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Chemistry LibreTexts

3.10: Additional Problems

  • Page ID
    194823
  • Exercise \(\PageIndex{1}\)

    For each of the following hand-drawn cartoons of IR spectra, identify the bond(s) that correspond to the indicated peaks.

    i.

    clipboard_e29a24b7efe23398a52c1eb45862460ef.png

    ii.

    clipboard_e91bfddf4e0f62cf662a50e56e1e19b5d.png

    iii.

    clipboard_eb25e694b22466ea0bd0f3eadf0ff6918.png

    iv.

    clipboard_ee9a5ffe5a3e6ba1e80d54bd1edc64869.png

    v.

    clipboard_e1578b81d59df5c062fa2a47d8b013be9.png

    vi.

    clipboard_e99a8ffbd4206182cf9d0eb08eed7a115.png

    vii.

    clipboard_ef21dcb32f485bf36c44af0dcabfaed67.png

    viii.

    clipboard_e7a56cbe3fe38c915e64f6c0687a51e71.png

    ix.

    clipboard_e098ba77cb146413edb5a768cf0128bdf.png

    x.

    clipboard_ebc60f5ef86de599a6aca1d21fb5ea6a9.png

    xi.

    clipboard_e927ebd439695423980ce6294f927a49e.png

    xii.

    clipboard_e27c3e5c25ea62ef32d197931fd0230e3.png

    xiii.

    clipboard_e85f601f6a229f9d7d1a0ab8ca502db37.png

    Answer

    i. a) O-H b) sp3 C-H c) C-O

    ii. a) sp2 C-H b) sp3 C-H c) C=C-H (oop bend)

    iii. a) N-H (two of them) b) C=O c) C-N

    iv. a) sp3 C-H b) H-C-H (CH2 bend)

    v. a) sp3 C-H b) C=O c) H-C-H (CH2 bend)

    vi. a) sp C-H b) sp3 C-H c) C=C

    vii. a) N-H b) sp3 C-H

    viii. a) C=O b) C-O

    ix. a) N-H (two of them) b) sp3 C-H

    x. a) sp3 C-H b) aldehyde C-H c) C=O

    xi. a) H-C-H (CH2 bend) b) C-O

    xii. a) O-H (very broad in CO2H) b) sp3 C-H c) C=O d) C-O

    xiii. a) sp2 C-H b) sp3 C-H c) aromatic overtones d) C=C

    Exercise \(\PageIndex{2}\)

    For each of the following hand-drawn cartoons of IR spectra, identify the functional group suggested by the spectrum.

    a)

    clipboard_e88c9d2456209ad3c3b50f8e882e8bb0a.png

    b)

    clipboard_e6672edb7548cd41c3c78ac05687ab500.png

    c)

    clipboard_efec70943c858961c180af9b5a0342504.png

    d)

    clipboard_e518b8bd04c52bc897bf1387cc1725b09.png

    e)

    clipboard_ec8ce64d530ded7dd56f14e330cc65809.png

    f)

    clipboard_e05df77b81e509094249c72d73d8dd497.png

    g)

    clipboard_e126cc694c2874237a983a47f11a8a1e8.png

    h)

    clipboard_eda86a9eaab8a3a4b1a58715fe42ae0a2.png

    i)

    clipboard_e27fa1270b8c3c51b0d929b12257eba67.png

    j)

    clipboard_e1a6a0ac4f576d3054466190a12ac9f5d.png

    k)

    clipboard_e791945bc56275deb4364d49b4b37239b.png

    Answer

    a) The rounded OH peak near 3300 cm-1 and the strong C-O peak near 1100 cm-1 suggest an alcohol.

    b) The sharp CH peaks below 3000 cm-1, the weak CH2 bending modes near 1500 cm-1 and the absence of any other features suggest an alkane.

    c) The strong C=O peak near 1700 cm-1 and the strong C-O peak near 1100 cm-1 suggest an ester.

    d) The strong C=O peak near 1700 cm-1 and the absence of additional features other than those associated with saturated hydrocarbons suggest a ketone.

    e) The CH peak above 3000 cm-1 and the strong oop bending modes below 1000 cm-1 suggest an alkene. The presence of two oop bending peaks may point to a terminal alkene (C=CH2).

