Haloalkanes and Haloarenes - MCQs with Explanations
1. What is true about DDT among the following?
- (a) Biodegradable pollutant
- (b) Greenhouse gas
- (c) Fertilizer
- (d) Non-biodegradable pollutant
Correct Answer: (d)
Explanation: DDT is a non-biodegradable pollutant, meaning it is not easily broken down by natural processes in the environment.
2. The strong nucleophile and the best leaving group in the following reaction are respectively: $CH_{3}X+Nu^{-}\longrightarrow CH_{3}-Nu+X^{-}$
- (a) $PhO^{-}, Cl^{-}$
- (b) $OH^{-}, Br^{-}$
- (c) $AcO^{-}, F^{-}$
- (d) $CH_{3}O^{-}, I^{-}$
Correct Answer: (d)
Explanation: The order of nucleophilicity is $CH_{3}O^{-} > OH^{-} > PhO^{-} > AcO^{-}$. Among halide ions, the order of leaving group ability is $I^{-} > Br^{-} > Cl^{-} > F^{-}$. Therefore, $CH_{3}O^{-}$ is the strongest nucleophile and $I^{-}$ is the best leaving group.
3. The halogen which is most reactive in the halogenation of alkanes under sunlight is:
- (a) F
- (b) Cl
- (c) Br
- (d) I
Correct Answer: (b)
Explanation: Under sunlight, chlorine is highly reactive for the free radical halogenation of alkanes. (Note: While Fluorine is more reactive, it is often too violent for standard sunlight-controlled reactions mentioned in this context).
4. Which of the following will have a meso isomer?
- (a) 2-Chlorobutane
- (b) 2-Hydroxypropanoic acid
- (c) 2,3-Dichloropentane
- (d) 2,3-Dichlorobutane
Correct Answer: (d)
Explanation: 2,3-Dichlorobutane contains a plane of symmetry. The upper half is the mirror image of the lower half, leading to internal compensation and optical inactivity, which defines a meso form.
5. $C_{6}H_{4}Cl_{2}(X) + Cl_{2} \xrightarrow{Fe} C_{6}H_{3}Cl_{3}(Y)$. If only one structural isomer of Y is formed, then X is:
- (a) o-isomer
- (b) p-isomer
- (c) m-isomer
- (d) none of these
Correct Answer: (b)
Explanation: In p-dichlorobenzene, all four remaining hydrogen positions are equivalent. Chlorinating it will only yield one possible structural isomer of trichlorobenzene.
6. Silver benzoate reacts with bromine to form:
- (a) Bromobenzene
- (b) Benzyl bromide
- (c) Benzoyl bromide
- (d) Benzoic acid
Correct Answer: (b)
Explanation: This is the Hunsdiecker reaction. Silver benzoate reacts with bromine to form bromobenzene, carbon dioxide, and silver bromide. (Note: Source label for (b) in hints matches the Hunsdiecker product, which is bromobenzene).
7. An unknown alkyl halide (A) reacts with alcoholic KOH to produce $C_{4}H_{8}$ which on ozonolysis gives one mole of propanone and one mole of formaldehyde. The structure of A is:
- (a) $CH_{3}-C(CH_{3})(Br)-CH_{3}$
- (b) $CH_{3}-CH(Br)-CH(Br)-CH_{3}$
- (c) $CH_{3}-CH_{2}-CH(Br)-CH_{3}$
- (d) $BrCH_{2}-CH_{2}-CH_{2}-CH_{2}Br$
Correct Answer: (a)
Explanation: Ozonolysis yielding propanone ($(CH_{3})_{2}CO$) and formaldehyde ($HCHO$) implies the alkene was 2-methylpropene. This alkene is formed by the dehydrohalogenation of tert-butyl bromide.
8. Chloroform on reaction with conc. $HNO_{3}$ gives an insecticide and war gas known as:
- (a) chloropicrin
- (b) nitromethane
- (c) picric acid
- (d) acetylene
Correct Answer: (a)
Explanation: Chloroform ($CHCl_{3}$) reacts with concentrated nitric acid to produce chloropicrin ($CCl_{3}NO_{2}$), which is used as an insecticide and was used as a war gas.
