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Hydrocarbons Class 11 Notes Chemistry Chapter 9


  • Compounds of carbon and hydrogen.
  • Classification of Hydrocarbons:
    Hydrocarbons Class 11 Notes Chemistry Chapter 9

Alkane

  • Open chain saturated hydrocarbon with general formula (CnH2n+2). 
  • All the C atoms are single bonded i.e. sp3 hybridised.

Conformations of Alkane

  • Conformations are the different arrangement of atoms that can be converted into one another by rotation about single bonds. 
  • Eclipsed Conformation: H atoms on two adjacent carbon atoms are closest to each other i.e. dihedral angle is 0.
    Hydrocarbons Class 11 Notes Chemistry Chapter 9
  • Staggered Conformation: H atoms on two adjacent carbon atoms are farthest to each other i.e. dihedral angle is 60.
    Hydrocarbons Class 11 Notes Chemistry Chapter 9

Preparation of Alkanes:

  • Reduction of Alkyl Halides:
    RX + Zn: + H+  → RH + Zn2+ + X-
    4RX + LiAlH4    → 4RH + LiX + AlX3 (X≠ F)                    
    RX + (n - C4H9)3 SnH  → R-H + (n - C4H9)3 SnX
  • Grignard Reagent:
    Hydrocarbons Class 11 Notes Chemistry Chapter 9         
  • Hydrogenation of Alkenes:
    Hydrocarbons Class 11 Notes Chemistry Chapter 9
  • Wurtz Reaction:
    2RX  + 2Na  → R-R + 2NaX
    2Na + 2CH3CH2CH2Cl  →  CH3CH2CH2CH2-CH2CH3 + 2NaCl
  • Corey House Reaction:
    Hydrocarbons Class 11 Notes Chemistry Chapter 9
  • Decarboxylation of a mixture of the sodium salt of a carboxylic acid:
    RCOONa +NaOH(CaO) → RH +  Na2CO3
  •  Kolbe's electrolytic method:
    2 RCOOK + 2H2O → R-R + 2CO2 + H2+ 2KOH

 Chemical Properties of Alkane

  • Direct Halogenation
    RH + X2→ RX + HX
    Order of Reactivity of X2:    F2 > Cl> Br2; I2 does not react
    a. Initiation Step
    Cl-ClHydrocarbons Class 11 Notes Chemistry Chapter 92Cl.
    b. Propagation Step
    H3C-H +Cl → H3C + H-Cl
    H3C + Cl-Cl → H3C-Cl +Cl
    c. Termination Step 
    Cl + Cl →Cl-Cl
    H3C + H3C → H3C-CH3
    Cl + H3C → Cl-CH3
  • Nitration 
    Nitration of alkane is made by heating vapours of alkanes and HNOat about 400oC to give nitroalkanes.
    This is also known as vapour phase nitration.
    Hydrocarbons Class 11 Notes Chemistry Chapter 9
  • Combustion:
    Alkanes burn readily with non luminous flame in presence of air or oxygen to give CO2 & water along with evolution of heat.
    C2H6 + 7O→ CO2 +6H2O + heat
  • Aromatization
    Alkanes having six to 10 carbon atoms are converted into benzene and its homologues at high pressure and temperature in presence of catalyst.
    Hydrocarbons Class 11 Notes Chemistry Chapter 9
  • Oxidization of 30 alkane:
    Tertiary alkanes are oxidized to tertiary alcohols by KMnO4
     R3CH + KMnO4 → R3COH

Alkene (olefins)

  • Open chain, Unsaturated hydrocarbons with general formula (CnH2n).
  • At least one  >c=c<  (double bond) group i.e. sp2 hybridisation, is present throughout the chain.
  • Allene: alkene molecule in which at least one C has double bonds with each of the adjacent carbon i.e. -c=c=c- group. 
  • Isomeric with saturated cycloalkanes.
    Hydrocarbons Class 11 Notes Chemistry Chapter 9

