Class 10 Chemistry Chapter Hydrocarbons Full Chapter Notes

Chapter: Hydrocarbons

Introduction:

Definition of Hydrocarbons:

Only carbon and hydrogen atoms containing organic compounds are called as hydrocarbons.

Types of Hydrocarbons:

Hydrocarbons are of three types namely; Alkanes, Alkenes and Alkynes. These three hydrocarbons can be differentiated from each other on the basis of the type of covalent bond present between their carbon atoms. However, we can say that;

Since only carbon and hydrogen atoms are found in organic substances, hydrocarbons are known as such.
They could have a ring structure or an open chain structure.

Moreover, hydrocarbons are classified into two classes.

Saturated organic compound
Un-saturated organic compounds

Saturated Hydrocarbons:

Saturated hydrocarbons are hydrocarbons in which all carbon atoms are connected by a single covalent bond, either in a straight chain structure or a ring shape.
There won’t be any double or triple covalent bonds in saturated hydrocarbons, only single covalent bonds
For example, alkanes

Unsaturated Hydrocarbons:

Unsaturated hydrocarbons are hydrocarbons in which at least one double or triple covalent bond is present between any of two carbon atoms.
For example, Alkenes have at least one double bond between any two carbon atoms and alkynes have at least one triple bond between any two carbon atoms.

Explanation of alkane, alkene and alkyne:

Alkanes:

Alkanes, also known as saturated hydrocarbons, are hydrocarbons in which all carbon atoms are connected by a single covalent bond, either in a straight chain structure or a ring shape.
There won’t be double or triple covalent bonds in saturated hydrocarbons, only single covalent bonds.
For example,
CH₄, CH₃-CH₃, CH₃-CH₂-CH₂-CH₃ etc.

The general formula for alkanes are CnH2n+2
n is the number of carbon atoms n = 1, 2, 3, 4………..
If n = 1 then according to general formula, C₁H_{2 x 1 + 2}= C₁H₂₊₂ = C₁H₄ = CH₄ (Methane), same for carbon 2, 3, and so on.

Alkenes:

Alkenes, also known as “unsaturated hydrocarbons,” are hydrocarbons in which at least two carbon atoms are connected together by a double covalent bond.
For example,
CH₂=CH₂, CH₃-CH=CH-CH₃etc.
The general formula for alkenes are CnH2n
n is the number of carbon atoms n = 1, 2, 3, 4………..
If n = 2 then according to general formula, C₂H_{2 x 2}= C₂H₄ (Ethene), same for carbon 3, 4, 5 and so on.

Alkynes:

Alkynes, as well known as “unsaturated hydrocarbons” are hydrocarbons in which at least two carbon atoms are linked together by a triple covalent bond.
For example,

The general formula for alkynes is C_nH_2n-2
n is the number of carbon atoms n = 1, 2, 3, ……………….
If n = 2 then according to general formula, C₂H_{2 x 2 – 2}= C₂H_4-2= C₂H₂(Ethyne), same for carbon 3, 4, 5, and so on.

Nomenclature:

An international union of pure and applied chemistry designed rules for deriving names of the three types of hydrocarbons namely; alkanes, alkenes and alkynes.
The number of carbon atoms in the hydrocarbon chain determines the name of the compound.
The IUPAC name has two components.

Prefix
Suffix

Prefix:

It refers to how many carbon atoms there are in the hydrocarbon chain.
The prefixes are shown in the table below according to how many carbon atoms are in the hydrocarbon chain.

Prefix	Number of Carbon atoms
Meth-	1
Eth-	2
Prop-	3
But-	4
Pent-	5
Hex-	6
Hept-	7
Oct-	8
Non-	9
Dec-	10

Suffix:

It identifies the hydrocarbon class.
The prefix “ane” is used with alkanes.
The prefix “ene” is used for alkenes
Prefix “yne” is used for alkynes.

IUPAC rules for assigning names to three types of hydrocarbons:

Alkanes:

If a substance contains four carbon atoms. Then, how will we name this compound in accordance with IUPAC rules?

