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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.
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.
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 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.
For the ordinary ester, the carbon nucleophile of a grignard reagent attacks the carbonyl carbon and moves the C=O pi pair onto oxygen. Collapse of the tetrahedral intermediate expels ethoxide, transiently giving . This ketone is more electrophilic than the starting ester because it lacks an alkoxy group's stabilizing resonance donation. A second equivalent of the grignard reagent therefore adds to it before isolation:The negatively charged intermediates are associated with magnesium counterions. Merely reducing the amount of reagent to one equivalent does not generally trap the ketone selectively; it can leave unreacted ester together with the double-addition product.
In the first successful preparation, magnesium insertion into the primary C–Br bond gives n-butylmagnesium bromide. The carbon chain has four carbons, so its acetylation produces hexan-2-one, , not pentan-2-one. Attack on acetic anhydride forms . Coordination of the anhydride-derived oxygens to magnesium can stabilize this tetrahedral addition complex. Under suitably cold, controlled-addition conditions it survives sufficiently to delay release of the free ketone until aqueous work-up has destroyed the remaining grignard reagent. Protonation followed by collapse releases acetate/acetic acid and the ketone.
This interpretation has direct experimental precedent: Melvin S. Newman and William T. Booth Jr. obtained methyl ketones by cold addition of grignard reagents to acetic anhydride, attributing selectivity to a persistent addition complex. The temperature and reagent-addition regime are genuine conditions on the explanation, rather than proof that an anhydride can never undergo overaddition; the exam scheme omits them.[ref]
The Preparation of Ketones from Grignard Reagents by Melvin S. Newman and William T. Booth Jr.. 1945, printed p. 154: proposed intermediate I and low-temperature dependence
For the benzonitrile route, n-butyl attacks the nitrile carbon and one C≡N pi pair moves to nitrogen, producing the magnesium ketimine salt . This anionic nitrogen-containing intermediate resists a second nucleophilic addition much more than a neutral carbonyl does. The ketone appears only during acidic aqueous work-up, after organomagnesium has been quenched. Protonation gives the iminium ion; water attacks its carbon, proton transfers give a carbinolamine, and C–N cleavage from its nitrogen-protonated form produces the protonated ketone and ammonia, followed by deprotonation. In the acidic mixture ammonia is present largely as ammonium. This sequence explains why nitrile addition stops at a ketone without requiring the ketone itself to be resistant to grignard reagents.
The atom-labelled hydrolysis sequence isWater's lone pair attacks the iminium carbon while its C=N pi pair moves to nitrogen; the carbinolamine oxygen then reforms C=O while the C–N pair moves to the departing protonated amino group. Final deprotonation gives the neutral ketone. The arrows involving protonation and hydrolysis also exchange protons with the acidic aqueous reservoir; the condensed sequence tracks the carbon–nitrogen and carbon–oxygen transformations rather than displaying every solvent molecule.
The oxygen substituent changes position through a halohydrin–epoxide sequence. First, hydroxide attacks the acetate carbonyl of , giving a tetrahedral intermediate. Collapse expels the substrate alkoxide, and proton transfer gives plus acetate. This is saponification, breaking the acyl–oxygen bond rather than replacing bromide directly with hydroxide.
A small equilibrium population of undergoes intramolecular SN2 reaction at the adjacent primary carbon: the O lone pair attacks and its C–Br pair leaves with bromide. The result is 2,2-Dimethyloxirane. Addition of HBr protonates the ring oxygen. bromide then attacks preferentially at the more substituted carbon, where the protonated ring has greater carbocation character, breaking that C–O bond and leaving oxygen attached to the former primary carbon:The middle formula denotes a three-membered epoxide, not an open-chain ether. Acidic opening of this highly unsymmetrical epoxide has the opposite regioselectivity to a simple basic nucleophilic opening at its less substituted carbon.
Finally, the primary alcohol oxygen attacks acetyl chloride. The tetrahedral intermediate collapses with chloride departure, and triethylamine removes the O–H proton and captures HCl. The product is . oxygen is carried through the epoxide to the other carbon; it is not necessary to invent a methyl migration or a carbon-skeleton rearrangement.
Label the acetyl methyl , the methylene , and the terminal methyl in . The assignments areThe NMR multiplet centre of the NMR quartet is the mean of its outer lines, ppm. Its adjacent lines differ by 0.024 ppm; the NMR triplet has the same spacing. ThereforeThe two groups, through vicinal coupling, split one another into the expected NMR quartet and NMR triplet. The acetyl methyl has no resolved neighbouring proton across its adjacent carbonyl and appears as a NMR singlet. The integrals sum to eight protons.
A is pentane-2,3-dione, . Its index of hydrogen deficiency is , accounted for by two ketone carbonyls. The two resonances near 200 ppm, together with the three saturated carbon signals and the proton integrals, identify an unsymmetrical adjacent diketone rather than an ester or an alkene-containing structure.
