JEE Main Study Notes for Hydrocarbons: Revision Notes, Tricks and Solved Questions

Hydrocarbon is one of the favorite topics that is asked in the JEE Main. It is covered under organic chemistry part. Students can expect around 2-3 questions carrying 8 marks from this topic. Hence the weightage of this topic is around 2-3% in JEE Main Chemistry Syllabus.

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A hydrocarbon is an organic compound consisting of only hydrogen and carbon atoms. They are group 14 hybrids, which implies they contain atoms of hydrogen, as well as of the carbon 14 group; carbon, silicon, germanium, tin, and lead. Carbon contains 4 electrons, which means it can make 4 bonds to be stable. Another type of hydrocarbons are aromatic hydrocarbons, which include alkanes, cycloalkanes, and alkyne-based compounds. 

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Classification of Hydrocarbons

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• JEE Main Study Notes for Equilibrium
• JEE Main Study Notes for s block Elements
• JEE Main Study Notes for States of Matter

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.

• Staggered Conformation: H atoms on two adjacent carbon atoms are farthest to each other i.e. dihedral angle is 60.

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:

• Hydrogenation of Alkenes:

• Wurtz Reaction:

2RX + 2Na → R-R + 2NaX

2Na + 2CH3CH2CH2Cl → CH3CH2CH2CH2-CH2CH3 + 2NaCl

• Corey House Reaction:

• 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

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Chemical Properties of Alkane

Direct Halogenation

RH + X2→ RX + HX

Order of Reactivity of X2: F2 > Cl2 > Br2; I2 does not react

a. Initiation Step

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 HNO3 at about 400oC to give nitroalkanes.

This is also known as vapour phase nitration.

Combustion

?Alkanes burn readily with non-luminous flame in presence of air or oxygen to give CO2 & water along with evolution of heat.

C2H6 + 7O2 → 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. 

 

Oxidization of 30 alkane

Tertiary alkanes are oxidized to tertiary alcohols by KMnO4

 R3CH + KMnO4 → R3COH

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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.

Geometric Isomers:

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

Heats of Hydrogenation: Heat of hydrogenation increases with increase in stability of alkene.

Order of heat of hydrogenation: 1-Butene> cis-2-Butene > trans-2-Butene

Order of stability: 1-Butene> cis-2-Butene > trans-2-Butene

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Preparation of Alkenes

1. Cracking of petroleum:

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 ofan H is 3° > 2° > 1°

4. Reduction of alkynes:

 

Chemical Properties of Alkenes

1. Electrophilic Polar Addition Reactions

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2. Addition of Hydrogen Halides to Alkenes: Markovnikov’s Addition:

R - CH = CH2 + HBr → R – CHBr – CH3

Mechanism:

R-CH =CH2 +HBr → R–CH+ -CH3 +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 

CH3·CHCH2Br + HBr→ CH3CH2CH2Br + Br.

Termination:

2RO. → R - O - O - R

Br. + Br.→Br2

3. Addition of Water to Alkenes: Acid Catalyzed Hydration

4. Oxymercuration-Demercuration:

Examples:

5. Hydroboration-Oxidation

Examples:

6. Halogen Addition in Non-polar Solvent

7. Halogen Addition in Aqueous Medium

8. Syn – Hydroxylation: Formation of di-oles

9. Ozonolysis of Alkenes

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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 of Alkyne

1. Dehydrohalogenation of vic-Dihalides or gem-Dihalides 

 
 

2. Dehalogenation of vic-Tetrahalogen Compounds

 

3. Alkyl Substitution in Acetylene; Acidity of º C-H

4. From Calcium Carbide

CaC2 +2H2O → Ca(OH)2+ C2H2

5. Kolbe’s Electrolysis

 

Chemical Properties of Alkyne

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

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). :

Benzene (C6H6)

1. Structure:

 2. Chemical Reactions of Benzene:

Anti-aromatic Hydrocarbons

Highly unstable compounds.

