NCERT Solutions for Class 11 Chemistry Chapter 13

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Chapter 13 – The Hydrocarbons 

NCERT Solutions for Class 11 Chemistry Chapter 13

• Hydrocarbon

A compound of hydrogen and carbon is called a hydrocarbon.

• Saturated Hydrocarbon

If the hydrocarbon contains C—C single bonds only.

Example: 

Ethane CH3—CH3 

• Unsaturated Hydrocarbon

• Aromatic Hydrocarbon

The Benzene and its derivatives are known as aromatic compounds.

Example:

• Alicyclic Compounds

Cyclic compounds consisting of carbon atoms only are known as carbocyclic or alicyclic compounds.

• Heterocyclic Compounds

Cyclic compounds where the ring atoms are of some element like N, S or O and carbon. They are known as heterocyclic compounds.

• Alkanes

The simplest organic compounds created from only carbon and hydrogen only are called Alkanes.

The general formula of Alkanes is CnHC2n+2 (where n donates = 1, 2, 3, etc.)

Single covalent bonds bond the C atoms in their molecules. As the carbon skeleton of alkanes is completely saturated with hydrogens, they are also known as saturated hydrocarbons. Alkanes have strong C —H and C —C bonds. Hence, this class of hydrocarbons are comparatively chemically inert. So, they are often called paraffin. 

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Structure

The carbon of Methane forms single bonds with four hydrogen atoms, so it is called Methane. It has a tetrahedral structure. 

• Nomenclature Guidelines

Use the following step-by-step process to write the IUPAC names from the structural formulas of the hydrocarbons. Consider the following structural formula:

• Step 1. The first step is to identify the longest chain in the hydrocarbon: longest chain here has seven carbons. The seven carbon chain is heptane.

• Step 2. Number the chain: The chain has to be numbered from left to right, which will give the lowest numbers to the alkyl group.

• Step 3. Identify the alkyl group in the hydrocarbon. So, there are two methyl groups at C-2 and C-3, there is one ethyl group of C-4.

• Step 4. So, the IUPAC name, in this case, is 4-Ethyl-2,3-dimethyl heptane. Points to note: 

(a) Always use commas to separate numbers. 

(b) Hyphens are used to separate names from numbers.

(c) Prefixes di, tri is not taken into consideration in naming substituent names.

• New Man Projections

The molecule is looked at the C—C bond head-on in this projection

• Relative Stability of Conformations

When Ethane is in the staggered form, then there are maximum repulsive forces and stability of the molecule and minimum energy. On the other hand, the eclipsed way the electron clouds of the C-H bond come closer to each other, which will result in increasing the electron cloud repulsion. Hence, the molecule will have more energy and less stability.

• Alkenes

Alkenes are hydrocarbons that have a double bond (C=C) in their C atoms 

The general formula:

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Structure

Let us consider (H2C=CH2) for visualizing the orbital make up of alkenes.

In ethylene, the C atoms are sp2 hybridized in the ethylene- The bond attaches them. 

The bond derives from the overlap of two sp2 hybrid orbitals. 

The π bond results from the overlap of the unhybridized p-orbitals. Ethylene is a planar molecule.

Points to Be Noted

(i) The carbon-carbon double bond in alkenes consists of one σ and one π-bond.

(ii) Alkanes are less reactive, and alkenes are more reactive due to the availability of n electrons.

• Nomenclature

In the IUPAC system

(i) The longest ‘ C chain of which double bond is a part has to be identified.

(ii) This chain is seen from the end closer to the double bond, and its position is defined by the number of the C atom not which the double bond originates. 

(iii) First, mention the name of the parent alkene along with the position number of the double bond and then the names of the other substitutes prefixed to it. 

(iv) The last one of the corresponding alkane is replaced by-a diene to get the name if there is more than one double bond. 

• Isomerism

Structural Isomerism: Structural isomers are those isomers in which the atoms are organized in a different order but having the same molecular formula. Ethene and propene do not show any structural isomers; however, butene has three structural isomers. 

Among them, two are straight-chain structures but having different positions of double bonds in the molecules. 

Geometrical Isomerism: They are molecules that are locked into their spatial positions concerning one another because of the double bond or a ring structure. For example, consider the following two molecules.

This rotation about the carbon-carbon double bond results in geometrical isomerism. An alkene with a formula RCH=CHR can have two stereoisomers, based on if the two alkyl groups are on the opposite or same sides of the double bond. 

But they are called cis-isomer if they are on the same side and they are called trans-isomer if they are on opposite sides. 

• Alkynes

Alkynes are defined by the occurrence of a triple bond in the molecule.

CnH2n-2 is the general formula.

The primary member of this is acetylene, HC=CH, and so they are known as the Acetylenes.

Structure: Consider ethyne (HC=CH) for visualizing the orbital make up of ethyne. The c atoms are sp hybridized in the ethyne. They are bonded by a σ-bond and two π-bonds.

The σ -bond comes from the overlap of two sp hybrid orbitals. 

The π bonds are made from the different overlap of the two p-orbitals from the two adjacent C atoms.

