Hydrocarbons: Class 11 Chemistry NCERT Chapter 13

Key Features of NCERT Material for Class 11 Chemistry Chapter 13 – Hydrocarbons 

In the last chapter 12, you learned about Organic Chemistry – Some Basic Principles and Techniques. In this chapter: Hydrocarbons, you will learn about the three significant pollutants-carbon monoxide, carbon dioxide, and hydrocarbons.

With the steep ascent of pollution levels in our environment, it is significant that we keep ourselves educated about it. One primary concern for us to know is the thing that precisely is adding to this pollution. 

Quick Revision notes 

  • Hydrocarbon 

A compound of carbon and hydrogen is known as a hydrocarbon. 

  • Saturated Hydrocarbon 

A hydrocarbon is supposed to be saturated on the off chance that it contains just C—C single bonds. 

For instance: Ethane is CH3—CH3

  • Unsaturated Hydrocarbon 
  • Aromatic Hydrocarbon

Benzene and its derivatives come under aromatic compounds.

Example:

  • Alicyclic Compounds

The Cyclic compounds which contain only carbon are called alicyclic or carbocyclic compounds

  • Heterocyclic Compounds

The Cyclic compounds in which the ring atoms are carbon and some other element (For instance, N, S, or O) are called heterocyclic compounds.

  • Alkanes

Alkanes are the most straightforward organic compounds made of carbon and hydrogen as it were. 

They have the overall equation CnHC2n+2 (where n = 1, 2, 3, and so forth.) 

The carbon atoms in their molecules are attached to one another by single covalent bonds. Since the carbon skeleton of alkanes is completely saturated with hydrogens, they are additionally called saturated hydrocarbons. Alkanes contain stable C — C and C — H bonds. Accordingly, this class of hydrocarbons is moderately chemically inert. Thus they are here and there alluded to as paraffin (Latin parum affinis = little liking). Initial three individuals from this class can be spoken to as

Structure:

In methane, carbon structures single bonds with four hydrogen atoms. All H—G—H bond edges are of 109.5°. Methane has a tetrahedral structure. C—C and C—H bonds are framed by a head-on covering of sp3 hybrid orbitals of carbon and Are orbitals of hydrogen atoms. 

  • Nomenclature Guidelines 

Utilize the accompanying bit by bit strategy to compose the IUPAC names from the fundamental equations. Think about the accompanying basic equation:

Step 1.   Identify the longest chain: In the example, the longest chain has seven carbons, and heptane is the seven carbon chain.

Step 2. Number the chain: The numbering of the chain is from left to right. This gives the least numbers to the joined alkyl gathering. 

Step 3. Recognize the alkyl gathering: There are two methyl bunches at C-2 and C-3, there is one ethyl gathering of C-4. 

Step 4. Compose the IUPAC name: For this situation, the IUPAC name is 4-Ethyl-2,3-dimethyl heptane. Continuously remember (a) Numbers are isolated from one another by commas. (b) Numbers are separated from names by hyphens, (c) Prefixes di, and tri are not considered in alphabetizing substituent names. 

  • Newman Projections 

In this projection, the atom is seen at the C—C bond head-on.

  • Relative Stability of Conformations

In a staggered type of ethane, there are most extreme horrendous powers, the least vitality, and the greatest security of a molecule. Then again, when the tremendous structural changes in the overshadowed structure, the electron billows of the carbon-hydrogen bonds come nearer to one another subsequent in increment in electron cloud shocks, the molecule needs to have more vitality and in this way has lower steadiness

Torsional Angle: Magnitude of torsional strain relies on the point of turn about the C—C bond. This point is likewise called a dihedral edge or torsional edge. 

  • Alkenes

Alkenes are hydrocarbons that contain a carbon-carbon twofold bond (C=C) in their molecule. 

They have the overall equation 

Structure

Allow us to consider (H2C=CH2) for outlining the orbital make up of alkenes. 

In ethylene, the carbon atoms are sp2 hybridized-They are connected to one another by a bond and a σ bond. 

The bond results from the cover of two sp2 hybrid orbitals. The π bond is shaped from the protection of the unhybridized p-orbitals. Ethylene is a planar molecule.

Focuses to be noted 

(I) The carbon-carbon twofold bond in alkenes is composed of one σ and one π-bond. 

(ii) Alkenes are more receptive than Alkanes. This is because of the accessibility of n electrons. 

  • Nomenclature 

In IUPAC framework 

(I) The name of the hydrocarbon depends on the parent alkene having the longest’ carbon chain’ of which a two-fold bond is separated. 

