Class 11 Hydrocarbons

The world around us is made up of different atoms that combine to form complex molecules. We may not be able to experience them through the naked eyes, but they are a vital part of various day-to-day things. You can learn about the same in-depth in the 13th chapter of the NCERT Chemistry class 11, Hydrocarbons. The compounds that are solely made of carbon and hydrogen atoms are referred to as Hydrocarbons. Hydrocarbon is a pre-eminent topic of organic chemistry, that is even crucial for various competitive exams. So, let’s begin our journey for class 11 hydrocarbons and discuss all the important topics. 

Types of Hydrocarbons

The unique compounds that are composed of only carbon and hydrogen atoms are called hydrocarbons. They are commonly colourless with only faint odours. Some daily-life examples of hydrocarbons are petroleum gas, CNG, LPG, etc. Based on the structure, hydrocarbons can be divided into two categories, as follows:

Acyclic or Open-Chain Hydrocarbons

Consisting of an open chain of carbon atoms in their molecules, they are also known as Aliphatic Hydrocarbons. These are further classified into three categories:

• Alkanes 
• Alkenes 
• Alkynes

Cyclic or Closed-Chain Hydrocarbons

These compounds contain a closed chain or ring of carbon atoms in their molecule. They are further divided into:

Alicyclic Hydrocarbons
Hydrocarbons that contain a ring of three or more carbon atoms and have properties similar to those of aliphatic hydrocarbons are called Alicyclic Hydrocarbons.

Aromatic Hydrocarbons
As per class 11 hydrocarbons chapter, these hydrocarbons differ from the above ones as aromatic hydrocarbons contain at least one aromatic ring.

Hydrocarbons: Alkanes

Alkanes fall under the saturated hydrocarbon category as they comprise single carbon-carbon bonds. An example of this is methane (CH₄), which is a gas obtained in coal mines. Hydrocarbons are inactive under normal conditions, and they don’t react with acids and bases. 

The generalised formula for alkanes is C_nH_2n+2, where n stands for the number of carbon atoms and 2n+2 for the number of hydrogen atoms in the molecule. 

Examples of Alkanes are C₂H₆ (Ethane), C₃H₈ (Propane), etc.

Physical Properties of Alkanes

Being likely compounds, alkanes show similar types of physical properties. The physical properties of alkanes that are mentioned in class 11 hydrocarbons are:

• Boiling Points: The boiling point of straight-chain alkanes increases fairly regularly with an increase in their molecular mass. There is an increase of nearly 20 to 30 K for the addition of each CH₂ group
• Melting Points: The melting points of alkanes increase with an increase in carbon content
• Solubility: Alkanes which are predominantly non-polar are insoluble in polar solvents but are highly soluble in non-polar solvents
• Density: The densities of alkanes increase with an increase in the molecular mass

Uses of Alkanes

Alkanes are important in manufacturing various solvents and other compounds. Their common uses are given below:

• Methane in the form of natural gas is used as a fuel
• Methane is also used for manufacturing halogen-containing compounds which are used as solvents
• Higher alkanes such as gasoline, kerosene oil, diesel, etc. are widely used

Nomenclature and Isomerism

According to the class 11 hydrocarbons, Nomenclature is a collection of rules to formulate precise names for chemical compounds, while Isomerism refers to more than one compound having the same chemical formula, but varying chemical structures. Structures that vary in the chain of carbon atoms are called chain Isomers.

For Example: The first three alkanes, namely Methane, Ethane, and Propane, have only one structure, but higher alkanes can have more than one structure. 

If you write down the structures for C₄H₁₀, the four carbon atoms of C₄H₁₀ can be connected either in a continuous chain or with a branched-chain. This shows us that even though Butane has the same molecular formula (C₄H₁₀ ), it differs in chemical properties like boiling point, melting point, and so on. They are known as isomers of Butane. 

The compounds can be classified on the basis of the number of carbon atoms connected to a carbon atom. The compounds termed as primary (1°) have a carbon atom connected to no other carbon atom or only one carbon atom. The secondary (2°) carbons are when the carbon atom is attached to two carbon. The tertiary (3°) when the carbon is attached to three carbon atoms, and quaternary (4°) when it is attached to four carbon atoms. 

Hydrocarbons: Alkenes

Now in our class 11 hydrocarbons notes, we will explore Alkenes. Alkenes are generally acyclic, unsaturated hydrocarbons containing a carbon-carbon double bond and are also known as Olefins.

The general formula for alkenes is C_nH_2n, where n stands for the number of carbon atoms and 2n for the number of hydrogen atoms in the molecule. 

Isomerism of Alkenes

The Alkene compounds vividly show isomerism. Alkenes show both structural and geometrical Isomerism, as explained below:

• Structural Isomerism: Other than ethene and propene, alkenes containing four or more carbon atoms can show both position and chain isomerism
• Geometrical Isomerism: Isomers having an identical structural formula, but varying in their corresponding spatial arrangement of atoms or groups around the double bond of carbon atoms are referred to as geometrical isomers

Physical Properties of Alkenes

Mentioned below are the physical properties of Alkenes:

• Physical state, Color, Smell: The first three members of the alkene family namely Propene, Ethene, and Butene are colourless gases, while the next fourteen members, i.e., from C₅to C₁₈ are liquid and the higher ones are solid. Except for Ethene which has a pleasant smell, all other alkenes are colourless and odourless gases
• Solubility: Alkenes are insoluble in water but are fairly soluble in non-polar solvents.
• Boiling Points: The boiling points of alkenes regularly increases with an increase in their molecular mass

Uses of Alkenes

Derived from the class 11 hydrocarbons chapter, the common uses of alkenes are as follows:

• The lower members of the alkene family are used as fuels
• Alkenes and substituted alkenes upon Polymerization form many useful polymers
• Ethene is employed for the preparation of ethyl alcohol and ethylene glycol (also known as antifreeze)
• Ethylene is also used in oxygen 
• Ethylene flame is used for cutting and welding of metals

Hydrocarbons: Alkynes

Alkynes are acyclic unsaturated hydrocarbons containing a carbon-carbon triple bond. Alkynes are even called acetylenes. Alkynes can be classified as terminal or non-terminal alkynes based on the triple bond present at the end of the chain.

