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Heterogeneous Catalysis with Elemental Boron and with Nickel Borides.

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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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