Hydrocarbons (Class 11 NCERT) Revision Notes

 Short Q&A for Hydrocarbons (Class 11 NCERT)

Part 1: Alkanes (Saturated Hydrocarbons)

Q. No.	Question	Answer
1	Define a Hydrocarbon.	Organic compounds composed only of Carbon and Hydrogen atoms.
2	What is the general formula for Alkanes?	Cn​H2n+2​.
3	What is the hybridization of carbon atoms in alkanes?	sp3.
4	What is the geometry around each carbon atom in alkanes?	Tetrahedral (≈109.5∘ bond angle).
5	What is Wurtz Reaction used for?	The synthesis of higher alkanes by coupling two alkyl halides with Na metal in dry ether.
6	What is the limitation of the Wurtz reaction?	It fails for synthesizing unsymmetrical alkanes (forms a mixture of three products).
7	What are the products when sodium acetate is heated with soda lime (Decarboxylation)?	Methane (CH4​) and Na2​CO3​.
8	What is the process of forming methane from aluminum carbide and water?	Al4​C3​+12H2​O→4Al(OH)3​+3CH4​ (Hydrolysis).
9	What is the Sabatier-Senderens Reaction?	Hydrogenation of alkenes or alkynes over Ni catalyst to form alkanes.
10	Why are alkanes often called paraffins?	Because of their low affinity towards chemical reagents (least reactive).
11	What type of reactions do alkanes primarily undergo?	Free Radical Substitution reactions (e.g., halogenation).
12	Write the three steps of the free radical substitution mechanism.	Initiation, Propagation, and Termination.
13	Which is the most reactive halogen in the halogenation of alkanes?	F2​ (Fluorination is explosive).
14	What is Isomerisation?	Conversion of n-alkanes to branched alkanes using AlCl3​/HCl at high T.
15	What is the product of controlled oxidation of methane with Cu/Ag catalyst at high T,P?	Methanol (CH3​OH).
16	What is the main characteristic of the cracking of alkanes?	Breaking large alkanes into smaller alkanes and alkenes at high temperature.
17	What is the main product of the complete combustion of alkanes?	CO2​ and H2​O.
18	What is the configuration of the preferred conformation of ethane?	Staggered conformation (most stable).
19	What is the energy difference between the staggered and eclipsed conformations of ethane called?	Torsional Strain.

Part 2: Alkenes (Unsaturated Hydrocarbons)

Q. No.	Question	Answer
20	What is the general formula for Alkenes?	Cn​H2n​ (containing one double bond).
21	What is the hybridization of carbon atoms involved in the double bond?	sp2.
22	What is the geometry around the double-bonded carbons?	Trigonal Planar (≈120∘ bond angle).
23	How many σ and π bonds are in the double bond?	One σ and one π bond.
24	Why are alkenes more reactive than alkanes?	Due to the presence of the weak π-bond, which is easily cleaved.
25	What type of reaction is characteristic of alkenes?	Electrophilic Addition Reactions.
26	What is the product of the addition of H2​O to ethene (in the presence of H+)?	Ethanol (CH3​CH2​OH).
27	State the rule governing the addition of unsymmetrical reagents to unsymmetrical alkenes.	Markownikoff's Rule.
28	What is the major product of HBr addition to propene?	2-Bromopropane (CH3​CH(Br)CH3​).
29	What type of intermediate is formed during the electrophilic addition of HBr?	Carbocation (R+).
30	What is the rule that explains the minor product of HBr addition in the presence of peroxide?	Anti-Markownikoff's Rule (or Peroxide Effect).
31	What is the intermediate in the addition of HBr under peroxide conditions?	Free Radical.
32	What is the test used to distinguish between saturated and unsaturated hydrocarbons?	Bromine water test (Unsaturated decolorizes the brown color).
33	What is the product when an alkene reacts with Br2​ in CCl4​?	A Vicinal Dibromide (1,2-dibromoalkane).
34	Name the reagent used for the hydroxylation of alkenes (cis-addition).	Cold, dilute, aqueous KMnO4​ (Bayer's reagent).
35	What is the final product of the ozonolysis of but-2-ene?	Ethanal (CH3​CHO).
36	What is Ozonolysis used for in organic chemistry?	To locate the position of the double bond.
37	What is Geometrical Isomerism (Cis-Trans isomerism)?	Isomerism arising due to restricted rotation around a C=C bond.
38	Which is generally more stable, Cis or Trans isomer?	Trans isomer (due to reduced steric repulsion).
39	What is the type of elimination reaction used to prepare alkenes from alkyl halides?	β-Elimination or Dehydrohalogenation.

