Preparation of Alkanes and Cycloalkanes:

Alkanes are generally attained from natural sources: petroleum and natural gas. However, synthetic methods are more practical whenever a pure alkane is required. Alkanes can be prepared from: (a) alkenes or an alkynes, (b) alkyl halides, and (c) carboxylic acids. We have already studied about the preparation of alkanes and cycloalkanes in our previous classes. Here we remind only the significant methods. Common methods for the preparation of alkanes and cycloakanes are summarized in Table.

Table: Reactions for the preparation of alkanes and cyclokanes

Alkanes:

Wurtz reaction:

In the Wurtz reaction, an alkyl halide is treated by sodium in the presence of dry ether. The consequence is the joining of the 2 alkyl groups from 2 molecules of alkyl halide through the loss of halogens.

 2RX + 2Na → RR + 2NaX

This reaction is useful only when 2 identical alkyl halide molecules are employed. When a mixture of 2 different alkyl halides is utilized, a mixture of three different alkane is attained. For instance, if we take a mixture of bromomethane and bromoethane, we will get 3 different products, viz.

CH₃Br + C₂H₅Br + 2Na   → CH₃-C₂H₅ + 2NaBr

CH₃Br + CH₃Br + 2Na →CH₃-CH₂ + 2NaBr

C₂H₅Br + C₂H₅Br + 2Na →C₂H₅-C₂H₅ + 2NaBr

The division of these is a mixture into individual alkane is quite hard. Therefore, the Wurtz reaction between 2 different alkyl halides is generally useless in practice. Whenever a single alkyl halide is utilized, the synthesized hydrocarbon encloses an even number of carbon atoms. In other terms, we can say that Wurtz reaction is suitable for the preparation of only those alkanes that have an even number of carbon atoms. As illustrate above, the major difficulty through the Wurtz reactions is the formation of many side products when an alkane by odd number of carbon atoms is desired.

Kolbe's electrolytic method:

Whenever a concentrated solution of sodium or potassium salt of a carboxylic acid is electrolyzed, an alkane is shaped. The process is recognized as Kolbe's electrolytic method.

RCOOK + RCOOK + H₂O → RR + 2CO₂ + H₂ + 2KOH

                                          at anode           at cathode

 The following mechanistic pathway demonstrates this process:

 CH₃COOK →CH₃COO^- +K⁺

      at cathode

In case a mixture of salts of two carboxylic acids is electrolysed, a mixture of alkanes is formed:

                              R'R'

R'COOK+R''COOK R"R" + CO₂ +H₂ +KOH

                              R"R"

This reaction has limited synthetic applications because of the formation of many side products as a result of other reactions of the free radicals formed.

Hydrogenation of Unsaturated Hydrocarbons:

Alkanes or cycloalknes can be prepared via hydrogenation of unsaturated hydrocarbons using platinum or palladium as a catalysts. The general reaction to the reduction of alkene is:

                        Pd or Pt

RCH=CHR + H2  R CH₂CH₂R'

      alkane                           alkane

                         Pd or Pt

RC ≡ CR + 2H2  RCH₂CH₂R

     alkyne                          alkene

Hydrogenation of an alkene can as well be carried out via using nickel catalyst but relatively higher temperature and pressure are needed for this reaction. This reaction is termed Sabatier Senderen's reaction. An instance is following below:

                                   Ni

CH₂CH=CH₂ + H₂   CH₃CH₂CH₃

    1-propene                              propane

This is a extremely helpful synthetic process and the yield is nearly 100%.

Reduction of Alkyl Halides:

Alkanes can as well be prepared via the reduction of alkyl halides through diverse methods. Reducing agents like zinc and acetic acid and zinc-copper couple give good yields of alkanes.

     reducing

RX zRH

       agent

Lithium aluminium hydride, LiAlA₄, is an excellent reducting agent. Thought it decreases many unsaturated functional groups, that as CO₃, C=N, etc, it does not attack isolated double bond or triple bond. Dry ether is the usually utilized solvent. For instance,

Alkyl halide in ether reacts with magnesium to form alkyl magnesium halide (Grignard reagent) which, on treatment by water or dilute acid, decomposes to provide alkanes. We will take up the preparation and properties of Grignard reagents.

Hydrogenation of an alkene can as well be carried out via using nickel catalyst but relatively higher temperature and pressure are needed for this reaction. This reaction is termed Sabatier-Senderen's reaction. An instance is following below:

                                   Ni

CH₂CH=CH₂ + H₂   CH₃CH₂CH₃

  l-propene                          propane 

This is an extremely useful synthetic process and the yield is nearly 100%.

Decarboxylation of the Carboxylic Acids:

Alkanes might be prepared by decarboxylation of carboxylic acids via heating a mixture of the sodium salt of a carboxylic acid by soda lime. Soda lime is a mixture of NaOH and CaO. The active ingredient NaOH , CaO assists in keeping the reaction mixture porous.

