Chapter: Sulfur, Silicon, and Phosphorus in Organic Chemistry

1. Introduction: The Roles of Sulfur, Silicon, and Phosphorus

While organic chemistry primarily focuses on carbon, hydrogen, oxygen, and nitrogen, elements from the third row and beyond, particularly sulfur, silicon, and phosphorus, play increasingly vital roles. Their unique electronic structures, larger atomic radii, and ability to form d-orbital interactions (for S, P, Si) lead to distinct chemical properties and reactivity patterns compared to their second-row counterparts (O, N, C). They are crucial in:

• Biological Systems: Phosphorus (DNA, RNA, ATP, phospholipids), Sulfur (amino acids, enzymes).
• Organic Synthesis: Powerful reagents, catalysts, and protecting groups.
• Materials Science: Polymers, semiconductors.

2. Sulfur in Organic Chemistry

Sulfur (Group 16) is below oxygen in the periodic table. It can exhibit various oxidation states and expand its octet.

2.1. Thiols (Mercaptans, R-SH)

• Structure and Properties: Analogous to alcohols, but with an -SH group.
  • Acidity: Thiols are significantly more acidic than alcohols (pKa ≈10−11 for aliphatic thiols, vs. ≈16−18 for alcohols). This is because the larger size of sulfur (compared to oxygen) allows for better delocalization/polarizability of the negative charge in the thiolate anion (RS−), stabilizing it more effectively.
  • Odor: Many thiols have strong, often unpleasant, odors.
• Synthesis:
  • From alkyl halides: R-X+NaSH→R-SH+NaX (Often problematic with polyalkylation).
  • Better method: R-X+thiourea→[isothiouronium salt]NaOH, H2​O​R-SH.
• Reactions:
  • Oxidation to Disulfides: Thiols readily oxidize to disulfides (R-S-S-R). This is a key reaction in biochemistry (e.g., disulfide bridges in proteins, like cysteine).
    • 2 R-SHOxidizing agent (e.g., I2​,O2​, DMSO)​R-S-S-R+2 H+
  • Alkylation (Formation of Thioethers): Thiolates are excellent nucleophiles for SN​2 reactions.
    • RS−+R’-X→R-S-R’+X−

2.2. Thioethers (Sulfides, R-S-R’)

• Structure and Properties: Analogous to ethers, but with a sulfur atom replacing oxygen.
  • Bond Angles: C-S-C bond angle is typically closer to 100∘ than 109.5∘.
• Synthesis: Williamson ether synthesis equivalent using thiolates (see above).
• Reactions:
  • Oxidation: Unlike ethers, thioethers are easily oxidized due to the availability of sulfur’s d-orbitals and its greater polarizability.
    • Sulfoxides (R2​S=O): Partial oxidation (e.g., by H2​O2​, NaIO4​). Sulfoxides are polar and often chiral (if R = R’). DMSO is dimethyl sulfoxide.
    • Sulfones (R2​SO2​): Further oxidation (e.g., by H2​O2​, peroxiacids). Sulfones are highly polar.
  • Alkylation (Formation of Sulfonium Salts): Sulfur is nucleophilic and can react with alkyl halides to form sulfonium salts (R3​S+X−).
    • R2​S+R’-X→R2​S+R’ X−
    • Sulfonium Ylides: Important intermediates from sulfonium salts (e.g., in cyclopropanation).

2.3. Sulfonic Acids (R-SO3​H) and Derivatives

• Sulfonic Acids: Strong acids (pKa ≈−1 to 0), much stronger than carboxylic acids, due to extensive resonance stabilization of the sulfonate anion (R-SO3−​).
  • Synthesis: Aromatic sulfonic acids are typically made by direct sulfonation of aromatic rings.
• Sulfonyl Chlorides (R-SO2​Cl): Formed from sulfonic acids. React with amines to form sulfonamides.
  • Sulfonamides (R-SO2​NR’2​): Stable, often crystalline, used as drugs (sulfa drugs) and as protecting groups for amines. Their N-H proton can be acidic.
  • Sulfonate Esters (R-SO2​OR’): Alkyl and aryl sulfonates are excellent leaving groups in substitution and elimination reactions (e.g., tosylate, mesylate, triflate).