    f) The sharp N-H peaks near 3200 cm-1 and the strong C=O peak near 1600 cm-1 suggest an amide. the presence of two N-H peaks rather than one points to a primary amide (O=C-NH2).

    g) The small, triangular "sharktooth" peak near 3200 cm-1 suggests an amine. With just one N-H peak, this is probably a primary amine (R-NH2).

    h) The strong C-O peak near 1100 cm-1 suggests an ether.

    i) The C-H peaks above 3000 cm-1 and the oop bending modes below 1000 cm-1 certainly suggest double bonds. The progression of tooth-like "aromatic overtones" between 1600 and 2000 cm-1 strongly indicates a substituted benzene.

    j) The small, triangular "sharktooth" peaks near 3200 cm-1 suggest an amine. With two N-H peaks, this is probably a secondary amine (R2NH).

    l) The broad, deep OH peak between 3300 cm-1 and 2600 cm-1 and the C=O peak near 1700 cm-1 suggests a carboxylic acid. The O-H peak of a carboxylic acid is often missed; it is moved to lower frequency by hydrogen bonding.

    Exercise \(\PageIndex{3}\)

    a) Identify at least three important peaks/bonds in each of the following IR spectra.

    b) Identify the functional group present in each of these samples. See the Functional Group Section

    for help.

    c) Draw a possible structure for each of these compounds (there may be many, many correct answers).

    problem i)

    clipboard_ea984bf1f58377694c53d186fb367ab19.png

    problem ii)

    clipboard_e6fd461e1710c288cfe4d2c99cf2d141c.png

    problem iii)

    clipboard_ea4b385f48dcc37b4d453dbf8ab5c4a6d.png

    problem iv)

    clipboard_e6c7384e3e598edd84fc278d855db2446.png

    problem v)

    clipboard_e38334e4971ce27b0ee25e12ee53da429.png

    Answer

    i)

    a)

    3200 cm-1 (sharp): N-H

    2900 cm-1 (sharp): Csp3-H

    1600 cm-1 (broad): probably NH2 bending

    b) A primary amine.

    c)

    ii)

    a)

    3400 cm-1 (strong, broad): O-H

    2900 cm-1 (sharp): Csp3-H

    1050 cm-1 (strong): probably C-O

    b) Alcohol.

    c)

    iii)

    a)

    2900 cm-1 (sharp): Csp3-H

    1700 cm-1 (strong): C=O

    1200 and 1000 cm-1 (strong & medium): C-O

    b) Ester

    c)

    iv)

    a)

    3050 cm-1 (sharp): Csp2-H

    1700 cm-1 (strong): C=O

    1200 and 1000 cm-1 (strong & medium): C-O

    b) Ester; probably contains an aromatic as well

    c)

    v)

    a)

    2750 and 2650 cm-1 (medium, sharp): C-H of an aldehyde

    1700 cm-1 (strong): C=O

    Below 1000 cm-1 (strong & medium): oop bends

    b) Aldehyde; probably contains an aromatic as well

    Exercise \(\PageIndex{4}\)

    Sketch an approximate IR spectrum for each of the following compounds:

    clipboard_e782416e0d284b127ff26e920943427d3.png

    Exercise \(\PageIndex{5}\)

    The oop bends are sometimes useful in distinguishing substitution pattrens around a benzene ring. Using the spectra of o-, m-, and p-xylene, formulate some guidelines about what the oop bends look like when substituents are one, two or three carbons away on a benzene ring.

    clipboard_ea1f2605e7b48c81835ac78f5a8090dec.png

    clipboard_ef981c25c859dfd3d372a05f9d5350b28.png

    Figure \(\PageIndex{1}\): IR spectrum of m-xylene.

    Source: SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology of Japan, 14 July 2008)

    clipboard_eec569787f4d28a0d994986b031bfd462.png

    Figure \(\PageIndex{2}\): IR spectrum of o-xylene.

    Source: SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology of Japan, 14 July 2008)

    clipboard_e46d91f49224c22023a5aeeeac4ed3816.png

    Figure \(\PageIndex{3}\): IR spectrum of p-xylene.

    Source: SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology of Japan, 14 July 2008