 RCOONa + NaOH    RH + Na₂CO₃

This procedure of eliminating CO2 from a carboxylic acid is recognized as decarboxylation. The alkanes so produced have one carbon atom less than the original acid. The new hydrogen atom in the product is derived from soda lime. Even though methane is obtained from ethanoic acid in good yield, other acids give only 10-20% of the corresponding hydrocarbon. Sometimes decarboxylation of the acid itself is more effective than that of its salt. The direct decarboxylation of a carboxylic acid can be carried out via heating it with an organic base, theser as pyridine using copper chromite (CuO.Cr₂O₃) as catalyst

                                  CuO.CR 2O₃

CH₃CH₂COOH + C₅H₅N    CH₃CH₃ + CO₂

    Propanoic       pyridine                 ethane

         acid

Next we discuss two methods of preparation of cycloalkanes.

Preparation of Cycloalkanes:

(i)  When 1, 5-dihalogen derivatives of alkanes are treated by sodium or zinc, the analogous cycloalkane is formed, for example 1, 5-dibromopentane would shape the cyclopentane.

(ii)  Whenever the calcium or barium salt of a dicarboxylic acid is distilled, a cyclic ketone is formed, for example, barium adipate gives cyclopentanone

Salt of dicarboxylic acid                             cyclopentanone

A cyclic ketone can be decreased into the analogous cycloalkane using zinc amalgam and concentrated hydrochloric acid (Clemmenson reduction).

Reactions of Alkanes:

Alkanes are comparatively unreactive to most of the general reagents. It is hard to describe the terms 'reactive' and 'unreactive', because a compound might be reactive under one set of conditions and unreactive under the other. This reactivity or unreactivity might be illustrated via considering the nature of C - C and C - H bonds present in their molecules. Since the electronegativities of carbon and hydrogen don't differ noticeably, the bonded electrons in C -H are more or less similarly shared between them. Therefore, C - H bonds encountered in alkanes are approximately nonpolar and similar is true of C - C bonds. Therefore, polar and ionic reagents discover no sites to attack an alkane molecule. Alkanes undergo mostly substitution reactions that can be illustrated using free radical chain mechanism. Such reactions occur in the presence of UV light or at a high temperature or in the presence of certain free radical initiators such as peroxide.  The chemical reactions that occur in the presence of light are termed photochemical reactions. In substitution reactions, one or more of the H atom(s) of alkanes are substituted via halogen or some another groups. Several significant reactions of alkanes are following in Table.

Table:  Reactions of Alkanes

Let us discuss these reactions in detail.

Halogenation

Halogenation of alkane is one of the most important reactions of alkanes. It is defined as the replacement of hydrogen atom(s), from an alkane molecule, by halogen atom(s)

                  Δ/hv

RH + X₂   RX + HX  

The reaction does not occur in dark but a vigorous reaction takes place whenever the mixture of alkane and halogen is depiction to light or heated to a high temperature. But in most cases, the reaction is of limited synthetic value because a mixture of products is obtained. Multiple substitutions may occur. For example, chlorination of methane produces a mixture of chloromethane, dichloromethane, trichloromethane and tetrachloromethane.

                     hv

CH₄ + Cl₂    CH₃Cl

                             chloromethane

                        hv

CH₃Cl + Cl₂   CH₂Cl₂

                              dichloromethane 

                        hv

CHCl₂ + Cl₂ CHCl₃

                              Trichloromethan

                           hv

CHCl₃^- + Cl₂     CCl₄

                                tetra chloromethane

The yield of the monosubstituted product may increase by using an excess of alkane. Similarly, a cycloalkane reacts with halogen to give halocyclalkane, for example:-

The mechanism of halogenation is supposed to involve the following

Steps:

In the first step, the halogen molecule undergoes homolysis forming free radicals. This step it called chain initiation:

             hv/Δ

(i)  X2  2X

In the next step, the halogen atom abstracts a hydrogen atom from the alkane molecule thereby producing an alkyl radical.

(ii)  RH + X ^·  HX + R^·

The alkyl radical on collision with another molecule of halogen abstracts a  halogen atom from it  generating a molecule of the alkyl halide and a halogen atom. These two steps are called propagation. They are repeated in sequence till the reactants are consumed.

 (iii)  X2 + R^· RX + X^·

Finally, the above chain might be terminated via coupling of any 2 radicals. This step is recognized as termination. 

(iv)  X^·+ X^· XX

        R^·+ X^· RX

        R^· + R^· RR

The array of reactivity of halogen in halogenation of alkanes is:

 F₂ > Cl₂ > Br >I₂

Though, the fluorination reaction is as well violent to be practical, and iodine actually doesn't react at all.

Nitration:

Alkanes mainly the higher member, can be nitrated by nitric acid at a temperature of 675-775 K. Like halogenation, it is as well a free radical reaction.

 RH + HNO₃  RNO₂ + H₂O

In this reaction, the product it generally a mixture of nitroalkanes including those through smaller carbon chain than the parent alkane.

