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Modern Methods for Electrical Indication of Titrations.

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On peroxidation of unsaturated compounds, several isomeric alkyl hydroperoxides may arise, according to the
number of double bonds. The cause of this is the mesomerism
of allyl radicals formed by reaction (b), e.g. from methyl
3-butenoate ( I ) and crotonate (2).
The so-called halogen-induced oxidation, hitherto incorrectly interpreted, has been clarified. @-Bromo hydroperoxides are obtained from olefins, HBr, and oxygen [41.
[Lectures at Magdeburg and Berlin (Germany),
January 6th and January 21st, 19661
[VB 987j293 IE]
German version: Angew. Chem. 78, 496 (1966)
Although the butenoate ( 1 ) is more easily oxidized, the
hydroperoxide derived from (3) is the main product obtained
from the two esters. The formation of isomeric alkyl hydroperoxides makes it difficult to study the aerial oxidation of
natural unsaturated oils and rubbers, so experiments to this
end have been carried out with unsaturated model substances 121. We were able to synthesize allyl hydroperoxide [31;
it proved to be surprisingly stable.
[l] A . Rieche, Angew.Chem.50, 520 (1937); A.Rieche, E.Schmitz,
and M . Schulz, Z. Chem. 3, 443 (1963).
121 A . Rieche, M . Schulz, H.-E. Seyfarth, and G . Gottschalk,
Fette, Seifen, Anstrichmittel 64, 198 (1962).
[3] H.-E. Seyfarth, J . Henkel, and A . Rieche, Angew. Chem. 77,
1078 (1965); Angew. Chem. internat. Edit. 4, 1074 (1965).
[4] A . Rieche, M . Schulz, and K . Kirschke, Angew. Chem. 77,219
(1965); Angew. Chem. internat. Edit. 4, 244 (1965).
Modern Methods for Electrical Indication of
Titrations
G . Kruft, Frankfurt/Main (Germany)
Ozonolysis of olefins in the presence of amines afforded
I
geminal hydroperoxyamines RHN-C-OOH
I
(by way of the
zwitterion >C+-OO-), which are also obtained from Schiff
bases and H202; they are precursors of 2-alkyloxaziridines.
On treatment with ammonia and ozone, olefins give isooximes (3-substituted oxaziridines) (“ammozonization”).
Cyclohexanone isooxime was isolated as its N-benzoyl
derivative ( 4 ) , which is also formed on ozonization of bi(cyclohexylidene) in the presence of benzamide.
Peroxidation of nitrogen compounds occurs at the carbon
atom next to the nitrogen atom. Tetrahydroisoquinoline and
azepine give peroxides ROOR directly, by way of hydroperoxides ROOH. Hydrazones and hexanolactam give
hydroperoxides ; pyrrolidone gives H202 and succinimide.
Cyclic peroxides such as ( 5 ) and (6) are obtained from pdiketones and “triacetylmethane” by means of H202. Like
acetals, compounds of type (5) readily undergo exchange of
OH groups for OOH or OOR by way of cations stablized by
mesomerism. The symmetrical peroxide (6) has a high
melting point and is chemically unusually stable, although
explosive.
Preparative use of peroxides has received new impulses by
the discovery of “fragmentable” peroxides. Polar cleavage
and synchronous electron shifts lead, by reaction (e), to only
a few fragments, whereas radical decomposition of peroxides
yields a large number of products.
This method leads, for instance, to quantitative cleavage of
2,3,4,6-acetylated t-butyl glucosyl peroxide into arabinose,
formic acid, and t-butyl alcohol (a new method of sugar
degradation).
524
Of the modern procedures for electrical determination of
endpoint those involving noble-metal electrodes polarized
by a few pA of direct current were discussed. These electrodes
differ from the potentiometric ones (which respond to chemical events occurring during the titration without drawing
current) in that their potential is determined only by their
state of polarization, although this in thurn is a fuction
of chemical events during the titration. Against this loss in
specificity is to be set the advantage that polarized indicator
electrodes are universally applicable - for redox titrations as
well as for argentometric, complexometric, and even acidimetric titrations.