3. Silicon in Organic Chemistry

Silicon (Group 14) is below carbon. It forms longer and weaker bonds to carbon than carbon-carbon bonds, and its electronegativity difference with carbon leads to reverse polarity.

3.1. Organosilicon Compounds

• C-Si Bond: Longer and weaker than C-C. Silicon is less electronegative than carbon, so the C-Si bond is polarized Cδ−−Siδ+. This makes the carbon nucleophilic.
• Reactivity: The reactivity of organosilicon compounds often involves the formation of strong Si-O bonds.

3.2. Silyl Ethers (R-OSiR’3​) as Protecting Groups

• Formation: Alcohols react with silyl chlorides (R’3​SiCl) in the presence of a base (e.g., triethylamine, imidazole) to form silyl ethers.
• Stability: Silyl ethers are stable to many reagents (e.g., Grignards, LiAlH4​, weak acids, weak bases).
• Deprotection: Readily cleaved by fluoride ions (F−, e.g., using TBAF – tetrabutylammonium fluoride) because of the extremely strong Si-F bond. This allows selective deprotection.

3.3. Peterson Olefination

• Reaction: A method for synthesizing alkenes from aldehydes or ketones using α-silyl carbanions.
• Mechanism:
  1. An α-silyl carbanion (from deprotonation of an α-silyl alkane) attacks a carbonyl compound (aldehyde or ketone).
  2. Formation of a β-hydroxysilane intermediate.
  3. Elimination of the OSiR3​ and OH groups to form an alkene. The stereoselectivity (cis/trans) depends on whether it’s acid- or base-catalyzed:
     • Acid-catalyzed: Favors E-alkene.
     • Base-catalyzed: Favors Z-alkene.

3.4. Silicones (Polysiloxanes)

• Structure: Polymers with repeating silicon-oxygen chains (-Si-O-Si-O-) with organic groups attached to silicon.
• Properties: Chemically inert, heat resistant, water repellent.
• Applications: Sealants, lubricants, medical implants.

3.5. Hiyama Coupling

• Reaction: A Palladium-catalyzed cross-coupling reaction that forms a new C-C bond between an organosilicon reagent and an organic halide (typically aryl or vinyl halides).
• Advantages: Organosilicon reagents are relatively non-toxic and easy to handle compared to some other organometallics.

4. Phosphorus in Organic Chemistry

Phosphorus (Group 15) is below nitrogen. It can also expand its octet and form very strong P=O bonds.

4.1. Phosphines (R3​P)

• Structure and Properties: Analogous to amines, but with phosphorus replacing nitrogen.
  • Basicity/Nucleophilicity: Phosphines are less basic than comparable amines, but they are generally more nucleophilic than amines due to their larger size and more polarizable lone pair, which allows for better orbital overlap with electrophiles. They are also less prone to inversion than amines.
  • Steric Bulk: Often used as ligands in transition metal catalysis due to their tunable steric and electronic properties (e.g., triarylphosphines like PPh3​).

4.2. Phosphonium Salts (R4​P+X−)

• Formation: Formed by the reaction of phosphines with alkyl halides (similar to amine alkylation).
  • R3​P+R’-X→R3​P+R’ X−
• Key Intermediate: Precursors to Wittig reagents.

4.3. Phosphorus Ylides (Phosphoranes, R3​P=CR”2​)

• Structure: A neutral molecule containing a positively charged phosphorus atom directly bonded to a negatively charged carbon atom (a carbanion). It’s typically represented with a double bond between P and C (P=C) due to resonance (ylide form and ylene form).
• Formation: Generated by deprotonating a phosphonium salt with a strong base (e.g., n-butyllithium, NaH).
  • R3​P+CH2​R’ X−Base​R3​P=CHR’+Base-H
• Reactivity: The carbanionic carbon is a strong nucleophile.

