Alkanes: Preparation, Decarboxylation, Kolbe’s Electrolysis, Physical Properties, Chemical Properties, Practice Problems, FAQs

Hydrocarbons have been playing an important role in our daily lives for a very long time. A significant portion of these compounds is saturated hydrocarbons.
Can you guess some saturated hydrocarbons which we can find around us? Let me give you a hint! 
CNG, which we use as a fuel for transport majorly contains methane and ethane which are alkanes. Similarly LPG or cooking gas majorly contains propane and butane. 

So in this article, you will get a deep understanding of alkanes and their preparation and properties.

Table of Contents

• Alkanes
• Preparation of alkanes from alkenes and alkynes
• Preparation of ethane from ethene
• Preparation of alkanes from alkyl halides
• Decarboxylation by soda-lime
• Kolbe’s Electrolysis
• Physical Properties of Alkanes
• Chemical Properties of Alkanes
• Practice Problems
• Frequently asked questions

Alkanes

The aliphatic saturated hydrocarbons are known as alkanes. The general formula for an alkane is CnH2n+2Here, n is the number of carbon atoms.

Examples: CH₄, C₂H₆,C₃H₈,C₄H₁₀,C₅H₁₂,C₁₀H₂₂, etc.

The simplest alkane is methane, which is an important constituent of CNG. Alkanes are also known as paraffins because they have less affinity toward many reagents.
Here, the low reactivity does not mean that they show lower reactivity with all reagents in general.

For example, CNG is made from alkanes. It shows a violent reaction to heating.

Preparation of alkanes from alkenes and alkynes

Catalytic hydrogenation of alkenes and alkynes involves the addition of H₂ in the presence of a catalyst (Ni/Pt/Pd). Here, H_2(g) acts as a reducing agent.

Catalyst: In general, it is a reagent that increases the speed of a reaction without getting consumed in the chemical reaction. However, in certain cases, a catalyst can also decrease the speed of a reaction. 

Catalysts Pt and Pd catalyze the reaction at room temperature. However, catalyst Ni requires a relatively higher temperature and pressure.

Catalytic addition of hydrogen is a syn addition reaction.

Examples:

1. Ethene gives ethane on reduction with H2 in the presence of a metal catalyst (Ni/Pt/Pd).

2. Prop-1-yne gives propane on reduction with H2 in the presence of a metal catalyst (Ni/Pt/Pd).

Preparation of ethane from ethene

Hydrogen (H₂) and alkene are first adsorbed on the surface of the metal catalyst. Then, the partial making and breaking of the bonds occur (The π bond of alkene, σ bond of hydrogen, H₂, breaks partially, while the new C–H bonds are formed partially in the transition state) and give a four-membered cyclic transition state. Hydrogens are added to the same side of the double bond, i.e., syn addition takes place. Finally, the alkane is formed, which gets desorbed from the surface of the metal catalyst.

Preparation of alkanes from alkyl halides

For the preparation of alkane, alkyl halides can be treated with either Zn or dil. HCl or Na, dry ether.

a) By reduction of an alkyl halide with Zn/dil. HCl.
The number of carbon atoms in the product remains the same as that of the reactant (alkyl halide). In this reaction, the halogen atoms (Cl, Br, and I) are substituted by H.
Alkyl fluorides (R–F) cannot be reduced using Zn, dil. HCl, as C–F bonds are very strong.

Examples:

1. Reduction of methyl chloride with Zn/dil. HCl produces methane and zinc chloride.

2. Reduction of n-propyl chloride with Zn/dil HCl produces propane and zinc chloride.

3. By reduction of alkyl halides using Na, dry ether (Wurtz reaction)
Wurtz reaction: The Wurtz reaction is an organic chemical coupling reaction in which sodium metal reacts with two molecules of alkyl halides in a solution of dry ether to form a higher alkane along with a compound sodium halide.

Example:

Preparation of Alkanes from Carboxylic Acids

Alkanes can be prepared from carboxylic acids using two methods:

Decarboxylation by soda-lime

Decarboxylation is the process of removal of CO₂. And in the soda-lime process, the carboxylic acid loses CO₂ molecule.

Reagents used: NaOH (Caustic soda) + CaO (Quick lime)
When carboxylic acids or their salts are heated with an approximately 1 : 3 mixture of CaO and NaOH i.e., soda lime, an alkane is formed due to the removal of CO2. This process of elimination of carbon dioxide from a carboxylic acid is known as decarboxylation.

The alkanes produced contain one C atom less than that in the carboxylate ion. So, decarboxylation is a step-down reaction.

Reaction Involved:

Mechanism of the reaction:

1. In the first step, the electrophilic carbon of the C=O is being attacked by the hydroxide ion  (OH^-), and the electron density moves towards the oxygen atom from the pi bond of C=O, i.e., the negative charge appears on the oxygen atom.

