Lab Activity 1: IR Spectroscopy

Organic chemists use the following methods to help them characterize (identify) compounds: melting point, chemical derivative, infrared (IR) spectroscopy, ultraviolet-visible (UV-VIS) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and mass spectrometry (MS).

The last four techniques require a small amount of sample and a spectrum is quickly and easily obtained. IR and UV-VIS spectrometers are relatively inexpensive. A typical NMR spectrometer is very expensive and usually beyond the budget of most community colleges; however, a low cost desktop NMR spectrometer recently came to market and we will use one in Lab Activity 7. IR, UV-VIS, and NMR are non-destructive whereas MS is destructive to compounds. Each technique by itself may not give enough information to determine the structure of a substance. Used in conjunction with each other, these characterization techniques are a very powerful way to characterize a compound. Bonding and Structure and Characterization by Infrared (IR) Spectroscopy. An IR spectrum tells you the bond types in a molecule. The vibrational motions of a molecule are studied using IR spectroscopy. IR can be understood by thinking of a molecule as atoms connected (bonded) together with springs. (See your molecular model kit.) If you stretch the spring by pulling on one atom and then letting go, the spring stretches (contracts and expands) like a bobble-head doll. This is a stretching mode of that bond. The amount of energy required to stretch the spring (bond) depends on the strength of the spring. Single bonds (weak springs) require less energy that double bonds (strong springs) to stretch. For example, IR shows a carbon-carbon double bond stretch at approximately 1650 cm-1 (cm-1 is the unit for wavenumbers; a wavenumber is equal to 1/wavelength) and a carbon-carbon triple bond stretch at 2200 cm-1. Also, the mass of the atom connected to the spring (bond) determines the amount of energy required to stretch it. The heavier the atom, the more energy is needed to stretch the bond. A carbon-hydrogen bond stretches at 2900 cm-1; an oxygen-hydrogen bond stretches at 3500 cm-1. IR is very useful in identifying functional groups in organic molecules. For example, an alcohol contains a C-O-H group. An IR spectrum of an alcohol will show the C-O stretch at approximately 1100 cm-1 and the O-H stretch at approximately 3500 cm-1. An IR spectrum plots % transmittance on the y-axis and energy in wavenumbers on the x-axis. A wavenumber is the traditional unit used in IR spectroscopy and is the reciprocal of wavelength. Note the units of wavenumbers are cm-1. Molecules not only stretch but also bend and wag. Each of these vibrational modes is detected by IR.

What causes a molecule to vibrate? As you know from your study of the atom in Chem 1A, when a photon of light is absorbed by an atom, an electron undergoes a transition from a lower energy state, usually the ground state, to a higher energy state (excited state). An electron in an excited state does not stay there very long (unstable) and will undergo another transition from its excited state to a lower energy state. When a substance absorbs IR radiation (heat), an electron undergoes a transition between vibrational energy states and the molecule vibrates (stretches, bends, wiggles). Materials Molecular Models Bruker FT-IR CH3OH, C2H5OH, acetone, hexane, cyclohexene, toluene 1. a. Most organic compounds are _____ (ionic/molecular) compounds. b. What chemical force exists between atoms in an organic compound? 2. a. See the picture of a spectrophotometer in Figure 1.

Figure 1. Spectrophotometer components (http://www.di.uq.edu.au/sparqspectro). The general method to measure an absorption spectrum is:

