Question

Work out a synthesis of each of the following compounds based on one of the available silicon compounds ${\mathrm{SiF}}_4$, ${\mathrm{SiCl}}_4,{\mathrm{HSiCl}}_2,{\mathrm{H}}_2{\mathrm{SiCl}}_2,{\left({\mathrm{CH}}_3\right)}_2{\mathrm{SiCl}}_2,{\left({\mathrm{CH}}_3\right)}_3\mathrm{SiCl}$ (a) ${\left({\mathrm{C}}_2{\mathrm{H}}_5\right)}_3\mathrm{SiF}$ (b) ${\left({\mathrm{C}}_6{\mathrm{H}}_5\right)}_2\mathrm{SiClH}$ (c) $(\mathrm{n}-{\mathrm{C}}_4{\mathrm{H}}_9)\mathrm{Si}{\left({\mathrm{CH}}_3\right)}_2\mathrm{H}$ (d) ${\mathrm{Cl}}_3{\mathrm{SiCH}}_2{\mathrm{CH}}_2\mathrm{CH}={\mathrm{CH}}_2$ (e) ${\mathrm{Cl}}_3{\mathrm{SiCH}}_2{\mathrm{CH}}_2{\mathrm{SiCl}}_3$ (f) ${\left({\mathrm{C}}_6{\mathrm{H}}_5\right)}_3{\mathrm{SiCH}}_2{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{Si}{\left({\mathrm{CH}}_3\right)}_3$

Short answer

(a) $(C_2{H}_5{)}_{3}SiF$: Start with $(C{H}_3{)}_{3}SiCl$, react with $C_2{H}_{5}ONa$, then with $HF$. (b) $(C_6{H}_5{)}_{2}SiClH$: Start with $(HSiC{l}_2)$, react with $2C_6{H}_{5}Li$. (c) $(n-C_4{H}_9)Si(C{H}_3{)}_{2}H$: Start with $(C{H}_3{)}_{3}SiCl$, react with $n-C_4{H}_{9}Li$. (d) $C{l}_{3}SiC{H}_{2}C{H}_{2}CH=C{H}_2$: Start with $(SiC{l}_4)$, react with $C{H}_2=CHC{H}_{2}MgCl$. (e) $C{l}_{3}SiC{H}_{2}C{H}_{2}SiC{l}_3$: Start with $(SiC{l}_4)$, react with $C{H}_{2}C{H}_{2}MgCl$, then with $SiC{l}_4$. (f) $(C_6{H}_5{)}_{3}SiC{H}_{2}C{H}_{2}C{H}_{2}Si(C{H}_3{)}_3$: Start with $((C_6{H}_5{)}_{3}SiCl)$ and $((C{H}_3{)}_{3}SiCl)$, react each with $C{H}_{2}C{H}_{2}C{H}_{2}MgCl$, then combine the Grignard products.

Step-by-step solution

(a) Synthesis of $(C_2{H}_5{)}_{3}SiF$

Step 1: Select the starting material We begin with trichlorosilane $(C{H}_3{)}_{3}SiCl$ as the starting material, because it already has three ethyl groups attached to the silicon atom. Step 2: Replace the chlorine atom Perform a nucleophilic substitution reaction by reacting $(C{H}_3{)}_{3}SiCl$ with sodium ethoxide $(C_2{H}_{5}ONa)$ to replace the chlorine with an ethoxide group: $(C{H}_3{)}_{3}SiCl+C_2{H}_{5}ONa\to (C{H}_3{)}_{3}SiO{C}_2{H}_5+NaCl$ Step 3: Replace the ethoxide group with a fluoride atom Perform another nucleophilic substitution reaction, this time with hydrofluoric acid $(HF)$, to replace the ethoxide group with a fluoride atom: $(C{H}_3{)}_{3}SiO{C}_2{H}_5+HF\to (C_2{H}_5{)}_{3}SiF+HO{C}_2{H}_5$