CH₃-CH₂-CH₂-CH₃

Solution:

Verify the prefix in the table above for four carbon atom
Prefix for four carbon atom is But-
Verify each bond in a compound
When the compounds only have one type of covalent bond (single covalent bond). Consequently, the compound is an alkane
The alkane family’s suffix is “ane”
We’ll write the prefix first, then the suffix
Therefore, the IUPAC name of the compound will be butane

Alkenes:

If a substance contains four carbon atoms and one double covalent bond. Then, how will we name this compound in accordance with IUPAC rules?

CH₃-CH=CH-CH₃

Solution:

Verify the prefix in the table above for four carbon atom
Prefix for four carbon atom is But-
Verify each bond in a compound
The compound have one double covalent bond. Consequently, the compound is an alkene
The alkene family’s suffix is “ene”
We’ll write the prefix first, then the suffix
Therefore, the IUPAC name of the compound will be butene

Alkynes:

If a substance contains four carbon atoms and one triple covalent bond. Then, how will we name this compound in accordance with IUPAC rules?

Solution:

Verify the prefix in the table above for four carbon atom
Prefix for four carbon atom is But-
Verify each bond in a compound
The compound have one triple covalent bond. Consequently, the compound is an alkene
The alkyne family’s suffix is “yne”
We’ll write the prefix first, then the suffix
Therefore, the IUPAC name of the compound will be butyne

Electron dot and cross structure for alkane:

The electron dot and cross structure uses dots and crosses to represent covalent bonds. Carbon valence electrons are shown in this diagram as dots, while hydrogen valence electrons are shown as crosses.

Method to draw electron dot and cross structure for alkanes:

Alkane’s carbon atoms should be written in a straight line.
Outline the four valence electrons of each carbon atom as dots.
Draw the hydrogen valence electron in the form of a cross so that it is parallel to the dot of a carbon atom. Draw the number of hydrogen atoms present in the alkane around the carbon atoms in such a way that one cross of hydrogen atom arrived in front of one dot of carbon.

For example:

Electron dot and cross structure:

Methane is the first member of the alkane family, and its electron dot and cross structural formulas are shown below:

Structural formula for methane:

Molecular formula for methane:

CH₄

Methods for synthesis of alkanes:

By Hydrogenation of alkene and alkyne:

By adding hydrogen to ethyne in the presence of a Ni catalyst, we shall first hydrogenate an alkyne at 200–300 ^oC. One of the pi bonds in ethyne will disintegrate during hydrogenation, turning it into ethene. Then, by breaking a second pi bond, ethane is produced by hydrogenating the resultant ethene at 200–300 oC in the presence of a Ni catalyst. We can utilize Pt and Pd catalysts at room temperature in place of Ni catalysts.

For example:

Mechanism:

Mechanism of reaction is given below in which we can see that,

Step 1:

In alkyne hemolytic fission occurs of one C-C pi bond and of H-H sigma bond in a hydrogen molecule
During homolytic fission, each of the atoms participating in the formation of a bond will take up one of its shared electrons.
As a result, the free radical formation will take place (a free radical is an atom with one unpaired electron)
So, the unpaired electrons at C number 1 will form a sigma bond with an unpaired electron of one hydrogen radical and the unpaired electron at C number 2 will form a sigma bond with an unpaired electron of another hydrogen radical and will lead to the formation of an alkene.

Step 2:

In alkene hemolytic fission occurs of one C-C pi bond and of H-H sigma bond in the hydrogen molecule
During homolytic fission, each of the atoms participating in the formation of a bond will take up one of its shared electrons.
As a result, the free radical formation will take place (a free radical is an atom with one unpaired electron)
So, the unpaired electrons at C number 1 will form a sigma bond with an unpaired electron of one hydrogen radical and the unpaired electron at C number 2 will form a sigma bond with an unpaired electron of another hydrogen radical and will lead to the formation of alkane.