Part 3: Alkynes (Triply Bonded Hydrocarbons)

Q. No.	Question	Answer
40	What is the general formula for Alkynes?	Cn​H2n−2​ (containing one triple bond).
41	What is the hybridization of carbon atoms involved in the triple bond?	sp.
42	What is the geometry around the triple-bonded carbons?	Linear (180∘ bond angle).
43	How many σ and π bonds are in the triple bond?	One σ and two π bonds.
44	Why are terminal alkynes (R−C≡C−H) acidic?	The sp hybrid C atom is highly electronegative (50% s-character), making the C-H bond highly polar.
45	Name the reaction used to test for terminal alkynes.	Reaction with Tollen's reagent (Ag+) or ammoniacal Cu2​Cl2​ (Cu+) to form metal acetylides.
46	Are non-terminal alkynes (e.g., but-2-yne) acidic?	No, they lack the terminal C-H bond.
47	What is the product of the addition of one mole of HBr to ethyne?	Vinyl Bromide (CH2​=CHBr).
48	What is the final product of the hydration (addition of H2​O) of ethyne?	Ethanal (Acetaldehyde, CH3​CHO).
49	What is the intermediate formed during the hydration of alkynes?	An Enol (which rapidly tautomerizes to a ketone or aldehyde).
50	What is the product when ethyne is passed through a red-hot iron tube?	Benzene (C6​H6​) (Cyclic Polymerisation).
51	What is the reagent used to convert an alkyne to a Cis-Alkene?	Lindlar's Catalyst (Pd/BaSO4​ poisoned with sulfur or quinoline).
52	What is the reagent used to convert an alkyne to a Trans-Alkene?	Na/liquid NH3​ (Birch Reduction).
53	Which is generally less reactive toward addition, Alkenes or Alkynes?	Alkynes (initial addition is harder due to high C-C bond energy).
54	What is the product of the addition of 2 moles of HBr to ethyne?	1,1-Dibromoethane (CH3​CHBr2​).

Part 4: Arenes (Aromatic Hydrocarbons)

Q. No.	Question	Answer
55	What is the simplest aromatic hydrocarbon?	Benzene (C6​H6​).
56	What is the hybridization of all carbon atoms in benzene?	sp2.
57	What is the geometry around each carbon atom in benzene?	Trigonal Planar (forming a hexagonal ring).
58	What is the Resonance Energy of benzene?	The difference in energy between the resonance hybrid and the most stable canonical form (≈150 kJ/mol).
59	What does the high resonance energy indicate about the stability of benzene?	Benzene is highly stable (thermodynamic stability).
60	What is the characteristic reaction type of aromatic hydrocarbons?	Electrophilic Substitution Reactions.
61	Give the name of the intermediate formed in electrophilic substitution reactions of benzene.	σ-complex or Arenium ion.
62	What is the purpose of FeCl3​ or AlCl3​ in the halogenation of benzene?	To generate the stronger electrophile (Cl+).
63	Name the electrophile in the nitration of benzene.	Nitronium ion (NO2+​).
64	Name the electrophile in the sulphonation of benzene.	Sulphur Trioxide (SO3​).
65	Name the reaction where an alkyl group is introduced onto the benzene ring.	Friedel-Crafts Alkylation.
66	Name the reaction where an acyl group is introduced onto the benzene ring.	Friedel-Crafts Acylation.
67	What is the major limitation of Friedel-Crafts Alkylation?	Polyalkylation (the product is more reactive than the reactant).
68	What is the product of the complete catalytic hydrogenation of benzene?	Cyclohexane (C6​H12​).
69	What is the product of the reaction of benzene with Cl2​ under UV light?	Benzene Hexachloride (BHC or Lindane), an addition product.
70	What type of groups are ortho and para directing?	Electron-donating groups (+M or +I groups, e.g., -OH,-CH3​).
71	What type of groups are meta directing?	Electron-withdrawing groups (−M or −I groups, e.g., -NO2​,-COOH).
72	What is the directing effect of the −CH3​ group on the benzene ring?	Ortho and Para directing (weakly activating).
73	What is the directing effect of the −NO2​ group on the benzene ring?	Meta directing (strongly deactivating).
74	Are halogens (-X) activating or deactivating toward electrophilic substitution?	Deactivating (due to strong −I effect), but ortho-para directing (+M effect).
75	How is Benzene prepared from Phenol?	By reduction using Zinc Dust (Zn).
76	How is Toluene oxidized to form Benzoic Acid?	Using acidified KMnO4​ (Side chain oxidation).

