(a) Write the structures of nucleosides and nucleotides, and discuss the primary structures of DNA and RNA. (10 marks) (b) Write…

Part (a): Nucleic Acid Structures and Primary Organization

A nucleoside consists of a nitrogenous base (purine or pyrimidine) linked via an N-glycosidic bond to a pentose sugar (ribose or deoxyribose). In a nucleotide, a phosphate group is esterified to the 5'-hydroxyl of the sugar. The primary structure of DNA is a polynucleotide chain where deoxyribose units are joined by 3',5'-phosphodiester bonds, forming a sugar-phosphate backbone. The bases (Adenine, Guanine, Cytosine, Thymine) project inward. RNA is structurally similar but utilizes ribose (with a 2'-OH group) and replaces Thymine with Uracil. This 2'-OH makes RNA more chemically reactive and less stable than DNA. Both adopt a 5' to 3' polarity, defined by the direction of the phosphodiester linkage.

Part (b): Photolysis of 2-Methylcyclohexanone

Upon irradiation, 2-methylcyclohexanone undergoes Norrish Type I cleavage. The carbonyl group absorbs light to reach an excited singlet state, which intersystem crosses to a triplet state. This leads to homolytic cleavage of the C-C bond alpha to the carbonyl, generating a 1,4-biradical intermediate. In the solution phase, this biradical can undergo two main pathways:

1. Fragmentation: The biradical collapses to form a carbonyl compound and an alkene. Specifically, cleavage between C1 and C2 yields cyclopentanecarboxaldehyde and ethene (or related fragments depending on the specific bond broken).
2. Hydrogen Abstraction: In solvents capable of H-donation, the radical center at C4 can abstract a hydrogen atom, leading to a saturated ketone or alcohol after further oxidation/reduction steps. However, the primary products in non-hydrogen-donating solvents are the cleavage products: a dicarbonyl species or an alkene-carbonyl pair. The key intermediate is the 1,4-diradical, which is responsible for the loss of ring integrity.

Part (c): Synthesis and Reaction Identification

(i) Synthesis from Acetylene: Although the target compound for (i) is not explicitly named in the text, a standard synthesis from acetylene involves:

1. Deprotonation of acetylene with NaNH₂ in liquid NH₃ to form sodium acetylide.
2. Alkylation with a suitable alkyl halide (e.g., ethyl bromide) to form 1-butyne.
3. Further deprotonation and alkylation if a longer chain is required.
4. Partial hydrogenation (Lindlar’s catalyst) to form the cis-alkene, or full hydrogenation to the alkane.
5. Oxidation or hydration depending on the final functional group required.

(ii) Identification of W, X, Y, Z:

• W: Toluene reacts with NBS (N-bromosuccinimide) in the presence of light (hv) and dibenzoyl peroxide. This is a radical benzylic bromination. The product W is Benzyl bromide (C₆H₅CH₂Br).
• X: Toluene reacts with CrO₃, Ac₂O, and H₃O⁺. This is the Etard reaction (or similar oxidation of methyl group to aldehyde). The product X is Benzaldehyde (C₆H₅CHO).
• Y: Benzaldehyde (X) reacts with Ac₂O, NaOAc, and heat. This is the Perkin reaction, which condenses an aromatic aldehyde with acetic anhydride in the presence of a carboxylate salt to form an α,β-unsaturated acid. The product Y is Cinnamic acid (Ph-CH=CH-COOH).
• Z: Cinnamic acid (Y) is treated with excess LiAlH₄. LiAlH₄ reduces the carboxylic acid group to a primary alcohol. The double bond remains intact under standard conditions. The product Z is 3-phenylpropan-1-ol (Ph-CH=CH-CH₂OH).

Part (d): Spectroscopic Distinctions

(i) IR Distinction of Amines: Primary amines (R-NH₂) exhibit two sharp peaks in the N-H stretching region (3300–3500 cm⁻¹) due to symmetric and asymmetric stretching vibrations, appearing as a doublet. Secondary amines (R₂NH) have only one N-H bond, resulting in a single peak (singlet) in this region. Tertiary amines show no N-H stretching peaks.

(ii) PMR Distinction of Alcohols:

• Primary Alcohols (R-CH₂-OH): The OH proton appears as a singlet (exchangeable). The adjacent CH₂ protons appear as a triplet (if coupled to CH₃) or multiplet. The OH signal disappears upon D₂O shake.
• Secondary Alcohols (R₂CH-OH): The OH proton is a singlet. The CH proton appears as a multiplet (often a quartet or multiplet depending on neighbors).
• Tertiary Alcohols (R₃C-OH): The OH proton is a singlet. There is no proton on the carbon bearing the OH group. The adjacent protons (if any) will show coupling patterns typical of alkyl chains, but no direct coupling to an OH-bearing carbon proton. The key distinction is the absence of a CH-OH proton signal in tertiary alcohols compared to the distinct CH-OH signal in primary and secondary alcohols.