4.4. The Wittig Reaction

• Reaction: A powerful and widely used method for synthesizing alkenes from aldehydes or ketones using a phosphorus ylide.
  • Overall: R3​P=CR”2​+R’2​C=O→R”2​C=CR’2​+R3​P=O (triphenylphosphine oxide)
• Mechanism:
  1. Nucleophilic Attack: The carbanionic carbon of the ylide attacks the carbonyl carbon of the aldehyde or ketone, forming a zwitterionic intermediate called a betaine.
  2. Ring Closure: The betaine rapidly closes to form a four-membered cyclic intermediate called an oxaphosphetane.
  3. Elimination: The oxaphosphetane rapidly breaks down to yield the alkene and triphenylphosphine oxide (Ph3​P=O). The strong driving force for the reaction is the formation of the very stable P=O double bond.
• Stereoselectivity:
  • Stabilized Ylides: (e.g., those with electron-withdrawing groups on the ylide carbon) tend to be less reactive and often favor the formation of the E-alkene (trans).
  • Unstabilized Ylides: (e.g., simple alkyl ylides) tend to be more reactive and often favor the formation of the Z-alkene (cis). This is often explained by the lowest energy pathway through the oxaphosphetane intermediate.

4.5. Phosphates and Phosphites

• Phosphates: Esters of phosphoric acid (H3​PO4​). Found ubiquitously in biochemistry (DNA, RNA backbone; ATP as energy currency; phospholipids in cell membranes).
• Phosphites: Esters of phosphorous acid (H3​PO3​). Used in organic synthesis (e.g., Arbuzov reaction).

5. Key Comparisons and Distinctions

• Sulfur vs. Oxygen: Larger size, less electronegative, can expand octet (valence shell expansion), can form disulfides, more readily oxidized, thiols are more acidic than alcohols, thioethers are more nucleophilic than ethers.
• Silicon vs. Carbon: Larger size, less electronegative, forms polarized C-Si bonds (Cδ−−Siδ+), strong affinity for oxygen (Si-O bonds), silyl ethers used as protecting groups, Peterson olefination for alkene synthesis.
• Phosphorus vs. Nitrogen: Larger size, less basic but more nucleophilic (due to polarizability), can expand octet, forms stable P=O bonds (driving force for Wittig), phosphonium ylides (Wittig reagents) for alkene synthesis.

Multiple Choice Questions (MCQ) on Sulfur, Silicon, and Phosphorus in Organic Chemistry

Instructions: Choose the best answer for each question.

1. Which of the following statements about thiols compared to alcohols is generally true? a) Thiols are less acidic than alcohols. b) Thiols are more acidic than alcohols. c) Thiols have higher boiling points than alcohols. d) Thiols are stronger bases than alcohols.

2. What is the approximate pKa of a typical aliphatic thiol? a) 5 b) 10-11 c) 16-18 d) 25

3. The strong acidity of thiols compared to alcohols is primarily attributed to: a) Hydrogen bonding in thiols. b) The smaller size of sulfur. c) The greater polarizability and larger size of sulfur, stabilizing the thiolate anion. d) The higher electronegativity of sulfur.

4. What is the product when two thiol molecules undergo oxidation? a) Sulfone b) Sulfoxide c) Disulfide d) Thioether

5. Thioethers are analogous to which class of oxygen-containing compounds? a) Alcohols b) Ethers c) Aldehydes d) Ketones

6. Unlike ethers, thioethers can be easily oxidized to form: a) Alkenes and water. b) Thiols and disulfides. c) Sulfoxides and sulfones. d) Esters and carboxylic acids.

