Write equations for and explain the use of tetrabutyl...

Write equations for and explain the use of tetrabutyl ammonium chloride, $\mathrm{Bu}_4 \mathrm{~N}^{+} \mathrm{Cl}^{-}$, to facilitate the reaction between 1-heptyl chloride and cyanide ion. $$ \begin{aligned} \mathrm{Bu}_4 \mathrm{~N}^{+} \mathrm{Cl}^{-}+\mathrm{Na}^{+} \mathrm{CN}^{-} & \longrightarrow \mathrm{Bu}_4 \mathrm{~N}^{+} \mathrm{CN}^{-}+\mathrm{Na}^{+} \mathrm{Cl}^{-} \quad \text { (in water) } \\ n-\mathrm{C}_7 \mathrm{H}_{15} \mathrm{Cl}+\mathrm{Bu}_4 \mathrm{~N}^{+} \mathrm{CN}^{-} & \longrightarrow n-\mathrm{C}_7 \mathrm{H}_{15} \mathrm{CN}+\mathrm{Bu}_4 \mathrm{~N}^{+} \mathrm{Cl}^{-} \quad \text { (in nonpolar phase) } \end{aligned} $$ The phase-transfer catalyst, $\mathrm{Bu}_4 \mathrm{~N}^{+} \mathrm{Cl}^{-}$, reacts with $\mathrm{CN}^{-}$to form a quaternary cyanide salt that is slightly soluble in the organic phase because of the bulky, nonpolar, butyl groups. Reaction with $\mathrm{CN}^{-}$to form the nitrile is rapid because it is not solvated or ion-paired in the organic phase; it is a free, strong nucleophile. The phase-transfer catalyst, regenerated in the organic phase, returns to the aqueous phase, and the chain process is propagated.

Write equations for and explain the use of tetrabutyl ammonium chloride, $\mathrm{Bu}_4 \mathrm{~N}^{+} \mathrm{Cl}^{-}$, to facilitate the reaction between 1-heptyl chloride and cyanide ion.
$$
\begin{aligned}
\mathrm{Bu}_4 \mathrm{~N}^{+} \mathrm{Cl}^{-}+\mathrm{Na}^{+} \mathrm{CN}^{-} & \longrightarrow \mathrm{Bu}_4 \mathrm{~N}^{+} \mathrm{CN}^{-}+\mathrm{Na}^{+} \mathrm{Cl}^{-} \quad \text { (in water) } \\
n-\mathrm{C}_7 \mathrm{H}_{15} \mathrm{Cl}+\mathrm{Bu}_4 \mathrm{~N}^{+} \mathrm{CN}^{-} & \longrightarrow n-\mathrm{C}_7 \mathrm{H}_{15} \mathrm{CN}+\mathrm{Bu}_4 \mathrm{~N}^{+} \mathrm{Cl}^{-} \quad \text { (in nonpolar phase) }
\end{aligned}
$$

The phase-transfer catalyst, $\mathrm{Bu}_4 \mathrm{~N}^{+} \mathrm{Cl}^{-}$, reacts with $\mathrm{CN}^{-}$to form a quaternary cyanide salt that is slightly soluble in the organic phase because of the bulky, nonpolar, butyl groups. Reaction with $\mathrm{CN}^{-}$to form the nitrile is rapid because it is not solvated or ion-paired in the organic phase; it is a free, strong nucleophile. The phase-transfer catalyst, regenerated in the organic phase, returns to the aqueous phase, and the chain process is propagated.

Key Concepts

Ion Exchange and Solvation Effects

Ion exchange and solvation play crucial roles in phase-transfer processes. In aqueous solvents, ionic species are heavily solvated, which can inhibit their reactivity by stabilizing them through ion pairing. However, when these ions are transferred into an organic phase via a phase-transfer catalyst, they experience diminished solvation, making them more nucleophilic and readily reactive. The exchange between the paired ions is facilitated by the catalyst, which regenerates in the process, allowing the reaction to continue efficiently.

Phase-Transfer Catalysis

Phase-transfer catalysis is a technique used to facilitate reactions between reactants located in different phases, typically water and an organic solvent. In these reactions, an agent, often a quaternary ammonium salt, transfers an ionic reactant from the aqueous phase into the organic phase where it can react with an organic substrate. This method circumvents the solubility issues of ionic compounds in nonpolar solvents and enables efficient chemical transformations.

Quaternary Ammonium Salts

Quaternary ammonium salts, such as tetrabutyl ammonium chloride, serve as effective phase-transfer catalysts due to their structure, which includes both a charged ammonium head and bulky, nonpolar alkyl groups. The charged head binds ionic species, while the nonpolar alkyl chains impart solubility in organic solvents. This dual solubility allows the catalyst to shuttle ions from the aqueous phase to the organic phase, thus promoting reactions that would otherwise be hindered by phase separation.

Nucleophilic Substitution (SN2) Reactions

Nucleophilic substitution reactions, particularly SN2 mechanisms, involve the attack of a nucleophile on an electrophilic carbon and the displacement of a leaving group. In the context of a phase-transfer catalyzed reaction, the ion transferred into the organic phase acts as a free, unsolvated nucleophile, which is typically much more reactive. This enhanced reactivity leads to a rapid substitution reaction with the organic substrate, such as an alkyl halide, resulting in product formation.

Transcript

00:01 This is problem 105 from chapter 16, and i'm going to write out the equation they give us.

00:09 So it's iodine plus chloromethane.

00:15 So ch3, cl, x, h, h, 3, i, plus chlorine.

00:35 Okay, so a is asking us to draw the wedge bond of chloroform and show the most effective direction of the iodide ion.

00:50 So a wedge bond is basically a 3d version of the molecules, and i'm going to draw, there's a straight black line.

01:00 That means the bond is on plane of the molecule, and there's a wedge and dash line, and that shows the bonds are either above.

01:08 Or below the plane.

01:12 So the iodine ion is a nucleophile which means that it donates its electrons.

01:19 So this is what the wedge plane is going to look like.

01:22 So we're going to have an iodine.

01:25 The iodine has eight electrons so we're going to draw eight dots around it.

01:34 And then we have my draw the chloroflorn.

01:37 So we have a line here, there, we have a wedge here.

01:46 And we have the dash line, we have h, the hydrogen here, and chlorine.

02:07 So what's going to happen? i'm going to draw an arrow.

02:10 So this iodine is going to go to the central of the bond, and then this, the central of the bond is going to attack the chlorine, and this is going to transform into something like this, mov -x...

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