Lecture 02: Chapter 14: Organic Chemistry - Infrared Spectroscopy

ORGANIC CHEMISTRY Diagnostic Region Fingerprint Region Chapter 14 – Part 1 Infrared Spectroscopy and Mass Spectrometry  Spectroscopy Technique used to determine the structure of a compound 2 ...

ORGANIC CHEMISTRY Diagnostic Region Fingerprint Region Chapter 14 – Part 1 Infrared Spectroscopy and Mass Spectrometry  Spectroscopy Technique used to determine the structure of a compound 2  The Electromagnetic Spectrum Different regions of the Electromagnetic Spectrum are used to probe different features of molecular structure 3 Technique used to Determine the Structure of a Compound Types of Spectroscopy: Infrared (IR) Spectroscopy: – Use Infrared region. – Used to determine the functional groups. Nuclear Magnetic Resonance (NMR) Spectroscopy: – Use Radio Wave region. – Used to determine the arrangement of all carbon and hydrogen atoms in the compound. UV-VIS spectroscopy: – Use Ultraviolet and Visible regions. – Used to determine any conjugated  system present in the compound. Mass Spectrometry (MS): – Used to determine the molecular weight of the compound. 4  IR Spectroscopy IR radiation→ generally causes molecular vibration. Different kinds of vibration → stretching and bending. We will focus on stretching vibrations → Bond Stretching Frequency 5 Identification of Functional Groups with IR Spectroscopy Bonds in molecules absorb IR radiation. Each type of bond will absorb a characteristic frequency. So, can determine which types of bonds are present in a molecule. 6  IR Spectrum Plot of percent transmittance as a function of frequency. Units of frequency in IR are called wavenumbers, cm–1 (reciprocal centimeters) Wavenumbers are proportional to frequency and energy. All the signals → point down. No two molecules will give exactly the same IR spectrum (except enantiomers) v v= c 7  Regions of the Spectrum The wavenumbers range from 400 to 4000 cm−1 The region between 400 and 1500 cm–1 → Fingerprint Region Has the most complex vibrations. The region between 1500 and 4000 cm–1 → Diagnostic Region Use this area to identify the functional groups in the molecule. 8  Characteristics of IR Spectrum Every signal (peak, band) in an IR spectrum has three characteristics: Intensity Shape Wavenumber 9  IR Spectrum: Signal Intensity Some signals is very strong compared to other signals. Because some bonds absorb IR radiation very efficiently compared to others. of bond dipole movement strength Signal intensity → depends on the ______________________________ 10  IR Spectrum: Signal Intensity Bonds with larger dipole moments → strong signal Bonds with smaller dipole moments → weak signal ↑ > 11  Class Activity Determine which C=C double bond will produce a stronger IR signal. 7 -  IR-Active and IR-Inactive Signals Bonds with dipole moments → generally produce signal in IR spectrum → IR-active Bond with no dipole moment → No signal in IR spectrum → IR-inactive Which of the C=C bond is expected to have IR-active signal? t at M 20 M - 1 - M - - ~ e -bonddpols moment - one o - act sig  IR Spectrum: Signal Shape broad Compounds with H-bonding → generally __________ signal 14  IR Spectrum: Wavenumber Frequency (wavenumber) of the stretching vibration depends on; Bond strength The masses of the atoms sharing the bond 15  Wavenumber Effect of Bond Strength: higher IR frequency Stronger bond → appears at _______ Weaker bond → appears at ________ lower IR frequency Bond strength: 1100 cm–1 E 1600 cm–1 2200 cm–1 Effect of Atomic Mass: higher Lighter atoms in the bond → appears at ________ IR frequency lower IR frequency Heavier atoms in the bond → appears at ________ 700 cm–1 - 1100 cm–1 2200 cm–1 3000 cm–1 #  Wavenumber Diagnostic Region Fingerprint Region 17  Carbon–Carbon Bond Stretching Stronger bonds absorb at higher frequencies C—C 1200 cm–1 C=C 1600 – 1700 cm–1 CC 2100 – 2200 cm–1 Diagnostic Region Fingerprint Region 18  Carbon–Carbon Double Bond Stretching Conjugation lowers the frequency: – isolated C=C 1640 – 1700 cm–1 > – conjugated C=C 1600 – 1640 cm–1 ength resonance water – aromatic C=C ~1600 cm–1 19  Class Activity Identify which carbon carbon double bond will exhibit a signal at lower frequency. isolated (C lowers cm conjugation frequency 1000-1700 the < conjugated c= ↳O 20  Carbon–Carbon Triple Bond Stretching Co · Match the C≡C signals with the corresponding alkyne signal around 2200 cm–1 no signal around 2200 cm–1 3 3 22 , M) M is small or zero ⑭ ferminal alkyne