The assignments are the carbonyl-adjacent methyl at , the methylene at , and the terminal ethyl methyl at ppm. The two carbonyl carbons give and ppm. Their individual ordering is not uniquely established by the supplied one-dimensional spectra; either ordering between the methyl-side and ethyl-side carbonyls should be kept tentative without a correlation or another independent assignment. In the displayed attached proton test phase convention, the two methyls have one sign, while the methylene and the nonprotonated carbonyls have the opposite sign. The solvent signal near 77 ppm is not a sixth carbon of A.
Original structural assignment of the five carbon environments in A; the two carbonyl shifts are an unordered pair
. Details. The large vinyl couplings are 17.1 Hz trans and 10.5 Hz cis; the O-methylene-to-vinyl coupling is 6.0 Hz. At this field, convert every separation by . Comparing the centres of corresponding terminal-proton submultiplets givesThe repeated 0.005-ppm fine spacing from allylic scalar coupling and geminal coupling gives the magnitudesFor example, splitting by a geminal proton and two equivalent long-range-coupled methylene protons, all with the same magnitude, produces overlapping lines with intensities . The 6-Hz splitting of creates two such small NMR triplets centred at 4.580 and 4.560 ppm. An independent check comes from the internal-vinyl multiplet: a centre at 5.919 ppm with splittings 17.1, 10.5 and 6.0 Hz, the last to two equivalent protons, reproduces all twelve labelled line positions. The spectrum determines these coupling magnitudes, not their algebraic signs.
Use for the acetyl methyl, for the O-bound methylene group pair, for the internal vinyl proton, and for the terminal vinyl protons respectively trans and cis to . The assignments areThe two terminal protons are diastereotopic: one is cis and the other trans to , so they have different shifts and different vicinal couplings. Each terminal proton also couples through geminal coupling to the other and through long-range scalar coupling to the two protons. When those small couplings coincide, an underlying doublet of doublets of NMR triplets collapses to the observed NMR doublet of apparent NMR quartets. The O-bound protons are enantiotopic and have chemical equivalence in NMR in this achiral environment; they are split by and by the two terminal protons. The integrated terminal envelope contains two protons, not two equivalent protons at one shift.
The mechanism is nucleophilic addition followed by elimination of methoxide. An ethoxide oxygen lone pair attacks the carbonyl carbon, while the C=O pi pair moves to oxygen:The tetrahedral intermediate collapses when its anionic oxygen reforms the carbonyl and the C–OMe bond breaks with its electron pair going to the methoxy oxygen:lithium is a coordinating counterion; it does not supply an electron pair to the acyl carbon. proton exchange with ethanol connects the methoxide produced to the ethoxide catalyst pool and gives methanol, so the net solvent-mediated exchange is methyl acetate plus ethanol to ethyl acetate plus methanol. The reversibility of the elementary acyl steps is important: changing an alkoxy group is not automatically an irreversible reaction.
ethanol undergoes proton transfer with the methoxide produced. The oxygen lone pair of methoxide attacks the hydroxyl proton of ethanol, while the ethanol O–H bond pair returns to its oxygen:The alcohol acidities are similar, so this is an equilibrium, not an overwhelmingly one-way deprotonation determined solely by intrinsic acidity. The large ethanol reservoir and initially small methanol concentration nevertheless matter. This exchange means that lithium ethoxide need not be consumed stoichiometrically in the net transesterification. Neutral ethanol can also participate in proton shuttling; it is not acting as a reducing reagent or as a substrate for elimination under these conditions.
The third electron must enter the higher, degenerate and partly antibonding orbital pair rather than the already filled fully bonding orbital. At an equilateral geometry the configuration becomes . This weakens the collective bonding and creates orbital degeneracy, so a distortion can lower the singly occupied component and localize bonding. Along the ground-state reaction coordinate, formation of a normal two-electron H–H bond with a separated hydrogen atom becomes favourable.
The result is ground-state dissociation to , not loss of a proton from the cation or formation of three isolated atoms. One should not claim that adding one electron exactly cancels two bonding electrons: a simple three-orbital count alone does not predict a zero bond order or prove dissociation. The important qualitative changes are occupancy of an antibonding/degenerate level and access to a lower-energy dimer-plus-atom arrangement. A complete dissociation statement requires the potential-energy surface, including nuclear repulsion; electronically excited neutral H states are a separate issue.
The first product is , with the original alkene geometry retained. methoxide attacks the primary saturated carbon bearing bromine from the backside. Its oxygen lone pair forms the new C–O bond as the C–Br bonding pair leaves with bromide. The other bromine is vinylic, directly bonded to an sp carbon: ordinary SN2 reaction is unfavourable there, and ionization would require an unstable vinyl cation. Thus the primary alkyl bromide is replaced selectively, rather than replacing the vinylic halide or moving the double bond. There is no observable inversion label at the attacked carbon because it is a methylene.
The second product is sodium 4-propoxybutane-1-sulfonate, . sodium hydride first removes the alcohol proton, giving propoxide and hydrogen. The propoxide oxygen then attacks the primary ring carbon bonded to the ring oxygen; simultaneous cleavage of that carbon–oxygen bond opens the sultone. The departing ring oxygen remains attached to sulfur as part of the resonance-stabilized sulfonate anion. The carbon attached directly to sulfur is not the substitution site, because cleavage there would not expel the same stable sulfonate leaving group. This is carbon-centred ring opening of a cyclic sulfonate ester, not addition across an S=O bond.
