Slide 1

Slide 2 C-H Stretch 2962 and 2872 cm -1 C-H in CH 3 strong 2926 and 2853 cm -1 C-H in CH 2 strong 2890 cm -1 tertiary C-H weak All 10 cm -1 2720 cm -1 C-H stretch in aldehydes Slide 3 C-H Bend ~ 1450 cm -1 Asymmetric methyl bend Methylene scissoring ~ 1380 cm -1 Symmetric methyl bend Slide 4 Slide 5 Slide 6 Slide 7 Alkene and alkyne C-H bonds display sharp stretching absorptions in the region 3100- 3000 cm-1. The bands are of medium intensity and are often obscured by other absorbances in the region (i.e., OH). Slide 8 Carbon-carbon double bond stretching occurs in the region around 1650-1600 cm -1. Generally sharp, medium intensity. Trans substituted bonds absorb much less than cis Aromatic compounds will typically display a series of sharp bands in this region. Slide 9 Vibration position (as well as strength) depends on substitution Slide 10 Slide 11 Triple bond stretching absorptions occur in the region 2400-2200 cm -1. Absorptions from nitriles are generally of medium intensity and are clearly defined. Alkynes absorb weakly in this region unless they are highly asymmetric Symmetrical alkynes do not show absorption bands. Slide 12 Terminal acetylenic bond Slide 13 Alcohols and amines display strong broad O-H and N-H stretching bands in the region 3400-3100 cm -1. The bands are broadened due to hydrogen bonding and a sharp 'non-bonded' peak can often be seen at around 3400 cm -1. Slide 14 Carbon-oxygen single bonds display stretching bands in the region 1200-1100 cm -1. The bands are generally strong and broad. You should note that many other functional groups have bands in this region which appear similar. Slide 15 Slide 16 Carbonyl stretching bands occur in the region 1800-1700 cm-1. The bands are generally strong and broad. Carbonyl compounds which are more reactive in nucleophilic addition reactions (acyl halides, esters) are generally at higher cm -1 than simple ketones and aldehydes. Amides are the lowest, absorbing in the region 1700-1650 cm-1. Slide 17 Band Shifts Influenced by molecular environment Stronger bonds absorb at higher frequency In general the carbonyl of an aldehyde is at higher frequency than carbonyl of ketone Shift depends on Physical state of sample Substitution effects Conjugation H-bonding Slide 18 Frequency increases in polar solvent because of reduced intermolecular interactions shifts of up to ~25 cm -1 Normal ketone carbonyl is at 1715 cm -1 If the -substituent is electron withdrawing, the bond will tighten, so there will be an increased force constant so band will be at higher frequency. Slide 19 O-H gets broader and moves to lower frequency as the amount of H-bonding increases Carboxylic acid O-H is particularly broa Slide 20 Slide 21 Slide 22 Slide 23 A shoulder band usually appears on the lower wavenumber side in primary and secondary liquid amines arising from the overtone of the NH bending band: this can confuse interpretation. Note the spectrum of aniline, below.) The NH bending vibration of primary amines is observed in the region 1650-1580 cm -1. Usually, secondary amines do not show a band in this region and tertiary amines never show a band in this region. (This band can be very sharp and close enough to the carbonyl region to cause students to interpret it as a carbonyl band.) Another band attributed to amines is observed in the region 910-665 cm -1. This strong, broad band is due to NH wag and observed only for primary and secondary amines Slide 24 Slide 25 Slide 26 Slide 27 Slide 28 Slide 29 Slide 30 Slide 31 Slide 32 Slide 33 ESTERS Slide 34 Slide 35 Amines Primary amine asymmetric and symmetric stretch see two bands Secondary amine only one band N-H in- plane bend strong ~ 1600 cm -1 N-H out- of -plane bend 750 850 cm -1 Very broad for primary amines Slide 36 1550, 1380 cm -1 Slide 37 Slide 38 Slide 39 Slide 40 Slide 41