(a) Predict the products in the following reactions : (i) PhCO–CH₂CH₂CH₂CH₃ (valerophenone) + CH₃CO–CH₂CH₂–CD₂–CH₃…

(a)(i) Norrish type II photolysis of the mixed ketones: a crossover test

Both ketones carry γ-hydrogens, so on n→π* excitation each undergoes a Norrish type II reaction. The excited C=O oxygen abstracts a γ-H through a six-membered (1,5-H shift) transition state, giving a 1,4-biradical (ketyl radical plus γ-alkyl radical). The biradical then either cleaves the Cα–Cβ bond (β-scission) to an enol and an alkene, or closes to a cyclobutanol (Yang cyclisation, minor).

• Valerophenone, PhCO–CH₂CH₂CH₂CH₃: γ-H abstraction from C-4, then cleavage, gives the enol PhC(OH)=CH₂, which tautomerises to acetophenone, together with propene. A little 2-methyl-1-phenylcyclobutanol forms as the minor product.
• Hexan-2-one-5,5-d₂, CH₃CO–CH₂CH₂–CD₂–CH₃: the γ-carbon carries only D, so the oxygen abstracts D. Cleavage gives the enol CH₃C(OD)=CH₂, which ketonises to acetone-d₁, CH₃COCH₂D, and the alkene CH₂=CD–CH₃ (propene-2-d).

The deuterium ends up only in the fragments of the labelled ketone. Acetophenone and propene from valerophenone contain no D, and the acetone carries exactly one D. There is no crossover, so the hydrogen transfer is intramolecular, through the six-membered cyclic transition state of each molecule, not by free radicals exchanging H between molecules. The C–D abstraction is also slower (primary isotope effect), so the labelled ketone reacts less efficiently.

(a)(ii) Photolysis of 4,5-dimethylheptan-2-one

The ketone is CH₃CO–CH₂(α)–CH(CH₃)(β, C-4)–CH(CH₃)(γ, C-5)–CH₂CH₃. It has two kinds of γ-H: one tertiary H at C-5 and three primary H on the C-4 methyl group. The tertiary C–H is weaker and gives the more stable radical, so abstraction occurs mainly there.

• Major products (type II cleavage): the 1,4-biradical with the radical at C-5 cleaves C3–C4. This gives the enol of acetone, which tautomerises to acetone, and 3-methylpent-2-ene, CH₃CH=C(CH₃)CH₂CH₃ (E and Z).
• Minor products: (1) Yang cyclisation of the same biradical gives 2-ethyl-1,2,3-trimethylcyclobutan-1-ol as a mixture of diastereomers. (2) Abstraction of a primary H from the C-4 methyl group gives acetone and 3-methylpent-1-ene.

Chirality of the recovered ketone. Some biradicals simply return the H from the ketyl O–H to C-5 and regenerate the ketone. In the biradical, C-5 is a planar radical, so the H can come back to either face. If a single stereoisomer is photolysed, the recovered ketone is therefore epimerised at C-5 (the γ-stereocentre). C-4 (β) never becomes sp², so its configuration is retained. The recovered material is a mixture of C-5 epimers that is still optically active because of C-4. This loss of configuration at the γ-carbon is evidence for the reversible 1,4-biradical.

(b)(i) Ziegler–Natta polypropylene

Catalyst: TiCl₄ supported on MgCl₂ (or TiCl₃) activated by AlEt₃, with an internal Lewis-base donor for stereocontrol. Polymerisation is run at about 60–80 °C and modest pressure.

Mechanism (Cossee–Arlman):

1. AlEt₃ alkylates titanium, creating a Ti–C bond next to a vacant coordination site.
2. Propylene coordinates to the vacant site through its π bond.
3. Migratory insertion: the growing chain moves onto the coordinated alkene through a four-centre transition state, forming a new C–C bond and leaving a vacancy where the chain used to sit.
4. Steps 2 and 3 repeat. Insertion is 1,2 (head-to-tail), and the chiral surface site makes each monomer present the same enantioface, so the product is isotactic PP.
5. The chain ends by β-hydride elimination or by chain transfer to H₂, which is used to control molecular mass.

Advantages over conventional free-radical polymerisation: propylene does not give useful high polymer by radical routes (allylic H abstraction stops the chain), whereas Ziegler–Natta gives high-molecular-mass, stereoregular, crystalline PP with Tₘ about 160–165 °C. It also works at low temperature and pressure, gives linear chains with little branching, and allows control of tacticity and molar mass.

(b)(ii) Perlon (nylon-6) from ε-caprolactam

Perlon is made by ring-opening polymerisation of ε-caprolactam, usually started with a little water at about 250 °C.

1. Water hydrolyses some lactam to 6-aminohexanoic acid, H₂N(CH₂)₅COOH.
2. The NH₂ group of this acid attacks the carbonyl carbon of another caprolactam. The tetrahedral intermediate collapses, the ring C–N bond breaks, and the chain grows by one unit while a new terminal NH₂ is formed.
3. Repetition, together with condensation between chain ends, gives –[NH–(CH₂)₅–CO]ₙ–, nylon-6 (Perlon).

Anionic polymerisation with a strong base (NaH or Na) and an N-acyl-lactam activator is the faster alternative.

(c)(i) OsO₄ dihydroxylation, then periodate cleavage

The only C=C in the substrate is in the side chain, cyclopropyl–CH=CH–CH₂CH₂CH₂Ph. The lactone and the cyclopropane are untouched.

• X: OsO₄ in pyridine adds to the alkene by a concerted [3+2] cycloaddition to give a cyclic osmate(VI) ester. Both C–O bonds form on the same face. Aqueous acid hydrolyses the osmate ester and releases the syn-1,2-diol, cyclopropyl–CH(OH)–CH(OH)–CH₂CH₂CH₂Ph. The two faces of the alkene give diastereomeric diols.
• HIO₄ forms a cyclic periodate ester with the vicinal diol and cleaves the C–C bond between the two carbinol carbons.
• Y: the tricyclic lactone carrying a cyclopropanecarbaldehyde (–CHO) at the former alkene carbon.
• Z: 4-phenylbutanal, OHC–CH₂CH₂CH₂Ph.

(c)(ii) Baeyer–Villiger oxidation of 3-phenylbutan-2-one

The substrate is one enantiomer of Ph–CH(CH₃)–CO–CH₃.

Mechanism: m-CPBA adds to the protonated carbonyl to give the Criegee intermediate. The group anti-periplanar to the O–O bond migrates from carbon to oxygen in a concerted step while m-chlorobenzoate leaves. Migratory aptitude is tertiary > secondary (benzylic) > phenyl > primary > methyl.

• Major product: the secondary benzylic group migrates, giving 1-phenylethyl acetate, CH₃CO–O–CH(CH₃)Ph. The migrating carbon keeps its bonds during the shift, so the reaction proceeds with retention of configuration. The (S)-ketone gives (S)-1-phenylethyl acetate, and the (R)-ketone gives the (R)-acetate.
• Minor product: methyl migration gives methyl 2-phenylpropanoate, Ph–CH(CH₃)–CO–OCH₃, also with the stereocentre unchanged, since that carbon is not involved.