7. Dimethyl sulfoxide (DMSO) is an example of a: a) Thiol b) Thioether c) Sulfoxide d) Sulfone

8. What is formed when a thioether reacts with an alkyl halide? a) Sulfone b) Sulfonium salt c) Sulfoxide d) Disulfide

9. Which of the following is typically a very strong acid and is a derivative of sulfur? a) Thiol b) Sulfide c) Sulfonic acid d) Disulfide

10. Sulfonate esters (e.g., tosylates) are well-known for their role as: a) Good nucleophiles. b) Good leaving groups. c) Strong bases. d) Oxidizing agents.

11. Which of the following statements is true about the carbon-silicon bond? a) It is typically shorter and stronger than a C-C bond. b) Silicon is more electronegative than carbon, making it Cδ+−Siδ−. c) Silicon is less electronegative than carbon, making it Cδ−−Siδ+. d) It is entirely non-polar.

12. Silyl ethers are commonly used in organic synthesis as: a) Oxidizing agents for alcohols. b) Protecting groups for alcohols. c) Strong bases for deprotonation. d) Reducing agents for aldehydes.

13. What reagent is typically used to deprotect silyl ethers (i.e., remove the silyl group to regenerate the alcohol)? a) Strong acid (e.g., HCl) b) Strong base (e.g., NaOH) c) Fluoride ions (e.g., TBAF) d) Oxidizing agents (e.g., KMnO4​)

14. The Peterson olefination is a method for synthesizing which functional group? a) Alcohols b) Ketones c) Alkenes d) Carboxylic acids

15. What type of intermediate is involved in the Peterson olefination reaction? a) Carbocation b) Radical c) β-hydroxysilane d) Oxaphosphetane

16. Silicones (polysiloxanes) are polymers characterized by repeating units of: a) Carbon-silicon chains. b) Silicon-oxygen chains. c) Silicon-nitrogen chains. d) Silicon-sulfur chains.

17. The Hiyama coupling is a palladium-catalyzed cross-coupling reaction that forms new C-C bonds using which type of organometallic reagent? a) Organolithium b) Organomagnesium (Grignard) c) Organocopper (Gilman) d) Organosilicon

18. How do phosphines compare to amines in terms of nucleophilicity? a) Phosphines are generally less nucleophilic. b) Phosphines are generally more nucleophilic. c) They have comparable nucleophilicity. d) Phosphines are electrophilic, not nucleophilic.

19. What is the general product when a phosphine reacts with an alkyl halide? a) Phosphine oxide b) Phosphonium salt c) Phosphorus ylide d) Alkyl phosphate

20. What is a phosphorus ylide (phosphorane) typically represented as? a) R3​P b) R4​P+X− c) R3​P=CR”2​ d) R3​P=O

21. What is the key reaction that phosphorus ylides are used for in organic synthesis? a) Aldol condensation b) Wittig reaction c) Diels-Alder reaction d) Friedel-Crafts alkylation

22. In the Wittig reaction, what type of bond is formed between the phosphorus ylide and the carbonyl compound? a) A new carbon-carbon single bond. b) A new carbon-carbon double bond. c) A new carbon-oxygen double bond. d) A new carbon-phosphorus bond.

23. What is the main driving force for the Wittig reaction? a) The formation of a stable carbocation. b) The release of energy from forming a highly stable P=O double bond. c) The stability of the phosphorus ylide. d) The formation of a very stable alkane.

24. What is the four-membered cyclic intermediate formed during the Wittig reaction mechanism? a) Betaine b) Oxaphosphetane c) Carbene d) Ylide

25. Which type of phosphorus ylide is generally associated with favoring the formation of the Z-alkene (cis) in the Wittig reaction? a) Stabilized ylides b) Unstabilized ylides c) Aryl ylides d) Cyclic ylides

26. Phosphates are esters of which acid? a) Phosphorous acid b) Hypophosphorous acid c) Phosphoric acid d) Pyrophosphoric acid

27. What is the common biochemical role of phosphates? a) Catalyzing redox reactions. b) Acting as structural components in cell walls. c) Energy currency (ATP), genetic material backbone (DNA, RNA), cell membrane components (phospholipids). d) Building blocks for proteins.

