Classification of Hydrocarbons
Organic chemistry is the branch of chemistry that deals with the reactions, structures, and properties (physical and chemical) of organic compounds that contain carbon atoms and covalent bonds (a chemical bond that involves sharing of electrons between atoms).
Any group of organic chemical compounds made up only of the atoms carbon (C) and hydrogen (H) is called hydrocarbons. The carbon atoms bond together to form the compound’s structure, and the hydrogen atoms bond to it in multiple ways. Petroleum and natural gas are mostly made up of hydrocarbons. They’re used as fuels and lubricants, as well as raw materials for plastics, fibres, rubbers, solvents, explosives, and industrial chemicals.
Hydrocarbons are abundant in nature. They are found in trees and plants as well as fossil fuels, for example, in the form of carotenes, which are pigments found in carrots and green leaves. A hydrocarbon polymer, a chainlike molecule made up of numerous units linked together, makes up more than 98% of natural crude rubber. Individual hydrocarbons’ shapes and chemistry are mostly determined by the types of chemical bonds that connect the atoms of their constituent molecules.
What are Hydrocarbons?
Hydrocarbons are made of two words hydro and carbon, in which hydro represents hydrogen elements and carbon represents carbon elements, so hydrocarbons can be easily defined as compounds that are formed from hydrogen and carbon molecules only.
Hydrocarbons can exist in a straight chain as well as in circular structures. These hydrocarbons are used in a different process to make new materials.
Hydrocarbons are divided into four subcategories: alkanes, alkenes, alkynes, and aromatic hydrocarbons. Alkanes, alkenes, alkynes, and aromatic hydrocarbons have simple or moderately complex structures. The study of hydrocarbons can reveal information on the chemical characteristics and manufacture of other functional groups.
CxHy is the molecular formula for these substances. Hydrocarbons can be found in both plants and trees. Carotenes, for example, are an organic pigment found in green leaves and carrots. Hydrocarbons are present in nature in different forms like fossil fuels which consist of crude oil, petroleum, kerosene, liquefied petroleum gas (LPG), compressed natural gas (CNG) etc are all contain hydrocarbons. This means hydrocarbons are a great source of energy.
Properties of Hydrocarbons:
- The melting and boiling points of hydrocarbon molecules are affected by the size of the molecules. As a result, at room temperature, certain hydrocarbons are gases while others are liquids or solids.
- They consist of strong carbon-hydrogen bonds.
- Molecules have strong attractive molecular forces.
- Non-polarity refers to the absence of oppositely charged sides in hydrocarbon molecules. As a result, they are polar compounds and do not dissolve in water. Hydrocarbons, on the other hand, have a natural tendency to reject water. This is why floor wax and other related products contain them.
Classification and Types of Hydrocarbons
Hydrocarbons were formerly classed as either aliphatic or aromatic by older scientists. The sources and characteristics were used to classify the hydrocarbons. As a consequence, it was discovered that aliphatic hydrocarbons were formed by the chemical degradation of fats or oils, whereas aromatic hydrocarbons included components formed by the chemical degradation of plant extracts. However, we now categorise hydrocarbons based on their structure rather than their origin.
Saturated hydrocarbons are known as Alkanes. These hydrocarbons contain single bonds of carbon-carbon and carbon-hydrogen. These hydrocarbons form chain structures that are open in nature. These hydrocarbons don’t contain any double or triple bonds. Saturated hydrocarbons are known as the simplest hydrocarbons. The general formula for saturated hydrocarbons is CnH2n+2.
Physical Properties of Saturated Hydrocarbons (Alkanes)
- Saturated Hydrocarbons (Alkanes) are colourless and odourless in nature.
- Alkanes are non-polar molecules.
- The First 4 members of alkanes exist in the gaseous phase. The next thirteen members from fifth to seventeenth exists in the liquid phase.
- Alkanes containing more than 18 carbon atoms are solid in nature.
- Intermolecular van der Waals forces exist in alkanes. Alkanes with stronger intermolecular van der Waals forces have higher boiling points.
- For the same reason as stated above, the melting points of alkanes follow a similar trend to boiling points. That is, the larger the molecule (everything else being equal), the higher the melting point.
