cl- + Cl2C:}$

The latter then adds to the $\ce{C=C}$ double bond.

The same is true of the carboxylate.

No! In this case, decarboxylation (loss of $\ce{CO2}$) is the first step. The fate of the carbanion is however as described above.

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  • $\begingroup$ Why does decarboxylation occur? Aren't carboxylate ions stable? Is that why heat is required? $\endgroup$ – RobChem Jan 24 '15 at 15:11
  • $\begingroup$ @RobChem You're right. Usually, they are stable. Decarboxylation may occur when a carboxylate is oxidized to an acetoxy radical, which is not the case here, or when when a resulting carbanion is stabilized by electron-withdrawing substituents. And yes, in your case, heat is necessary for the decarboxylation of the trichloroacetate. $\endgroup$ – Klaus-Dieter Warzecha Jan 24 '15 at 15:25

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