User:Sfs031317/SandBox4
四溴甲烷，又稱為四溴化碳、R-10B4及UN 2516，是一種鹵代烴（化學式：CBr4）。它既可以被視為一種鹵代烴、鹵代甲烷、全溴化碳，也可以被視為一種無機化合物。
物理性質[编辑]
四溴甲烷擁有兩個同分異構物：分別為crystaline II or β below 46.9 °C (320.0 K) and crystalline I or α above 46.9 °C. 單斜晶系 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[编辑]
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.
化學反應[编辑]
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]
用途[编辑]
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.
參考資料及注釋[编辑]
- ^ 1.0 1.1 F. Brezina, J. Mollin, R. Pastorek, Z. Sindelar. Chemicke tabulky anorganickych sloucenin (Chemical tables of inorganic compounds). SNTL, 1986.
- ^ 2.0 2.1 N. N. Greenwood, A. Earnshaw. Chemie prvku (Chemistry of the Elements). Informatorium, Prague, 1993年.
- ^ 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
參見[编辑]
外部連結[编辑]