Aqueous base selectively converts the methyl ester to a carboxylate, and brief acidification gives the corresponding monoacid while retaining the tert-Butyl ester. At the methyl-ester end, a hydroxide lone pair attacks the acyl carbon and the carbonyl pi pair moves to oxygen. The tetrahedral intermediate collapses as the oxygen anion reforms C=O and the acyl C–OMe bond breaks to methoxide. proton transfer then gives the carboxylate and methanol. carboxylate formation makes the net saponification effectively irreversible under the basic conditions; protonation protonates that carboxylate.
The tert-Butyl ester reacts more slowly under these conditions because its bulky alkoxy group hinders acyl attack, and tert-butoxide is a stronger base and poorer leaving group than methoxide. The selectivity is kinetic, not an assertion that tert-butyl esters can never undergo base hydrolysis. The work-up should be sufficiently mild and brief to avoid the separate acid-labile pathway.
Anhydrous trifluoroacetic acid instead removes the tert-butyl group, leaving the methyl ester. protonation activates the ester; in a useful reactive protonation form the alkoxy oxygen bears the proton. The O–C(tert-butyl) bond then cleaves with its electron pair going to oxygen, producing the carboxylic acid and a tert-butyl-cation-like fragment. Loss of a methyl proton gives 2-methylpropene and regenerates the acid catalyst. Carbonyl-oxygen protonation and charge delocalization can precede cleavage; drawing alkoxy-oxygen protonation is a transparent way of tracking the bonds broken. The key distinction is alkyl–oxygen cleavage here versus acyl–oxygen cleavage in the base reaction. Methyl-cation formation is strongly unfavourable, and without water ordinary acid-catalysed methyl-ester hydrolysis is not available. No water is required to account for the tert-butyl deprotection or its alkene by-product.
Build the phenethyl grignard reagent, use one portion to make a phenethyl-substituted ester, and add two further equivalents to that ester. Write ; the target is , so all three carbon substituents must be phenethyl, not benzyl.
hydroboration followed by oxidation of styrene gives : use in a suitable ether, followed by alkaline hydrogen peroxide. The Anti-Markovnikov addition regiochemistry is essential; acid-catalysed Markovnikov addition of water would give the wrong alcohol. Convert the primary alcohol to with phosphorus tribromide, then form with magnesium in dry ether. The organomagnesium reagent must be prepared and used without a protic solvent.
carboxylate one portion with carbon dioxide and acidify to obtain , then esterify with ethanol under acid catalysis to give . Treat the isolated ester with at least two equivalents of another portion of and then perform an aqueous acidic work-up:The first addition to the ester gives a ketone after ethoxide elimination; the second addition gives the tertiary alkoxide. Work-up supplies its OH proton. The extra central carbon comes from carbon dioxide, and each original styrene contributes one intact eight-carbon phenethyl unit. The resulting carbon count is , matching the target skeleton.
The assignments are : ppm, H, doublet of quartets; : ppm, H, doublet of quartets; : ppm, H, quartet; : ppm, H, triplet. The chemical shift of a multiplet is its centre, rather than its most downfield line.
Each vinylic proton has scalar spin-spin coupling to the other vinylic proton and to three equivalent spin- fluorine nuclei. The first interaction produces a doublet, and the second a quartet: eight lines overall, partly interleaved for . The two chemical shifts differ because the vinylic sites have different substituents.
One fluorine-19 NMR resonance, integrating to three equivalent fluorine nuclei, appears as a doublet of doublets with and . This predicts four principal lines of equal intensity, from coupling to and , respectively. Relative to the unknown resonance centre, their offsets are and .
Equivalent fluorine nuclei do not split one another. This prediction assumes ordinary proton-coupled acquisition; proton decoupling would collapse these four principal lines to a singlet. The proton and carbon spectra do not determine the numerical fluorine chemical shift. Weak natural-abundance carbon-13 satellites could accompany the principal pattern.
For a hydrogen-like ion, the Coulomb potential is stronger by the nuclear atomic number , and the energy levels scale as :Here is the number of protons in the atomic nucleus; the ion has exactly one electron, so its net electric charge is . The zero of energy is a separated stationary electron and atomic nucleus at infinite distance. This is the nonrelativistic hydrogenic approximation using the supplied ; a more precise isotope-specific result also corrects the reduced mass.
The products are hexanenitrile, , and ethyl triphenylmethyl ether, . The nitrile carbon comes from cyanide, so replacing chloride in the five-carbon chain gives a six-carbon product.
For the primary chloride, the carbon-end lone pair of attacks the carbon carrying Cl from the back side while the C–Cl bond pair moves onto Cl. The single concerted nucleophilic substitution step forms the C–C bond and releases . Attack through the carbon end gives the nitrile, not an isocyanide.