28. Which element, when part of an organic molecule, can exhibit a wide range of oxidation states and readily expand its octet? a) Carbon b) Nitrogen c) Oxygen d) Sulfur

29. The reaction of an alkyl halide with thiourea, followed by hydrolysis, is a common method for synthesizing: a) Thioethers b) Disulfides c) Thiols d) Sulfones

30. Which of the following is a key characteristic of sulfoxides like DMSO? a) They are non-polar. b) They are good reducing agents. c) They are polar and can be chiral. d) They readily undergo substitution at sulfur.

31. In the Peterson olefination, what does acid-catalysis typically favor? a) Z-alkene b) E-alkene c) Alkyne d) Alcohol

32. What is the property of F− that makes it particularly effective in deprotecting silyl ethers? a) Its strong acidity. b) Its ability to form an extremely strong Si-F bond. c) Its large size. d) Its oxidizing power.

33. The betaine intermediate in the Wittig reaction is a: a) Neutral compound with a double bond. b) Zwitterionic compound. c) Carbocation. d) Radical.

34. Compared to amines, phosphines are: a) More basic and less nucleophilic. b) Less basic and more nucleophilic. c) Both more basic and more nucleophilic. d) Both less basic and less nucleophilic.

35. What class of compounds contains repeating silicon-oxygen chains and is known for chemical inertness and heat resistance? a) Polyethers b) Polyesters c) Silicones d) Polysulfides

36. Sulfonamides are derivatives of sulfonic acids that are often used in pharmaceuticals. Their N-H proton can be: a) Highly basic. b) Acidic. c) Inert. d) Non-existent.

37. Which of the following organosilicon compounds is typically used in the Hiyama coupling? a) Silyl ethers b) Organosilanes (R-SiR’3​) c) Silanols d) Silicon dioxide

38. The Wittig reaction converts a carbonyl group (aldehyde or ketone) into a(n): a) Alcohol b) Carboxylic acid c) Alkene d) Amine

39. The ability of sulfur and phosphorus to expand their octet is due to the presence of: a) Empty p-orbitals. b) Empty d-orbitals. c) Lone pairs of electrons. d) High electronegativity.

40. Which reaction sequence would be suitable for converting an alkyl halide into a thiol without significant polyalkylation? a) R-X+NaOH b) R-X+NaSH c) R-X+thiourea→isothiouronium saltNaOH, H2​O​ d) R-X+H2​S

