Preparation of Alkanes

Think of yourself on that journey, with the sunroof open, your favorite music playing back-to-back, cruising down the highway. Those compounds that fuel the energy to power the engine of your car are called alkanes. These alkanes are sometimes called paraffins; they're a very important class of compounds that contribute to some of the most basic functionalities for car moving, home heating, and industry power generation.

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This Story also Contains

1. Understanding Alkanes
2. Methods of Preparation
3. Preparation of Alkanes (Grignard Reagent)
4. Preparation of Alkanes (Corey-House Reaction, Reduction of Alkyl Halides by LiAlH4, Wurtz Reaction)
5. Preparation of Alkanes (Decarboxylation and Kolbe's Electrolysis)
6. Conclusion

Alkanes are not only practically important but also important in the sense of preparation, as they introduce the basis of the study found in organic chemistry. Alkanes are the simplest class of hydrocarbons, consisting solely of carbon and hydrogen atoms. The carbons are linked in a chain and the hydrogens are attached to the carbons by single bonds, as pictured above. The general formula for alkanes is given as C_nH_2n+indicating they are saturated with hydrogen atoms bonded to each available carbon atom. Introduction

Understanding Alkanes

Alkanes are also sometimes called paraffins and are defined as saturated hydrocarbons with the general formula $\mathrm{CnH} 2 \mathrm{n}+2$ This general description refers to the idea that the carbon atoms connect only via single covalent bonds, which makes them quite stable and not very reactive. So, the simplest alkane would be methane$\mathrm{CH}_4$ and then ethane C₂H₆,propaneC₃H_8, and so on. A number of chemical reactions amalgamate various reactions aimed at preparing alkanes from simpler compounds or other hydrocarbons. Among these methods are hydrogenation of the alkenes, reduction of the alkyl halides, and decarboxylation of the carboxylic acids in key ways.

Dihydrogen gas adds to alkenes and alkynes in the presence of finely divided catalysts like platinum, palladium, or nickel to form alkanes. This process is called hydrogenation. These metals adsorb dihydrogen gas on their surfaces and activate the hydrogen–hydrogen bond. Platinum and palladium catalyse the reaction at room temperature but relatively higher temperature and pressure are required with nickel catalysts.

$\begin{array}{ll}\mathrm{CH}_2=\mathrm{CH}_2+\mathrm{H}_2 \xrightarrow{\mathrm{Pt} / \mathrm{Pd} / \mathrm{Ni}} \mathrm{CH}_3-\mathrm{CH}_3 \\ \text { Ethene } & \text { Ethane }\end{array}$

$\begin{array}{ll}\mathrm{CH}_3-\mathrm{C} \equiv \mathrm{C}-\mathrm{H}+2 \mathrm{H}_2 \stackrel{\mathrm{Pt} / \mathrm{Pd} / \mathrm{Ni}}{\longrightarrow} \mathrm{CH}_3-\mathrm{CH}_2-\mathrm{CH}_3 \\ \text { Propyne } & \text { Propane }\end{array}$

It is to be noted that the highly strained ring of cyclopropane under ring opening upon catalytic hydrogenation to form propane.

Methods of Preparation

Preparation of Alkanes (Hydroboration Reaction)

BH₃ when added to an alkene and then is oxidized, will give an alcohol. After it is reduced, it can be converted to an alkane. As for example, ethene or C₂H₄ , when added to BH₃ , after oxidation and reduction, the product will be ethane or $\mathrm{C}_2 \mathrm{H}_6: \mathrm{C} _2 \mathrm{H}_4+\mathrm{BH}_3\rightarrow\mathrm{C}_2 \mathrm{H}_5 \mathrm{~B} \rightarrow \mathrm{C}_2 \mathrm{H}_ 5 \mathrm{OH} \rightarrow \mathrm{C}_2 \mathrm{H}_6$

Diborane(B₂H₆) adds to an olefinic bond-forming trialkyl borane which on treatment with acetic acid or propionic acid yields the corresponding alkane.

$\mathrm{R}-\mathrm{CH}=\mathrm{CH}_2 \xrightarrow{\mathrm{B}_2 \mathrm{H}_6}\left(\mathrm{R}-\mathrm{CH}_2-\mathrm{CH}_2-\right)_3 \mathrm{~B} \xrightarrow[\left(\mathrm{H}^{+}\right)]{\mathrm{CH}_3 \mathrm{COOH}} \mathrm{R}-\mathrm{CH}_2-\mathrm{CH}_3$

It is an important method for preparing alkane from an alkene. Methane cannot be prepared by this method.