The other sp hybrid orbital makes a σ bond with another carbon or hydrogen atom. 

Ethyne is a linear molecule.

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Points to Be Noted

(i) The carbon-carbon triple bond consists of one σ and two π bonds in alkynes.

(ii) Like alkenes, more exposure of π electrons makes the alkylene undergo different reactions.

Nomenclature

IUPAC System

It is obtained by omitting the suffix-ane of the parent alkane and an addition of a new suffix-yne. 

Carbon chain having the triple bond is – numbered from the end closest to this bond. The position of the triple bond is defined by prefixing the number of carbon before the name of the alkyne.

Preparation

From calcium carbide: Ethyne is prepared when calcium carbide is mixed with water. 

The preparation of calcium carbide is as follows:

From Vicinal Dihalides: The alcoholic potassium hydroxide undergoes dehydrohalogenation when reacted with vicinal dihalides. One molecule of hydrogen halide is dropped to make alkyl halide which, when treated with sodamide results in the formation of alkyne.

• Aromatic Hydrocarbons

These hydrocarbons are also called ‘arenes’. Mostly such compounds contain a benzene ring.

Benzenoids are the category of aromatic compounds having benzene rings. On the other hand, aromatic compounds not having the benzene rings are called non- benzenoids. 

Structure of Benzene: C6H6 is the molecular formula for Benzene. This shows that Benzene is too unsaturated. 

The Kekule structure given by Kekule shows the chances of two isomeric 1,2-dibromobenzene. From one of them is When the bromine atoms are attached to the double bond C atoms. However, in the other one, they are attached to the single bond C.

Benzene Was an Amalgamation of These Two Forms

However, this structure was not successful in explaining the unique stability and its inclination to substitution reaction rather than addition reactions.

Resonance Structure of Benzene: Resonance is a phenomenon where two or more structures can be used for a substance involving the same positions of atoms. 

Benzene's Kekule’s structures (1) and (2) show the resonance structures. The original structure – of the molecule is represented by the mixture of these two structures.

Orbital structure of Benzene: All C atoms in Benzene are sp2 hybridized. The hybrid sp2 orbitals overlap with each other and also with s orbitals of the six hydrogen atoms results in the formation C—C and C—H σ-bonds.

X-ray diffraction data show that Benzene is a planar molecule. The data indicate that all the six Carbon bonds length is of the same order (139 pm) which is intermediate between carbon-carbon single bond (154 pm) and a carbon-carbon double bond (133 pm). Hence, the presence of the double bond in Benzene proves the idea of the reluctance of Benzene to show additional reactions under average condition. Therefore, it justifies the peculiar behaviour of Benzene.

Aromaticity: The property of the sp2 hybridized planar rings where the p orbitals permit the cyclic delocalization of π electrons.

Conditions for Aromaticity:

(i) An aromatic compound is planar and cyclic. 

(ii) Every atom has a p orbital in an aromatic ring. These p orbitals should be parallel so that a consistent overlapping is made around the ring.

(iii) The cyclic π molecular orbital (electron cloud) which is formed by the overlap of p orbitals should contain (4n + 2) π electrons. 

Where n donates any integer (0, 1, 2, 3, etc.). 

This rule is called the Huckel rule.

Preparation of Benzene: There are some laboratory methods for preparing Benzene. 

Physical Properties of Benzene

(i). It is a colourless liquid.

(ii) It is not soluble in water. However, it is soluble in alcohol, ether, chloroform etc.

(iii). It is a suitable solvent for many inorganic and organic substances, e.g., resins, fats, sulphur and iodine.

(iv) It produces a luminous flame while burning in contrast to alkanes and alkenes which usually have a bluish flame.

Chemical Properties

Benzene goes through the following chemical reactions.

(i) Electrophilic Substitution Reaction

(ii) Addition Reaction

Electrophilic Substitution Reactions

When too much of chlorine is treated with Benzene in the presence of ALCL3, then Benzene can be chlorinated to hexachlorobenzene (C6Cl6)

Carcinogenicity and Toxicity: Some polynuclear hydrocarbons having more than two Benzene rings mix together and become more toxic, and they have cancer-producing properties. The incomplete combustion of some organic materials like tobacco, coal and petroleum, etc. leads to the formation of this.

Hydrocarbons: They are compounds of carbon and hydrogen only.

Alkane is an open chain saturated compound. 

Unsaturated Compound - Alkynes Aromatic Compound—Benzene and its derivatives and Alkenes are unsaturated compounds, and they are weak acids.

Conformation: The Spatial arrangements made by rotation around sigma bonds.

Eclipsed Conformation: The more repulsion between bond pairs of electrons makes it less stable.

Staggered: The lesser repulsion between bond pairs of electrons makes it more stable.

Key Features of NCERT Solutions for Class 11 Chemistry Chapter 13

Check NCERT Chemistry solutions for Class 11 Chapter 13 offered by Vedantu to know more about hydrocarbons The proper explanation of all types of hydrocarbons and their nomenclature in the notes will improve your knowledge about hydrocarbons. Some of the important features of downloading NCERT solutions Class 11 chemistry hydrocarbons notes are: 

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