1、 Organic Spectroscopic Analysis Structural Analysis by IR IR Absorption of Functional Groups IR spectroscopy for structure analysis IR Absorption of Functional Groups 官能团的红外吸收=O-H and N-H Stretching Both of them occur around 3300 cm-1,but they look different.Alcohol O-H,broad with rounded tip.Primar
2、y amine(RNH2),broad with two sharp spikes.Secondary amine(R2NH),broad with one sharp spike(尖刺).No signal for a tertiary amine(R3N).O-H Stretch of a Carboxylic Acid This O-H absorbs broadly,2500-3500 cm-1,due to strong hydrogen bonding.Carbon-Hydrogen Stretching Bonds with more s character absorb at
3、a higher frequency.sp3 C-H,just below 3000 cm-1(to the right)sp2 C-H,just above 3000 cm-1(to the left)sp C-H,at 3300 cm-1 Carbon-Carbon Bond Stretching Stronger bonds absorb at higher frequencies:C-C 1200 cm-1 C=C 1660 cm-1 C C 2200 cm-1(weak or absent if internal)Conjugation(共扼)lowers the frequency
4、:isolated C=C 1640-1680 cm-1 conjugated C=C 1620-1640 cm-1 aromatic C=C approx.1600 cm-1 An Alkane IR Spectrum just below 3000 cm-1 An Alkene（烯烃）IR Spectrum just above 3000 cm-1 Carbonyl Stretching The C=O bond of simple ketones,aldehydes,and carboxylic acids absorb around 1710 cm-1.Usually,its the
5、strongest IR signal.Carboxylic acids will have O-H also.Aldehydes have two C-H signals around 2700 and 2800 cm-1.A Ketone IR Spectrum An Aldehyde IR Spectrum Strengths and Limitations The functional group is usually indicated.The absence of a signal is definite proof that the functional group is abs
6、ent.Correspondence with a known sample s IR spectrum confirms the identity of the compound.IR alone cannot determine a structure.Some signals may be ambiguous.You have the IR spectra for an organic liquid that was found on the clothes of a criminal defendant.Questions&Problems The crime scene was he
7、avily contaminated with a mixture of hydrocarbons,but there were no ketones,aldehydes,acids or alcohols present.Questions&Problems 1.Is the defendants clothing contaminated with a hydrocarbon?2.Would this spectral evidence put the suspect at the crime scene?Justify your answer using IR functional group information.Questions&Problems