It is to be noted that the B atom attaches to the less hindered carbon atom in the first step while the H atom attaches to the adjacent C atom containing the double bond.

In the next step, the BH₂ group is replaced by the H of the acid $\left(\mathrm{CH}_3 \mathrm{COOH}\right.$ or $\left.\mathrm{H}_2 \mathrm{SO}_4\right)$

The mechanism of this reaction is beyond the scope of the syllabus

Preparation of Alkanes (Grignard Reagent)

Synthesis of Alkanes by the Grignard Reagent
Grignard reagents, RMgX, react with water to give alkanes. It is thus possible to prepare alkanes by this method from alkyl halides. Example :$\mathrm{CH}_3 \mathrm{MgBr}+\mathrm{H} 2 \mathrm{O} \rightarrow \mathrm{CH}_4+\mathrm{Mg}(\mathrm{OH}) \mathrm{Br}$

Alkyl magnesium halides(RMgX) are called Grignard reagents. These undergo double decomposition reactions with water or ammonia or alcohol or amine having active H atom(attached to strongly electronegative O, N, S, or F and triple bond, etc.) to give alkane corresponding to an alkyl group of Grignard reagent. The reaction occurs as follows:

$\mathrm{CH}_3 \mathrm{MgBr}+\mathrm{C}_2 \mathrm{H}_5 \mathrm{OH} \longrightarrow \mathrm{CH}_4+\left(\mathrm{C}_2 \mathrm{H}_5 \mathrm{O}\right) \mathrm{MgBr}$

A similar reaction occurs with other sources of acidic hydrogen or acids

$\mathrm{CH}_3 \mathrm{MgBr}+\mathrm{H}_2 \mathrm{O} \longrightarrow \mathrm{CH}_4+(\mathrm{OH}) \mathrm{MgBr}$

$\mathrm{CH}_3 \mathrm{MgBr}+\mathrm{NH}_3 \longrightarrow \mathrm{CH}_4+\left(\mathrm{NH}_2\right) \mathrm{MgBr}$

$\mathrm{CH}_3 \mathrm{MgBr}+\mathrm{RNH}_2 \longrightarrow \mathrm{CH}_4+(\mathrm{RNH}) \mathrm{MgBr}$

$\mathrm{CH}_3 \mathrm{MgBr}+\mathrm{R}-\mathrm{C} \equiv \mathrm{C}-\mathrm{H} \longrightarrow \mathrm{CH}_4+(\mathrm{R}-\mathrm{C} \equiv \mathrm{C}) \mathrm{MgBr}$

$\mathrm{CH}_3 \mathrm{MgBr}+\mathrm{CH}_3 \mathrm{COOH} \longrightarrow \mathrm{CH}_4+\left(\mathrm{CH}_3 \mathrm{COO}\right) \mathrm{MgBr}$

It is to be noted that Grignard reagents are not stable in Protic Solvents like Water or Ethanol and require aprotic solvents like Ether or Tetrahydrofuran (THF) for their synthesis and reactions.

Preparation of Alkanes (Corey-House Reaction, Reduction of Alkyl Halides by LiAlH₄, Wurtz Reaction)

Corey House Synthesis
It is suitable for the preparation of alkanes with the odd number of carbon atoms by the following S_N² mechanism.

$
\mathrm{R}-\mathrm{X} \xrightarrow[\text { Ether }]{\mathrm{Li}} \mathrm{R}-\mathrm{Li} \xrightarrow{\mathrm{CuI}} \mathrm{R}_2 \mathrm{CuLi} \xrightarrow[\mathrm{S}_{\mathrm{N}^2}]{\mathrm{R}^{\prime} \mathrm{X}} \mathrm{R}-\mathrm{R}^{\prime}+\mathrm{R}-\mathrm{Cu}+\mathrm{LiX}
$

For example:

$
\mathrm{CH}_3 \mathrm{I}+\left(\mathrm{CH}_3 \mathrm{CH}_2\right)_2 \mathrm{CuLi} \longrightarrow \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}_3+\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{Cu}+\mathrm{LiI}
$

It is to be noted that in this method of preparation, tertiary halides should be avoided as they may lead to the formation of Alkenes via the elimination mechanism

Reduction by LiAlH₄
In this reaction, alkyl halides are reacted with LiAlH₄ (a strong reducing reagent) to reduce to alkanes. The reaction occurs as follows:

$\mathrm{R}-\mathrm{X}+\mathrm{LiAlH}_4 \rightarrow \mathrm{R}-\mathrm{H}$

It is to be noted that in this method of preparation, tertiary halides should be avoided as they may lead to the formation of Alkenes via the elimination mechanism

Wurtz reaction
Alkyl halides on treatment with sodium metal in dry ethereal (free from moisture) solution give higher alkanes. This reaction is known as the Wurtz reaction and is used for the preparation of higher alkanes containing an even number of carbon atoms.