Platinum foil electrodes 10 to 100 mm2 in size and polarized
with currents between about 0.3 and 3 FA or potentials up to
several hundred mV have proved valuable. Au, Ag, and
Au-amalgam electrodes can also be used successfully. If
polarization is effected with a constant current, the indication
is described as voltametric, if with constant polarization
potential as arnperometric (or as dead-stop technique if only
small polarization potentials are applied).
Details were discussed for the less well-known voltametry.
The following two forms of indication technique are differentiated : A) Both electrodes are polarized and function
simultaneously for potential measurement. B) Only one of
the two polarized electrodes is a measuring electrode; it
operates in conjunction with an additional, unpolarized
electrode, such as a calomel electrode, also placed in the
solution to be analysed.
For electrochemically reversible titration reactions the technique A gives a titration curve (potential/volume curve)
characterized by a sharp peak; for irreversible systems it
gives a steeply rising curve that somewhat resembles a potentiometric curve in appearance. The maximum or point of
inflection corresponds to the equivalence point.
Independently of the reversibility of the chemical process,
technique B leads to a steeply rising potential-volume curve;
the point of inflection does not correspond exactly to the
equivalence point. The deviation may be positive or negative
according to whether the indication process occurs at the
anode or the cathode, and increases with increasing dilution
of the solution being analysed. This technique is thus useful
for studying electrode reactions rather than for determination
of endpoints.
Independently of which technique is used, voltametric indication involves a connection between polarization current density at the electrodes and concentration of the sohtion being analysed. Conditions are optimum at a polarization current density of about 0.05-0.1 pAjmm2 in titration
with 0.1 N solutions, or about 0.02-0.05 pA/mm2 or 0.005
to 0.02 pA/mmZ for titration with 0.01 N or 0.001 N solutions, respectively.
Angew. Chem. internat. Edit.
Yol. 5 (1966)
No. 5
Voltametric indication (technique A) is usually superior to
potentiometric indication, giving readily interpretable
curves for titration of Ce(IV) or V(V) with Fe(l1) even in
0,0001 N solutions. Complexometric titration of e.g. Cu,
Pb, Zn, or Ca, which cannot be followed directly by the
potentiometric method, can be carried out so precisely that
the resulting accuracy of analysis exceeds that of the usual
color-indicator methods.
[Lecture at Clausthal-Zellerfeld (Germany), Feb. 4th, 19661
[VB 985,’285 IE]
German version: Angew. Chem. 78, 551 (1966)
Rearrangements of or-Substituted Aldehydes
A . Kirrmann, Paris (France)
Attack of nucleophilic reagents o n or-halogenated aldehydes
takes place primarily at the carbonyl group. With the
nucleophilic reagents OH-, OR-, NHz-, or CN-, it can lead
to the products of Favorski rearrangements or to stable
epoxides, e.g. with alkoxides or, particularly, cyanides. The
substituted aldehydes (substituent: SH, SR, SAr, or NRz)
that are first formed are often not very stable and rearrange
to ketones, sometimes spontaneously, e.g. RCH(NR2)CHO
+ R-CO-CHz-NR2,
or o n acid (substituent: OH, OR,
SR) or basic catalysis (substituent: OH, OAc), or on reaction
with amines by way of enediamines in accordance with
reaction (a) :
C~HS-CH(NR~)-CHO + C ~ H ~ - C ( N R ~ ) T C H - N R ~
--f
C6Hz-CO-CH2-NH2
(a)
Correspondingly substituted epoxides are often very unstable and rearrange thermally to aldehydes (substituent :
OAc, CI), or catalytically [*I to ketones (substituent : OR,
C1) [see reaction (b)]. The catalytic rearrangement proceeds
by way of enols.
R-CO-CHzCl
Catalysis
t---
-
R-CH -CH-CI
A
-+
‘
0
’
R-CHCI-CHO
(b)
The reaction with alkalis and amines is bimolecular only
for branched chloro aldehydes. An intermediate formed from
two molecules of aldehyde must be assumed in the case of
unbranched or-halogeno aldehydes.
