Haloalkanes and Haloarenes (Class 12 NCERT) Revision Notes

 over 100 short question-and-answer pairs covering the essential concepts, preparation, properties, and reactions from the NCERT Class 12 Chemistry Chapter:

Short Q&A for Haloalkanes and Haloarenes (Class 12 NCERT)

Part 1: Classification and Nomenclature

Q. No.	Question	Answer
1	Define a Haloalkane (Alkyl Halide).	A hydrocarbon where one or more H atoms of an alkane are substituted by X (F,Cl,Br,I).
2	Define a Haloarene (Aryl Halide).	A hydrocarbon where one or more H atoms of an aromatic ring are substituted by X.
3	Classify haloalkanes based on the number of halogen atoms.	Mono-, Di-, Tri-, and Polyhalogen compounds.
4	Classify CH3​CH2​Cl based on the C atom bearing the Cl atom.	Primary (1∘) alkyl halide.
5	Classify CH3​CH(Br)CH3​ based on the C atom bearing the Br atom.	Secondary (2∘) alkyl halide.
6	Classify the C−X bond in Vinyl Halides.	sp2−X bond (X attached to a C=C carbon).
7	Classify the C−X bond in Benzyl Halides.	sp3−X bond (X attached to an sp3 C adjacent to a benzene ring).
8	Classify the C−X bond in Allyl Halides.	sp3−X bond (X attached to an sp3 C adjacent to a C=C bond).
9	What is the common name for CHCl3​?	Chloroform (Trichloromethane).
10	What is the common name for CH2​Cl2​?	Methylene Chloride (Dichloromethane).

Part 2: Nature of $\text{C}-\text{X}$ Bond and Physical Properties

Q. No.	Question	Answer
11	What is the nature of the C−X bond (polar or non-polar)?	Polar (Halogens are more electronegative than Carbon).
12	How does the polarity of the C−X bond change from C−F to C−I?	Decreases (F is most electronegative).
13	How does the bond length of the C−X bond change from C−F to C−I?	Increases (I atom size is largest).
14	How does the bond strength of the C−X bond change from C−F to C−I?	Decreases (C−I is weakest).
15	Why are haloalkanes only sparingly soluble in water?	They cannot form strong H-bonds with water molecules strong enough to break existing H-bonds in water.
16	How do the boiling points of haloalkanes compare to the parent alkanes?	Higher (due to greater size, mass, and polarity leading to stronger van der Waals forces).
17	Rank the boiling points: R−F,R−Cl,R−Br,R−I.	R−I>R−Br>R−Cl>R−F (Increases with molar mass).
18	How does the boiling point of isomers change with branching?	Decreases (due to reduced surface area for van der Waals forces).
19	How does the density of alkyl halides change with the size of the halogen atom?	Increases (RI>RBr>RCl).

Part 3: Preparation of Haloalkanes and Haloarenes

Q. No.	Question	Answer
20	How are alkyl chlorides prepared from alcohols using HCl?	Groove's Process (using anhydrous ZnCl2​ catalyst).
21	What is the best reagent for preparing alkyl chlorides from alcohols?	Thionyl Chloride (SOCl2​) (Darzen's process).
22	Why is Darzen's process considered the best for preparing R−Cl?	The byproducts (SO2​ and HCl) are gaseous and escape easily, leaving a pure product.
23	How is R−I prepared from R−OH?	By heating with NaI or KI in the presence of H2​SO4​ or red phosphorus and I2​.
24	Write the reaction for the preparation of alkyl halides from alkenes following Markownikoff's rule.	Addition of HX (HCl,HBr,HI).
25	What is the product when propene reacts with HBr in the presence of peroxides?	1-Bromopropane (Anti-Markownikoff's addition).
26	What is the reaction used for synthesizing alkyl fluorides?	Swarts Reaction (e.g., CH3​Br+AgF→CH3​F+AgBr).
27	What is the reaction used for synthesizing alkyl iodides?	Finkelstein Reaction (e.g., R−Cl+NaI→R−I+NaCl).
28	What is the name of the reaction used for the preparation of aryl chlorides/bromides from diazonium salts?	Sandmeyer Reaction (using Cu2​X2​/HX).
29	What is the name of the reaction used for the preparation of aryl chlorides/bromides from diazonium salts using copper powder?	Gatterman Reaction (using Cu powder /HX).
30	How is iodobenzene prepared from aniline?	By treating diazonium salt with KI (requires no Cu+ catalyst).
31	How are aryl fluorides prepared?	Balz-Schiemann Reaction (treating diazonium salt with HBF4​).

