Alkanes II
Preparation & Reactions
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| Unit 4
Alkanes II Preparation & Reactions |
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Industrial Source
The principal source of alkanes is petroleum, together with the accompanying natural gas. Anaerobic decay and millions of years of time and pressure have transformed the complex organic compounds that were once living and breathing and animals into a mixture of alkanes, ranging in size from one C atom (methane) to 30 or 40 carbon atoms. Forming along with the alkanes, and particularly abundant in California petroleum, are the cycloalkanes, known in the petroleum industry as napthalenes.
The other fossil fuel, coal, is a potential second source of alkanes. Processes are being developed to convert coal, through hydrogenation, into gasoline and fuel oil. It is also a potential source of natural gas.
Natural gas contains only the more volatile alkanes of low molecular weight. It consists chiefly of methane and progressively smaller amounts of ethane, propane and the higher alkanes. The propane-butane fraction is separated from the more volatile components by liquefaction, compressed into air cylinders, and sold as bottled gas in areas not served by a gas utility.
Petroleum is separated by distillation into the various fractions listed here.
Because of the relationship between boiling point and molecular weight, this amounts to a rough separation according to carbon number. Each fraction is still a very complicated mixture, however, since it contains alkanes of a range of carbon numbers and isomers. The practical use of each fraction depends upon its physical properties, such as volatility and viscosity. It matters little whether is is a complicated mixture or a pure single hydrocarbon compound.
Certain petroleum fractions are converted into other kinds of chemical compounds. Catalytic isomerization changes straight-chain alkanes into branched-chain alkanes. The cracking process converts higher alkanes into smaller smaller alkanes and alkenes, and thus increases the yield of gasoline. In addition, the alkenes thus formed are the most important raw material for the large-scale synthesis of organic compounds. The process of catalytic reforming converts alkanes and cylcoalkanes into aromatic hydrocarbons and thus provides the chief raw material for the large-scale synthesis of another broad class of organic compounds.
~~~~ Preparation ~~~~
I. Hydrogenation of Alkenes
Of all the methods we will discuss here, the method hydrogenation of alkenes is by far the most important technologically. When shaken under a slight pressure of hydrogen gas in the presence of a small amount of catalyst, alkenes are converted smoothly and quantitatively onto alkanes of identical carbon skeleton.
The method is limited only by the availability of the proper alkene. But as we shall see, this is not a very serious limitation, as synthetic alkenes are easily prepared by a number of various laboratory techniques -- primarily from alcohols which are themselves readily synthesized.
II. Reduction of Alkyl Halides
a) Hydrolysis of a Grignard Reagent
When a solution of an alkyl halide in dry ethyl ether si allowed to stand over shavings of metallic magnesium, a vigorous reaction occurs. The solution turns cloudy, begins to boil, and the magnesium metal gradually disappears. The resulting solution is known as the Grignard reagent. It is one of the most useful and versatile reagents known to the organic chemist.
The Grignard reagent has the general formula RMgX, and the general name alkylmagnesium halide. The C-Mg bond is covalent but highly polar, with the C atom pulling electrons from the electropositive Mg. Alternatively, the Mg-H bond is essentially ionic. Thus,
R : Mg+ , X-
Since Mg becomes bonded to the same C atom that previously held an H atom, the alkyl group remains intact during the preparation of the reagent (no rearrangement). Thus, an n-propyl halide yields an n-propyl metal halide. Similarly, an isopropyl halide yields an isopropyl metal halide.
This is the most common and best-known member of a broad class of substances we call organometallic compounds. In these compounds, the C atom has been known to bond to almost any metal known (e.g. Mg, Li, K, Na, Zn, Hg, Pb, etc.). Whatever the metal, it is less electronegative than carbon, and the carbon-metal bond is highly polar. Organometallic compounds owe their usefulness largely to their ability to serve as a source from which carbon is readily transferred with its valence electrons intact.
The Grignard reagent is highly reactive. it reacts with numerous inorganic compounds including water, carbon dioxide, oxygen, and most organic compounds. It is extremely useful in organic synthesis.
The reaction with water to form an alkane is typical of the behavior of the Grignard reagent toward acids. In view of the marked carbanion character of the alkyl group, we may consider the Grignard reagent to be the magnesium slat, RMgX, of hte extremely weak acid, R-H. The following reaction:
RMGX + HOH = R-H + Mg(OH)X
is simply the displacement of the weaker acid, R-H, form its salt by the stronger acid, HOH. Furthermore, an alkane is such a weak acid that it is displaced form the Grignard reagent by compounds that we might ordinarily consider to be very weak acids themselves (or possibly not acids at all). Any compound containing an H atom attached to an O tom or an N tom is tremendously more acidic than an alkane, and can therefore decompose the Grignard reagent. Examples are ammonia and methyl alcohol:
RMGX + NH3 = R-H + Mg(NH2)X
RMGX + CH3OH = R-H + Mg(OCH3)X
For the preparation of an alkane, one acid is as good as another. So we naturally choose water as the most available and convenient.
b) Reduction by Metal and Acid
Reduction of an alkyl halide, either via the Grignard reagent or directly with metal and acid, involves the replacement of a halogen X atom by a hydrogen H atom. In either case, the carbon skeleton remains intact. This method has about the same applicability as the previous method, since, like alkenes, alkyl halides are generally prepared form alcohols. Where either method could be used, the hydrogenation of alkenes would probably be preferred because of its simplicity and higher yield.
