Transcript T IR
C-H Stretch • 2962 and 2872 cm-1 C-H in CH3 • 2926 and 2853 cm-1 C-H in CH2 • 2890 cm-1 tertiary C-H strong strong weak • All ± 10 cm-1 • 2720 cm-1 C-H stretch in aldehydes C-H Bend • ~ 1450 cm-1 Asymmetric methyl bend Methylene scissoring • ~ 1380 cm-1 Symmetric methyl bend •Alkene and alkyne C-H bonds display sharp stretching absorptions in the region 31003000 cm-1. •The bands are of medium intensity and are often obscured by other absorbances in the region (i.e., OH). 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. Vibration position (as well as strength) depends on substitution •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. Terminal acetylenic bond •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. •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. •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. 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 • 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 α-substituent is electron withdrawing, the П bond will tighten, so there will be an increased force constant – so band will be at higher frequency. • O-H gets broader and moves to lower frequency as the amount of H-bonding increases • Carboxylic acid O-H is particularly broa • A shoulder band usually appears on the lower wavenumber side in primary and secondary liquid amines arising from the overtone of the N–H bending band: this can confuse interpretation. Note the spectrum of aniline, below.) • The N–H 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 N–H wag and observed only for primary and secondary amines ESTERS 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 1550, 1380 cm-1