Codex @codex  0

Carbon-13 NMR spectra: 1 = C, 2 = A, 3 = B. Approximate shifts, read from the printed axes, are shown in the annotated structures.

The two methyls in A are chemically distinct because one is cis and the other trans to each substituent at the other alkene carbon. Their two measured shifts are secure; assigning an individual methyl to a particular side of the drawn double bond requires an additional stereospecific shift argument or experiment, so their spatial interchange is left explicit.

The strong $1724\mathrm{c}{\mathrm{m}}^{-1}$ absorption is the carbonyl stretching vibration of a saturated ketone. Its exact wavenumber depends on substitution and measurement conditions; the important discrimination is that it is far below the high-frequency acyl chloride band in B and is accompanied by a carbonyl carbon-13 NMR resonance near 200 ppm.

A is 3-Chlorobutan-2-one, $\mathrm{C}{\mathrm{H}}_3\mathrm{C}(=\mathrm{O})\mathrm{CH}(\mathrm{Cl})\mathrm{C}{\mathrm{H}}_3$. Counting chlorine as one hydrogen in the index of index of hydrogen deficiency gives $(2\times 4+2-7-1)/2=1$. A single carbonyl accounts for that unsaturation; the resonance at $203.2$ ppm identifies a ketone, while the $59.0$ ppm methine bears chlorine.

The carbon-13 NMR assignments are ${\delta }_C=203.2$ for $\mathrm{C}=\mathrm{O}$, $25.7$ for $\mathrm{C}{\mathrm{H}}_3\mathrm{CO}$, $59.0$ for $\mathrm{CHCl}$ and $20.1$ ppm for $\mathrm{CH}(\mathrm{Cl})\mathrm{C}{\mathrm{H}}_3$. The four distinct carbon environments agree with the unsymmetrical structure. The small signal near 77 ppm belongs to the usual deuterated chloroform solvent, rather than to a fifth carbon of A.

B is 2-Methylpropanoyl chloride, $(\mathrm{C}{\mathrm{H}}_3{)}_2\mathrm{CHC}(=\mathrm{O})\mathrm{Cl}$. The single index of index of hydrogen deficiency is supplied by the carbonyl. Its relatively high-frequency infrared spectroscopy band identifies an acyl chloride, while two equivalent methyl groups explain why four carbons produce only three carbon-13 NMR resonances.

Assign ${\delta }_C=178.1$ to the acyl chloride carbonyl, $46.3$ to the methine and $18.9$ ppm to both methyl groups. Their chemical equivalence in NMR follows from the symmetry of the isopropyl fragment in this achiral environment.

The $1621\mathrm{c}{\mathrm{m}}^{-1}$ absorption is predominantly the carbon–carbon double-bond stretching vibration of the electron-rich vinyl ether. resonance donation from an oxygen lone pair changes both its frequency and its electric dipole moment variation during stretching. There is no carbonyl in the assigned structure, consistent with the absence of a carbon-13 NMR signal in the usual ketone or acyl chloride region.

C is 2-Chloroethyl vinyl ether, $\mathrm{C}{\mathrm{H}}_2=\mathrm{CH}-\mathrm{O}-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}{\mathrm{H}}_2\mathrm{Cl}$. There is one alkene and no carbonyl. Assign the carbon-13 NMR resonances asThe strongly differentiated pair of alkene shifts is characteristic of a vinyl ether; interpreting $151.1$ ppm as a carbonyl would leave the unusually shielded unsaturated $87.4$ ppm carbon and the three distinct vinylic protons unexplained.

The carbon spectra are A → 5, B → 8, C → 6, and D → 7. Number each acyclic acid or ester chain from its carbonyl carbon. The assignments below account for every carbon-13 NMR environment.

For A, both ketone carbonyl carbons have $\delta =208.3$ ppm and all four methylene carbons have $\delta =36.5$ ppm. Molecular symmetry and rapid conformational averaging explain why six carbons give only two resonances.

For B, written $\mathrm{HOOC}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}-\mathrm{C}{\mathrm{H}}_3$, assign $\delta ({\mathrm{C}}_1)=172.8$, $\delta ({\mathrm{C}}_2)=118.1$, $\delta ({\mathrm{C}}_3)=147.4$, $\delta ({\mathrm{C}}_4)=129.7$, $\delta ({\mathrm{C}}_5)=140.8$, and $\delta ({\mathrm{C}}_6)=18.1$ ppm. The alternation among vinylic chemical shifts reflects polarization of the conjugated chain, especially the deshielded carbon beta to the carboxyl group.