In order to generate alkanes from alkynes, hydrogenation is performed two times.
It’s because the first hydrogenation breaks the first pi bond, turning alkynes into alkenes.
The second pi bond in an alkene is broken by hydrogenation a second time, turning it into an alkane.
Therefore, this is the reason we hydrogenate twice.

By reduction of alkyl halides:

In this process, Zn and aqueous hydrochloric acid react.
This process produces two nascent hydrogen atoms, also known as atomic hydrogen (The hydrogen atom produced during a reaction and reacts immediately).
This developing hydrogen then reacts with the alkyl halide.
This means that alkyl halide is reduced by nascent hydrogen (The addition of hydrogen is called reduction).
In this procedure, aqueous acetic acid (CH₃COOH_(aq)) is also employed in place of aqueous hydrochloric acid (HCl_(aq)).

Mechanism:

Methods for the synthesis of alkenes:

By dehydration of alcohol:

Dehydration refers to the removal of water.
In this process, alcohol is passed over heated Al₂O₃ between 340-450 ^oC to evaporate water from it.
This reaction produces an alkene and a water molecule as its byproduct.
In this reaction, the catalyst is Al₂O₃.
Instead of Al₂O₃ (alumina), we can use H₃PO₄ (phosphoric acid), P₄O₁₀ (phosphorus pentoxide), and concentrated Sulphuric acid.

Note:

Never forget that during hydrogenation, a hydroxyl atom will be removed from one carbon atom, followed by the removal of a hydrogen atom from a carbon atom next to the hydroxyl bonded carbon atom.
A single (same) carbon atom should never have its hydroxyl and hydrogen atoms removed.

Example 1:

Mechanism of reaction is given below in which;

The mechanism of a reaction is illustrated below in which you can see that how Al2O3 reacts with ethanol and form ethene from it.

Example 2:

Mechanism of reaction is given below in which;

The mechanism of a reaction is illustrated below in which you can see how H₂SO₄ reacts with ethanol and form ethene from it.

By dehydrohalogenation of alkyl halides:

In this chemical process, the hydrogen halide from the alkyl halide is removed.
Dehydrohalogenation is the reaction’s scientific name, which stands for the elimination of the hydrogen and halide group (de means removal).
Here, alcohol-infused KOH is used to perform the removal.
Always remember that hydrogen and halogen both will be removed from two adjacent carbon atoms not from a single carbon atom.
From one C atom halide group will be removed and from another adjacent C atom hydrogen atom will be removed.

Mechanism of reaction is given below in which we can see that;

In a reaction, alcoholic KOH is utilized as a base (OH^–)
This base will extract the proton from an alkyl halide
When the base will extract the proton of 1st carbon atom then, the sigma bond between carbon and hydrogen atom will shift between CH2-CH2, resulting in the formation of a pi bond between these two carbon atoms.
Alkene is created when a proton is removed and a sigma bond is moved.
The halide ion will react with the K⁺ of alcoholic KOH to generate KCl, whereas the proton will react with the hydroxyl group OH^– of alcoholic KOH to form H₂O.

Methods for synthesis of alkynes:

By dehydrohalogenation of vicinal di-halides:

According to the method’s name, dehydrohalogenation, the vicinal di-halide will be freed of its hydrogen and halide ions.

What is vicinal di-halide, exactly?

A compound in which two halogen atoms are attached with two covalently bounded carbon atoms is called vicinal di-halide.
In this method, Hydrogen and halogen atom both will not be removed from one single C atom.
Removal will be like, hydrogen atom will be removed from one carbon atom and halogen atom will be removed from next adjacent carbon atom.

Mechanism of a reaction is given below in which we can see that,

Step 1:

Alcoholic KOH is used in a reaction which acts as a base OH-
This base will extract the proton from 1st carbon of vicinal di-halide
When the base will extract the proton of 1st carbon atom then, the sigma bond between the carbon and hydrogen atom will shifts between CH-CH, resulting in the formation of pi bond between these two carbon atoms.
Removal of proton and shifting of sigma bond leads to the formation of vinyl chloride
The proton will react with hydroxyl group OH^– of alcoholic KOH and will form H₂O
The halide will react with K⁺of alcoholic KOH and will form KCl.