Number of π electrons in ring = 4n. 

Example: 

 

Previous Year Solved Questions

Question: The main product of the following reaction is:

Solution: (2)

Question: One mole of symmetrical on ozonylysis gives two moles of an aldehyde having a molecular mass of 44u. The alkaline is:

1) Ethene

2) Propene

3) 1-Butene

4) 2-Butene

Solution: (4) 

Question: Ozonylysis of an organic compound gives formaldehyde as one of the products. This confirms the presence of:

1) an isopropyl group

2) an acetylenic triple bond

3) two ethylenic double bonds

4) a vinyl group

Solutions: (4)

Question: 2-Hexyne gives trans-2-Hexene on treatment with:

1) LiAlH4

2) Pt/H2

3) Li/NH3

4) Pd/BaSO4

Solution: (3)

Question: The major organic compound formed by the reaction of 1,1,1-tricholoroethane with silver powder is:

1) 2-Butyne

2) 1-Butyne

3) Acetylene

4) Ethene

Solution: (1)

Revision Tips for Hydrocarbons

1. The below figure will show the basic outline for the methods of preparation of Alkanes

 

2. Alkenes can be prepared by Dehydrohalogenation of alkyle halides. Ease of dehydration follows the sequence 30 >20 >10 for alkyle halides. The reaction is stereo-selective and favors more stable stereo-isomers

3. Cycloadditions: A useful synthetic reaction cycloaddition of an alkene, called dienophile, to conjugate diene by 1,4-addition. This reaction is known as Diels-Alder reaction

Tips and Tricks for Hydrocarbons

• The strong oxidizing agents are: KMnO4 and K2Cr2O7
• Mild oxidizing agents are CrO4, Ammoniacal AgNO3, Benedicts Solution, Fehling’s Solution, Cu or CuO at 573K, Bromine water etc.
• Any carboxylic acid derivative can be converted back to the carboxylic acid by hydrolysis.
• Haloalkanes can give out possible functional group. They can also help increasing the chain size. So you might convert your starting material to haloalkanes and then go to the desired product.
• Grignard reagent gives you much-needed Alkyl neuclophile, which can be used at appropriate places
• Decarboxylation and ozonylysis could go-to methods for decreasing the numbers of carbon atoms
• Saytezeff/Markovnikov’s rules must be kept in mind while dealing with alkenes

2 Month Chemistry Study plan for JEE Main

July 5 to 15, 2020

Focus on physical chemistry as it has the most weightage in the Chemistry part of JEE Main exam. Go through the important topics like Atomic Structure, Equilibrium, Electrochemistry etc first. Them go ahead with doing chapters like Surface chemistry and solid state since they have less weightage. You can choose 2-3 topic each day and then revise later at the end of the week. It is advisable to go through NCERT books first before taking other reference books.

July 16 to 26, 2020

Focus on organic chemistry. Start with the important chapters that have more weightage i.e. Aromatic Compounds, Alkyl Halides, Alcohol and Ether. After completing these, move on to less important chapters like Carbonyl Compounds, Biomolecules etc. For organic chemistry, stick to NCERT books and get the concepts clear. Revise during weekend and polish the concepts

July 27 to August 8, 2020

Focus on inorganic chemistry in these days. Go through important chapters like p block, s bock, chemical bonding which have more weightage in the JEE Main exam. After these, move on to chapters like Metallurgy and Qualitative Analysis. For inorganic chemistry also, use NCERT books.

August 9 to 20, 2020

Take this week out for practicing questions only. Revise all the concepts again if the first 2 days and then move on with solving questions. Take questions from previous year papers and other sample papers of chemistry and solve them. Keep a check on the time that you take for solving and your accuracy. Try to go through the topics in which you lag.

August 21 to 31, 2020

Focus only on giving mock test in this week and final revision. Try to take one mock test every day.

September 1 to 6, 2020

Research and analysis of the ongoing papers.