The general formula is C_nH_2n-2, where n stands for the number of carbon atoms and 2n-2 for the number of hydrogen atoms in the molecule.

Isomerism of Alkynes

The varied types of isomerism shown by the alkyne compounds are-

• Position Isomerism: The first two members of Alkynes, i.e., Propyne and Ethyne, exist in only one form. The higher alkynes after ethyne (from butyne) exhibit position isomerism because of the different position of the triple bond on the carbon chain
• Chain Isomerism: Alkynes that have five or more carbon atoms show chain isomerism due to the different structures of the carbon chain
• Functional Isomerism: Alkynes are the functional isomers of dienes, the compounds containing two double bonds
• Ring Chain Isomerism: Alkynes exhibit ring chain isomerism with cycloalkenes

Physical Properties for Alkynes 

Just like alkanes and akenes, alkynes also exhibit some physical features. Straight from the NCERT class 11 hydrocarbons chapter, here are the physical properties of alkynes-

• Physical State: The first three members of the alkyne family (propyne, ethyne, and butyne) are colourless gases, the next eight are liquid, while the higher ones are solids
• Smell: All the alkynes are odourless, however, acetylene has a garlic smell due to the presence of Phosphine as an impurity
• Melting and Boiling Points: The melting and boiling points of alkynes are slightly higher than those of the corresponding alkanes and alkenes due to the triple bond
• Solubility: Alkynes are non-polar and insoluble in water, but they easily dissolve in organic solvents
• Density: The densities of alkynes increase with increasing molecular size, but they are lighter than water 

Uses of Alkynes

Alkynes are used in many industries to manufacture a variety of products. Some of the prominent uses are:

• Alkynes are utilised for producing polymers like PVC or synthetic rubber products
• They are even employed to artificially ripe fruits
• Ethyne is adopted for brewing several organic solvents

Practice MCQ

1. Arrange the following in decreasing order of their boiling points.
   (A) n–butane
   (B) 2–methylbutane
   (C) n-pentane
   (D) 2,2–dimethylpropane
   • (a) A > B > C > D
   • (b) B > C > D > A
   • (c) D > C > B > A
   • (d) C > B > D > A 
2. Arrange the halogens F₂, Cl₂, Br₂, I₂, in order of their increasing reactivity with alkanes.
   • (a) I₂ < Br₂ < Cl2 < F₂
   • (b) Br₂ < Cl₂ < F2 < I₂
   • (c) F₂ < Cl₂ < Br2 < I₂
   • (d) Br₂ < I₂ < Cl2 < F₂
3. The position of double bond in alkenes can be located by :
   • (a) Hydrogenation of oil
   • (b) Ozonolysis
   • (c) Photolysis
   • (d) Hydration
4. A dibromo derivative of an alkane reacts with sodium metal to form an alicyclic hydrocarbon. The derivative is ______. 
   • (a) 2, 2-dibromobutane 
   • (b) 1, 1-dibromopropane
   •  (c) 1, 4-dibromobutane  
   • (d) 1, 2-dibromoethane
5. Some meta-directing substituents in aromatic substitution are given. Which one is most deactivating?
   • (a) –C ≡ N
   • (b) -SO3H
   • (c) -COOH
   • (d) -NO2
6. Nitrobenzene on reaction with conc. HNO3/H2SO4 at 80 – 100°C forms which one of the following products?
   • (a) 1, 2-Dinitrobenzene
   • (b) 1, 3-Dinitrobenzene
   • (c) 1, 4-Dinitrobenzene
   • (d) 1, 2, 4-Trinitrobenzene
7. A dibromo derivative of an alkane reacts with sodium metal to form an alicyclic hydrocarbon. The derivative is ______. 
   • (a) 2, 2-dibromobutane 
   • (b) 1, 1-dibromopropane
   •  (c) 1, 4-dibromobutane  
   • (d) 1, 2-dibromoethane
8. The angle strain in cyclobutane is
   • (a) 24°44
   • (b) 29°16
   • (c) 19°22
   • (d) 9°44
9. The first fraction obtained during the fractionation of petroleum is:
   • (a) Gasoline
   • (b) Diesel Oil
   • (c) Hydrocarbon Gases
   • (d) Kerosene Oil
10. The increasing order of reduction of alkyl halides with zinc and dilute HCl is
    • (a) R–Cl < R–I < R–Br
    • (b) R–Cl < R–Br < R–I
    • (c) R–I < R–Br < R–Cl
    • (d) R–Br < R–I < R–Cl

As this chapter is one very important and fundamental topic of class 11 chemistry, we hope that through our detailed notes on class 11 hydrocarbons, you have understood it clearly. For career-related queries and trending career advice, reach out to our experts at Leverage Edu. Hurry Up! Book and e-meeting.