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Tetrabromomethane: Wikis

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Tetrabromomethane
Identifiers
CAS number 558-13-4 Yes check.svgY
PubChem 11205
EC number 209-189-6
UN number 2516
RTECS number FG4725000
SMILES
InChI
Properties
Molecular formula CBr4
Molar mass 331.63 g/mol
Appearance Colorless solid
Density 3.42 g/cm3
2.961 g/cm3 (liquid, 100 °C)
Melting point

91.0 °C

Boiling point

189.5 °C

Vapor pressure 5.33 kPa (96.3 °C)
Structure
Crystal structure Monoclinic
Molecular shape Tetrahedral
Hazards
MSDS ICSC 0474
EU Index Not listed
Flash point Non-flammable
Related compounds
Related bromomethanes Bromomethane
Dibromomethane
Bromoform
Related compounds Tetrafluoromethane
Tetrachloromethane
Tetraiodomethane
 Yes check.svgY (what is this?)  (verify)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Tetrabromomethane, CBr4, also known as carbon tetrabromide, is a carbon bromide. Both names are acceptable under IUPAC nomenclature, depending on whether it is considered as an organic or an inorganic compound.

Contents

Physical properties

Tetrabromomethane has two isomers: crystaline II or β below 46.9 °C (320.0 K) and crystalline I or α above 46.9 °C. Monoclinic allotrope has space group C2/c with lattice constants: a = 20.9, b = 12.1, c = 21.2 (.10-1 nm), β = 110.5°.[1] Bond energy of C-Br is 235 kJ.mol-1.[2]

Due to its symmetrically substituted tetrahedral structure, its dipole moment is 0 Debye. Critical temperature is 439 °C (712 K) and critical pressure is 4.26 MPa.[1]

Plastic crystallinity

The high temperature α phase is known as a plastic crystal phase. Roughly speaking the CBr4 are situated on the corners of the cubic unit cell as well as on the centers of its faces in an fcc arrangement. It was thought in the past that the molecules could rotate more or less freely (a 'rotor phase'), so that on a time average they would look like spheres. Recent work [3] has shown however that the molecules are restricted to only 6 possible orientations (Frenkel disorder). Moreover, they cannot take these orientations entirely independently from each other because in some cases the bromine atoms of neighboring molecules would point at each other leading to impossibly short distances. This rules out certain orientational combinations when two neighbor molecules are considered. Even for the remaining combinations displacive changes occur that better accommodate neighbor to neighbor distances. The combination of censored Frenkel disorder and displacive disorder implies a considerable amount of disorder inside the crystal which leads to highly structured sheets of diffuse scattered intensity in X-ray diffraction. In fact it is the structure in the diffuse intensity that provides the information about the details of the structure.

Chemical reactions

In combination with triphenylphosphine, CBr4 is used in the Appel reaction, which converts alcohols to alkyl bromides. It is significantly less stable than lighter tetrahalomethanes.

It is prepared by methane bromination using HBr or Br2. It can be also prepared by more economical reaction of tetrachloromethane with aluminium bromide at 100 °C.[2]

Uses

It is used as a solvent for greases, waxes and oils, in plastic and rubber industry for blowing and vulcanization, further for polymerization, as a sedative and as an intermediate in manufacturing agrochemicals. Due to its non-flammability it is used as an ingredient in fire resistant chemicals. It is also used for separating minerals because of its high density.

References

  1. ^ a b F. Brezina, J. Mollin, R. Pastorek, Z. Sindelar. Chemicke tabulky anorganickych sloucenin (Chemical tables of inorganic compounds). SNTL, 1986.
  2. ^ a b N. N. Greenwood, A. Earnshaw. Chemie prvku (Chemistry of the Elements). Informatorium, Prague, 1993.
  3. ^ Coupled orientational and displacive degrees of freedom in the high-temperature plastic phase of the carbon tetrabromide α-CBr4 Jacob C. W. Folmer, Ray L. Withers, T. R. Welberry, and James D. Martin. Physical Review B 77 in press

External links

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