                        HNO₃

CH₃CH₂CH₃      CH₃CH₂CH₂NO₂ + CH₂CHCH₃ + CH₃CH₂NO₂ + CH₃NO₂

Propane                         l-nitropropane          2-nitropropane     nitroethane    nitromethane

Isomerisation:

The molecular rearrangement of one compound into another compound or into more than one compound is termed isomerisation. The straight chain alkanes are converted into branched chain isomers in the presence of aluminum chloride and hydrogen chloride.

                                                                CH₃

                             AlCl₃ - HCl                  |

CH₃CH₂CH₂CH₃                    CH₃CHCH₃

    Butane                                        2-methylpropane

Similarly, other less branched alkanes isomerise to more branched ones. 

 Therefore :

     CH₃                                       CH₃        CH₃

      |                                            |              |

 CH₃CHCH₂CH₂CH₃   CH₃CH - CH₃CHCH₃

  2-methylpentane                    2, 3-methylpropane

Isomerization is used to increase the branched chain content of lower alkanes produced by cracking.

Aromatization:

This is a procedure of translating aliphatic or acyclic compounds to aromatic hydrocarbons. Alkanes by 6 or more carbon atoms, whenever heated strongly under pressure in the presence of a catalyst, provide aromatic hydrocarbons. This method involves cyclisation, isomerization and dehydrogenation. Aromatization of gasoline increases their octane number from 40 - 95 since unsaturated hydrocarbons are better fuels.

Catalytic aromatization in the occurrence of platinum is sometimes referred to as platforming or hydroforming. This method as well constitutes a value method for commercial production of such hydrocarbons.

Pyrolysis:

This is a procedure of decomposing an organic substance via heating it to high temperature in the absence of oxygen. The word pyrolysis is in use from the Greek words pyro (fire) and lysis (disintegration). The pyrolysis of alkanes, particularly wherever petroleum is concerned, is recognized as cracking. Whenever an alkane is heated to about 775-875 K, it decomposes into smaller molecules. For instance, on cracking propane, the possible products are:

CH₃CH = CH₂ + H₂   ←   CH₃CH₂CH₃ →   CH₄    +   CH₂ = CH₂

Propane                            propane                 methane         ethane

Large quantities of high boiling fractions of petroleum are converted into low boiling gasoline via cracking. Propane and hydrogen are produced from propane as a consequence of fission of C- H linkages. In the case of higher alkanes, fission of C - C linkages occurs more readily. The presence of catalysts similar to oxides of chromium, vanadium and molybdenum, though, accelerates the fission of C - H linkage. Pyrolysis in the presence of a catalyst is utilized in the manufacture of alkenes.

The mechanism of cracking is at rest obscure, but it is believed to be a free radical as illustrated below:

CH₃CH₂CH₃ CH₃CH₂CH₂ + H  CH₃CH = CH₂ + H ₂

         ↓

CH₃CH₂ + CH₃       CH₂=CH₂ + CH₄

The products formed during cracking of alkanes depend upon: (i) the structure of alkane, (ii) the pressure employed, and (iii) the presence or absence of a catalyst.

Combustion:

Alkane's burn in excess of air or oxygen to provide carbondioxide and water. This reaction is recognized as combustion and is the most significant of all their reactions. Combustion is highly exothermic and accounts for their utilize as precious fuels. It is a free radical chain reaction and requires an extremely high temperature for its initiation. Once the reaction is started, the subsequent chain-carrying steps proceed readily by the evolution of a large amount of energy. For instance, the heat of combustion of pentane is 2549 kJ mol^-1.

 CH₃CH₂CH₂CH₂CH₃ + 8O₂  5CO₂ + 6H₂O + 2549 kJ mol^-1

The large quantity of heat evolved can be a source of extensive power. Therefore, the utilize of petrol, diesel etc., as fuels in internal combustion engines. The burning of alkanes as well produces carbon black that is utilized in the manufacture of Indian ink, printer's ink, black pigments and as filler in rubber compounding.

Reactions of Small Ring Compounds:

Let us now learn the reactions characteristic of small ring compounds, these as cyclopropane and cyclobutane. In addition, the free radical substitution reactions that are characteristic of cycloalkanes and of alkanes, in common, cyclopropane and, to several extents, cyclobutane undergo certain addition reactions. We will remind from our earlier classes that the bonding in cyclopropane and cyclobutane is not as strong as that in higher homologues. Therefore, the bonds in cyclopropane and cyclobutane are vulnerable to attack via certain consequences. The sp³ orbitals of the carbon atoms in cyclopropane can't undergo complete overlap through each other since the angles between the carbons atoms of cyclopropane are geometrically needed to be 600. The ring sigmas bonds of cyclopropane are, thus, less stable than sp³ sigma bonds that contain the normal tetrahedral angle. Such addition reactions destroy the cyclopropane and cyclobutane ring system, and yield open chain products. Some instances are following below: 

In each of such reactions, a carbon-carbon bond is broken and the 2 atoms of the reagent appear at the terminal carbon atoms.

Cyclobutane doesn't undergo most of the ring opening reactions of cyclopropane; it gets hydrogenated under vigorous situations. So we can see that cyclobutane undergoes addition reactions less readily than cyclopropane.

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