For C, written $\mathrm{HOOC}-\mathrm{C}\equiv \mathrm{C}-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}{\mathrm{H}}_3$, assign the carbonyl at $158.5$, the adjacent sp carbon at $72.9$, the other sp carbon at $92.6$, and the terminal methyl at $13.4$ ppm. The two methylene resonances are $20.7$ and $21.0$ ppm. A plausible assignment is ${\mathrm{C}}_4=20.7$ and ${\mathrm{C}}_5=21.0$ ppm, but their ordering is tentative from the supplied shift list alone; a correlation experiment would resolve it.

For D, written $\mathrm{C}{\mathrm{H}}_3\mathrm{CH}=\mathrm{CHC}(=\mathrm{O})\mathrm{OCH}=\mathrm{C}{\mathrm{H}}_2$, the carbonyl is at $163.2$ ppm. The crotonyl beta-CH, alpha-CH and methyl carbons are at $148.0$, $121.7$ and $18.2$ ppm, respectively. The O-bound vinyl CH is at $141.3$ ppm and its terminal $\mathrm{C}{\mathrm{H}}_2$ is at $97.4$ ppm. The two sides of the ester oxygen therefore produce quite different vinylic environments.

Basic methanol exchanges the ester alkoxy group while retaining the acetal; dilute aqueous acid removes the acetal while retaining the ethyl ester on the stated reaction timescale. An acetal lacks an electrophilic carbonyl and is stable to ordinary base: expelling methoxide directly would require a poor leaving group. The ester can instead undergo nucleophilic acyl substitution through a carbonyl-derived tetrahedral intermediate.

Under acid, protonating an acetal oxygen converts an alkoxy substituent into departing methanol, and the resulting oxocarbenium is stabilized by its remaining oxygen. Water then drives acetal hydrolysis toward the aldehyde. Ester hydrolysis is also possible under acid, but is slower under the conditions implied by the product data. The retained ethyl group and six distinct carbon-13 NMR environments support E rather than the carboxylic acid that prolonged hydrolysis could produce.

D is $(\mathrm{MeO}{)}_2\mathrm{CHC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{O}}_2\mathrm{Me}$, and E is $\mathrm{OHCC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{O}}_2\mathrm{Et}$. D results from transesterification; E results from acid-catalysed acetal hydrolysis.

For D, methoxide attacks the ester carbonyl carbon and moves the carbonyl $\pi$ pair to oxygen. Collapse of the tetrahedral reaction intermediate reforms C=O and expels ethoxide. Proton transfer with the methanol solvent regenerates methoxide. Excess methanol favours replacement of the ethyl ester by a methyl ester; the dimethyl acetal survives these basic conditions.

For E, protonate one acetal oxygen, then break its C–O bond with the pair going to oxygen, releasing methanol and forming an oxocarbenium ion. Water attacks that carbon. Deprotonation gives a hemiacetal; protonation of its remaining methoxy oxygen permits loss of a second methanol. The hydroxyl oxygen's lone pair forms C=O during this loss, and deprotonation gives the aldehyde, regenerating the acid catalyst.

The spectroscopy is consistent with these products. D has one ester carbonyl near $1745\mathrm{c}{\mathrm{m}}^{-1}$ and six carbon-13 NMR environments: acetal CH, two distinct chain $\mathrm{C}{\mathrm{H}}_2$ sites, carbonyl, ester $\mathrm{OC}{\mathrm{H}}_3$, and the two equivalent acetal $\mathrm{OC}{\mathrm{H}}_3$ groups. Thus seven carbon atoms give six resonances. E has separate ester and aldehyde carbonyl bands, approximately $1745$ and $1730\mathrm{c}{\mathrm{m}}^{-1}$, and six distinct carbon atoms: two carbonyls, two chain methylenes and the two ethyl carbons.

Irradiation that removes resolved scalar spin-spin coupling to protons from another nucleus’s spectrum. Other couplings, such as carbon–fluorine coupling in a proton-decoupled carbon-13 NMR spectrum, can remain.