Answer Key with Explanations

1. b) Thiols are more acidic than alcohols.
   • Explanation: Thiols are significantly more acidic than alcohols due to the larger size and greater polarizability of the sulfur atom, which better stabilizes the negative charge of the thiolate anion.
2. b) 10-11.
   • Explanation: Aliphatic thiols typically have pKa values around 10-11, making them more acidic than water (pKa 15.7) or alcohols (pKa 16-18).
3. c) The greater polarizability and larger size of sulfur, stabilizing the thiolate anion.
   • Explanation: The negative charge on the larger sulfur atom in the thiolate anion is more diffuse and better accommodated than on the smaller oxygen atom of an alkoxide, leading to greater stability and thus higher acidity.
4. c) Disulfide.
   • Explanation: Thiols readily undergo oxidation to form disulfides (R-S-S-R), a reaction that is reversible and important in biological systems.
5. b) Ethers.
   • Explanation: Thioethers (R-S-R’) are structural analogs of ethers (R-O-R’), with sulfur replacing oxygen.
6. c) Sulfoxides and sulfones.
   • Explanation: Unlike relatively unreactive ethers, thioethers can be easily oxidized at the sulfur atom due to the availability of d-orbitals, forming sulfoxides (one oxygen) and further to sulfones (two oxygens).
7. c) Sulfoxide.
   • Explanation: Dimethyl sulfoxide (DMSO) has a sulfur atom double-bonded to one oxygen and single-bonded to two methyl groups, fitting the definition of a sulfoxide.
8. b) Sulfonium salt.
   • Explanation: The nucleophilic sulfur of a thioether can attack an alkyl halide in an SN​2-like reaction to form a positively charged sulfonium salt (R3​S+X−).
9. c) Sulfonic acid.
   • Explanation: Sulfonic acids (R-SO3​H) are strong acids, comparable in strength to inorganic acids like H2​SO4​, due to extensive resonance stabilization of their conjugate base, the sulfonate anion.
10. b) Good leaving groups.
    • Explanation: Sulfonate esters (e.g., tosylates, mesylates, triflates) are highly stable anions due to resonance, making them excellent leaving groups in substitution (SN​1, SN​2) and elimination reactions.
11. c) Silicon is less electronegative than carbon, making it Cδ−−Siδ+.
    • Explanation: Silicon is less electronegative than carbon. This means electrons in a C-Si bond are drawn towards carbon, creating a partial negative charge on carbon and a partial positive charge on silicon.
12. b) Protecting groups for alcohols.
    • Explanation: Silyl ethers are widely used to protect alcohol functional groups because they are stable under many reaction conditions and can be selectively removed later.
13. c) Fluoride ions (e.g., TBAF).
    • Explanation: Silyl ethers are uniquely deprotected by fluoride ions. This is due to the extremely strong bond that silicon forms with fluorine, which provides a strong driving force for the reaction.
14. c) Alkenes.
    • Explanation: The Peterson olefination is a synthetic method specifically designed to create carbon-carbon double bonds (alkenes) from carbonyl compounds and α-silyl carbanions.
15. c) β-hydroxysilane.
    • Explanation: The Peterson olefination mechanism involves the formation of a β-hydroxysilane intermediate, which then undergoes elimination to form the alkene.
16. b) Silicon-oxygen chains.
    • Explanation: Silicones, or polysiloxanes, are polymers characterized by a backbone of alternating silicon and oxygen atoms (-Si-O-Si-O-) with organic groups attached to the silicon atoms.
17. d) Organosilicon.
    • Explanation: The Hiyama coupling is a Palladium-catalyzed cross-coupling reaction that uses organosilicon reagents (e.g., organosilanes or silanols) to form new carbon-carbon bonds.
18. b) Phosphines are generally more nucleophilic.
    • Explanation: While phosphines are less basic than comparable amines, their lone pair on the larger, more polarizable phosphorus atom makes them more nucleophilic, better able to attack electrophiles.
19. b) Phosphonium salt.
    • Explanation: Phosphines react with alkyl halides to form phosphonium salts (R4​P+X−), similar to the alkylation of amines.
20. c) R3​P=CR”2​.
    • Explanation: A phosphorus ylide (or phosphorane) is a neutral molecule represented with a formal double bond between phosphorus and carbon, where the carbon has a negative charge and the phosphorus has a positive charge (often shown in resonance forms).
21. b) Wittig reaction.
    • Explanation: Phosphorus ylides are the key reagents used in the Wittig reaction, which is a powerful method for converting aldehydes and ketones into alkenes.
22. b) A new carbon-carbon double bond.