(i) Measure the background spectrum. Why is a background spectrum needed? (ii) Add your sample and measure a spectrum. What substances are absorbing light in this spectrum? (iii) How is running an absorption spectrum similar to measuring the mass of a solid? b. In an IR spectrum (see Figure 1 below), % transmittance (% IR wavelength that is transmitted through the sample, i.e., the IR wavelengths that are not absorbed by the sample) is plotted on the y-axis and wavenumber is plotted on the x-axis. A wavenumber equals 1/wavelength. E = hυ (E = energy in J, h = Planck’s constant, and υ = frequency in 1/sec) and υ = 1/λ (λ = wavelength in m). As wavenumbers increase, does E increase or decrease? 3. When a substance is exposed to IR radiation, the bond that connects two atoms vibrates, i.e., it stretches or bends. a. (i) Using Molecular Models, represent a carbon-carbon single bond by connecting one spring between two balls. (ii) Using Molecular Models, represent a carbon-carbon double bond by connecting two springs between two balls. With one hand on each ball, stretch each bond by pulling gently. Which bond requires more energy to stretch? b. Is a C-C bond stronger, weaker, or the same strength as a C=C bond? In other words, does it take the same amount, more, or less energy to break a C-C bond compared to a C=C bond? c. Is a C-C bond stronger, weaker, or the same strength as a C-O bond? d. Will you observe a C-C bond peak at a higher or lower wavenumber than a C=C bond peak in an IR spectrum? 4. An IR spectrum tells you the bond types in a molecule. Enter “IR correlation table” using your favorite search engine. Use the information from the IR correlation table to complete Table 1. Peak sizes: strong means big peak and weak means small peak; peak shape: sharp means narrow and broad means wide. Table 1. IR peaks and bond types Bond Type IR peak in wavenumbers (cm-1) Peak size (strong, medium, or weak); peak

shape (sharp or broad) C-H stretch C-H bend C-C < 1000 Not interpretively useful C=C C≡C C-O C=O O-H

5. Use and operation of the IR; IR spectrum interpretation. a. To learn about IR, you will measure the IR spectra of 5 or 6 organic compounds and correlate the IR bands to bond type and functional group. (i) Using the Bruker FT-IR, measure the IR spectrum of your assigned organic solvent. Sign up for the IR on the signup sheet. Your instructor will show the first group how to use the IR. This group will show the second group how to use the IR and so on. Work with your lab partner. Your IR spectrum should have a flat baseline and well defined peaks. If your spectrum does not look “good”, measure another IR spectrum. See Figures 1 and 2.

Measure a “good” IR spectrum of your solvent. Compare the IR spectra:

Figure 1. IR spectra (taken from Figure 2. White noise (taken from http://orgchemboulder.com/Technique/Procedures/IR/index.shtml) https://en.wikipedia.org/wiki/White_noise) The “excellent” spectrum has a flat baseline. If your spectrum looks like this, usually it The “pitted plate” and “cloudy plate” spectra have baselines that mean you used too much sample or not drift and broader peaks due to poor sample preparation. enough sample. (ii) On the shared Google sheet, list each peak and the wavenumber on Table 2. Table 2. IR data for various organic solvents. Solvent Chemical

formula Structure Bond types IR peaks, cm-1 Peak intensity

(strong, medium, weak) E.g., water H2O H-O-H O-H 3200-3400 strong methanol ethanol acetone hexane cyclohexene toluene

(iii) For the six compounds, which compounds have the same peaks? Which ones have different peaks? Match each peak to a specific bond type. (iv) Can IR spectroscopy be used to distinguish between methanol and ethanol? Give reasons. Waste Disposal: clean the IR sample disk and tip with acetone after you measure your IR spectrum. 6. IR spectra of three organic compounds are shown below. Choices: cyclohexane, cyclohexanol, cyclohexanone, methyl cyclohexane. a. Draw the structure of each compound: cyclohexane, cyclohexanol, cyclohexanone, and methyl cyclohexane. b. Interpret each IR spectrum by matching the peak to the bond type in your structure. Summarize your IR interpretation in a table similar to Table 2. c. Determine the identity of the compound for each spectrum.

IR Spectrum 1

IR Spectrum 2

IR Spectrum 3

7. Draw the structures and compare the IR spectra of cyclohexene and toluene. Each compound has a C=C bond. The C=C bond peak is at a higher energy in cyclohexene than toluene. Is the C=C bond in cyclohexene stronger, weaker, or the same strength as the C=C bond in toluene? Give reasons.