Page 365 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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468 Organic Chemistry, Compound Detection

groups in the molecules of such parent compounds is the R OH R O COR
most significant source of difficulty associated with their
(R CO) 2 O

GC analysis. R NH −−−−−→ R NH COR
R =aryl

Carboxyl, hydroxyl, carbonyl, and amino groups, be-
cause of their polarity and tendency to form hydrogen R SH R S COR
bonds, are responsible for the low volatility of the com-
Silyl derivatives are probably the most commonly used
pound and for other phenomena (such as strong absorption
for protecting functional groups of volatile substances
on the stationary phase) that make direct GC either diffi-
in gas chromatography. Trimethylsilyl (TMS) derivatives
cult or impossible. The separation of closely related com-
can be prepared by the reaction of trimethylsilylating
pounds is easier after their conversion into suitable deriva-
agents such as trimethylsilyl chloride with groups con-
tives. For example, the sensitive and labile prostaglandins
taining active hydrogen.
such as PGF 2α are converted to their trimethylsilyl deriva-
tives which are stable for GC analysis. OH O Si(CH 3 ) 3
The combination of GC with MS (GC–MS) for which
COOH COO Si(CH 3 ) 3
special derivatives are being developed, gives character-
istic fragments that make identification and quantitative Me 3 SiCl
SH −−−−→ S Si(CH 3 ) 3
evaluation easier. Derivatives commonly used to protect
thesegroupsareusuallylesspolarthantheoriginalgroups. NH 2 NH Si(CH 3 ) 3
Efforts aimed at developing a single procedure in which NH N Si(CH 3 ) 3
several or all functional groups in the molecule could be
converted into a suitable derivative in one reaction step
led to the development of methods for the preparation of 1. Cyclic Derivatives
silyl, acyl, isopropyl, and other derivatives.
If two or more functional groups which should be pro-
Esters are common derivatives of carboxyl groups.
tected occur in the molecule of the substrate, blocking can
Methyl esters are the most often used as they have a suf-
be accomplished with a bi-functional reagent, thus pro-
ficient volatility even for the chromatography of higher
ducing a cyclic product. Cyclic boronates, for example,
fatty acids contained in fats. A number of methods have can be used for the GC of compounds containing cis-diol
been developed for their preparation, often exclusively for groups in the 1,2- and 1,3-positions.
the purpose of GC determination.
Elegant methods for esterification with diazomethane
and methanolic solutions of BF 3 or BCl 3 are fairly
widespread. Reactions with methanol can also be cat-
alyzed with HCl or H 2 SO 4 . Methyl esters can also be
prepared by pyrolysis of tetramethylammonium salts in
the inject port, and by esterification on an ion exchanger: 2. Separation of Enantiomers

Two basic approaches to the separation of enantiomers
+ −
R COOH + CH 2 N N → R COOCH 3 + N 2 ↑ of amino acids have been applied: (1) amino acid deriva-
tives are chromatographed on optically active stationary
+ − +
−
R COOH + (CH 3 ) 4NOH → RCOON(CH3 ) 4 phases such as N-acyl alkyl esters, ureides or N-acyl alkyl
esters of dipeptides and (2) GC separation is performed

→ RCOOCH 3 + (CH 3 ) 3 N ↑
on conventional stationary phases and the derivatives of
amino acids are prepared by reaction with optically active
Ethers are useful for protecting hydroxyl groups. Hy- reagents. The bifunctionality of amino acids offers the use
droxyl groups of high molecular weight compounds, such of either optically active esters or acyl derivatives.
as sugars and sterols, are converted into ethers by reaction
with methyl iodide in the presence of silver oxide.
3. Cyclodextrins
The cyclodextrins (cycloamyloses) are torus-shaped
2ROH + 2CH 3 I + Ag O → 2R OCH 3
2
molecules consisting of 6, 7, 8, and 9 α-1,4-linked
+ 2AgO + H 2 O D-glucopyranose units, α, β, γ , and δ-cyclodextrin (α-
CD [Fig. 7], β-CD, γ -CD, and δ-CD). Cyclodextrins
Aryl derivatives are common for hydroxy, amino and thiol have a hydrophobic center and a hydrophilic outer sur-
groups. face. In aqueous solution both the inner and the outer