Step 2:

Alcoholic KOH is used in a reaction which acts as a base OH-
This base will extract the proton from 2nd carbon of vinyl chloride
When the base will extract the proton of 2^nd carbon atom then, the sigma bond between carbon and hydrogen atom will shifts between CH=CH, resulting in the formation of pi bond between these two carbon atoms.
Removal of proton and shifting of sigma bond leads to the formation of alkyne
The proton will react with hydroxyl group OH^– of alcoholic KOH and will form H₂O
The halide will react with K⁺of alcoholic KOH and will form KCl.

2. By dehalogenation of tetra-halides:

In this method, as the name indicates (dehalogenation) means halogen atom will be removed from tetra-halides.

Now what is Tetra-halide?

A compound in which four halogen atoms are attached with two covalently bounded carbon atoms is called tetra-halide.
In this method, in one step two halogen atoms will be removed at a time.
But remember two halogen atoms will not be removed from a single C atom.
The removal will be like; one halogen atom will be removed from one C atom and second halogen atom will be removed from next adjacent C atom.
Same for the next two halogen atoms.

Mechanism of a reaction is given below in which we can see that;

Two Zn metal atoms are used in a reaction which helps in the abstraction of a halogen atom
This Zn metal atom will abstract two halogen atoms from 1st and 2nd carbons of tetra-halide
In the first step, the Zn metal atom will attack the halogen atom bound to 1st carbon atom.
As a result, the sigma bond between C and Cl will shift between C-C atoms.
This bond shifting leads to the removal of the halogen atom from 2nd C atom.
Removal of a halogen atom from 2nd C atom leads to the formation of 1,2-Dichlororthane and ZnCl2
In the second step, the second Zn metal will attack the halogen atom bound to 2nd C atom.
As result, sigma bond between C and Cl will shift between C-C atoms.
This bond shifting leads to the removal of the halogen atom from 1st C atom.
Removal of a halogen atom from 1^st C atom leads to the formation of alkyne and ZnCl₂.

Properties of Hydrocarbons:

Alkanes:

Properties:

Alkanes are saturated hydrocarbons
1st four members of the alkane family e.g. methane, ethane, propane and butane are colorless and odorless gases
The next two members of alkane family e.g. pentane and hexane are colorless and odorless liquids
And remaining members next to hexane e.g. heptane, octane, Nonane and Decane and so on are colorless and odorless solids
Alkanes are non-polar in nature
Their non-polar nature is due to very less electronegativity difference between C and H atom or due to the absence of an electronegative atom
Density of alkanes is less than water
Due to their non-polar nature alkanes are insoluble in water according to like dissolve like rule, polar substances dissolve in polar solvents and non-polar substances dissolve in non-polar solvents
Water is polar solvent. Therefore alkanes (non-polar) are insoluble in water
Alkanes are soluble in non-polar solvents e.g. ether and benzene etc.
Due to non-polar nature, alkanes are good solvents and used in useful reactions
They do not react with ionic compounds
Hexane is the member of alkane family and used for oil extraction from cotton seed, soya beans and corn etc.
Due to saturated nature addition reactions are not possible in alkanes due to strong sigma bonds whose breakage is not easy. Therefore, only substitution reactions are possible in it.

Alkenes:

Properties:

Alkenes are unsaturated hydrocarbons
1st three members of the alkene family e.g. Ethene, propene and butene are gases
The members of alkene family next to butene e.g. pentene, hexene, heptene, octene, nonene, decene, undecene, dodecene, tridecene, tetradecene and pentadecene are liquids
And remaining members next to pentadecene are solids
Alkenes are non-polar in nature
Their non-polar nature is due to very less electronegativity difference between C and H atom or due to the absence of electronegative atom
Density of alkenes is less than water
Due to non-polar nature alkenes are insoluble in water according to like dissolve like rule, polar substances dissolve in polar solvents and non-polar substances dissolve in non-polar solvents
Water is polar solvent. Therefore alkenes (non-polar) are insoluble in water
Alkenes are soluble in non-polar solvents e.g. ether, alcohol and benzene etc.
Due to non-polar nature, alkenes are good solvents and used in useful reactions
They do not react with ionic compounds
Due to un-saturation (presence of double covalent bond, one sigma and one pi bond) they perform addition reactions
The pi bond is weak bond and can be easily break down. Therefore, alkenes undergo addition reaction easily by breaking old pi bond and can make new strong sigma bonds with other atoms

