化学专业英语之烷烃
ALKANES
Number of Isomers
The compounds now assigned the generic name alkane are also referred to as saturated hydrocarbons and as paraffin hydrocarbons. The word paraffin, from the Latin parum affinis(slight affinity)refers to the inert chemical nature of the substances and is applied also to the wax obtanable from petroleum and consisting of a mixture of higher alkanes.
Derivation of the formulas of the pentanes (3 isomers), hexanes (5). and heptanes(9) has already demonstrated the sharp ri in diversity with increasing carbon content.
Normal Alkanes
Successive members of the ries differ in composition by the increment CH2 and form a homologous ries. Thus heptane and octane are homologous hydrocarbons; icosane is a higher homolog of methane.
Saturated Unbranched — Chain Compounds and Univalent Radicals
The first four saturated unbranched acyclic hydrocarbons are called methane .ethane, propane and butane. Names of the higher members of this ries consist of a numerical term, followed by "-ane" with elision of terminal "a" from the numerical term. Examples of the names are shown in the table below. The generic name of- saturated acyclic hydrocarbons (branched or unbranched) is “alkane.”
Examples of names: (n = total number of carbon atoms)
n | | n | | n | |
1 | Methane | 15 | Pentadecane | 29 | Nonacosane |
2 | Ethane | 16 | Hexadecane | 30 | Triacontane |
3 | Propane | 17 | Heptadecane | 31 | Hentriacontane |
4 | Butane | 18 | Octadecane | 32 | Dotriacontane |
5 | Pentane | 19 | Nonadecane | 33 | Tritriacontane |
6 | Hexane | 20 | Icosane | 40 | Tetracontane |
7 | Heptane | 21 | Henicosane | 50 | Pentacontane |
8 | Octane | 22 | Docosane | 60 | Hexacontane |
9 | Nonane | 23 | Tricosane | 70 | Heptacontane |
10 | Decane | 24 | Tetracosarre | 80 | Octacontane |
11 | Undecane | 25 | Pentacosane | 90 | Nonacontane |
12 | Dodecane | 26 | Hexacosane | 100 | Hectane |
13 | Tridecane | 27 | Heptacosane | 132 | Dotriacontahectane |
14 | Tetradecane | 28 | Octacosane | | |
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Saturated branched acyclic hydrocarbon is named by prefixing the designations of the side chains to the name of the longest chain which is numbered from one end to the other by Arabic numerals, the direction being so chon as to give the lowest numbers possible to the side chains. When ries of locants containing the same number of terms are compared term by term, that ries is "lowest" which contains the lowest number on the occasion of the first difference1. This principle is applied irrespective of the nature of the substituents.
The prence of identical unsubstituted radicals is indicated by the appropriate multiplying prefix di-, tri-, tetra-, penta- , hexa- , hepta-, octa- , nona-, deca , etc.
Univalent radicals derived from saturated acyclic hydrocarbons by removal of hydrogen from a terminal carbon atom are named by replacing the ending " — ane" of the name of the hydrocarbon by "—yl". The carbon atom with the free valence is numbered as 1. As a class, the radicals are called normal, or unbranched chain, alkyls.
Stability. — Alkanes are relatively inert, chemically, since they are indifferent to reagents which react readily with alkenes or with alkynes. n-Hexane, for example, is not attacked by concentrated sulfuric acid, boiling nitric acid, molten "sodium hydroxide, potassium per
manganate, or chromic acid; with the exception of sodium hydroxide, the reagents all attack alkenes at room temperature. The few reactions of which alkanes are capable require a high temperature or special catalysis.
Halogenation. —If a test tube containing n-hexane is put in a dark place and treated with a drop of bromine, the original color will remain undiminished in intensity for days. If the solution is expod to sunlight, the color fades in a few minutes. and breathing across the mouth of the tube produces a cloud of condensate revealing hydrogen bromide as one reaction product. The reaction is a photochemical substitution:
Chlorination of alkanes is more general and more uful than bromination and can be effected not only photochemically but also by other methods.
Light initiates chlorination of an alkane by converting chlorine molecules into chlorine atoms by a process of hemolysis, in which a covalent bond is vered and one electron is
retained by each of the atoms forming the bond: Cl:Cl —→Cl • + C1• . A chlorine atom has an odd, or unpaired electron and is a free radical. Becau of the tendency of atoms to attain their normal valence shells, any free radical is a highly reactive species. Photochemical chlorination proceeds through a succession of free radicals; it is a free radical chain reaction. The chain initiating step (1 ), hemolytic fission of chlorine molecules, produces chlorine free radicals; in chain propagating steps, a chlorine radical attacks a molecule of alkane to produce hydrogen chloride and an alkyl radical (2), which in turn attacks a chlorine molecule to produce a chloroalkane and a chlorine radical (3). Since chlorine-radicals required in step (2) are regenerated in step (3), the two reactions together constitute a chain which, if both reactions proceeded with perfect efficiency, would be lf-propagating without further requirement of light energy, The efficiency. however. is not perfect, for chlorine radicals can recombine (4), combine with alkyl radicals ( 5), or dissipate energy by collision with the flask wails. Hence continued radiation is required to maintain an adequate supply of initiating radicals. The chain initiating step requires input of light energy amounting to + 242.8kJ/mole. Step (2), howev
er, is exothermic, since the energy required to break the C — H bond is less than the bond energy of H — Cl. The cond chain propagating step (3) is likewi exothermic, and indeed chlorination of an alkane can proceed explosively.
Cracking. —Heated to temperatures in the range 500 ~ 700°, higher alkanes undergo pyrolytic rupture or cracking to mixtures of smaller molecules, some saturated and some unsaturated. Unsaturated hydrocarbons produced by lective cracking of specific petroleum fractions are uful in chemical synthesis. Cracking ruptures carbon — carbon rather than carbon —hydrogen bonds becau the energy required to break the C —C bond is 247kJ. /mole, whereas the C — H bond energy is 364kJ/mole.
Oxidation. —The reaction of hydrocarbons with oxygen with the output of energy is the basis for u of gasoline as fuel in internal combustion engines. The energy relea on burning a given hydrocarbon is expresd as the heat of combustion in terms of kJ/mole.
Incomplete combustion of gaous hydrocarbons is important in the manufacture of carbon blacks, particularly lampblack, a pigment for ink, and channel black, ud as a fille
r in rubber compounding. Natural gas is ud becau of its cheapness and availability; the yield of black varies with the type of gas and the manufacturing process but usually is in the range of 2~6% of the theoretical amount.
