A is 3-Chlorobutan-2-one, . Counting chlorine as one hydrogen in the index of index of hydrogen deficiency gives . A single carbonyl accounts for that unsaturation; the resonance at ppm identifies a ketone, while the ppm methine bears chlorine.
The carbon-13 NMR assignments are for , for , for and ppm for . 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.
The strong 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.
Write the proton-labelled structure as . The assignments areThe NMR quartet is centred at ppm; each adjacent line differs by ppm. The NMR doublet has the same spacing, ppm. At 300 MHz,Three equivalent methyl protons split into a NMR quartet; the single methine proton splits into a NMR doublet. No resolved long-range scalar coupling splits the acetyl methyl, giving its NMR singlet. The integrals total seven protons.
B is 2-Methylpropanoyl chloride, . 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 to the acyl chloride carbonyl, to the methine and ppm to both methyl groups. Their chemical equivalence in NMR follows from the symmetry of the isopropyl fragment in this achiral environment.
The strong band is an acyl chloride carbonyl stretching vibration. Compared with an ester, donation from a chlorine lone pair into the carbonyl antibonding molecular orbital is relatively ineffective, while chlorine's inductive effect withdraws electron density. The resulting stronger effective carbonyl force constant places the absorption above an ordinary saturated ketone band. Bond polarity alone is not a universal quantitative frequency rule.
The two methyl groups give one six-proton NMR doublet centred at ppm. The methine gives a one-proton NMR septet centred near ppm. The latter is split by six equivalent methyl protons, producing the first-order intensity pattern.This follows exactly from the methyl doublet spacing, , and approximately from the successive septet spacings, which alternate between and ppm within the precision of the printed labels. There is no methylene triplet or quartet: the six-proton integral and septet establish the branched skeleton.
Independent original carbon and proton assignments for B; both equivalent methyl groups couple to the methine
. Details. 
C is 2-Chloroethyl vinyl ether, . 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 ppm as a carbonyl would leave the unusually shielded unsaturated ppm carbon and the three distinct vinylic protons unexplained.
The 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.
Let be the internal vinylic proton, the terminal proton trans to , the terminal proton cis to , the oxygen-bound methylene and the chlorine-bound methylene. The assignments areFor the doublet of doublets, pairing corresponding lines gives Hz and Hz. The larger value identifies trans vinylic coupling; the smaller identifies cis vinylic coupling. The resonance confirms the large splitting, approximately Hz, and has a small geminal coupling to .
The small spacings are not perfectly reciprocal if treated as exact First-order NMR multiplets: the lines imply Hz, whereas the lines imply Hz and Hz. Thus report Hz, Hz and – Hz, rather than asserting incompatible exact constants. Printed precision and departures from a simple first-order pattern limit this line-picking estimate; a full spin-system fit would be needed for refined values. Splittings alone do not establish the signs of these couplings.
The two methylene groups split one another into approximate NMR triplets, with Hz; the oxygen-bound triplet gives approximately the same value from its rounded line labels. The three vinylic protons plus two methylenes give the required seven-proton integral. The terminal vinylic protons are diastereotopic, because replacing one or the other creates different alkene geometries.
Original carbon and separate proton assignments for C, including trans, cis, geminal and methylene coupling paths
. Details. 
The vinyl ether has an important resonance contributorDonation from oxygen into the conjugated system increases electron density at the terminal unsaturated carbon, helping explain its unusually shielded ppm and its terminal protons near 4.1–4.2 ppm. The directly oxygen-bound unsaturated carbon is strongly deshielded at ppm; its proton at ppm also experiences the oxygen substituent and alkene magnetic anisotropy. These trends are consistent with the whole delocalized electronic structure; a chemical shift is not simply a meter for an atom's partial atomic charge.
For the saturated part, the stronger local deshielding by oxygen places at , , while lies at and ppm. The two terminal vinylic protons have different geometrical relationships to the oxygen substituent and therefore different chemical shifts as well as different vicinal couplings.
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