Geometric Isomers:
Hydrocarbons Class 11 Notes Chemistry Chapter 9
Z is used if the higher - priority substituents on each C are on the same side of the double bond. letter E is used if they are on opposite sides
Hydrocarbons Class 11 Notes Chemistry Chapter 9         

Heats of Hydrogenation: Heat of hydrogenation increases with increase in stability of alkene.
Hydrocarbons Class 11 Notes Chemistry Chapter 9       
Order of heat of hydrogenation: 1-Butene> cis-2-Butene > trans-2-Butene
Order of stability: 1-Butene> cis-2-Butene > trans-2-Butene

Preparation of Alkenes:
1. Cracking of petroleum:
Hydrocarbons Class 11 Notes Chemistry Chapter 9
2. Dehydrohalogenation of alkyl halides:   RCH2CH2X + alc.KOH → RCH = CH2
3. Dehydration of Alcohols:
Saytzeff Rule: In dehydration and dehydrohalogenation the preferential order for removal of an H is 3° > 2° > 1°
Hydrocarbons Class 11 Notes Chemistry Chapter 9
 4. Reduction of alkynes:
Hydrocarbons Class 11 Notes Chemistry Chapter 9

Chemical Properties:
1. Electrophilic Polar Addition Reactions

Reagent
Product
Name
Structure
Name
Structure
Halogens
(Cl2, Br2 only)
X:X
Ethylene dihalide
CH2XCH2X
Hydrohalic acids
H:X
Ethyl halide
CH3CH2X
 
Hypohalous acids
 
X:OH
 
Ethylene halohydrin
 
CH2XCH2OH
 
Sulfuric acid (cold)
 
H:OSO2OH
 
Ethyl bisulfate
 
CH3CH2OSO3H
 
Water (dil. H3O+)
 
H:OH
 
Ethyl alcohol
 
CH3CH2OH
Borane
 
H2B:H
Ethyl borane
(CH3CH2BH2) → (CH3CH2)3B
Peroxyformic acid
 
     H:O-OCH=O    
(HCO3H)
Ethylene glycol
CH2OHCH2OH


2. Addition of Hydrogen Halides to Alkenes: Markovnikov’s Addition:
R - CH = CH2 + HBr → R – CHBr – CH3
Mechanism:
R - CH = CH2 + HBr → R – CH+ - CH +Br-
R – CH+ - CH3 + Br- → R – CHBr - CH3
Anit- Markovnikov’s Addition (Peroxide Effect):
R - CH = CH2 + HBr + (C6H5CO)2O2 → R – CHBr – CH3
Mechanism
Initiation:
R - O - O - R  →  2RO
RO. +  HBr  →  Br.   +   ROH
Propagation
CH3CH = CH2   + Br →   CH3•CH - CH2Br
CH3CHCH2Br + HBr→ CH3CH2CH2Br + Br
Termination:
2RO  →  R - O - O - R  
Br + Br→Br2


3. Addition of Water to Alkenes: Acid Catalyzed Hydration:
Hydrocarbons Class 11 Notes Chemistry Chapter 9


4. Oxymercuration-Demercuration:
Hydrocarbons Class 11 Notes Chemistry Chapter 9
Examples:
Hydrocarbons Class 11 Notes Chemistry Chapter 9


5. Hydroboration-Oxidation:
Hydrocarbons Class 11 Notes Chemistry Chapter 9
Examples:
Hydrocarbons Class 11 Notes Chemistry Chapter 9


6. Halogen Addition in Non-polar Solvent:
Hydrocarbons Class 11 Notes Chemistry Chapter 9


7. Halogen Addition in Aqueous Medium:
Hydrocarbons Class 11 Notes Chemistry Chapter 9


8. Syn – Hydroxylation: Formation of di-oles.
Hydrocarbons Class 11 Notes Chemistry Chapter 9


9. Ozonolysis of Alkenes:
Hydrocarbons Class 11 Notes Chemistry Chapter 9 

Alkyne

  • Saturated open chain hydrocarbon with general formula (CnH2n-2).
  • At least one -c≡c-  (triple bond) group i.e. sp hybridisation, is present throughout the chain.
  • Physical properties of alkynes are similar to those of the corresponding  alkenes