Chemical Properties of Saturated Hydrocarbons (Alkanes)
- On heating, alkanes react with oxygen and produce carbon dioxide and water. Let’s take methane (CH4) first member of alkanes and heat it.
CH4(gas) + 2O2(gas) → CO2(gas) + 2H2O(liquid)
- Hydrogen atoms can be replaced by fluorine, chlorine, bromine, iodine means with halogen group to form new compounds, this process is known as Halogenation. The halogenation process occurs in the presence of sunlight.
CH4 + Cl2 → CH3Cl (Chloromethane) + HCl
- Straight-chain alkanes are heated in the presence of a platinum catalyst during the isomerization and reformation processes. Isomerization converts alkanes into branched-chain isomers. The alkanes undergo reformation to become cycloalkanes or aromatic hydrocarbons, emitting hydrogen as a byproduct. Both of these methods increase the substance’s octane number. Butane is the most common alkane that undergoes isomerization, as it produces a large number of branched alkanes with high octane values.
Preparation of Saturated Hydrocarbons (Alkanes)
Alkanes can be prepared by a number of methods, some of them are described below:
- From unsaturated hydrocarbons
Unsaturated hydrocarbons are alkenes or alkynes, which can be converted to alkanes by adding hydrogen gas in the presence of a catalyst (like nickel), this process of alkane formation is known as Hydrogenation.
C2H4 (Ethene) + H2 → C2H6 (Ethane)
- From alkyl halides
Alkanes can also be prepared from alkyl halides by reacting them with hydrogen in the presence of zinc.
CH3Cl + H2 Zn→ CH4 (Methane) + HCl
Unsaturated hydrocarbons are organic molecules composed completely of carbon and hydrogen atoms that have a double or triple bond between two neighbouring carbon atoms. Alkenes are hydrocarbons that contain at least one double bond between two adjacent carbon atoms, whereas alkynes are hydrocarbons that contain a carbon-carbon triple bond. Saturated and unsaturated hydrocarbons have different chemical formulas, as shown below.
Unsaturated Hydrocarbons are of two types :
Alkenes: Unsaturated hydrocarbons contain Alkenes. Alkenes are oil forming compounds so these are known as Olefins. These hydrocarbons contain single bonds of carbon-carbon and carbon-hydrogen along with at least one double bond. These hydrocarbons also form chain structures that are open in nature.
The general formula for Alkenes: CnH2n
Physical Properties of Alkenes
- Unsaturated Hydrocarbons (Alkenes) are colourless and odourless in nature except for Ethene which has a sweet smell.
- Alkenes have stronger odours than their comparable alkanes.
- Alkenes are polar molecules.
- The physical state is determined by molecular mass: the simplest alkenes, like the corresponding saturated hydrocarbons, are gases at room temperature.
- The first 3 members of alkenes exist in the gaseous phase.
- The next fourteen members from fourth to seventeenth exists in the liquid phase.
- Alkenes containing more than 14 carbon atoms are solid in nature.
- These have high melting and boiling points.
Chemical Properties of Alkenes
- Alkenes have the property to perform addition reactions so it adds hydrogen molecules to form alkanes.
C2H4 (Ethene) + H2 → C2H6 (Ethane)
- Hydrogen atoms can be replaced by fluorine, chlorine, bromine means with halogen group to form new compounds, called dihalides except for iodine as iodine does not show an addition reaction.
C2H4 (Ethene) + Br2 → C2H4Br2 (Dibromoethane)
Preparation of Alkenes
Alkenes can be prepared by a number of methods, some of them are described below:
- From Alkynes: Unsaturated hydrocarbons (Alkenes), can be prepared from alkynes by adding hydrogen gas in the presence of a catalyst (like palletised charcoal), this catalyst is also known as Lindlar’s Catalyst.
C2H2 (Ethyne) + H2 Pd/C→ C2H4 (Ethene)
- From alkyl halides: Alkenes can also be prepared from alkyl halides by heating them in the presence of potash dissolved in alcohol so to remove the halogen molecule from alkyl halides.
C2H5Cl → C2H4 (Ethene)
Alkynes: Unsaturated hydrocarbons contain Alkynes also. These hydrocarbons contain single bonds of carbon-carbon and carbon-hydrogen along with at least one triple bond between two carbon atoms. These hydrocarbons also form chain structures that are open in nature. The first member of the alkyne series is Ethyne also known as Acetylene.
