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Chapter 12: Q.12P (page 635)

Predict the characteristic infrared absorptions of the functional groups in the following molecules.

(a) cyclopentene

(b) hexan-2-ol

(c) hexan-2-one

(d) hex-1-yne

(e) dimethylamine

(f) hexanoic acid

(g) hexanenitrile

(h) methyl acetate

(i) hexanamide.

Short Answer

Expert verified

Answer

a. Cyclopentene has only C-H stretch due to double bond which shows IR absorption at around 3000-2800 cm-1.

b. In Hexan-2-ol, alcohol is present. Hence, it shows IR absorption at around 3700-3500 cm-1.

c. In Hexan-2-one, aliphatic ketone is present. Hence, it shows IR absorption at around 1725-1705 cm-1.

d. Hex-1-yne shows presence of triple bond and therefore, it shows a weak band at 2100cm-1.

e. Dimethylamine has amine group and it shows IR absorption at around 3500-3300 cm-1 due to N-H stretching.

f. Hexanoiccid has carboxylic acid group and hence, it shows IR absorption band at 1720cm-1. Due to the presence of 鈥揙H and CH, streching peaks at 2900cm-1 are observed.

g. Hexanenitrile has 鈥揅N group and hence, it shows absorption at 2200-2300cm-1.

h. Methylacetate has ester group and it shows absorption at 1750-1735cm-1.

i. Hexanamide has amide group and it shows absorption at 1690-1730cm-1.

Step by step solution

01

 IR spectrum characteristic peaks

The IR spectrum is highly helpful in predicting the functional group present in the given organic molecule. Every functional group has its own characheristic IR frequency.

02

 Explanation

a) Cyclopentene only has C-H stretch due to double bond which shows IR absorption at around 3000-2800 cm-1.

b) In Hexan-2-ol, alcohol is present. Hence, it shows IR absorption at around 3700-3500 cm-1.

c) In Hexan-2-one, aliphatic ketone is present and hence, it shows IR absorption at around 1725-1705 cm-1.

d) Hex-1-yne shows presence of triple bonds and therefore, it shows weak band at 2100 cm-1.

e) Dimethylamine has amine group and shows IR absorption at around 3500-3300 cm-1 due to N-H stretching.

f) Hexanoiccid has carboxylic acid group ahd hence, it shows IR absorption band at 1720cm-1. Due to the presence of 鈥揙H and CH streching peaks at 2900 cm-1 is observed.

g) Hexanenitrile has 鈥揅N group and hence, it shows absorption at 2200-2300cm-1.

h) Methylacetate has ester group and it shows absorption at 1750-1735cm-1.

i)Hexanamide has amide group and it shows absorption at 1690-1730cm-1.

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Most popular questions from this chapter

Question: (Another true story.) A student who was checking into her lab desk found an unlabeled sample from a previous student. She was asked to identify the sample. She did an IR spectrum and declared, 鈥淚t looks like an alkane.鈥� But it seemed too reactive to be an alkane, so she did a GC鈥揗S. The mass spectrum is shown next. Identify the compound as far as you can, and state what part of your identification is uncertain. Propose fragments corresponding to the numbered peaks.

Question: (A true story.) While organizing the undergraduate stockroom, a new chemistry professor found a half-gallon jug containing a cloudy liquid (bp 100鈥�105 掳C), marked only 鈥淪TUDENT PREP.鈥� She ran a quick mass spectrum, which is printed below. As soon as she saw the spectrum (without even checking the actual mass numbers), she said, 鈥淚 know what it is.鈥�

(a) What compound is the 鈥渟tudent prep鈥�? Any uncertainty in the structure?

(b) Suggest structures for the fragments at 136, 107, and 93. Why is the base peak (at m/z 57) so strong?

A laboratory student added 1-bromobutane to a flask containing dry ether and magnesium turnings. An exothermic reaction resulted, and the ether boiled vigorously for several minutes. Then she added acetone to the reaction mixture and the ether boiled even more vigorously. She added dilute acid to the mixture and separated the layers. She evaporated the ether layer, and distilled a liquid that boiled at 143C鈭�. role="math" localid="1649504068494" GC-MSanalysis of the distillate showed one major product with a few minor impurities. The mass spectrum of the major product is shown here.

(a) Draw out the reactions that took place and show the product that was formed.

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Account for the peaks at m/z 87, 111, and 126 in the mass spectrum of 2,6-dimethylheptan-4-ol.

Question: A common lab experiment is the dehydration of cyclopentanol tocyclopentene.

a. Explain how you could tell from the IR spectrum whether your product was pure cyclopentene, pure cyclopentanol or a mixture of cyclopentene and cyclopentanol. Give approximate frequencies for distinctive peaks.

b. Explain why mass spectrometry might not be a good way to distinguish cyclopentene from cyclopentanol.
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