Part 4: Nucleophilic Substitution (SN​) Reactions

Q. No.	Question	Answer
32	What is the characteristic reaction type of haloalkanes?	Nucleophilic Substitution (SN​) Reactions.
33	Define a Nucleophile.	An electron-rich species that attacks the electron-deficient carbon of the C−X bond.
34	What is the full form of SN​2?	Bimolecular Nucleophilic Substitution.
35	How many steps are involved in the SN​2 mechanism?	One step (a single transition state).
36	What is the role of the concentration of the nucleophile in the SN​2 reaction rate?	Rate depends on the concentration of the nucleophile (Rate∝[R−X][Nu−]).
37	What is the stereochemical outcome of an SN​2 reaction?	Inversion of Configuration (Walden Inversion).
38	Rank the reactivity of alkyl halides in SN​2 reactions: 1∘,2∘,3∘.	CH3​X>1∘>2∘>3∘ (Less steric hindrance is faster).
39	What is the full form of SN​1?	Unimolecular Nucleophilic Substitution.
40	How many steps are involved in the SN​1 mechanism?	Two steps (Carbocation formation, followed by nucleophilic attack).
41	What is the role of the concentration of the nucleophile in the SN​1 reaction rate?	Rate is independent of the concentration of the nucleophile (Rate∝[R−X]).
42	What is the stereochemical outcome of an SN​1 reaction?	Racemisation (due to the planar carbocation intermediate).
43	Rank the reactivity of alkyl halides in SN​1 reactions: 1∘,2∘,3∘.	3∘>2∘>1∘ (Stability of the carbocation is key).
44	What type of solvent favors the SN​1 reaction?	Polar Protic Solvents (e.g., H2​O,Alcohols), which stabilize the carbocation.
45	What type of solvent favors the SN​2 reaction?	Polar Aprotic Solvents (e.g., Acetone,DMSO), which do not solvate the nucleophile strongly.
46	Which is a better leaving group: Cl− or I−?	I− (larger size, weaker C−I bond, and more stable anion).
47	When an alkyl halide reacts with KCN, what is the product?	Alkyl Cyanide (R−CN).
48	When an alkyl halide reacts with AgCN, what is the major product?	Alkyl Isocyanide (R−NC) (due to the covalent nature of Ag−CN bond).
49	What is an Ambident Nucleophile?	A nucleophile that can attack through two different sites (e.g., CN−,NO2−​).
50	What is the product when ethyl chloride reacts with aqueous KOH?	Ethanol (CH3​CH2​OH) (Substitution).

Part 5: Elimination and Reactions of Haloarenes

Q. No.	Question	Answer
51	What is the side reaction that competes with substitution in alkyl halides?	Elimination Reaction (E)
52	What type of reagent favors elimination over substitution?	A Bulky, Strong Base and High Temperature.
53	What is β-Elimination (or Dehydrohalogenation)?	Removal of a hydrogen atom from the β-carbon and the halogen atom from the α-carbon, forming an alkene.
54	State Saytzeff's Rule (or Zaitsev's Rule).	In dehydrohalogenation, the preferred product is the most substituted alkene (more stable).
55	What is the full form of E1?	Unimolecular Elimination.
56	What is the full form of E2?	Bimolecular Elimination.
57	Which type of reaction results in a trans-alkene being the major product?	E2 (due to the required anti-periplanar geometry of H and X).
58	Why are Haloarenes less reactive toward nucleophilic substitution than haloalkanes?	Partial double bond character of C−X due to resonance, making the bond harder to break.
59	How does the sp2 hybridization of carbon in haloarenes affect reactivity?	sp2 carbon is more electronegative, holding the bonding electrons tighter and making C less positive (harder for Nu− attack).
60	Name the only functional group that is ortho/para directing but deactivating toward electrophilic substitution.	Halogen (-X)
61	What reaction condition is required to replace the Cl atom in chlorobenzene with OH−?	High temperature (623 K) and high pressure (300 atm) (Dow's Process).
62	What is the effect of an electron-withdrawing group (like −NO2​) on the SN​ reactivity of haloarenes?	Increases the reactivity (especially at o and p positions) by stabilizing the carbanion intermediate.
63	What is the product when chlorobenzene is treated with Na metal in dry ether?	Biphenyl (Fittig Reaction).
64	What is the product when methyl iodide reacts with chlorobenzene and Na in dry ether?	Toluene (Wurtz-Fittig Reaction).
65	What is the product of the catalytic hydrogenation of vinyl chloride (CH2​=CHCl)?	Chloroethane (CH3​CH2​Cl).

