In each of the sets that follow, match the proper compound with the appropriate

Question

In each of the sets that follow, match the proper compound with the appropriate set of IR peaks and give the rationale for your assignment. dodecane, 1 -decene, 1-hexyne, 1, 2-dimethylbenzene 3311(s), 296 l(s), 2119(m) cm^-1 3020(s), 2940(s), 1606(s), 1495(s), 741(s) cm^-1 3049(w), 2951(m), 1642(m) cm^-1 2924 (s), 1467(m) cm^-1 phenol, benzyl alcohol, methoxybenzene 3060(m), 2835(m), 1498(s), 1040(s) cm^-1 3370(s), 3045(m), 1595(s), 3330(br, s), 3030(m), 2950(m), 1454(m), 1223(s) cm^-1 2-pentanone, acetophenone, 2-phenylpropanal, heptanoic acid, 2-methylpropanamide, phenyl acetate, 1-aminooctane 3070(m), 2978(m), 2825(s), 2720(m), 1724(s)cm^-1 3372(m), 3290(m), 2925(s) cm^-1 3070(w), 1765(s), 1215(s), 1193(s) cm^-1

Explanation / Answer

Ans a)

Dodecane: 2924(s), 1467(m) cm-1

Dodecane IR spectrum will show peaks around 2924cm-1(s) due to C-H stretching vibration of alkane (sp3 carbon). Peak around 1467cm-1 can be attributed to sp3 C-H bending vibrations.

1-decene: 3049(w), 2951(m), 1642(m) cm-1

1-decene IR spectrum will show peaks around 3049cm-1 due to sp2 C-H stretching vibration of alkene. Peak around 2951 cm-1 is due to sp3 C-H stretching and peak around 1642cm-1 is due to C=C stretching of alkene.

1-hexyne: 3311(s), 2961(s), 2119(m) cm-1

1-hexyne IR spectrum will show peaks around 3311cm-1 due to presence of alkyne sp hybridized C-H stretching.

2961cm-1 absorption will be due to sp3 C-H stretching. Absorption around 2119 cm-1 will be due to alkyne C=C

stretching vibrations.

1,2-dimethyl benzene : 3020 (s), 2940(s), 1606(s), 1495(s), 741(s) cm-1

IR spectrum of 1,2-dimethyl benzene will show peak around 3020 cm-1 due to presence of sp2 C-H stretching of aromatic ring. 2940 cm-1 absorption will be due to sp3 C-H stretching of methyl protons. Peak around 1606cm-1 and 1495cm-1 will be due to C=C stretching. Peak around 741cm-1 will be due to 1,2-disubstituted benzene ring.

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And b)

Phenol: 3370(s), 3045(m), 1595 (s), 1224(s) cm-1

Phenolic OH gives peak around 3370cm-1 due to O-H stretching. Aromatic C-H stretching gives peak around 3045cm-1. Peak around 1595cm-1 is due to C=C stretching. 1224cm-1 peak is due to C-O stretching vibrations.

benzyl alcohol: 3330(br,s), 3030(m), 2950(m), 1454(m), 1223(s) cm-1

Benzylic OH group gives broad peak around 3330cm-1. Aromatic C-H stretching gives peak around 3030cm-1. Aliphatic sp3 C-H stretching gives peak around 2950 cm-1. Peak around 1454cm-1 is due to C-H bending. 1223cm-1 peak is due to C-O stretching vibrations.

methoxybenzene: 3060(m), 2835(m), 1498(s), 1247(s), 1040(s) cm-1

Aromatic C-H stretching gives signal around 3060 cm-1. C-H stretching of methoxy group gives peak around 2835cm-1. C=C stretching gives signal around 1498 cm-1. 1247cm-1 and 1040cm-1 peaks are due to C-O stretching.

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Ans c)

1-aminooctane : 3372(m), 3290 (m), 2925 (s) cm-1

Primary amine gives two peaks at 3372 cm-1 and 3290cm-1. sp3 C-H stretching gives signal around 2925cm-1.

2-phenylpropanal : 3070(m), 2978(m), 2825(s), 2720(m), 1724(s) cm-1

Aromatic C-H stretching gives peak around 3070cm-1. sp3 C-H gives peak around 2978cm-1. Aldehyde C-H gives signal around 2825cm-1 and 2720cm-1. Aldehyde C=O gives signal around 1724cm-1.

phenylacetate : 3070(w), 1765(s), 1215(s), 1193(s) cm-1

Aromatic C-H stretching gives peak around 3070cm-1. Ester C=O gives signal around 1765cm-1. C-O stretching gives signal around 1215cm-1 and 1193cm-1

  

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