March 2011 - Career Point

The compound B 2 H 6 . 2NH 3 is ionic, and comprises
[H 3 N → BH 2 ← NH 3 ] + and [BH 4 ] – ions. On heating,
it forms borazine.
Boron nitride is a white slippery solid. One B atom
and one N atom together have the same number of
valency electrons as two C atoms. Thus boron nitride
has almost the same structure as graphite, with sheets
made up of hexagonal rings of alternate B and N
atoms joined together. The sheets are stacked one on
top of the other, giving a layer structure.
Borazine B 3 N 3 H 6 is sometimes called 'inorganic
benzene' because its structure shows some formal
similarity with benzene, with delocalized electrons
and aromatic character. Their physical properties are
also similar.
140ºC
3BCl 3 + 3NH 4 Cl B 3 N 3 H 3 Cl 3
Na[BH 4]
MeMgBr
B 3 N 3 H 6
B 3 N 3 H 3 (Me) 3
Hydride Anions of Aluminium :
Both Al and Ga hydride anions are obtained by the
reaction
4LiH + MCl 3 ⎯
Et 2
⎯→ ⎯
O LiMH 4 + 3LiCl
However, for AlH – 4 the sodium salt can be obtained
by direct interaction :
Na + Al + 2H 2
⎯
THF ⎯⎯⎯
→
150ºC/ 2000psi / 24th
⎯ NaAlH 4
The salt is obtained by precipitation with toluene and
can be converted efficiently to the lithium salt :
NaAlH 4 + LiCl ⎯
Et 2
⎯→ ⎯
O LiAlH 4 + NaCl(s)
The most important compound is lithium aluminum
hydride, LiAlH 4 , a nonvalatile crystalline solid, stable
below 120ºC, that is explosively hydrolyzed by
water. In the crystal there are tetrahedral AlH – 4 ions
with an average Al–H distance of 1.55 Å. The Li +
ions each have four near hydrogen neighbors
(1.88 – 2.00Å) and a fifth that is more remote
(2.16Å). Lithium aluminum hydride is soluble in
diethyl and other ethers and can be solubilized in
benzene by crown ethers. In ethers, the Li + , Na + , and
R 4 N + salts of AlH – 4 and GaH – 4 tend to form three
types of species depending on the concentration and
on the solvent, namely, either loosely or tightly
bound aggregates or ion pairs. Thus LiAlH 4 is
extensively associated in diethyl ether, but at low
concentrations in THF there are ion pairs. Sodium
aluminum hydride (NaAlH 4 ) is insoluble in diethyl
ether.
Carbon Family : Compounds with C—N Bonds;
Cyanides and Related compounds:
An important area of "inorganic" carbon chemistry is
that of compounds with C—N bonds. The most
important species are the cyanide, cyanate, and
thiocyanate ions and their derivatives.
1. Cyanogen. There are three known isomers of
composition C 2 N 2 :
N≡C–C≡N C=N–C≡N C=N–N=C
1 2 3
Isomer2, isocyanogen, and isomer 3, diisocyanogen,
have been detected by nmr and other spectroscopies;
isocyanogen is extremely unstable and polymerizes
above –80ºC. Isomer 1, cyanogen, is a flammable gas
which is stable even though it is unusually
endothermic (∆H f 298
0 = 297 kJ mol –1 ). It can be
prepared by oxidation of HCN using (a) O 2 with a
silver catalyst, (b) Cl 2 over activated carbon or silica,
or (c) NO 2 over calcium oxide-glass; the last reaction
allows the NO produced to be recycled :
2HCN + NO 2 → (CN) 2 + NO + H 2 O
Cyanogen can also be obtained from the cyanide ion
by aqueous oxidation using Cu 2+ (cf. the Cu 2+ – I –
reaction) :
Cu 2+ + 2CN – ⎯→ CuCN + ½(CN) 2
or acidified peroxodisulfate. A better procedure for
dry (CN) 2 employs the reaction
Hg(CN) 2 + HgCl 2 ⎯→ Hg 2 Cl 2 + (CN) 2
The cyanogen molecule, N ≡C–C ≡ N, is linear. It
dissociates into CN – radicals, and, like RX and X 2
compounds, it can oxidatively add to lower-valent
metal atoms giving dicyano complexes, for example,
(Ph 3 P) 4 Pd + (CN) 2 ⎯→ (Ph 3 P) 2 Pd(CN) 2 + 2PPh 3
A further resemblance to the halogens is the
disproportion in basic solution :
(CN) 2 + 2OH – ⎯→ CN – + OCN – + H 2 O
Thermodynamically this reaction can occur in acid
solution, but it is rapid only in base. Cyanogen has a
large number of reactions. A stoichiometric mixture
of O 2 and (CN) 2 burns, producing one of the hottest
flames (~ 5050 K) known from a chemical reaction.
Impure (CN) 2 can polymerize on heating to give a
polymer, paracyanogen which will depolymerize
above ~ 850ºC.
N N N N
C C C C
C C C C
N N N N
2. Hydrogen Cyanide : Like the hydrogen halides,
HCN is a covalent, molecular substance, but is a
weak acid in aqueous solution (pK = 9.0).
Proton transfer studies, however, show that as with
normal protonic acids, direct proton transfer to base B
B : + HCN ⎯→ BH + + CN –
occurs without participation of water.
XtraEdge for IIT-JEE 34 MARCH 2011