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5: Infrared Spectroscopy

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    332805
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    Infrared Spectroscopy

    Infrared spectroscopy measures the absorption of energy that matches the vibrational frequency. The energies are affected by the strength of the bond and the masses of the atoms.

    Definitions and Introduction to Hooke’s Law

    Hooke's Law: \(\displaystyle \nu = \frac{1}{2\pi} \sqrt{\frac{k}{m}}\)

    Where k = force constant

    m= mass

    \(\nu\) = frequency

    • What happens to \(\nu\) if k is increased?
    • What happens to \(\nu\) if m is increased?

     

    Hooke’s Law applied to IR Spectroscopy*

    *Watch a video on IR and Hooke’s Law. [meta: LINK MISSING]

    • Circle the two true statements:
      1. The stronger the spring, the higher (faster) the frequency of bouncing.
      2. The weaker the spring, the higher (faster) the frequency of bouncing.
      3. The heavier the weight, the higher (faster) the frequency of bouncing.
      4. The lighter the weight, the higher (faster) the frequency of bouncing.
    • Order the bonds shown below in order of strength:

    clipboard_e6d7d2d791ca6a2a62c98bfe8c47296d9.png

    • If we assume bonds are like springs, explain the frequencies in this table:
    Bond type Alkane Alkene Alkyne
    Absorbance* 1300cm-1 1600cm-1 2200cm-1

    *IR absorbance is typically measured in wavenumbers (proportional to Hz); another system for measuring frequency.

    • Explain the trends in this table. Hint: consider atomic weights of halides.
    Bond type C-F C-Cl C-Br C-I
    Absorbance (cm-1) 1400-1000 783-540 650-510 600-485

    IR can therefore be a useful tool for determining what types of bonds (functional groups are present in a sample.

     

    Infrared Spectroscopy: Functional Group Determination*

    IR can be a useful tool for determining what types of bonds (functional groups are present in a sample.

    * All spectra are either from SDBS (Japan National Institute of Advanced Industrial Science and Technology) or simulated.

    Hydrocarbons

    The spectrum of octane is shown below.

    clipboard_ebbd84e8be8311ab6ca77a1bfced86579.png

    • List the different types of bonds present in this structure.
    • How many peaks would you expect to see?

    clipboard_e1f1a7b0a77e1610368b9239ec85f0b92.png

    • Based on Hooke’s Law which peak corresponds to C-H stretches? Which peak corresponds to C-C stretches?

     

    Hydrocarbons (cont.)

    The spectrum of octene is shown below.

    clipboard_e50b21163079a652bedefd3605cf20aba.png

    • List the different types of bonds present in this structure.
    • There are two different types of C-H stretches: sp3 C-H and sp2 C-H. Based on the location of the sp3 C-H in octane, which of these is the new sp2 C-H?

    clipboard_e1d83bdf46fdb26cd3f6a1a631461709c.png

    • There are two different types of C-C stretches. C-C and C=C. Based on the location of the sp3 C-H in octane, which of these is the new C=C?

    clipboard_ebc3093a087760146841fbb5f29c8e1c2.png

     

    Alkene Analysis (cont.)

    The spectra of two different alkenes are shown below.

    Octene

    clipboard_eb60ffc11e7cf7ce7eeaeb787298bd54d.png

    clipboard_ec7067d5a430335c88cbe9400f8645ce1.png

    1, 11-dodecadiene

    clipboard_e25b0b0b85c21110b8ce2e891d3c67aae.png

    clipboard_e71faa30405025ec3b20609c8ad1876d2.png

    • How do these two spectra differ?
    • Can you tell how many C-C bonds or C=C are in a structure from the IR spectrum?

     

    Alcohols

    1-Octanol

    clipboard_eee1ff494d20fdde9eeaef295a8714c7d.png

    • Find the sp3 C-H and the C-C stretches in the spectrum below.

    There are two new stretches: O-H and C-O.

    Most OH stretches show up between 3200-3400 cm-1.

    • The shape of this OH peak is very distinctive. Describe this shape.

    clipboard_e8310f13900e292d13f7c6f1523273e65.png

    The C-O stretches show up between 1000-1300 cm-1.

    • Circle or highlight this peak.

    clipboard_eab1a70b8727af28f7ebb8729ba844ebd.png

     

    Ethers

    Ethers are similar to alcohols. Here is the spectrum of butyl ether.

    clipboard_e84a8b17699aa8bea17d8416e5e3296a3.png

    Ethers have sp3 C-H stretches. C-C stretches and a C-O stretch.

    • Find these key peaks in the spectrum below.
    • They lack a __________ stretch that is present in an alcohol.

    clipboard_e6b8a47bbe03bb38e57bf554276edaf1d.png

     

    Amines

    Amines are similar to alcohols, also. Let’s consider the spectrum of 1-hexylamine

    clipboard_e800cd8f83b6b5888b97fd19bd44eb7ee.png

    Amines have sp3 C-H stretches. C-C stretches and a C-N stretch.

