Organic Chemistry 1 · Molecular evidence and structure
Combined spectra to structure: practice problems
34 problems, each with its worked solution. Open one to read every step, or filter by tag.
Predict the product 5
- IR bands of pentan-2-onePredict which IR absorptions an acyclic ketone shows and which it lacks.
- Count ¹H signals in 2,2-dimethylpropan-1-olPredict the number of ¹H NMR signals from sets of equivalent hydrogens.
- ¹H NMR pattern of 4-chlorotoluene's aromatic ringPredict the aromatic ¹H NMR pattern of a para-disubstituted benzene.
- C₅H₁₀O ketone with two signalsUse symmetry and splitting to choose among C₅H₁₀O ketone isomers.
- C₇H₈O with no O–HPlace the oxygen of an aromatic C₇H₈O isomer from IR and ¹H NMR.
Classify 2
- C₄H₈O₂ with a very broad O–H bandClassify a C₄H₈O₂ compound from the shape and position of its O–H and C=O bands.
- Classify a C₄H₁₁N amine from its N–H bandsClassify an amine as primary, secondary or tertiary from the number of N–H stretching bands.
Compare 5
- Which C₃H₈O alcohol shows a septet?Compare 1-propanol and 2-propanol and name the ¹H signal that decides between them.
- A C₄H₁₀O alcohol with two ¹³C signalsCompare ¹³C signal counts of three C₄H₁₀O alcohols to pick the most symmetric one.
- C₃H₆Cl₂ isomers: two ¹³C signals, triplet and quintetChoose among the four dichloropropanes using ¹³C signal count and ¹H splitting together.
- C₄H₆: but-1-yne or but-2-yne from IR and ¹H NMRDistinguish terminal and internal C₄H₆ alkynes from a diene with IR and ¹H NMR.
- C₄H₈O₂: acid or ester?Separate carboxylic acid and ester isomers of C₄H₈O₂ using IR and ¹H NMR.
Calculate 4
- C₅H₈O with a strained carbonylCompute degrees of unsaturation for C₅H₈O and use IR to decide how they are spent.
- C₇H₇Br with a 2H singletUse the degree-of-unsaturation count for C₇H₇Br to expect a benzene ring, then place the bromine from IR and ¹H NMR.
- Degrees of unsaturation in C₅H₉N with a 2250 cm⁻¹ bandCompute degrees of unsaturation for a nitrogen-containing formula and assign them with IR.
- Degrees of unsaturation in C₈H₈O₂Compute degrees of unsaturation for a formula containing oxygen.
Find the error 3
- A pentanone misassignedDiagnose why pentan-2-one cannot produce a spectrum of only a quartet and a triplet.
- An equal-height pair at m/z 136 and 138Diagnose a formula error by reading the M and M+2 isotope pattern of bromine.
- Dichloroethane from an m/z 98/100/102 clusterCount chlorines from an M/M+2/M+4 cluster and place them with ¹H NMR.
Interpret the evidence 11
- A sharp band at 2250 cm⁻¹Identify the functional group responsible for a sharp 2250 cm⁻¹ band in C₄H₇N.
- C₄H₈O₂: quartet, singlet, tripletAssemble an ester from a quartet at 4.12 ppm, a singlet at 2.04 ppm and a triplet at 1.26 ppm.
- C₃H₇Cl: doublet and septetRecognise an isopropyl group from a 6H doublet and a 1H septet.
- C₈H₁₀: an aromatic ring and an ethyl groupDistinguish ethylbenzene from the xylenes using integration and splitting.
- C₄H₁₀O with no O–H bandCombine formula, IR and ¹H NMR evidence to identify a symmetric ether.
- M⁺ 58, base peak 43, one singletCombine mass-spectral, IR and ¹H NMR evidence to identify a C₃H₆O isomer.
- C₃H₇Br: which bromopropane?Tell the two bromopropanes apart from ¹H NMR signal count and splitting.
- C₄H₉Cl with one 9H singlet: tert-butyl chlorideIdentify a C₄H₉Cl isomer whose nine hydrogens are all equivalent.
- C₅H₁₃N with no N–H band: N,N-diethylmethylamineAssemble a tertiary amine from the nitrogen rule, a missing N–H band and ¹H NMR splitting.
- C₈H₉Br full unknown: 1-bromo-4-ethylbenzeneCombine MS, degrees of unsaturation, ¹H and ¹³C NMR to place substituents on a benzene ring.
- An M+2 peak one third of MIdentify a halogen from the M : M+2 intensity ratio.
Work backward 1
- Matching spectra to 1,3,5-trimethylbenzene (mesitylene)Predict the ¹H, ¹³C and MS data of a highly symmetric arene and match them to a data set.
Plan a synthesis 1
- Telling o-, m- and p-dichlorobenzene apart by ¹³C NMRChoose the one measurement that separates the three dichlorobenzene isomers.
Explain why 2
- Why ethanol’s CH₂ appears at 3.7 ppmExplain the downfield shift of hydrogens on a carbon bonded to oxygen.
- Why a terminal alkyne C–H sits near 2 ppmExplain the upfield shift of a terminal alkyne proton with magnetic anisotropy.
Stuck on your own homework?
Snap it — first scan free, no sign-up
Scanning every week? Scan Standard: for about problems