    • Explanation: The Wittig reaction’s primary synthetic utility is to create a new carbon-carbon double bond (an alkene) from a carbonyl compound.
23. b) The release of energy from forming a highly stable P=O double bond.
    • Explanation: The formation of the very strong and stable phosphorus-oxygen double bond (P=O, in triphenylphosphine oxide) provides a significant driving force that makes the Wittig reaction highly favorable.
24. b) Oxaphosphetane.
    • Explanation: In the Wittig reaction mechanism, the initial attack of the ylide forms a betaine, which then cyclizes to form a four-membered ring intermediate called an oxaphosphetane, prior to elimination.
25. b) Unstabilized ylides.
    • Explanation: Unstabilized (or non-stabilized) ylides are more reactive and typically favor the formation of the Z-alkene (cis) in the Wittig reaction, often due to kinetic control involving the formation of a less hindered oxaphosphetane.
26. c) Phosphoric acid.
    • Explanation: Phosphates are organic esters derived from inorganic phosphoric acid (H3​PO4​).
27. c) Energy currency (ATP), genetic material backbone (DNA, RNA), cell membrane components (phospholipids).
    • Explanation: Phosphates are fundamental in biochemistry, serving as the backbone of nucleic acids, the primary energy currency (ATP), and key components of cell membranes.
28. d) Sulfur.
    • Explanation: Sulfur (and phosphorus) can utilize their empty d-orbitals to expand their octet, allowing them to exhibit a wide range of oxidation states and bonding patterns. Carbon, nitrogen, and oxygen typically obey the octet rule.
29. c) Thiols.
    • Explanation: The reaction of an alkyl halide with thiourea, followed by hydrolysis, is a clean and effective method for synthesizing thiols while minimizing unwanted polyalkylation reactions that can occur with simple NaSH.
30. c) They are polar and can be chiral.
    • Explanation: Sulfoxides like DMSO are highly polar due to the sulfinyl group. If the two groups attached to sulfur are different, the sulfoxide can be chiral at sulfur.
31. b) E-alkene.
    • Explanation: In the Peterson olefination, acid-catalyzed elimination of the β-hydroxysilane intermediate typically proceeds to give the more stable E-alkene (trans).
32. b) Its ability to form an extremely strong Si-F bond.
    • Explanation: The bond between silicon and fluorine is one of the strongest single bonds known. This high bond strength provides a very powerful driving force for the reaction, making fluoride ions highly effective in cleaving Si-O bonds in silyl ethers.
33. b) Zwitterionic compound.
    • Explanation: The betaine intermediate in the Wittig reaction is a zwitterion, meaning it has both a positive and a negative charge on different atoms within the same molecule.
34. b) Less basic and more nucleophilic.
    • Explanation: This is a key distinction. Due to the larger size and greater polarizability of phosphorus, phosphines are less basic (less proton affinity) but more nucleophilic (better able to donate electrons to form new bonds) than amines.
35. c) Silicones.
    • Explanation: Silicones (polysiloxanes) are polymers characterized by repeating units of silicon and oxygen atoms in their backbone, along with organic substituents, giving them their characteristic inertness and heat resistance.
36. b) Acidic.
    • Explanation: The hydrogen atom directly attached to the nitrogen in a sulfonamide is acidic, due to the electron-withdrawing effect of the sulfonyl group, and can be removed by bases.
37. b) Organosilanes (R-SiR’3​).
    • Explanation: The Hiyama coupling utilizes organosilicon reagents, typically organosilanes or silanols, as the carbon source for forming the new C-C bond in a palladium-catalyzed cross-coupling reaction.
38. c) Alkene.
    • Explanation: The Wittig reaction is a powerful method for converting the carbonyl functional group (C=O) of an aldehyde or ketone into a carbon-carbon double bond (C=C).
39. b) Empty d-orbitals.
    • Explanation: Sulfur and phosphorus are in the third row of the periodic table and possess accessible empty d-orbitals in their valence shell. This allows them to accommodate more than eight electrons and expand their octet, leading to higher valencies and diverse bonding patterns.
40. c) R-X+thiourea→isothiouronium saltNaOH, H2​O​.
    • Explanation: This two-step method is generally preferred for synthesizing thiols from alkyl halides because the thiourea acts as a masked SH− equivalent, preventing the polyalkylation (i.e., formation of thioethers) that often occurs when using NaSH directly.