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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RX + Zn: + H+ → RH + Zn2+ + X-
4RX + LiAlH4 → 4RH + LiX + AlX3 (X≠ F)
RX + (n - C4H9)3 SnH → R-H + (n - C4H9)3 SnX
2RX + 2Na → R-R + 2NaX
2Na + 2CH3CH2CH2Cl → CH3CH2CH2CH2-CH2CH3 + 2NaCl
RCOONa +NaOH(CaO) → RH + Na2CO3
2 RCOOK + 2H2O → R-R + 2CO2 + H2+ 2KOH
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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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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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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:
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 |
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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
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 |
Examples:
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3. Alkyl Substitution in Acetylene; Acidity of º C-H
CaC2 +2H2O → Ca(OH)2+ C2H2
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
For being aromatic a hydrocarbon should
1. Structure:
2. Chemical Reactions of Benzene:
Highly unstable compounds.
Number of π electrons in ring = 4n.
Example:
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)
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
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.
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
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.
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.
Focus only on giving mock test in this week and final revision. Try to take one mock test every day.
Research and analysis of the ongoing papers.
*The article might have information for the previous academic years, please refer the official website of the exam.