Alkynes:

Properties:

Alkynes are un-saturated hydrocarbons
1st three members of alkyne family e.g. Ethyne, propyne and butyne are gases
The next eight members of alkyne family e.g. pentyne, hexyne, heptyne, octyne, nonyne, decyne, undecyne, dodecyne are liquids
And remaining members next to dodecyne are solids
Ethyne, a first member of alkyne has garlic like smell
Alkynes are non-polar in nature
Their non-polar nature is due to very less electronegativity difference between C and H atom or due to the absence of electronegative atom
Density of alkynes is less than water
Due to non-polar nature alkynes are insoluble in water according to like dissolve like rule, polar substances dissolve in polar solvents and non-polar substances dissolve in non-polar solvents
Water is polar solvent. Therefore alkynes (non-polar) are insoluble in water
Alkynes are soluble in non-polar solvents e.g. ether, alcohol and benzene etc.
They do not react with ionic compounds
Due to un-saturation (presence of triple covalent bond (one sigma and two pi bonds)) they perform addition reactions
The pi bonds are weak bonds and can be easily broken down. Therefore, alkynes undergo addition reactions easily by breaking old pi bonds and can make new strong sigma bonds with other atoms.

Reactions of Hydrocarbons:

Alkanes:

Reactions:

Halogenation reaction:

The reaction of an alkane and halogen is a substitution reaction.
In substitution reaction all of the H atoms of an alkane will replaced one by one with halogen radical and will leads to the formation of carbon tetrachloride.
This substitution reaction takes place in the presence of sunlight by the formation of alkyl and halide free radicals.
This reaction is also called the chlorination of methane.
The presence of sunlight is necessary because chlorine gives a slow reaction at room temperature.
Therefore, for its rapid chain reaction sunlight energy is necessary.
On the other hand, bromine is less reactive than chlorine that’s why bromine needs more energy e.g. high temperature or strong sunlight to convert into free radicals and perform chain reaction.
While, fluorine is highly reactive atom and react explosively.
Moreover, iodine is highly unreactive and did not perform this reaction. Reactivity order of halogens is F₂ > Cl₂> Br₂ > I₂ .

2. Combustion reaction:

A reaction which takes place in the presence of oxygen is called combustion reaction. Combustion is of two types.

Complete combustion reaction
Incomplete combustion reaction

Complete combustion reaction:

In complete combustion reaction methane burns with the help of excess oxygen and produces flame, carbon-dioxide, water and heat.
Mostly alkanes produces blue color flame on burning.
Complete combustion reaction and its mechanism is given below:

Incomplete combustion reaction:

Incomplete combustion occurs when there is deficiency of oxygen in an environment.
However, in incomplete combustion reaction methane burns with the help of limited oxygen and produces carbon-monoxide, water and C.
Incomplete combustion reaction is given below:

Methane:

Methane is commonly used as a fuel for automobiles (CNG) and as fuel in cylinders for burning is due to its low molecular weight or lighter weight.
Because combustion of low molecular weight alkanes can be controlled.
And these low molecular weight or light alkanes produce large amount of heat on combustion.
Moreover, they are cheaply and easily available.

So, these were the reasons that why we used methane most commonly as a fuel in cylinders and automobiles.

Alkenes:

Reactions:

Reaction with halogens:

Double covalent bond is present between any two carbon atoms in alkenes.
In this double covalent bond, one bond is sigma bond (strong bond) and another bond is pi bond (weak bond).
Therefore, due to presence of this weak pi bond, alkenes perform addition reaction.
Pi bond is very weak covalent bond and can be easily break down.
By breakage of this old pi bond alkene can easily convert to alkane by formation of two new sigma bonds with halogen atoms.