    • They also have an N-H stretch (or two) above 3200 cm-1.
    • Find these key peaks in the spectrum below.

    clipboard_ebc92a66a3597f35644ff213d2fd81774.png

    • The N-H stretch (or two) above 3200 cm-1 has a significantly different shape than the alcohol O-H stretch. Describe the difference.

     

    Carbonyls

    Aldehydes, such as 1-octanal, have sp3 C-H stretches and C-C stretches.

    clipboard_e71c04b6703dacff6f8d11e740f28e130.png

    All carbonyl functional groups have a distinctive C=O stretch that absorbs strongly around 1700 cm-1.

    • Find the sp3 C-H stretches and C-C stretches in the spectrum below.
    • Circle or label the C=O peak.

    clipboard_e58f35c41a8a2b2919100d6d1e9a18ed8.png

    Aldehydes also have an sp2 C-H stretch due to the aldehydic H. This peak usually has two bands around 2850 and 2750 cm−1.

    clipboard_e93da8f614089829024aa2a488e7f6db3.png

    • How would the spectrum of 2-octanone differ from 1-octanal?

     

    Similar Functional Groups

    • Label 1-2 relevant peaks that will help you distinguish these two functional groups.

    Esters vs Ketones

    clipboard_eeb904f4151c9fda844ebf2680207d7b9.png

    • Label 1-2 relevant peaks that will help you distinguish these two functional groups.

    Alcohol vs Carboxylic Acids

    clipboard_e6a047ef822b25c97d911af74e1135edc.png

     

    Summary: IR Key Ranges

    clipboard_ecd214eff5789a2a7bf81cd6b05852e75.png

     

    Summary: IR Key Ranges (cont)

    Functional Group Molecular Motion Wavenumber (cm-1)
    alkanes

    C-H stretch

    C-C stretch

    2950-2800

    1300

    alkenes =CH stretch 3100-3010
    C=C stretch (isolated) 1690-1630
    alkynes C=C-H stretch ~3300
    C,C triple bond stretch ~2150
    aromatics C-H stretch 3020-3000
    C=C stretch ~1600 & ~1475
    alcohols O-H stretch ~3650 or 3400-3300
    C-O stretch 1260-1000
    ethers C-O-C stretch (dialkyl) 1300-1000
    C-O-C stretch (diaryl) ~1250 & ~1120
    aldehydes C-H aldehyde stretch ~2850 & ~2750
    C=O stretch ~1725
    ketones C=O stretch ~1715
    C-C stretch 1300-1100
    carboxylic acids O-H stretch 3400-2400
    C=O stretch 1730-1700
    C-O stretch 1320-1210
    esters C=O stretch 1750-1735
    C-O-C stretch (acetates) 1260-1230
    C-O-C stretch (all others) 1210-1160
    amines N-H stretch (1 per N-H bond) 3500-3300
    C-N Stretch (alkyl) 1200-1025
    C-N Stretch (aryl) 1360-1250
    amides N-H stretch 3500-3180
    C=O stretch 1680-1630
    nitro -NO2 stretch

    1550-1490

    1355-1315

     

    Practice: IR Spectroscopy to Determine Functional Group

    • Identify the most likely functional groups in these spectra.

    clipboard_e5edbca42afe531444bab269846d1aff4.png

    clipboard_ecc1ddedcccca89dbfdf8efd9975e5fec.png

    clipboard_e227879b7dd083cdc206763fd5c64a08f.png

    clipboard_e64605279af0979bacfed3070cb0685de.png

    clipboard_ec2e1f21bc42bad56db23f566394fe68e.png

    clipboard_eb23e91206af66457d3a14396ae5ebfb1.png

    clipboard_ec25f458a0ea983c0fd3d7798a2de14b9.png

    clipboard_e788454c332fb4d5e18b4672729df7d1b.pngclipboard_ef9c7ccbb3aaa0a6a274da3fc7c8dfe15.png

     

    IR and Functional Group Determination

    Part 1. Match the following compounds with the corresponding spectra on the following pages.

    clipboard_ea1c79f1db23377a292bc4e2703c1c0f4.png

    Part 2. Label 2-3 relevant peaks for each IR spectrum with the correct bond type.

    clipboard_e98320c226ac3333e5c6c8afba918aeb8.png

    clipboard_eefe0a9ad77354c7e760138f79c626044.png

    clipboard_e1de091fc57bd7b89ea2d74945064d3cf.pngclipboard_e33d08b491d4e491e0d20e67b133f39b8.png

    clipboard_ee271df9dae5be26b7155733e20626648.png

    clipboard_efc4a19488d0f3806f50cb71ebdae8d10.png

    clipboard_e121102d575e2e08da2bd80c3fdfab3c6.png

     

    Infrared Spectroscopy: Analysis Beyond Stretching

    A molecule can vibrate in many ways, and each way is called a vibrational mode. Simple diatomic molecules have only one bond and only one vibrational band. If the molecule is symmetrical, the band is not observed in the IR spectrum. More complex molecules have more possible vibrations many peaks in their IR spectra.