$\begin{aligned} & \mathrm{CH}_3 \mathrm{Br}+2 \mathrm{Na}+\mathrm{BrCH}_3 \xrightarrow{\text { dry ether }} \mathrm{CH}_3-\mathrm{CH}_3+2 \mathrm{NaBr} \\ & \mathrm{C}_2 \mathrm{H}_5 \mathrm{Br}+2 \mathrm{Na}+\mathrm{BrC}_2 \mathrm{H}_5 \xrightarrow{\text { dry ether }} \mathrm{C}_2 \mathrm{H}_5-\mathrm{C}_2 \mathrm{H}_5\end{aligned}$

Wurtz Fittig Reaction

A modification of the Wurtz reaction is the Wurtz Fitting reaction in which an Alkyl halide and an Aryl halide on treatment with sodium metal in dry ether give substituted aromatic compounds. This reaction involves the coupling of an alkyl halide and an aryl halide.

$\begin{aligned} & \mathrm{PhBr}+2 \mathrm{Na}+\mathrm{BrCH}_3 \xrightarrow{\text { dry ether }} \mathrm{Ph}-\mathrm{CH}_3+2 \mathrm{NaBr} \\ & \mathrm{PhBr}+2 \mathrm{Na}+\mathrm{BrC}_2 \mathrm{H}_5 \xrightarrow{\text { dry ether }} \mathrm{Ph}-\mathrm{C}_2 \mathrm{H}_5+2 \mathrm{NaB}\end{aligned}$

Frankland reaction

Alkyl halides on treatment with Zinc metal give higher alkanes. This reaction is known as the Frankland reaction and is used for the preparation of higher alkanes containing an even number of carbon atoms.

$\begin{aligned} & \mathrm{CH}_3 \mathrm{Br}+\mathrm{Zn}+\mathrm{BrCH}_3 \xrightarrow{\text { dry ether }} \mathrm{CH}_3-\mathrm{CH}_3+\mathrm{ZnBr}_2 \\ & \mathrm{C}_2 \mathrm{H}_5 \mathrm{Br}+\mathrm{Zn}+\mathrm{BrC}_2 \mathrm{H}_5 \xrightarrow{\text { dry ether }} \mathrm{C}_2 \mathrm{H}_5-\mathrm{C}_2 \mathrm{H}_5+\mathrm{ZnBr}^2\end{aligned}$

Preparation of Alkanes (Decarboxylation and Kolbe's Electrolysis)

Decarboxylation of Fatty acids
When anhydrous sodium salt of fatty acid is fused with soda lime (NaOH +CaO) a paraffin dry ether having one carbon atom less than the fatty acid is acid. The reaction occurs as follows:

$\mathrm{RCOONa}+\mathrm{NaOH} \xrightarrow[\Delta]{\mathrm{CaO}} \mathrm{RH}+\mathrm{Na}_2 \mathrm{CO}_3$

For example:$\mathrm{CH}_3 \mathrm{COONa}+\mathrm{NaOH} \xrightarrow[\Delta]{\mathrm{CaO}} \mathrm{CH}_4+\mathrm{Na}_2 \mathrm{CO}_3$

The various important physical properties of alkanes are discussed below:

• State: Due to weak forces, the alkanes up to four carbon atoms, i.e., methane, ethane, propane, and butane are colorless, odorless gases but the next thirteen members are colorless, odorless liquids. Alkanes from C₁₈ onwards are colorless and odorless solids.
• Density: The density of alkanes increases very slowly with the rise of molecular mass until it becomes constant at about 0.8. Thus, all alkanes are lighter than water.
• Solubility: They are generally insoluble in polar solvents such as water but soluble in non-polar solvents like ether, carbon tetrachloride, benzene, etc. The solubility decreases with an increase in molecular mass.
• Boiling point: The boiling points of straight-chain or n-alkanes increase regularly with the increasing number of carbon atoms. Among the isomeric alkanes, the normal isomer has a higher boiling point than the branched-chain isomers. The greater the branching of the chain, the lower will be the boiling point.
• Melting point: The melting points of alkanes do not follow a very smooth gradation with the increase of molecular size. Alkanes with an even number of carbon atoms have a higher melting point than the next lower and next higher alkanes having an odd number of carbon atoms.