Use of epoxy nitriles permits the synthesis of e.g. or-fluoro
aldehydes that are difficultly accessible by other routes [see
reaction (c)].
R-CH‘
0
’
CH-CN
+
HF
+ R-CHF-CH(0H)-CN
+ R-CHF-CHO
(c)
[Lecture at Munich (Germany), February 15th, 19661
[VB 990 / 292 IE]
German version: Angew. Chem. 78, 551 (1966)
[*] Catalysts: Bronsted or Lewis acids.
Angew. Chem. internat. Edit. 1 VoI. 5 (1966) 1 No. 5
Heterogeneous Catalysis with Elemental Boron
and with Nickel Borides
H . J. Becher, Munster (Germany)
In continuation of previous experiments “1, comparative
measurements, at various streaming velocities, were made of
catalytic dehydration of 2-propanol vapor o n elemental
boron (prepared by various methods [*I) and o n technical
A1203 catalysts. The most active catalyst was finely divided
boron in the cc-modification, with a surface area of 15 to
30 mz/g; this was superior to the most active A1203 catalyst
by a factor of two in respect of amount of catalyst and by a
factor of 50 in respect of surface area.
Determination of the oxygen content and of the protons that
are exchangeable with D20 indicates that an average of one
O H group per boron atom is present in the surface of finely
divided or-boron. On dehydration of CD&H(OD)CD3 and
CH3CD(OH)CH3 on a-boron hardly any of the hydrogen
attached to secondary carbon is removed or exchanged, in
contrast to the deuterium and hydrogen of CD3 and CH3
groups. Exchange experiments between CH3CH=CH2 and
D20 at 300 “C also showed that the H atoms of CH2 and CH3
groups are exchanged preferentially in presence of thea-boron
catalyst; in absence of cc-boron there is n o exchange under
similar conditions. These observations accord with the
representation for catalytic dehydration on metal oxides put
forward by Encken and WickeL21; according to this model, finely divided boron is a weakly acidic, oxidic dehydration catalyst by virtue of the BOH groups on the
surface; the high activity of the BOH groups is to be emphasized. It seems to be intensified by the peculiar arrangement of boron atoms in the or-boron lattice[**] since boric
acid itself does not dehydrate 2-propanol under these conditions but on the contrary converts it into a n ester.
Nickel borides, Ni3B, NizB, and NIB, have a dehydrogenating
action o n alcohols. Their activities in heterogeneous gas
catalysis are less than that of finely divided nickel (nickel
carbonyl), but nickel boride electrodes have more favorable
properties than nickel electrodes [31 for anodic oxidation of
hydrogen and methanol in a fuel cell. Reaction of nickel
borides with aluminum at 675 “C leads to nickel-aluminum
phases and to nickel borides very rich in boron, one such
(NiBI2) having been characterized. These alloys can be
converted by NaOH solution into a mixture of Raney nickel
and NiBIP, and electrodes prepared therefrom are superior
to electrodes made from NiBlz-free Raney nickel for anodic
oxidation of methanol in fuel cells.
[Lecture at M x l (Germany), March 9th, 19661
[VB 989/291 IE]
German version: Angew. Chem. 78, 552 (1966)
[*] Samples: iinely powdered cc-boron made by reduction of
gaseogs BBr3 with H:; amorphous boron supplied by Borax
Consolidated Co., London, and Consortium fur electrochernischc
Industric, Munich.
[*“I Cf. B. F. Deckei and J . S. Kmper, Acta crystallogr. I 2 , 503
(1959).
[I] H . J . Becher, L . Marosi, and H , Widmann, Lecture at the
Westdeutsche Chemiedozententagung, Freiburg 1964; cf. Angew.
Chem. 76,574 (1964); Angew. Chem. internat. Edit. 3,644 (1964).
[2] E. Wicke, Angew. Chern. 59, 34 (1947).
[3] From measurements in the physikalisch-chemische Laboratorien of Robert Bosch GmbH., Stuttgart (Germany). - Cf.
H . Jahnke, Lecture at the Journees Internationales d’Etude des
Piles a Combustile, Brussels 1965.
525
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