Preparation
1. Dehydrohalogenation of vic-Dihalides or gem-Dihalides
Hydrocarbons Class 11 Notes Chemistry Chapter 9

2. Dehalogenation of vic-Tetrahalogen Compounds
Hydrocarbons Class 11 Notes Chemistry Chapter 9

3. Alkyl Substitution in Acetylene; Acidity of º C-H
 Hydrocarbons Class 11 Notes Chemistry Chapter 9

4. From Calcium Carbide:
CaC2 +2H2O →   Ca(OH)2+ C2H2

5. Kolbe’s Electrolysis:
Hydrocarbons Class 11 Notes Chemistry Chapter 9

Chemical Properties
1. Hydrogenation: RC ≡ CCH2CH3 + 2H2 →  CH3CH2CH2CH2CH3
2. Hydro-halogenation:
Markovnikov addition: RC≡CH +HBr → RCBr=CH2 +HBr→ RCBr2-CH3
Anti-markovnikov addition: RC≡CH +HBr +peroxide → RCH=CHBr
Hydrocarbons Class 11 Notes Chemistry Chapter 9  

Aromatic Hydrocarbons: 
For being aromatic a hydrocarbon should

  • be a cyclic compounds.
  • have planarity in geometry.  
  • have complete delocalization of electrons over ring.
  • follow Huckel Rule i.e. number of ?? electrons in ring = (4n+2).
    Hydrocarbons Class 11 Notes Chemistry Chapter 9

Benzene (C6H6)
1. Structure:
Hydrocarbons Class 11 Notes Chemistry Chapter 9
2. Chemical Reactions of Benzene:
Hydrocarbons Class 11 Notes Chemistry Chapter 9

Anti-aromatic Hydrocarbons:
Highly unstable compounds.
Number of π electrons in ring = 4n.
Example:
Hydrocarbons Class 11 Notes Chemistry Chapter 9

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FAQs on Hydrocarbons Class 11 Notes Chemistry Chapter 9

1. What are hydrocarbons?
Ans. Hydrocarbons are organic compounds made up of hydrogen and carbon atoms. They are the simplest form of organic compounds and can be found in various forms, such as methane, ethane, propane, and butane. Hydrocarbons are the main components of fossil fuels and are widely used as fuels and raw materials in various industries.
2. How are hydrocarbons classified?
Ans. Hydrocarbons are classified into two main types: aliphatic hydrocarbons and aromatic hydrocarbons. Aliphatic hydrocarbons are further divided into alkanes, alkenes, and alkynes based on the type of carbon-carbon bonds present. Aromatic hydrocarbons contain a benzene ring structure and are known for their distinct aroma.
3. What are the common uses of hydrocarbons?
Ans. Hydrocarbons have numerous uses in our daily lives. They are primarily used as fuels for transportation, heating, and electricity generation. Hydrocarbons are also used as raw materials in the production of plastics, synthetic fibers, detergents, lubricants, and various chemicals. Additionally, they are used in the manufacturing of pharmaceuticals and cosmetics.
4. How are hydrocarbons extracted from the Earth?
Ans. Hydrocarbons are extracted from the Earth through a process called drilling. Oil and gas wells are drilled into underground reservoirs where hydrocarbons are trapped. Once the well reaches the reservoir, the hydrocarbons flow to the surface under natural pressure or with the help of pumps. The extracted hydrocarbons are then processed and refined for various uses.
5. Are hydrocarbons harmful to the environment?
Ans. While hydrocarbons have many beneficial uses, their combustion releases carbon dioxide and other greenhouse gases, contributing to climate change. Additionally, the extraction, transportation, and refining of hydrocarbons can have negative environmental impacts, such as oil spills, air pollution, and habitat destruction. Therefore, it is important to develop sustainable alternatives and reduce our dependence on hydrocarbons to mitigate these environmental concerns.
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