Part (e): Spectral Data Matching

(i) IR: 1750 cm⁻¹; NMR: δ 2.0 (s, 3H), 5.1 (s, 2H), 7.3 (s, 5H)

• IR 1750 cm⁻¹ indicates an ester or ketone.
• NMR: 7.3 (s, 5H) indicates a monosubstituted benzene ring. 5.1 (s, 2H) indicates a benzylic CH₂ group (Ar-CH₂-X). 2.0 (s, 3H) indicates a methyl group adjacent to a carbonyl (CH₃-C=O).
• This matches Structure (4) Benzyl acetate (CH₃COOCH₂C₆H₅). The ester carbonyl is at ~1750 cm⁻¹.

(ii) IR: 1740 cm⁻¹; NMR: δ 3.5 (s, 3H), 3.6 (s, 2H), 7.4 (s, 5H)

• IR 1740 cm⁻¹ indicates an ester.
• NMR: 7.4 (s, 5H) is monosubstituted benzene. 3.6 (s, 2H) is a benzylic CH₂ (Ar-CH₂-C=O). 3.5 (s, 3H) is a methoxy group (O-CH₃).
• This matches Structure (2) Methyl 3-phenylpropanoate (C₆H₅-CH₂-CH₂-COOCH₃). Wait, let's re-evaluate. Structure (2) is Methyl 3-phenylpropanoate: Ph-CH₂-CH₂-COOCH₃. The CH₂ next to Ph is ~2.6-2.9, not 3.6. Let's look at Structure (1) Methyl 4-methylbenzoate: No, that has a para-methyl.
• Let's re-examine Structure (2): Methyl 3-phenylpropanoate. The protons are Ph-CH₂-CH₂-COOCH₃. The OCH₃ is ~3.7. The CH₂ next to C=O is ~2.7. The CH₂ next to Ph is ~2.9. This does not match 3.5/3.6 singlets.
• Let's look at Structure (4) Benzyl acetate again. Ph-CH₂-O-CO-CH₃. OCH₂Ph is ~5.1. CH₃ is ~2.0. This matches (i).
• Let's look at Structure (2) again. Is it possible the structure is Methyl 2-phenylacetate? Ph-CH₂-COOCH₃. NMR: Ph (7.4, 5H), CH₂ (3.6, 2H, s), OCH₃ (3.7, 3H, s). This matches (ii) perfectly. The provided description for (2) says "Methyl 3-phenylpropanoate" but the spectral data (3.5, 3.6 singlets) strongly suggests Methyl 2-phenylacetate. However, assuming the provided structure list is fixed, let's check Structure (1) Methyl 4-methylbenzoate: Ph (7.4, 4H, d), CH₃ on ring (2.4, 3H, s), OCH₃ (3.9, 3H, s). No.
• Let's check Structure (5) 3-phenylpropanoic acid: Ph-CH₂-CH₂-COOH. NMR: Ph (7.4, 5H), CH₂ (2.8, 2H, t), CH₂ (2.6, 2H, t), OH (12, 1H, s). This matches (iii).
• So (iii) is Structure (5).
• (i) is Structure (4).
• (ii) must be Structure (2) or (1). If (2) is actually Methyl 2-phenylacetate (common in such questions despite description errors), it fits. If we must stick to the description "Methyl 3-phenylpropanoate", it doesn't fit the singlets. However, Structure (1) Methyl 4-methylbenzoate has a para-methyl. The NMR for (ii) has 5H aromatic, implying monosubstituted. Structure (1) is disubstituted. Structure (2) is monosubstituted. The chemical shifts 3.5 and 3.6 are very close to Methyl 2-phenylacetate. Given the options, (ii) matches Structure (2) (assuming the description implies the 2-phenyl isomer or there is a typo in the description, as 3-phenylpropanoate would show triplets).
• (iii) IR 3200-2800 (broad OH) and 1700 (C=O). NMR: 12.0 (s, 1H) is COOH. 7.4 (s, 5H) is Ph. 2.75 (t, 2H) and 2.95 (t, 2H) are two CH₂ groups. This is Structure (5) 3-phenylpropanoic acid.

Conclusion: (i) matches (4). (ii) matches (2). (iii) matches (5). Structures (1) and (3) are distractors.