(b) Synthesis of $(C_6{H}_5{)}_{2}SiClH$

Step 1: Select the starting material We begin with dichlorosilane $(HSiC{l}_2)$ as the starting material, because it has a hydrogen atom and two chlorine atoms attached to the silicon atom. Step 2: Replace one chlorine atom with a phenyl group Perform a nucleophilic substitution reaction by reacting $(HSiC{l}_2)$ with two equivalents of phenyllithium $(C_6{H}_{5}Li)$ to replace two chlorine atoms with phenyl groups: $HSiC{l}_2+2C_6{H}_{5}Li\to (C_6{H}_5{)}_{2}SiClH+2LiCl$

(c) Synthesis of $(n-C_4{H}_9)Si(C{H}_3{)}_{2}H$

Step 1: Select the starting material We begin with chlorosilane $(C{H}_3{)}_{3}SiCl$ as the starting material, because it has three methyl groups and one chlorine atom attached to the silicon atom. Step 2: Replace one methyl group with a butyl group Perform a nucleophilic substitution reaction by reacting $(C{H}_3{)}_{3}SiCl$ with n-butyllithium $(n-C_4{H}_{9}Li)$ to replace the chlorine atom with a butyl group: $(C{H}_3{)}_{3}SiCl+n-C_4{H}_{9}Li\to (n-C_4{H}_9)Si(C{H}_3{)}_{2}H+LiCl$

(d) Synthesis of $C{l}_{3}SiC{H}_{2}C{H}_{2}CH=C{H}_2$

Step 1: Select the starting material We begin with silicon tetrachloride $(SiC{l}_4)$. Step 2: Perform a Grignard reaction Add allylmagnesium chloride $(C{H}_2=CHC{H}_{2}MgCl)$ as a Grignard reagent to replace a chlorine atom with the allyl group: $SiC{l}_4+C{H}_2=CHC{H}_{2}MgCl\to C{l}_{3}SiC{H}_{2}C{H}_{2}CH=C{H}_2+MgC{l}_2$

(e) Synthesis of $C{l}_{3}SiC{H}_{2}C{H}_{2}SiC{l}_3$

Step 1: Select the starting material We begin with silicon tetrachloride $(SiC{l}_4)$. Step 2: Perform a Grignard reaction Add chloromagnesium ethyl $(C{H}_{2}C{H}_{2}MgCl)$ as a Grignard reagent to replace a chlorine atom with an ethyl group attached to the silicon atom: $SiC{l}_4+C{H}_{2}C{H}_{2}MgCl\to C{l}_{3}SiC{H}_{2}C{H}_{2}MgCl+MgC{l}_2$ Step 3: Replace the magnesium atom React the product with silicon tetrachloride to replace the magnesium atom with a silicon atom and form the final product: $C{l}_{3}SiC{H}_{2}C{H}_{2}MgCl+SiC{l}_4\to C{l}_{3}SiC{H}_{2}C{H}_{2}SiC{l}_3+MgC{l}_2$

(f) Synthesis of $(C_6{H}_5{)}_{3}SiC{H}_{2}C{H}_{2}C{H}_{2}Si(C{H}_3{)}_3$

Step 1: Select the starting material We begin with trichlorosilane $((C_6{H}_5{)}_{3}SiCl)$ for the first part of the compound and chlorosilane $((C{H}_3{)}_{3}SiCl)$ for the second part. Step 2: Perform Grignard reactions For each starting material, perform a Grignard reaction with an ethylmagnesium chloride $(C{H}_{2}C{H}_{2}C{H}_{2}MgCl)$ to replace a chlorine atom with an allyl group: $(C_6{H}_5{)}_{3}SiCl+C{H}_{2}C{H}_{2}C{H}_{2}MgCl\to (C_6{H}_5{)}_{3}SiC{H}_{2}C{H}_{2}C{H}_{2}MgCl+MgC{l}_2$ $(C{H}_3{)}_{3}SiCl+C{H}_{2}C{H}_{2}C{H}_{2}MgCl\to (C{H}_3{)}_{3}SiC{H}_{2}C{H}_{2}C{H}_{2}MgCl+MgC{l}_2$ Step 3: Combine the two parts React the two Grignard products together to form the desired product: $(C_6{H}_5{)}_{3}SiC{H}_{2}C{H}_{2}C{H}_{2}MgCl+(C{H}_3{)}_{3}SiC{H}_{2}C{H}_{2}C{H}_{2}MgCl\to (C_6{H}_5{)}_{3}SiC{H}_{2}C{H}_{2}C{H}_{2}Si(C{H}_3{)}_3+MgC{l}_2$

Key concepts

Nucleophilic Substitution

Nucleophilic substitution is a fundamental class of reactions in organic chemistry where an electron-rich nucleophile selectively bonds with or attacks the positive or partially positive charge of an atom or a group of atoms to replace a leaving group. The nucleophile can be negatively charged or neutral with a lone pair of electrons it can donate. For instance, in organic synthesis, this reaction is crucial for modifying the backbone of a compound by exchanging different functional groups.