Kolbe's Electrolysis

Kolbe's electrolysis involves the electrolysis of carboxylic acids or their salts to form alkanes. For example, the electrolysis of sodium acetate produces ethane:

Sodium or potassium salts of carboxylic acids on electrolytic hydrolysis give alkanes at anode as follows:

$2 \mathrm{R}-\mathrm{COONa} \xrightarrow{\text { electrolytic hydrolysis }} \mathrm{R}-\mathrm{R}+2 \mathrm{CO}_2+2 \mathrm{NaOH}+\mathrm{H}_2 \uparrow$

For example:$2 \mathrm{CH}_3-\mathrm{COONa} \xrightarrow{\text { electrolytichydrolysis }} \mathrm{CH}_3-\mathrm{CH}_3+2 \mathrm{CO}_2+2 \mathrm{NaOH}+\mathrm{H}_2 \uparrow$

Relevance and Application
The preparation of alkanes is pertinent to the chemical industry because alkanes are found in large fractions of natural gas and liquefied petroleum gas, LP gas, which are used in a variety of fuelling processes. These include methane, propane, and butane. In this way, the use of alkanes as raw materials leads to the synthesis of a very wide range of chemicals in the petrochemical industry, from plastics through detergents to pharmaceuticals. Albeit of great importance in the petrochemical industry, knowledge of the methodology comprising the preparation of alkanes is also essential in academic research, since these processes are under research, possibly resulting in the development of more efficient and more sustainable chemical reactions.

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Some Solved Examples

Example 1
Question: Butene-1 may be converted to butane by reaction with which of the following?

1) $\mathrm{Zn}-\mathrm{HCl}$
2) $\mathrm{Sn}-\mathrm{HCl}$
3) $\mathrm{Zn}-\mathrm{Hg}$
4) $\mathrm{Pd} / \mathrm{H}_2$

Solution:

Alkenes from additional product with H₂ (alkanes) under pressure and in the presence of a catalyst (Ni, Pt, or Pd)

$\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}=\mathrm{CH}_2 \xrightarrow{\mathrm{H}_2 / \mathrm{Pd}} \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{CH}_3$

Hence, the answer is the option (4).

Example 2
Question: The treatment of $\mathrm{H}-\mathrm{C}=\mathrm{C}-\mathrm{H}$ with $2 \mathrm{H}_2$ in presence of Pd produces:

1) Alkane
2) gem-dihalide
3) Alcohol
4) Aldehyde

Solution

Dihydrogen gas when added to alkenes and alkynes in the presence of catalysts like platinum, palladium, or nickel forms alkanes. This process is called hydrogenation. These metals adsorb dihydrogen gas on their surfaces and activate the hydrogen–hydrogen bond. Pt and Pd catalyze the reaction at room temperature but relatively higher temperatures and pressure are required with nickel catalysts.

In the presence of Pd, the reaction is:

$\mathrm{H}-\mathrm{C} \equiv \mathrm{C}-\mathrm{H}+2 \mathrm{H}_2 \rightarrow \mathrm{CH}_3-\mathrm{CH}_3$

Hence, the answer is the option (1).

Example 3
Question:

In the following reaction

$\mathrm{CaC}_2+\mathrm{H}_2 \mathrm{O} \rightarrow$

The hybridization of Carbon atom in the product is

1) sp²
2) sp³
3) Both 1 & 2
4) sp

Solution:

Solution

Ethyne is prepared by treating calcium carbide with water.

$\mathrm{CaC}_2+2 \mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{C}_2 \mathrm{H}_2+\mathrm{Ca}(\mathrm{OH})_2$

Here both carbon atoms are sp hybridized.

Hence, the answer is the option (4).

Conclusion

Alkanes belong to one of the most essential classes of hydrocarbon compounds, responsible for a wide range of many proposed applications, such as fuels and chemical feedstocks. Major methods for the preparation of alkanes were discussed in the present paper, including hydroboration reactions, methods using Grignard reagents, the Corey-House reaction, methods of reduction of alkyl halides, the Wurtz reaction, decarboxylation, Kolbe's electrolysis. All have specialty types of reactions carried out with specialty importance towards synthetic application in several industries and academic disciplines. Understanding the methods in the making of these forms of substances unlocks a new frontier in chemical research and its application to the industry.