III. Coupling of Alkyl Halides w/ Organometallics
The coupling of alkyl halides with organometallic compounds is the only one of these methods of preparation in which C-C bonds are formed and a new, bigger carbon skeleton is generated. In order to make an alkane of higher carbon number that the starting material requires the formation of C-C bonds, most directly by the coupling together of two alkyl groups. The most versatile method of doing this is in the reaction between an organometallic complex R2CuLi and an alkyl halide R'X.
An alkyl lithium RLi is prepared form an alkyl halide RX in much the same way as a Grignard reagent. To it is added cuprous halide CuX, and then the second halide R'X. The alkane R-R' is synthesized form the two alkyl halides RX and R'X.
For good yields, R'X should be a primary halide. the alkyl group R in the organometallic may be primary, secondary, or tertiary. Although the mechanism is not understood very well, it is clear that the alkyl group R is transferred form copper -- taking a pair of elctrons with it -- and becomes attached to the alkyl group R' in place of the halide ion.
Common Reactions
The alkanes are sometimes referred to as paraffins, a name which they obtained due to their relatively low level of reactivity towards other elements and compounds. But in reality, reactivity depends upon the choice of reagent.
1) If alkanes are inert toward hydrochloric and sulfuric acids, they react readily with certain fluoric acids to yield a variety of products.
2) If alkanes are inert toward oxidizing agents like KMnO4 (permanganate) or NaCrO3 (dichromate), most of this section is dedicated to their oxidation by halogens.
3) Certain yeasts feed happily on alkanes to produce proteins.
4) Consider the inertness of a room containing a natural gas, air (or oxygen), and a lighted match. It does seem highly unlikely that there will be no pro-active result !
Still, on a relative basis, the reactivity of the alkane family is limited. Fluoric acids are anything but common. Halogenation requires heat or light. Combustion requires a flame or spark in order to initiate the reaction.
Much of the chemistry of alkanes (such as methane) involves free-radical chain reactions, which occur under vigorous conditions and often yield a mixture of products. Thus, as we shall see, they are the least desirable of all the organic precursors for use in the purpose of controlled organic synthesis of specific hydrocarbon compounds.
I. Halogenation
As we might expect, halogenation of the higher alkanes is essentially the same as the halogenation of methane. The main difference is the complications that arise due to structural variation in the form of isomers. [ See details under the preparation of Alkyl Halides - Unit 8 ].
More details to come soon on the:
1) Mechanism
2) Orientation
3) Relative Reactivities
4) Stability of Free Radicals
5) Transition States
6) Absence of Rearrangement
...........in the Halogenation reaction of alkanes (see Unit 1: Methane).
II. Combustion
The reaction of alkanes with oxygen to form carbon dioxide, water and heat is the primary reaction occurring in the internal combustion engine. Its tremendous practical importance is obvious.
The mechanism of the combustion reaction is extremely complicated and not yet fully understood. There seems to be no doubt, however, that it is a free-radical chain reaction. The reaction is extremely exothermic and yet requires a very high temperature (that of a flame: > 1000 degrees C) for its initiation. As in the case of chlorination, a great deal of energy is required for the bond-breaking that generates the initial reactive particles. Once this energy barrier is surmounted, however, the subsequent chain-propagating steps proceed readily and with the evolution of large amounts of energy.
III. Cracking (Pyrolysis)
Decomposition of a compound by the action of heat alone is a physio-chemical process known commonly as pyrolysis.
Pyrolysis is the chemical decomposition of organic materials by heating in the absence of oxygen or any other reagents, except possibly steam. Extreme pyrolysis, that leaves only carbon as the residue, is called carbonization. As such, pyrolysis is a means of reusing the scrap rubber in automobile tires. Pyrolysis in this context is the degradation of the rubber of the tire using heat in the absence of oxygen. It is used to recycle car tires rather than burn or bury them, which can have a detrimental effect on the environment.
The pyrolysis of alkanes, particularly when petroleum is concerned, is known as cracking. In thermal cracking. alkanes are simply passed through a chamber which has been previously heated a high temperature (> 1000 degrees C). Large alkanes are converted into smaller alkanes, alkenes, and hydrogen. This process yields predominantly ethylene (C2H4) together with small molecules.
In a modification called steam cracking, the hydrocarbon is diluted with steam, heated for a fraction of a second to 700 - 900 degrees C, and rapidly cooled or "quenched". Steam cracking is of great importance in the production of such compounds as ethylene, propylene, butadiene, isoprene and cyclopentadiene.
Another source of of smaller hydrocarbons is hydrocracking, which is carried out in the presence of a catalyst and hydrogen, under high pressures and at much lower temperatures (250 - 450 degrees C).
The low molecular weight alkenes which are obtained form these cracking processes can be separated and purified. They provide the most important raw materials for the large-scale synthesis of aliphatic hydrocarbon compounds.