Part 6: Important Concepts and Polyhalogen Compounds

Q. No.	Question	Answer
66	Why does iodide ion (I−) act as a good nucleophile?	It is large and easily polarizable, allowing it to form the transition state easily.
67	What is the full form of DDT?	Dichlorodiphenyltrichloroethane.
68	What is the major drawback of using DDT?	It is non-biodegradable and persists in the environment (causing bioaccumulation).
69	What is the common name and formula for Freon-12?	CCl2​F2​ (Dichlorodifluoromethane).
70	What is the main environmental concern related to Freons (CFCs)?	Ozone layer depletion in the stratosphere.
71	What is the use of Chloroform (CHCl3​)?	Primarily used as a solvent and historically as an anaesthetic.
72	Why must chloroform be stored in dark, brown bottles, filled completely?	To prevent oxidation by air/light, which forms the poisonous gas **Phosgene (COCl2​) **.
73	What is the product when Chloroform reacts with concentrated HNO3​?	Chloropicrin (CCl3​NO2​), an insecticide.
74	What is Iodoform (CHI3​)?	A yellow solid historically used as an antiseptic.
75	What is the common name for the reaction of Haloalkanes with Na metal in dry ether?	Wurtz Reaction (coupling to form higher alkanes).
76	What is the product when ethyl chloride is heated with Na metal?	Butane (CH3​CH2​CH2​CH3​).
77	What is the role of the Na metal in the Wurtz reaction?	It removes the halogen, forming an alkyl radical intermediate.
78	What is the product when Bromobenzene is nitrated?	p-Bromonitrobenzene (major product, Br is o/p directing).
79	Which one is faster: SN​1 or SN​2 reaction with Tertiary Butyl Bromide?	SN​1 (due to highly stable tertiary carbocation).
80	Which one is faster: SN​1 or SN​2 reaction with Methyl Bromide?	SN​2 (least steric hindrance).
81	What is the general electronic state of the transition state in the SN​2 mechanism?	sp2 hybridized C with a partial Nu−C bond and a partial C−X bond.
82	What is the structural feature that makes Vinyl Chloride unreactive towards SN​ reactions?	The C−Cl bond has partial double bond character due to resonance.
83	What is the major product of the reaction of 2-bromobutane with alcoholic KOH?	But-2-ene (Saytzeff's product).
84	What is the major product of the reaction of 2-bromobutane with bulky base (Potassium tert-butoxide)?	But-1-ene (Hoffman product, less substituted alkene).
85	What is the hybridization of the carbon atom in the Carbocation intermediate of the SN​1 reaction?	sp2 (planar geometry).
86	What is the term for a compound that rotates the plane of plane-polarized light?	Optically active (Chiral).
87	How is the C-X bond in Chloroform (CHCl3​) cleaved to test for Cl?	By heating with Na metal (Lassaigne's test) to form NaCl (ionic form).
88	What happens to the specific rotation of an optically active compound during Racemisation?	It becomes zero.
89	What is the term for a single starting material giving two or more products due to competing reactions?	Product distribution (governed by kinetics/thermodynamics).
90	How is Freon-12 synthesized?	By treating CCl4​ with SbF3​ (Swarts reaction).
91	What is the term for the process of separating enantiomers from a racemic mixture?	Resolution.
92	What is the geometry of the Transition State in the SN​2 mechanism?	Trigonal Bipyramidal.
93	Why is allyl chloride more reactive than propyl chloride toward SN​1 reactions?	Allyl carbocation (CH2​=CH−CH2+​) is stabilized by resonance.
94	What is the order of reactivity of alkyl halides towards SN​ reactions based on the leaving group?	R−I>R−Br>R−Cl>R−F.
95	Give one example of a chiral molecule containing Br.	2-Bromobutane (CH3​CH2​CH(Br)CH3​).
96	What is the relationship between the boiling point of o-dichlorobenzene and p-dichlorobenzene?	o- isomer has a lower BP (due to lower symmetry and dipole moment).
97	What is the relationship between the melting point of o-dichlorobenzene and p-dichlorobenzene?	p- isomer has a higher MP (due to higher symmetry and better crystal packing).
98	Why are aryl halides poor reducing agents?	The C−X bond is strong and the C atom is less electron deficient.
99	Name the major product when chloroform reacts with conc.HNO3​.	Chloropicrin (CCl3​NO2​).
100	What is the general rule for determining the dominant mechanism (SN​1/E1 vs SN​2/E2)?	SN​1/E1 is favoured by 3∘ substrates and polar protic solvents.
101	What is the term for the separation of a pair of enantiomers?	Resolution.
102	What is the key chemical feature necessary for a molecule to show optical activity?	Chiral carbon (asymmetric carbon).
103	What is the IUPAC name for DDT?	1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane.
104	What is the general name for the C-H acidity test used for terminal alkynes?	Test with AgNO3​/NH4​OH (Tollen's Reagent).
105	What is the role of anhydrous ZnCl2​ in the reaction of alcohol with HCl?	It acts as a Lewis Acid to help break the C−OH bond (stabilizing the transition state).
106	Why is NaI used for the preparation of R-I in the Finkelstein reaction?	NaI is soluble in acetone, driving the equilibrium forward by precipitating NaCl or NaBr.