Part 5: Preparation Methods (Comprehensive)

Q. No.	Question	Answer
77	Name a method to prepare alkanes from alkyl halides using zinc.	Reduction (R−X+Zn+H+→R−H).
78	What is the catalyst used for the Kolbe's Electrolysis?	No catalyst; it is an electrolytic reaction (Free Radical mechanism).
79	What is the limitation of Kolbe's electrolysis?	It cannot be used to prepare methane (the product is always R−R).
80	How is Ethene prepared from Ethanol?	Dehydration with conc. H2​SO4​ at 443 K.
81	How is Propene prepared from 1-Bromopropane?	By Dehydrohalogenation using alcoholic KOH.
82	What is the type of reagent used for the dehydrohalogenation reaction?	Alcoholic/ethanolic KOH (a strong base).
83	How is Ethyne prepared from Calcium Carbide (CaC2​)?	Hydrolysis (CaC2​+2H2​O→Ca(OH)2​+C2​H2​).
84	How is an alkyne prepared from a vicinal dihalide?	By Double Dehydrohalogenation using NaNH2​ (sodamide).
85	What is the use of Decarboxylation in preparation?	Reducing the number of C atoms in the product alkane by one.
86	How is Benzene prepared from ethyne?	Cyclic Polymerization in a red-hot iron tube.
87	How is Toluene prepared from Benzene?	Friedel-Crafts Alkylation using CH3​Cl/AlCl3​.
88	What is the Wurtz-Fittig reaction used for?	To prepare alkyl-substituted benzenes (e.g., toluene) from aryl and alkyl halides.

Part 6: Miscellaneous and Conceptual

Q. No.	Question	Answer
89	Define Aromaticity (Hückel's Rule).	The molecule must be planar, cyclic, fully conjugated, and have (4n+2)π electrons (where n=0,1,2,…).
90	Does cyclopropenyl cation follow Hückel's Rule?	Yes (n=0, 2π electrons).
91	Does cyclobutadiene follow Hückel's Rule?	No (it has 4π electrons, 4n system, thus anti-aromatic).
92	What is the term for a compound that does not follow Hückel's Rule but is fully conjugated and cyclic?	Anti-aromatic.
93	What is the stability order: Aromatic, Anti-aromatic, Non-aromatic?	Aromatic > Non-aromatic > Anti-aromatic.
94	What is the reason for the high boiling point of alkenes compared to alkanes of comparable mass?	Alkenes are slightly polar, leading to stronger dipole-dipole forces.
95	How does the boiling point of alkanes change with branching?	Decreases (due to decreased surface area and weaker van der Waals forces).
96	What is the role of the H+ ion in the electrophilic addition of HBr to alkenes?	It acts as the electrophile that initiates the reaction by attacking the π-bond.
97	What is the main cause of the formation of 2-Bromopropane (major product) from propene and HBr?	The formation of the more stable 2∘ carbocation intermediate.
98	What is the term for the replacement of a hydrogen atom in an aromatic ring by a halogen atom?	Halogenation.
99	Name a reagent that causes hydroxylation of alkenes through anti-addition.	Peroxyacids (e.g., RCO3​H) followed by hydrolysis.
100	What is the difference between elimination and substitution in alkyl halides?	Elimination forms a π-bond; Substitution replaces the halogen with another group.
101	What is the relationship between E1 and SN​1 reaction mechanisms?	Both share the same carbocation intermediate.
102	What is the effect of heat on the major product formation between E2 and SN​2 competition?	Higher temperature favors E2 (Elimination).
103	What is the product of propene reacting with Cl2​ at high T (773 K)?	Allyl Chloride (Substitution, not Addition).
104	What is the common name for the reaction of alkenes with O2​ in the presence of Ag at 523 K?	Epoxidation (forms an epoxide).
105	Which isomer is formed when propene is oxidized with KMnO4​ in hot, acidic solution?	CH3​COOH (Ethanoic acid) and CO2​ (from the CH2​ group).
106	Why does Cyclopentadiene act as a strong acid?	Its conjugate base, Cyclopentadienyl anion, is aromatic (highly stable).
107	What is the shape of the Cyclohexane ring that is most stable?	Chair form.
108	In the Kolbe's Electrolysis of sodium succinate, what is the product?	Ethene (C2​H4​).
109	What is the product of heating n-hexane with Cr2​O3​/Al2​O3​ at 773 K?	Benzene (Aromatization).
110	What is the difference between a meta-directing group's effect on reactivity and an ortho-para directing group's effect?	Meta groups deactivate the ring; O/P groups usually activate (except halogens).
111	What is the primary cause of the low reactivity of alkanes?	The presence of only strong σ bonds (C-C and C-H) and their non-polar nature.
112	What is the formula used to calculate the degree of unsaturation (Double Bond Equivalent)?	DBE=C−H/2−X/2+N/2+1.

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