Alkanes are organic compounds that contain only single bonds between carbon atoms and hydrogen atoms. They are generally unreactive because of the strength of their carbon-carbon and carbon-hydrogen bonds. However, there are some reactions that alkanes can undergo. In this article, we will study some reactions of alkanes like combustion, nitration, oxidation, etc.
Alkanes
Alkanes are a class of organic compounds that are composed only of carbon and hydrogen atoms, with single bonds between the carbon atoms.
They are sometimes referred to as "saturated hydrocarbons" because they contain the maximum number of hydrogen atoms possible for a given number of carbon atoms.
Alkanes are the simplest type of hydrocarbon and have the general molecular formula CnH2n+2, where "n" is the number of carbon atoms in the molecule.
1. Preparations of Alkanes
There are several ways to prepare alkanes, which are organic compounds that contain only single bonds between carbon atoms and hydrogen atoms. Some common methods include:
2. Catalytic Hydrogenation
This method involves the reaction of an unsaturated hydrocarbon (such as an alkene or alkyne) with hydrogen gas in the presence of a catalyst (such as palladium or nickel) to produce the corresponding alkane. This reaction is highly selective for the formation of the alkane product.
3. Reduction of Alkyl Halides
Alkyl halides (such as chloroalkanes or bromoalkanes) can be reduced with a reducing agent (such as lithium aluminum hydride or sodium borohydride) to produce the corresponding alkane. R – X + 2[H] → R – H + HX CH3I + 2[H] → CH4 + HI CH3CH2Br + 2[H] → CH3CH3 + HBr
4. Wurtz Reaction
This reaction involves the reaction of an alkyl halide with metallic sodium in the presence of anhydrous ether to produce an alkane. The reaction involves a radical mechanism and is useful for the synthesis of symmetrical alkanes.
5. Decarboxylation
Carboxylic acids can be decarboxylated with heat to produce an alkane and carbon dioxide. This method is useful for the synthesis of higher alkanes.
6. Clemmensen Reduction
This reduction involves the reaction of a ketone or aldehyde with amalgamated zinc in the presence of hydrochloric acid to produce the corresponding alkane.
7. Kolbe Electrolysis
When an aqueous solution of sodium or potassium salt of the carboxylic acid is electrolyzed, the alkane is evolved at the anode.Thus, the reaction may be written as: 2CH3 COOK + 2H2O → CH3CH3 + 2CO2 + H2 + 2KOH
2. Chemical Reactivity of Alkanes
2.1 Substitution Reaction
A substitution reaction occurs when a hydrogen atom in a hydrocarbon is replaced by an atom or a group of atoms. Alkanes only undergo substitution reactions because of the sigma bond between C -C and C -H. The following are some substitution reactions:
3.1.1 Halogenation
Alkanes can react with halogens (chlorine, bromine, or iodine) in the presence of ultraviolet light to produce alkyl halides. This reaction is an example of a radical substitution reaction, where a halogen atom replaces a hydrogen atom on the alkane. CH4 + Cl2 → CH3Cl + HCl CH4 +I2 → CH3I + HI
3.1.2 Nitration
Alkanes can react with nitric acid in the presence of sulfuric acid to produce nitroalkanes. This reaction is an example of an electrophilic substitution reaction, where the nitro group (NO2) replaces a hydrogen atom on the alkane. CH4 + HNO3 → CH3NO2 + H2O C6H14 HNO3 → C6H13NO2 + H2O
3. Two Oxidation Reaction
Alkanes can undergo oxidation reactions to form a variety of products.
3.2.1 Complete Oxidation or Combustion
Alkanes are highly flammable and will react with oxygen to produce carbon dioxide and water. This reaction is exothermic and releases a large amount of energy. C3H8 + 5O2 → 3CO2 + 4H2O 2C4H10 + 13O2 → 8CO2 + 10H2O
3.2.2 Incomplete
Combustion When the combustion is carried out in an insufficient supply of oxygen, they form carbon monoxide and carbon (carbon black or soot) CH4 + O2 → C + 2H2O 2CH4 + 3O2 → 2CO + 4H2O
3.2.3 Isomerization
Alkanes can undergo isomerization reactions, where the arrangement of atoms in the molecule is changed, but the number and type of atoms remain the same. This reaction can occur under certain conditions, such as high temperature and pressure.
3.2.4 Aromatization
The alkanes containing six or more carbon atoms when heated about 773 K under high pressure of 10 - 20 atm in the presence of the catalysts like oxides of chromium, molybdenum or vanadium supported on alumina get converted to aromatic compounds. This process is called aromatization or reforming.
3. Three Thermal Decomposition
pyrolysis When higher alkanes are heated to high temperatures (about 700-800 K) in the presence of alumina or silica catalysts, the alkanes break down into lower alkanes. C3H8 → C2H4 + CH4 or C3H6 + H2
Key Takeaways
Alkanes are saturated hydrocarbons with a single bond between C and C and C and H atoms.
Alkanes can be prepared by hydrogenation, Wurtz reaction, Clemmensen reduction, Kolbe’s electrolysis, etc.
Alkanes are generally unreactive, but they undergo reactions such as combustion, nitration, oxidation, halogenation, etc.
Conclusion
Understanding the reactions of alkanes is crucial for grasping their significance in both chemistry and practical applications. Their relatively stable nature and primary reactions, such as combustion and halogenation, underline their importance in various fields, including energy production and chemical synthesis.
If you have more questions about alkanes or their reactions, feel free to ask!
FAQs
1) What are alkanes?
Alkanes are a family of hydrocarbons, meaning they are molecules of only carbon and hydrogen atoms. They are also known as saturated hydrocarbons because they contain only single bonds between carbon atoms.
2) What is a combustion reaction of alkanes?
Combustion is a type of reaction where a substance reacts with oxygen to produce heat and light. When alkanes are burned in the presence of oxygen, they undergo a combustion reaction that produces carbon dioxide and water.
3) What is a halogenation reaction of alkanes?
Halogenation is a type of reaction where a halogen (such as chlorine or bromine) is added to an organic molecule. When alkanes are halogenated, the halogen replaces one or more of the hydrogen atoms in the molecule, producing a halogenated alkane.
4) What is a substitution reaction of alkanes?
Substitution is a type of reaction where an atom or group of atoms in a molecule is replaced by a different atom or group of atoms. When alkanes undergo substitution reactions, a functional group (such as a halogen or hydroxyl group) is added to the molecule by replacing one of the hydrogen atoms.
5) Are alkanes reactive?
Alkanes are generally unreactive due to their strong carbon-carbon and carbon-hydrogen bonds. However, they can undergo certain types of reactions (such as combustion and halogenation) under specific conditions.
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