(a) (i) Predict the products in the following reactions: Propose suitable mechanism to justify your answer. (ii) Giving…

(a)(i) Norrish Type II Photolysis The substrate is 3-methyl-2-pentanone (a γ-ketoalkane). Under hν, the carbonyl undergoes n → π^* excitation. The excited ketone abstracts a γ-hydrogen from the isopropyl group via a six-membered cyclic transition state, forming a 1,4-biradical. This intermediate undergoes β-scission of the C-C bond between the α- and β-carbons. The products are acetone (CH_3COCH₃) and 2-methyl-1,3-butadiene (isoprene). The mechanism involves homolytic cleavage driven by the stability of the resulting alkene and carbonyl.

(a)(ii) Chirality and Product Distribution Assuming the reaction proceeds via a radical pathway where the α-carbon becomes planar (sp2) in the biradical intermediate, the recovered reactant (if any unreacted starting material remains) retains its original chirality because the reaction is irreversible for the product-forming pathway, but any reversible H-abstraction would racemize the α-carbon. However, typically in Norrish II, the starting material is consumed. If we consider a competing Norrish Type I α-cleavage, it would yield acetyl and sec-butyl radicals. The major product is determined by the stability of the resulting alkene; isoprene (conjugated diene) is favored over non-conjugated alkenes. The chirality of the starting material is lost in the products as the stereocenter is destroyed during bond cleavage.

(b)(i) Ziegler-Natta Synthesis of Polypropylene Polypropylene (PP) is synthesized using a Ziegler-Natta catalyst, typically TiCl₄ supported on MgCl₂, activated by AlEt₃. The mechanism is coordination-insertion. Propylene coordinates to the vacant orbital of the titanium center. The C=C bond inserts into the Ti-C bond, extending the polymer chain. The catalyst controls stereochemistry: the rigid MgCl₂ surface and the specific geometry of the active site force propylene to approach in a specific orientation, leading to isotactic polypropylene (all methyl groups on the same side). Advantages over free-radical polymerization include: (1) production of stereoregular (isotactic) polymers with high crystallinity and melting point (~160°C), whereas free-radical polymerization yields atactic, amorphous, low-melting materials; (2) operation at low temperatures and pressures; (3) no need for initiators that leave impurities.

(b)(ii) Perlon (Nylon-6) Synthesis Perlon-6 is synthesized from ε-caprolactam via ring-opening polymerization. The mechanism is anionic (or cationic) chain growth.

1. Initiation: A small amount of water or base opens the lactam ring to form an amino-acid anion (or cation).
2. Propagation: The nucleophilic nitrogen of the opened ring attacks the carbonyl carbon of another caprolactam molecule, opening it and extending the chain.
3. Termination: Chain transfer or reaction with a terminating agent stops growth. The repeating unit is -[NH-(CH₂)₅-CO]-.

(c)(i) OsO4 Dihydroxylation and Cleavage The substrate is 2-methyl-1-phenylpropan-1-one? No, the question implies an alkene for OsO4. Let's assume the substrate is an alkene derived from the context or a standard alkene like 2-methyl-2-butene or similar. Wait, the prompt says "chiral ketone" in (c)(ii) but (c)(i) says "reactions". Let's look at the figure description for (c): "A reaction scheme showing a chiral ketone reacting with m-CPBA". This is (c)(ii). For (c)(i), the text says "Write the structures of the products X, Y and Z... (1) OsO4... (2) H+, H2O; X ->[HIO4] Y + Z". This implies the starting material is an alkene. Let's assume the alkene is 2-methyl-2-butene or a similar chiral alkene? OsO4 adds to alkenes. Let's assume the starting alkene is 2-methyl-2-butene (achiral) or a chiral alkene like 3-methyl-2-pentene. Let's assume the alkene is 2-methyl-2-butene for simplicity, or better, a generic chiral alkene R_1R_2C=CR_3R₄. Mechanism for X: OsO4 undergoes [3+2] cycloaddition with the alkene to form a cyclic osmate ester. Hydrolysis (H⁺, H_2O) cleaves the Os-O bonds, yielding a syn-1,2-diol (X) and regenerating OsO4 (or Os(VI) species). Cleavage: Periodic acid (HIO₄) cleaves the vicinal diol (X) via a cyclic periodate ester intermediate. The C-C bond breaks, forming two carbonyl compounds (Y and Z). If the alkene is 2-methyl-2-butene: X is 2-methyl-2,3-butanediol. Y is acetone, Z is acetaldehyde. If the alkene is chiral, the syn-addition creates new stereocenters. The diol X is a racemic mixture if the alkene is achiral, or diastereomers if chiral.

(c)(ii) Baeyer-Villiger Oxidation Substrate: 2-methyl-1-phenylpropan-1-one? No, the text says "Ph-C(=O)-CH3" in the prompt text but the figure description says "2-methyl-1-phenylpropan-1-one". Let's stick to the figure description: 2-methyl-1-phenylpropan-1-one (Ph-C(=O)-CH(CH₃)₂). Reagent: m-CPBA. Mechanism: The carbonyl oxygen attacks the peracid, forming a Criegee intermediate. The peracid leaves, and a group migrates from the carbonyl carbon to the adjacent oxygen. Regioselectivity: Migratory aptitude is Ph > t-Bu > i-Pr > Et > Me. Here, the groups are Phenyl and Isopropyl. Phenyl migrates preferentially. Major Product: Phenyl isopropyl ether? No, the oxygen inserts between C and Ph. So, Ph-O-C(=O)-CH(CH₃)₂ (Isopropyl benzoate). Minor Product: Ph-C(=O)-O-CH(CH₃)₂ (Isopropyl phenyl ketone? No, Isopropyl benzoate is the major. The minor is the ester where isopropyl migrates: Ph-C(=O)-O-CH(CH₃)₂ (Isopropyl benzoate? No, that's the same name structure? No. Let's name them properly. Substrate: Ph-C(=O)-CH(CH₃)₂. Migration of Ph: Product is Ph-O-C(=O)-CH(CH₃)₂ (Isopropyl benzoate). Migration of Isopropyl: Product is Ph-C(=O)-O-CH(CH₃)₂ (Isopropyl benzoate? No. Wait. If Ph migrates: The O inserts between C and Ph. Structure: Ph-O-C(=O)-R. This is an ester of benzoic acid. R is isopropyl. So Isopropyl benzoate. If Isopropyl migrates: The O inserts between C and Isopropyl. Structure: Ph-C(=O)-O-CH(CH₃)₂. This is also an ester of benzoic acid? No. Let's check the structure. Substrate: Ph-C(=O)-iPr.

1. Ph migrates: Ph-O-C(=O)-iPr. This is Isopropyl benzoate.
2. iPr migrates: Ph-C(=O)-O-iPr. This is Isopropyl benzoate. They are the same compound? No. Ph-O-C(=O)-iPr: The acyl group is isopropyl (propanoyl). The alkoxy group is phenyl. This is Phenyl isobutyrate. Ph-C(=O)-O-iPr: The acyl group is phenyl (benzoyl). The alkoxy group is isopropyl. This is Isopropyl benzoate. Migratory aptitude: Phenyl > Isopropyl. So Phenyl isobutyrate is the Major product. Isopropyl benzoate is the Minor product. Stereochemistry: The migrating group retains its configuration. Since the isopropyl group is not chiral (it has two methyls), there is no stereochemical issue. The product is achiral. The reaction is concerted, so no carbocation intermediate.