2. Shown on the next page are the IR spectra of the four isomeric butyl chlorides. The spectra...


2. Shown on the next page are the IR spectra of the four isomeric butyl chlorides. The spectra are clearly

different, as are their structures. Focus on two of the absorption regions in the spectra: {eq}\sim 1380 \ cm-1{/eq} and

{eq}\sim 525 cm-1{/eq}. Notice the variability in intensity and appearance of the peaks in those two regions for the

isomers. Mark these regions on the spectra on the next page.

Spectra A

Spectra B

a) For the two IR spectra below, specify in the white space on the spectra, whether they contain an

n-butyl, sec-butyl, isobutyl, or tert-butyl group. Circle the peaks that justify your choices using the

spectra on the next page for comparison. Remember to compare the two important absorption regions

specified above.

b) Which of these two spectra is of an aromatic compound? Mark the peaks that justify your choice.

Put all your answers on this page.

Infrared Spectroscopy

Infrared spectroscopy (IR) involves the interaction of infrared radiation with matter. Functional groups will behave (vibrate, stretch, flex, wiggle, basically move around) at different ranges on the spectra based on the type of functional group. This can be used to identify and study chemical substances.

The width and location of the peak in an IR is indicative of what functional group caused it. Alcohol and carboxylic acid peaks are very broad verses carbonyl peaks which are very narrow and sharp. Substituted benzene rings have peaks that correspond to the substitution pattern (Mono, para, meta, etc) in the fingerprint and overtone regions of the IR.

Aromatic rings have very characteristic peaks that will appear based on the substitution patter of the ring. These peaks appear in the fingerprint region of the IR (900-670{eq}cm^{-1} {/eq}) and have a very strong to moderate intensity. In the overtone region (2000-1650{eq}cm^{-1} {/eq}), a pattern can also form that will further support the aromatic substitution pattern, but these peaks are very weak. Since carbonyl peaks also appear in this region, the overtone pattern might be drowned out by more intense peaks. See the table below for the key peaks to look for.

Answer and Explanation:


In spectra A, it has 2 peaks in from 1400-1350{eq}cm^{-1} {/eq} and one peak around 550 {eq}cm^{-1} {/eq}. This matches with the same peaks in...

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Infrared Spectroscopy & Molecule Identification
Infrared Spectroscopy & Molecule Identification

from AQA A-Level Chemistry: Practice and Study Guide

Chapter 2 / Lesson 13

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