2. In the second step, the lone pair of oxygen again forms a double bond with carbon, thereby allowing the alkyl substituent to leave in the form of R^- (carbanion).

3. R^- combines with H of sodium bicarbonate, giving alkane as the final product.

Rate of decarboxylation by soda lime: During decarboxylation, the alpha carbon of the carboxylate ion acquires a negative charge. The more the stability of the carbanion, the faster will be the rate of decarboxylation.

Kolbe’s electrolysis

An aqueous solution of the sodium/potassium salt of a saturated carboxylic acid on electrolysis gives an alkane containing an even number of C atoms at the anode. The general chemical reaction is given as follows:

Mechanism:

1. The reaction is believed to occur through the following steps:
RCOO^-Na⁺ + RCOO^-Na⁺ --> 2RCOO^- + 2Na⁺
2. At anode (Oxidation)

3. At the cathode (Reduction)

Physical Properties of Alkanes

1. Boiling Point of Alkanes

• With the increase in the molecular weight, the Boiling point of alkane increases.
• Straight-chain alkanes have a higher boiling point than branched-chain alkanes due to increases in the van der Waal forces by an increase in the surface area in straight-chain alkanes.

2. Melting Point of Alkanes

• With the increase in the molecular weight, the Melting point of alkane also increases.

• Even-numbered alkanes have a higher melting point trend than odd-numbered alkanes, owing to the fact that even numbered alkanes pack well in the solid phase, generating a well-organized structure that is difficult to break.

3. Solubility of Alkanes

• Alkanes are nonpolar compounds with little difference in electronegativity between carbon and hydrogen and the covalent character of the C-C or C-H bond.
• Alkanes are hydrophobic by nature, which is why they are water-insoluble and are soluble in organic solvents.

Chemical Properties of Alkanes

Alkanes are also known as paraffin because they are the least reactive toward any reagent.

Alkanes are inert to most reagents under normal conditions. However, they can react under certain conditions.

The types of reactions that alkanes undergo are as follows:

• Halogenation reaction
• Combustion
• Controlled oxidation
• Isomerization
• Aromatization
• Reaction with steam
• Pyrolysis

Halogenation 

The reactions where one or more hydrogen atoms of an alkane are replaced by halogen(s) (F, Cl, Br, I) are known as halogenation reactions. Halogenation takes place in the following conditions:

• High-Temperature 
• Presence of UV light

Rate of reaction of alkanes with Halogens:

Fluorination of alkane is explosive, violent, and uncontrollable while Iodination is Very slow and reversible and carried out in the presence of oxidizing agents like HIO3/HNO3. But bromination and chlorination of alkanes occurs smoothly.

Rate of replacement of H from alkanes

The order of the rates of replacement of H from alkanes is: 3° > 2° > 1° > CH4

This is not the preferred way of producing alkyl halides from alkanes as multiple products are formed, resulting in low yield.

Examples:

Combustion

It is the process of heating a substance in the presence of sufficient air/O₂. The substance is completely oxidized to CO₂and H2O, accompanied by the evolution of a large amount of heat.
Combustion reactions are mostly exothermic in nature. 

General reaction for combustion is:

Example of Combustion Reaction:

Due to the large amounts of heat released during their combustion, alkanes are used as fuels.

Example: Butane is present in LPG.

Incomplete combustion

The combustion in the absence of sufficient amounts of air is known as incomplete combustion.
During the incomplete combustion of alkanes, carbon black is formed.

Heat of Combustion

Heat of combustion is the amount of heat released when a substance is subjected to combustion.

The factors affecting the heat of combustion are as follows:

1. The heat of combustion is directly proportional to the number of carbon atoms.

Example:

2. The heat of combustion is inversely proportional to the branching of alkanes (in isomers) because of the difference in the stability of alkanes. More the thermodynamic stability of alkane, the lesser will be the heat of combustion.

Example :

3. The heat of combustion per CH2 group is inversely proportional to the size of the ring (cycloalkane).

Example:

Cyclopropane is the most unstable due to the high ring strain (angle strain and torsional strain).
However, the overall heat of combustion increases as the size of the ring increases.

Controlled oxidation of Alkanes

It is the process of heating a substance in a regulated supply of O₂/air at a high pressure and in the presence of a suitable catalyst to produce various oxidation products.

Oxidation of Alkanes

Generally, alkanes resist oxidation, but alkanes having 3 H atoms can be oxidized to their corresponding alcohols heated by KMnO4.

Isomerization of n-Alkanes

On heating in the presence of anhydrous AlCl₃ and HCl(g), n-alkanes isomerize to form branched-chain alkanes.