internal alkyne 2200cm- signal around no signal or very weak cca 21  Carbon–Hydrogen Bond Stretching 2850 – 3000 cm–1 3000 – 3100 cm–1 ~3300 cm–1 A greater percent of s character in the hybrid orbitals → stronger C—H bond Bond strength: sp3c-Ac Acsp1-H 22  IR Spectrum of Alkanes An alkane will show frequencies for C—H and C—C only. The C—H stretching is a broad band between 2850 and 3000 cm–1, a band present in almost all organic compounds. " 23  IR Spectrum of Alkenes Characteristic signals: C═C signal at 1600 – 1700 cm–1 ═C—H signal at 3000 – 3100 cm–1 Notice that the bands of the alkane are present in the alkene. C= L 24  IR Spectra of Alkynes Characteristic signals: CC signal at 2100 – 2200 cm–1 C—H signal at 3300 cm–1 (sharp peak) for terminal alkynes Cl = C H - C-H no noch visible ECtetch 25  O—H and N—H Bond Stretching Both occur around 3300 cm–1, but they look different: – OH → broad signal with a rounded tip. – NH → broad with sharp spike(s) ~ 26  IR Spectrum of Alcohols Characteristic signals: Broad, rounded tip, strong O—H signal between 3200 and 3600 cm–1 i I C- H 27  IR Spectrum of Amines Characteristic signals: broad N—H signal between 3350 – 3500 cm–1 with sharp spike(s) – Primary amine (RNH2) is broad with two sharp spikes. – Secondary amine (R2NH) is broad with one sharp spike – There is no signal for a tertiary amine (R3N) because there is no N—H bond. 28  Carbonyl Compounds Carbonyl groups show characteristic peak around 1700 cm–1 Usually, the carbonyl is the strongest IR signal 29  IR Spectrum of Ketones Characteristic signals: C═O signal around 1720 cm–1 overtone - small signal 3440 around w * overtone C-H = 0 C 30  IR Spectrum of Aldehydes Characteristic signals: C═O signal around 1730 cm–1 two different bands for the O═C—H stretch at 2750 and 2850 cm–1 A overtone O -H c - H G 31  IR Spectrum of Carboxylic Acids Characteristic signals: C═O signal around 1715 cm–1 broad, strong O—H signal between 2500–3500 cm–1 Both peaks need to be present to identify the compound as a carboxylic acid 0 H - foroxylic i acid i frooxylic acid 32  Conjugated Carbonyl Compounds Conjugation lower the carbonyl frequency. 1680't + Micom 33  Class Activity The following compound contains two carbonyl groups. Identify which carbonyl group will exhibit a signal at lower wavenumber. O 34  IR Spectrum of Amides Characteristic signals: strong C═O signal around 1640–1680 cm–1 broad N—H stretching around 3300 cm–1 with sharp spikes Tertiary amides do not show N—H absorptions C-H CO N-H Because of the resonance conjugate, the carbonyl groups of amides absorb at particularly low IR frequencies: about 1640 to 1680 cm–1 35  Carbon—Nitrogen Bond Stretching C—N 1200 cm–1 C=N 1660 cm–1 Usually, strong signal CN above 2200 cm1 CC below 2200 cm–1 (usually moderate or weak) 36  IR Spectrum of Nitriles Characteristic signals: Intense and sharp CN signal around 2200 to 2300 cm–1 CN bonds are more polar than CC, so nitriles produce stronger absorptions than alkynes. iC-H SEN I I 37  When Problem Solving You don’t need to assign all the signals. Some signals may be ambiguous. For an IR band to be observed → the bond stretching, or bending must involve a change in dipole moment. Bonds with zero dipole moment → No signal in IR spectrum Your structure has a particular functional group → So, what IR bands will it show? You see an IR band at a certain frequency → So, what functional group is likely? Diagnostic Region Fingerprint Region 38  Class Activity Identify the structure below that is most consistent with the IR spectrum. A B D C E Gif 0 c= N-H-HHG C - H cn = C-H 2830+100 2300 - 2100 9000-3000 1895-1600 39  Class Activity Identify the structure below that is most consistent with the IR spectrum. A B C D E = C-H Cl 28 40  Class Activity Match each structure with its characteristic IR absorption peak. A. Sharp, 2254 cm -1 4 's -1 B. Very broad, centered about 3330 cm -1 C. Strong, slightly broadened, 1645 cm -1 D. Broad with spikes at 3367 and 3292 cm 3 -1 E. Strong, sharp 1717 cm 2 41  Using IR to Distinguish Between Two Compounds IR spectroscopy can often be used to determine the success of the reactions. 42  Class Activity How could you use IR spectroscopy to determine if the following reaction was successful? Use the following frequencies: 3300 cm-1 3100 cm-1 2250 cm-1 2150 cm-1 -1 1720 cm 1690 cm-1 1630 cm-1 -1 Look for the appearance of a signal around _____ 1630 and _____ 3100 cm , and -1 the disappearance of signal around _____ 2150 cm 43