    Some possible movements:

    clipboard_ea9a99cbed1eb98ea9bc2951342a85966.png

    So far, we have looked the primary stretching frequencies for polar bonds.

    There are some diagnostic bending and out-of-plane bending.

     

    Aromatic Substitution Patterns

    Here are typical out-of-plane (oop) bends for mono- and di-substituted aromatic compounds.

    clipboard_ef0510ac19364ae75764bf5d98af5d1c1.png

    • Determine the substitution pattern of the following compound based on oop bends.

    clipboard_e7cb82043b9f9d3c8d66849502f9450e6.png

     

    Aromatic Substitution Patterns (cont)

    • These are the spectra of the three isomers of dimethylbenzene (xylenes). Identify the ortho, meta, and para isomers.

    clipboard_e93d47911d20de345322c3e4b7ece4a56.png

    clipboard_e33011d32feba5e3587d017d1a5336fad.png

    clipboard_e056beb42490187e9feaacb1eb809aae4.png

     

    Alkene Substitution Patterns

    • Alkene molecules have C-H stretches above ____________ cm-1.
    • Alkene molecules have C-C stretches around ____________ cm-1.

    These two stretches are very typical for ALL alkene structures and do not provide any information about the substitution patterns but the out-of-plane (oop) bends can be diagnostic.

    Here are typical out-of-plane (oop) bends for cis, trans, and terminal alkenes.

    clipboard_ec57fc87048ecd5cce322aa6d15ffddae.png

    Tetrasubstituted alkenes have no hydrogens attached to the C=C moiety, and thus have no alkene C-H stretching or wagging peaks. They do have a C=C stretching peak, which appears from 1680 to 1660.

    • Is there a peak that would be diagnostic for a tetra-substituted alkene?

     

    Alkene Substitution Patterns

    • Determine the substitution pattern of the following isomers of hexenes based on oop bends. (trans-3-hexene, cis-3-hexene, and 1-hexene)

    clipboard_ecc1d3dc6df6003481e463e543e9fe0c5.png

    clipboard_e09bac8b8969c15933e82123ba1632e44.png

    clipboard_e67acd85a4dbd9a69b26419b1e6e3f4f7.png

     

    Amines

    N-H stretches fall in the same wavenumber range as O-H stretches, but that N-H stretches are easy to distinguish, because they will be less broad and less intense. This is due to N-H bonds being less polar than O-H bonds.

    • O-H and N-H stretches show up around ____________ cm-1.

    An amine does contain a C-N bond and there will be a C-N stretching peak as a result. However, C-N stretches are not very polar, so the peak intensities are not strong.

    • C-N stretches show up around ____________ cm-1.

    It is important to note that a primary amine has a scissors peak that appears from 1650 to 1580 cm-1. Sometimes, people will confuse this peak with an alkene.

    • In this spectrum of propylamine, identify the following peaks:
      • N-H stretch
      • C-N stretch
      • NH2 scissor

    clipboard_ec4a0cc1874489632d887254ccffa17fa.png

     

    Summary of Infrared Spectroscopy:

    • What information does IR provide?
    • What are the limitations of IR?
    • Briefly explain what information can be deduced from each of the IR regions below.

    clipboard_e76970343570ed16872d90a58593c8ebe.png

    • For each set of compounds shown below, explain how IR Spectroscopy could help you tell them apart.

    clipboard_e30488c6836930f63f41ee585c179684b.png

     

    Another IR Matching Problem

    Part 1. Identify the major functional group for the following compounds:

    clipboard_ed78bed3e6a2e8019c5284af95912893d.png

    Part 2. Label 2-3 relevant peaks for each IR spectrum with the correct bond type.

    Part 3. Match all of the IR spectra with the correct compound (from Part 1).

    clipboard_e2127305fe08a31db9e4ffad099bdafca.png

    clipboard_ef8b07ef0326af93b41d439c7dfa89dae.png

    clipboard_e84d4e48875d6efc7b00cbd69c009e6b1.png

    clipboard_e370b51d7c41f0e26debab49a1c450b6e.png

    clipboard_edf88f1229df1ccc6231671fa7fc6ac54.png

    clipboard_ef3299572e3692210f52cd1d85c54fab0.png

    clipboard_e91790bc6afbab311982c3c3c35d3de06.png


    This page titled 5: Infrared Spectroscopy is shared under a not declared license and was authored, remixed, and/or curated by Kate Graham.

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