The general formula for Alkynes: CnH2n-2
Physical Properties of Alkynes
- Unsaturated Hydrocarbons (Alkynes) are colourless and odourless in nature except for Ethyne which has odour.
- Alkynes are polar molecules.
- The first 3 members of alkynes exist in the gaseous phase.
- The next eight members from fourth to eleventh exists in the liquid phase.
- Alkynes containing more than 11 carbon atoms are solid in nature.
Chemical Properties of Alkynes
- Alkynes also show addition reactions like alkenes as alkynes have a triple bond so they have the tendency to add two molecules of hydrogen, halogen, hydrogen halides to form different compounds.
- Alkynes do not react with water.
- Alkynes show polymerisation means they can form long-chain carbon molecules.
Preparation of Alkynes
Alkynes can be prepared by a number of methods, some of them are described below:
- From calcium carbide: Alkynes can be prepared by adding water to calcium carbide which will result in the formation of calcium hydroxide and alkyne say ethyne.
CaC2 + 2H2O → Ca(OH)2 + C2H2 (Ethyne)
- From vicinal dihalides: Alkynes can be prepared from vicinal dihalides by reacting them with alcoholic potash to eliminate one molecule of halogen from dihalide to form alkenyl halide which then can be reduced to the alkyne.
The aromatic word comes from the word aroma which means pleasant smell so aromatic compounds have a pleasant odour. These hydrocarbons are also known as Arenes. Most of the aromatic hydrocarbons contain benzene rings so they are known as Benzenoids. There are some aromatic hydrocarbons that do not contain benzene rings so these are also known as Non-Benzenoids.
Physical Properties of Aromatic Hydrocarbons
- These chemicals are aromatic (additional stability granted by resonance)
- In these compounds, the carbon-to-hydrogen atom ratio is relatively high.
- Aromatic hydrocarbons exist in the gaseous phase and liquid phase.
- Aromatic hydrocarbons are colourless in nature.
- Aromatic hydrocarbons are insoluble in water.
Chemical Properties of Aromatic hydrocarbons
- Aromatic hydrocarbons are able to perform Electrophilic Substitution Reactions.
- Under some conditions, these hydrocarbons can also show addition and oxidation reactions.
- When aromatic hydrocarbons are burned, they produce a yellow, sooty blaze.
Preparation of Aromatic Hydrocarbon
- Reduction of Phenol: Aromatic hydrocarbons say benzene can be prepared by reducing phenol by zinc dust.
C6H6OH (Phenol) + Zn → C6H6 (Benzene) + ZnO
- Decarboxylation of aromatic acids: Aromatic hydrocarbons say benzene can be prepared by heating sodium salt of benzoic acid.
C6H6COONa + NaOH CaO→ C6H6 (Benzene)+ Na2CO3
Uses of Hydrocarbons
- Hydrocarbons are used in paints as a solvent.
- These are also used in grease and lubricant industries.
- Different types of drugs use hydrocarbons for their preparation.
- Hydrocarbons are used in the synthesis of different types of polymers.
- Fuels consist of main hydrocarbons.
- Natural gas is primarily composed of methane.
- Ethane and propane are either transformed into syngas or ethylene and propylene.
Question 1: Why Methene does not exist?
Methene does not exist as for alkenes to make there should be one carbon-carbon double bond so two carbon atoms are needed and this is the reason that methene which contains only a single carbon atom does not exist.
Question 2: What is Huckel Rule?
Aromatic compounds are compounds that contain one benzene ring, benzene rings contain electrons which should follow a rule called the Huckel rule. The presence of (4n+2) pi electrons in the ring is the Huckel rule.
Question 3: What is Ozonolysis?
Ozonolysis is the process of the addition of ozone molecules to the alkene to form ozonide, this process is useful in investigating the location of the double bond in alkenes.
Question 4: What is Pyrolysis?
Decomposition of higher alkanes into lower alkanes by the effect of heat is called pyrolysis, another name of pyrolysis is cracking.
Question 5: Why Methyne does not exist?
Methyne does not exist as for alkynes to make there should be one carbon-carbon triple bond so two carbon atoms are needed and this is the reason that methyne which contain only a single carbon atom does not exist.