(ii) This chain is numbered from the end close to the twofold bond, and its position is indicated by the quantity of the carbon particle not which the twofold bond starts, 

(iii) The name of the parent alkene with the position number of the twofold bond is composed first and afterward the names of different substituents prefixed to it.

(iv) When there are a few twofold bonds in a molecule, the ending-one of the corresponding alkane is supplanted by-a diene to get the name.

  • Isomerism

Structural Isomerism:  Ethene and propene do not have any structural isomers; however, there are three corresponding structures for butene.

Of these, two are straight-chain structures, and the only difference is in the position of double bonds in the molecules.

These are position isomers, and the third is known as the branched-chain isomer.(because it has branches).

Geometrical Isomerism: It is considered as a fact that a carbon-carbon double bond is made up of one σ bond and one π-bond. The π-bond presents free rotation about the double bond.

This presentation of rotation about the carbon-carbon double bond gives rise to the phenomenon of geometrical isomerism. An alkene having a formula RCH=CHR can have two stereoisomers, depending upon whether the two alkyl groups are on the same or opposite sides of the double bond. If they fall on the same side, then it is called cis-isomer. If they are on opposite sides, then it is called trans-isomer.

Due to different arrangements of atoms or groups in space, these isomers differ in their properties like melting point, boiling point, dipole moment, solubility, etc.

  • Alkynes

Alkynes are described by the nearness of a triple bond in the molecule. 

Their overall equation is CnH2n-2. 

The first and the most significant individual from this arrangement of hydrocarbons is acetylene, HC=CH, and subsequently, they are additionally called the Acetylenes. 

Structure: Let us think about ethyne (HC=CH) for delineating the orbital make up of ethyne. The carbon atoms have sp hybridization in ethyne. A σ-bond and two π-bonds interconnect them. 

The σ – bond results from the cover of two sp hybrid orbitals. The π bonds are framed from the different cover of the two p-orbitals from the two nearby carbon atoms. 

The other sp hybrid orbital of every carbon particle shapes a σ bond with another carbon or hydrogen molecule. Ethyne is a linear molecule.

Focuses to be noted:

(I) The carbon-carbon triple bond in alkynes is composed of one σ and two π bonds. 

(ii) Like alkenes, alkynes experience additional response. These responses are because of the accessibility of more uncovered π electrons. 

  • Nomenclature 

IUPAC System: The names of alkynes are gotten by eliminating the ending-ane of the parent alkane and using the suffix-yne. The Carbon chains, including the triple bond, are numbered from the end closest to this bond. The triple bond situation is indicated by prefixing the quantity of carbon preceding it to the name of the alkyne.

Preparation:

From calcium carbide: Ethyne is set up by treating calcium carbide with water. Calcium carbide is set up as follows:

From vicinal dihalides:

When responded with vicinal dihalides, alcoholic potassium hydroxide experiences dehydrohalogenation. One molecule of hydrogen halide is wiped out to frame alkyl halide, which gives alkyne treatment with sodamide.

  • Aromatic Hydrocarbons

These hydrocarbons are otherwise called ‘arenes’. A large portion of such compounds was found to contain a benzene ring. 

Aromatic compounds containing benzene ring are known as benzenoids, and those not containing a benzene ring are known as non-benzenoids. A few instances of arenes are given beneath.

Nomenclature and Isomerism: Benzene and The homologous are, for the most part, called by their common names, acknowledged by the IUPAC framework. The homologous of benzene having a solitary alkyl bunch are named as Alkyl benzenes

Structure of Benzene: 

By elemental investigation, it is discovered that the atomic recipe of benzene is C6H6. This indicates benzene is an exceptionally unsaturated compound. In 1865, Kekule gave the cyclic planar structure of benzene with six carbons with exchange twofold and single bonds.

The Kekule structure indicates the chance of two isomeric 1,2-bromobenzene. In one of the isomers, the bromine atoms are appended to the doubly fortified carbon atoms, while in the other, they are joined to the independently reinforced carbon. 

In fact, only one ortho-dibromobenzene could be prepared.

To get rid of this problem, Kekule suggested that benzene was a mixture of two forms.

Drawbacks of Kekule’s structure: Kekule’s structure of benzene neglected to clarify the special security and its inclination to replacement response than additional responses. 

Resonance Structure of Benzene: The marvel where at least two structures can be composed for a substance which includes indistinguishable places of atoms is called resonance. In benzene’s Kekule’s structures (1) and (2) speak to the resonance structures. The real structure – of the molecule is expressed to by hybrid of these two structures.

Orbital structure of benzene: Every one of the six carbon atoms in benzene is sp2 hybridized. The sp2 hybrid orbitals cover with one another and with s orbitals of the six hydrogen atoms shaping C—C and C—H σ-bonds.

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