28.12.2013 Views

preface to fifteenth edition

preface to fifteenth edition

preface to fifteenth edition

SHOW MORE
SHOW LESS

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

SPECTROSCOPY 7.53<br />

TABLE 7.23<br />

Absorption Frequencies of Carbonyl Bands (Continued)<br />

Groups Band,cm 1 Remarks<br />

Imides 9CO9N9CO9<br />

Cyclic six-ring<br />

Cyclic five-ring<br />

ca 1710 and<br />

ca 1700<br />

ca 1770 and<br />

ca 1700<br />

Shift of 15 cm 1 with , unsaturation<br />

Ureas N9CO9N<br />

RNHCONHR ca 1660<br />

Six-ring ca 1640<br />

Five-ring ca 1720<br />

Urethanes R9O9CO9N 1740–1690 Also shows amide II band when nonsubstituted<br />

on N<br />

Thioesters and Acids<br />

RCO9S9R<br />

RCOSH ca 1720 ,-Unsaturated or aryl acid or ester shifted<br />

about 25 cm 1<br />

RCOS9alkyl ca 1690<br />

RCOS9aryl ca 1710<br />

7.5.1 Intensities of Carbonyl Bands<br />

Acids generally absorb more strongly than esters,and esters more strongly than ke<strong>to</strong>nes or aldehydes.<br />

Amide absorption is usually similar in intensity <strong>to</strong> that of ke<strong>to</strong>nes but is subject <strong>to</strong> much greater<br />

variations.<br />

7.5.2 Position of Carbonyl Absorption<br />

The general trends of structural variation on the position of C"O stretching frequencies may be<br />

summarized as follows:<br />

1. The more electronegative the group X in the system R9CO9X9,the higher is the frequency.<br />

2. , Unsaturation causes a lowering of frequency of 15 <strong>to</strong> 40 cm 1 ,except in amides,where little<br />

shift is observed and that usually <strong>to</strong> higher frequency.<br />

3. Further conjugation has relatively little effect.<br />

4. Ring strain in cyclic compounds causes a relatively large shift <strong>to</strong> higher frequency. This phenomenon<br />

provides a remarkably reliable test of ring size,distinguishing clearly between four-,five-,<br />

and larger-membered-ring ke<strong>to</strong>nes,lac<strong>to</strong>nes,and lactams. Six-ring and larger ke<strong>to</strong>nes,lac<strong>to</strong>nes,<br />

and lactams show the normal frequency found for the open-chain compounds.<br />

5. Hydrogen bonding <strong>to</strong> a carbonyl group causes a shift <strong>to</strong> lower frequency of 40 <strong>to</strong> 60 cm 1 . Acids,<br />

amides,enolized -ke<strong>to</strong> carbonyl systems,and o-hydroxyphenol and o-aminophenyl carbonyl<br />

compounds show this effect. All carbonyl compounds tend <strong>to</strong> give slightly lower values for the<br />

carbonyl stretching frequency in the solid state compared with the value for dilute solutions.<br />

6. Where more than one of the structural influences on a particular carbonyl group is operating,the<br />

net effect is usually close <strong>to</strong> additive.

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!