Chemistry:Carbon tetrabromide

Carbon tetrabromide

Names
Preferred IUPAC name Tetrabromomethane
Other names Carbon(IV) bromide Carbon bromide, neutral (1:4) Carbon tetrabromide
Identifiers
CAS Number	558-13-4 Y
3D model (JSmol)	Interactive image
Abbreviations	R-10B4
Beilstein Reference	1732799
ChEBI	CHEBI:47875 Y
ChemSpider	10732 Y
EC Number	209-189-6
Gmelin Reference	26450
MeSH	carbon+tetrabromide
PubChem CID	11205
RTECS number	FG4725000
UNII	NLH657095L Y
UN number	2516
InChI InChI=1S/CBr4/c2-1(3,4)5 Y Key: HJUGFYREWKUQJT-UHFFFAOYSA-N Y
SMILES BrC(Br)(Br)Br
Properties
Chemical formula	CBr4
Molar mass	331.627 g·mol−1
Appearance	Colorless to yellow-brown crystals
Odor	sweet odor
Density	3.42 g mL−1
Melting point	94.5 °C; 202.0 °F; 367.6 K
Boiling point	189.7 °C; 373.4 °F; 462.8 K decomposes
Solubility in water	0.024 g/100 mL (30 °C)
Solubility	soluble in ether, chloroform, ethanol
Vapor pressure	5.33 kPa (at 96.3 °C)
Magnetic susceptibility (χ)	-93.73·10−6 cm3/mol
Refractive index (nD)	1.5942 (100 °C)
Structure
Crystal structure	Monoclinic
Coordination geometry	Tetragonal
Molecular shape	Tetrahedron
Dipole moment	0 D
Thermochemistry
Heat capacity (C)	0.4399 J K−1 g−1
Std molar entropy (So298)	212.5 J/mol K
Std enthalpy of formation (ΔfH⦵298)	26.0–32.8 kJ mol−1
Gibbs free energy (ΔfG˚)	47.7 kJ/mol
Std enthalpy of combustion (ΔcH⦵298)	−426.2–−419.6 kJ mol−1
Hazards
Safety data sheet	inchem.org
GHS pictograms
GHS Signal word	DANGER
GHS hazard statements	H302, H315, H318, H335
GHS precautionary statements	P261, P280, P305+351+338
NFPA 704 (fire diamond)	0 2 0
Flash point	noncombustible
Lethal dose or concentration (LD, LC):
LD50 (median dose)	56 mg kg−1 (intravenous, mouse) 100 mg kg−1 (oral, rat)
NIOSH (US health exposure limits):
PEL (Permissible)	none
REL (Recommended)	TWA 0.1 ppm (1.4 mg/m3) ST 0.3 ppm (4 mg/m3)
IDLH (Immediate danger)	N.D.
Related compounds
Other anions	Carbon tetrafluoride Carbon tetrachloride Carbon tetraiodide
Other cations	Silicon tetrabromide Germanium tetrabromide Tin(IV) bromide
Related alkanes	Dibromomethane Bromoform 1,1-Dibromoethane 1,2-Dibromoethane Tetrabromoethane
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

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Carbon tetrabromide, CBr₄, also known as tetrabromomethane, is a bromide of carbon. Both names are acceptable under IUPAC nomenclature.

Production

CBr₄ can be obtained by the bromination of methane. The byproducts include other brominated methanes (methyl bromide, dibromomethane and bromoform) and hydrogen bromide. This process is analogous to the chlorination of methane:

Br₂ + hν → 2 Br·;

Br· + CH₄ → ·CH₃ + HBr.

·CH₃ + Br₂ → CH₃Br + Br·.

CH₃Br + Br· → ·CH₂Br + HBr,

·CH₂Br + Br₂ → CH₂Br₂ + Br·,

CH₂Br₂ + Br· → ·CHBr₂ + HBr,

·CHBr₂ + Br₂ → CHBr₃ + Br·,

CHBr₃ + Br· → ·CBr₃ + HBr,

·CBr₃ + Br₂ → CBr₄ + Br·

Halogen exchange of carbon tetrachloride with aluminium bromide gives higher yields with aluminium chloride as the byproduct:

4 AlBr₃ + 3 CCl₄ → 4 AlCl₃ + 3 CBr₄

Physical properties

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

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.

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 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. Similarly, CBr₄ is used in combination with triphenylphosphine in the first step of the Corey-Fuchs reaction, which converts aldehydes into terminal alkynes. It is significantly less stable than lighter tetrahalomethanes. It is made via bromination of methane using HBr or Br₂. It can be also prepared by more economical reaction of tetrachloromethane with aluminium bromide at 100 °C.

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.

External links

• International Chemical Safety Card 0474
• NIOSH Pocket Guide to Chemical Hazards. "#0106". National Institute for Occupational Safety and Health (NIOSH). https://www.cdc.gov/niosh/npg/npgd0106.html. 
• MSDS at SIRI.org