Examples:

Aromatization

On heating to 773 K at 10-20 atm pressure in the presence of oxides of V/Mo/Cr supported over alumina, n-alkanes with six or more than six carbon atoms get dehydrogenated and cyclized to form benzene and its derivatives. The aromatization of n-alkanes is also known as reforming.

Examples:
Aromatization of n-hexane

Aromatization of n-heptane:

On heating to 773 K at 10-20 atm pressure in the presence of oxides of V/Mo/Cr, n-heptane gets dehydrogenated and cyclized to form toluene. A seven-membered ring is not formed.

Reaction with Steam

Methane reacts with steam at 1273 K in the presence of a nickel catalyst to form carbon monoxide and dihydrogen. This method is used in the industrial preparation of H₂.

Pyrolysis

On heating to a higher temperature and in the absence of air, higher alkanes decompose into lower alkanes, alkenes, etc. The decomposition reaction on the application of heat is known as pyrolysis or cracking of alkanes.

Example:
Pyrolysis of hexane:

Pyrolysis of dodecane (C₁₂H₂₆):

Practice Problems

Q1. What is the correct order for the rates of decarboxylation by soda lime for the given compounds?

Solution: In the given compounds, the alpha carbon is attached next to the benzene ring. Hence, it forms a benzylic anion, which is resonance stabilized. The more the electron-withdrawing groups attached to the benzene ring, the more is the dispersion of the negative charge of the carbanion and the higher the stability of the carbanion. Hence, the rate of decarboxylation reaction is faster. 

Out of the given para substituents, the -OCH3 group shows the +R effect, thus destabilizing the carbanions; the -CH3 group shows the +H and +I effects and makes the carbanion less stable.

Whereas, –Cl shows the +R and –I effects, the –I effect dominates, and the -NO₂ group stabilizes the carbanions by dispersing the negative charge through the –R effect.

Thus, the correct order of the rates of decarboxylation by soda lime is: (I) > (II) > (III) > (IV) > (V)

Q2. Arrange the following compounds in the order of their heat of combustion.

I.  

II.  

III. 

Solution: In the given structures, the ethyl group is common but the ring size varies from the seven-membered ring in structure (I), the six-membered ring in structure (II), to the five-membered ring in structure (III).

As the number of carbon atoms decreases, the heat of combustion also decreases.
So, the order of heat of combustion is: (I) > (II) > (III)

Q3. The salt of an unknown carboxylic acid forms cyclobutane on Kolbe’s electrolysis. What can be the carboxylic acid?

1. Hexanoic acid
2. Succinic acid
3. Adipic acid
4. Fumaric acid

Solution: A carboxylic acid having six carbon atoms is known as adipic acid or hexanedioic acid that forms cyclobutane by Kolbe’s electrolysis method.
Starting from the salt of adipic acid,

Hence, option (C) is the correct answer.

Q4. Why does the formation of compound A take place rather than compound B ?

Solution: After the hydrogenation of the inner ring, the four-membered rings in the product (A) become non-aromatic from anti-aromatic, while in the product (B), two rings remain anti-aromatic. So, the formation of compound (A) takes place rather than compound B.

Q5. Iodoethane and iodopropane are allowed to undergo the Wurtz reaction. Find the alkane that can be obtained in this reaction.

1. Butane
2. Propane
3. Pentane
4. Hexane

Solution: Wurtz reaction takes place when two moles of alkyl halide react with Na in the presence of dry ether to give alkane.

Therefore, options (A), (C), and (D) are the correct answers.

Q6. What is the order of heat evolved upon the catalytic hydrogenation of the given compounds with H₂?

A. 

B. 

C. 

D. 

Solution: Compounds (I), (II), (III), and (IV) have three, four, six, and ten α-hydrogens, respectively. The more the α-hydrogens, the more will be the number of hyperconjugative structures, and hence more stable will be the alkene. Thus, less heat of hydrogenation will be evolved.

So, the order of heat evolved will be as follows:
(I) > (II) > (III) > (IV)

Frequently asked questions

Question 1. What is the primary source of alkanes?
Answer: Fuels such as Petrol, Diesel, and LPG are obtained from the fractional distillation of crude oil and all these fuels contain majorly alkanes.

Question 2. Is it possible to convert alkenes to alkanes?
Answer: Yes, by Hydrogenation (reduction) of alkenes, multiple bonds of alkenes reduce to single covalent bonds to form alkanes.

Question 3. What is the IUPAC name for the simplest alkane?
Answer: Simplest alkane is methane and its IUPAC name is Methane.

Question 4. What is the general formula of alkanes?
Answer: The general formula of alkanes is C_nH_2n+2, where n starts from 1. The simplest alkane is Methane.

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