Chemistry 242 - Inorganic Chemistry II
Chapter 14 - Carbon

Introduction

The Chemistry and Physical Properties of Diamond Graphite and the Fullerenes

The allotropes of carbon have been covered in sufficient detail in Chapter 8. Skip this long section!

Carbides

These come in three general types:

Ionic carbides are formed by elements of groups 1, 2 and aluminum. The actual for of the carbon varies, for example, aluminum carbide, based on its hydrolysis product seems to contain "C4-" units:

Al4C3   +   6H2O      2Al(OH)3   +   3CH4

but calcium carbide seems to contain [CºC]2- units:

CaC   +   2H2O      Ca(OH)2   +   HCºH

Interstitial carbides are compounds of the transition metals with metallic properties and the C in tetrahedral holes in the metal atom lattice. The best known example is the extrememly hard tungsten carbide, WC, used in cutting tools.

Covalent carbides include B4C3 and SiC (carborundum - an abrasive with a diamondlike structure)

Carbon Monoxide

Carbon Dioxide and Carbonic Acid

Compounds with C-N Bonds: Cyanides and Related Compounds

Cyanogen (NºC–CºN) is a poisonous and flammable gas (bp -21 oC. Although its heat of fomation is strongly endothermic 297 kJ mol-1 is is fairly stable. Impure cyanogen polymerizes to form "paracyanogen":

Cyanogen is prepared by nitrogen dioxide catalysed oxidation of hydrogen cyanide by oxygen:

2HCN   +   NO2      (CN)2   +   NO   +   H2O

NO   +   ½O2      NO2

It is also formed by oxidation od CN- with Cu2+:

Cu2+   +   2CN-      CuCN   +   ½(CN)2

Notice the similarity between the above reaction and the one below:

Cu2+   +   2I-      CuI   +   ½(I)2

The term "pseudo halogen/halide" is often applied to molecules and derived ions such as (CN)2 and CN-. Notice also the existence of HCN which has its parallel in the hydrohalic acids and the reaction of cyanogen with base:

(CN)2   +   2OH-      CN-   +   OCN-   +   H2O

Compare:

(Cl)2   +   2OH-      Cl-   +   OCl-   +   H2O

The reaction of cyanogen with oxygen produces one of the hottest flames known at about 5000 oC.

Hydrogen cyanide boils at 25.6 oC. It is very poisonous and has an odour of almonds which not everyone can smell. It is a very good solvent due to its high dielectric constant, e = 107. It is made on an industrial scale (~300 000 tons in 1980) as follows:

CH4   +   3O2   +   2NH3      2HCN   +   6H2O    (Pt/Rh or Pt/Ir catalyst and 800 oC)

or

CH4   +   NH3      HCN   +   3H2    (Pt catalyst and 1200 oC)

Cyanides are made industrially via the calcium cyanamide salt by the processes:

CaC2   +   N2      CaNCN   +   C   (1100 oC
CaNCN   +   C   +   Na2CO3      CaCO3   +   2NaCN

or

NaNH2   +   C      NaCN   (500-600 oC)

(NCN2- which yields cyanamide itself, H2NCN, by hydrolysis of the salt, is isoelectronic with CO2.)

Cyanide is important, among other things, as a very good p-acceptor ligand like CO.

Compounds with C-S bonds

Carbon disulphide is perhaps the most important as a solvent and a synthetic reagent. It gives rise to other carbon sulphur compounds such as:

RO-   +   CS2      ROCS2-   (xanthates)

HS-   +   CS2      CS32-   (thiocarbonate)

R2HN   +   CS2      R2NCS2-   (dithiocarbamates)

These ions are important ligands for transition metals, and the dithiocarbamtes are sued as agricultural fungicides.