Reaction with chlorine:

A balanced chemical reaction and mechanism is given below in which we can see that,

In this reaction as the name indicates (reaction with halogens), halogens will react with an alkene.
Here, pi bond in alkene will break by homolyticaly and both carbon atoms will take back their shared electrons.
In this way, each C atom will convert to alkyl free radicals by gaining one unpaired electron.
Similarly, in Cl2 molecule homolytic fission will occur and two chlorine free radicals will be produced.
These two chlorine free radicals will react with two alkyl free radicals (unpaired electrons present at two carbon atoms in alkene) and will lead to the formation of 1,2-dichloromethane.

Reaction with Bromine:

Reaction and its mechanism is given below.

This reaction and its mechanism is same as for mentioned above reaction of alkene with chlorine. Just use Br₂ molecule instead of chlorine.

Reaction with KMnO₄:

This reaction is also known as Bayer’s test. This test is used for identification of presence of double bond in a substance.
In this reaction KMnO₄ react with alkene in the presence of water molecules and produces ethylene glycol by addition of two hydroxyl groups to alkene. Ethylene glycol is used as an anti-freeze agent.

Reaction and its mechanism is given below in which we can see that how alkene reacts with KMnO₄ and form ethylene glycol.

alkene reacts with KMnO4 and form ethylene glycol

Alkynes:

Reactions:

Addition of halogens:

Triple covalent bond is present between any two carbon atoms in alkynes.
In this triple covalent bond, one bond is a sigma bond (strong bond) and another two bonds are pi bonds (weak bonds).
Therefore, due to the presence of these weak pi bonds, alkynes perform addition reactions.
Pi bond is a very weak covalent bond and can be easily broken down.
By fission of this old pi bond alkyne can easily convert to alkane through two-step reaction by the formation of four new sigma bonds with halogen atoms.

Reaction with chlorine:

Reaction and mechanism is given below in which we can see that,

In this reaction as the name indicates (reaction with halogens), halogens will react with an alkyne.
Here in the first step, one pi bond in alkyne will break homoliticaly and both carbon atoms will take back their shared electrons.
In this way, both C atoms involved in this pi bond will convert to alkyl free radicals by gaining an unpaired electron.
Similarly, in Cl₂ molecule homolytic fission will occur and two chlorine free radicals will be produced.
These two chlorine free radicals will react with alkyl free radicals (two unpaired electrons present on two carbon atoms in alkene) and will lead to the formation of 1,2-dichloroethene.
In the second step, repeat the same reaction.
React 1,2-dichloroethene with Cl₂ molecule in the same way and convert it into 1,1,2,2-tetrachloroethane.

2.Reaction with KMnO₄:

This reaction is used to identify the presence of a triple bond in a substance.
In the first step, alkaline KMnO4 reacts with an alkyne in the presence of water molecules and produces tetrahydroxy ethane by the addition of four hydroxyl groups to the alkyne.
Then in the second step, by removal of two water molecules tetrahydroxy ethane convert into glyoxal.
This glyoxal on oxidation converts into oxalic acid.

Reaction and its mechanism is illustrated below in which we can see how alkyne reacts with KMnO₄ and form oxalic acid.

Chapter: Hydrocarbons (Meanings of Difficult Words)

Words	Meaning
Dissipating	Disappear
Treated	Reacted
Referred	Called
Illustrated below	As mentioned below / Given below / shown below
Fission	Break
Demonstrate	Illustrate/explain
Covalent connection	Covalent bond
Extract	Remove
Diversity	Variety / types
For instance	For example
Manufacturing	Synthesis / production / preparation
Chopped	Break / Cut into small pieces
Utilized	Used
Accomplished	Fulfil / Achieve
Starting ingredient	Starting material
Comprehend	Understand / include / comprise
Categorized	Classify
Discriminate	Differentiate
Distinction	Differentiation
Illustration	Demonstration / Explanation
Few instances	Few examples