Improvements in understanding can be achieved by highlighting the two main types of nucleophilic substitution reactions: SN1 and SN2. SN1 involves a two-step mechanism with a carbocation intermediate, suitable for tertiary carbon centers, while SN2 involves a one-step mechanism with a backside attack, ideal for primary carbon centers. The choice of the reaction path depends on the structure of the molecule and the conditions of the reaction. For example, the synthesis of $C_2{H}_5{)}_{3}SiF$ incorporates a SN2 reaction, where the backside attack is preferred due to the primary silicon center present in the starting material $C{H}_3{)}_{3}SiCl$.

A deeper insight into the selection of nucleophiles, such as sodium ethoxide for its ability to displace a chlorine atom effectively, can help students understand how the efficiency of a reaction can hinge on the right choice of reagents.

Grignard Reaction

The Grignard reaction is a tool of immense importance in organic synthesis. It involves the creation of a Grignard reagent, which is essentially a magnesium halide adduct with an alkyl or aryl group. These reagents are highly nucleophilic and can attack electrophiles, particularly carbonyl groups, leading to the formation of alcohols, carboxylic acids, and other carbon-carbon bond forming reactions.

To improve comprehension, one should note that Grignard reactions require anhydrous conditions because Grignard reagents react with water, deactivating their reactivity. The synthesis of $C{l}_{3}SiC{H}_{2}C{H}_{2}CH=C{H}_2$ and $C{l}_{3}SiC{H}_{2}C{H}_{2}SiC{l}_3$ showcases the use of Grignard reagents to form complex carbon-silicon compounds, demonstrating versatility. Additionally, an understanding of the controlled reaction environment necessary to prevent side reactions or Grignard reagent deactivation is essential for the successful application of this reaction.

Synthesis Strategies

Synthesis strategies involve a systematic approach to constructing complex molecules from simpler ones, using a wide array of reactions. The choice of strategy involves understanding the reactivity of functional groups, protection and deprotection of groups, regioselectivity, and stereoselectivity. These aspects dictate the direction and the steps in a synthetic route.

Improvement in grasping synthesis strategies can be achieved by studying retrosynthesis, where you work backwards from the final compound to simpler starting materials. Such a method is very pragmatic and aligns with how chemists tackle the synthesis of complex molecules. For example, when synthesizing ${\left({\mathrm{C}}_6{\mathrm{H}}_5\right)}_3{\mathrm{SiCH}}_2{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{Si}{\left({\mathrm{CH}}_3\right)}_3$, retrosynthesis can be particularly beneficial in planning the sequence of Grignard reactions to achieve the optimal pathway. Recognizing intermediate structures and functional groups' reactivity helps in piecing together the synthetic puzzle.

Silicon Compound Chemistry

Silicon compound chemistry, or organosilicon chemistry, deals with compounds that contain carbon-silicon bonds. Silicon compounds have intriguing properties—due to their similarity to carbon—they can enhance stability, increase resistance to thermal or chemical decomposition, and improve material properties such as flexibility and permeability.

For better understanding, students should familiarize themselves with silicon's ability to form four stable covalent bonds and how that underpins the versatility in organosilicon compounds. For instance, the silicon starting materials employed in the textbook exercise, such as ${\mathrm{SiCl}}_4$, ${\mathrm{HSiCl}}_2$, and ${\left({\mathrm{CH}}_3\right)}_3\mathrm{SiCl}$, establish the foundation for the diverse transformations possible in synthesizing targeted silicon-containing organic compounds. Attention to the roles of silicon as both a electrophile and a stabilizing element can deeply enrich students' comprehension of the organic synthesis involving these materials.