9. Vicinal and geminal dihalides can be distinguished by:
- (a) KOH (aq.)
- (b) KOH (alc.)
- (c) Zn dust
- (d) none of these
Correct Answer: (a)
Explanation: Aqueous KOH reacts with vic-dihalides to form diols (glycols), while it reacts with gem-dihalides to form aldehydes or ketones.
10. The reactivities of methyl chloride (A), propyl chloride (B) and chlorobenzene (C) are in the order:
- (a) $A > B > C$
- (b) $C > B > A$
- (c) $A > C > B$
- (d) $B > A > C$
Correct Answer: (a)
Explanation: Reactivity decreases with increasing size of the alkyl group for primary halides ($CH_{3}Cl > C_{3}H_{7}Cl$). Chlorobenzene is the least reactive among these because of the resonance stabilization of the C-Cl bond.
11. Consider the following reaction: $CH_{3}-CH(D)-CH(CH_{3})_{2} + Br^{\cdot} \rightarrow X + HBr$. Identify structure X.
- (a) $CH_{3}-CH(D)-CH(CH_{3})-CH_{2}^{\cdot}$
- (b) $CH_{3}-CH(D)-C^{\cdot}(CH_{3})_{2}$
- (c) $CH_{3}-C^{\cdot}(D)-CH(CH_{3})_{2}$
- (d) $CH_{3}-CH(D)-CH(CH_{3})^{\cdot}$
Correct Answer: (b)
Explanation: Bromine radicals are highly selective and prefer to abstract a hydrogen from a tertiary carbon to form the most stable $3^{\circ}$ free radical.
12. $PhOH + SOCl_{2} \xrightarrow{\text{Pyridine}} \text{Product}$. Which statement is true?
- (a) Retention of configuration
- (b) Inversion of configuration
- (c) Inversion and retention both
- (d) None of the above
Correct Answer: (b)
Explanation: When $SOCl_{2}$ is used in the presence of pyridine, a stronger nucleophile ($Cl^{-}$) attacks from the back, resulting in the inversion of configuration.
13. $C_{2}H_{5}Br \xrightarrow{\text{aq. KOH}} \text{Product-I}$; $C_{2}H_{5}Br \xrightarrow{\text{alc. KOH}} \text{Product-II}$. The correct statement is:
- (a) Product-I is obtained by elimination
- (b) Product-II is obtained by substitution
- (c) Product-I is $C_{2}H_{4}$, Product-II is $C_{2}H_{6}O$
- (d) Product-I is an isomer of dimethyl ether, Product-II is the dehydrated form of Product-I.
Correct Answer: (d)
Explanation: Product-I is ethanol ($C_{2}H_{5}OH$), which is a functional isomer of dimethyl ether. Product-II is ethene ($C_{2}H_{4}$), which can be considered the dehydrated product of ethanol.
14. Reactivity of 2-bromo-2-methylbutane(I), 1-bromopentane(II) and 2-bromopentane(III) towards $S_{N}2$ is:
- (a) I > II > III
- (b) I > III > II
- (c) II > III > I
- (d) II > I > III
Correct Answer: (c)
Explanation: For $S_{N}2$ reactions, reactivity follows the order: $1^{\circ} > 2^{\circ} > 3^{\circ}$. Thus, 1-bromopentane ($1^{\circ}$) > 2-bromopentane ($2^{\circ}$) > 2-bromo-2-methylbutane ($3^{\circ}$).
15. Fluorobenzene can be synthesised in the laboratory:
- (a) by heating phenol with HF and KF
- (b) from aniline by diazotisation followed by heating diazonium salt with $HBF_{4}$
- (c) by direct fluorination of benzene with $F_{2}$
- (d) by reacting bromobenzene with NaF
Correct Answer: (b)
Explanation: This is the Balz-Schiemann reaction. Aniline is converted to benzene diazonium chloride, then reacted with $HBF_{4}$ to form benzene diazonium tetrafluoroborate, which decomposes on heating to yield fluorobenzene.
16. Structural and configurational isomers of $C_{5}H_{9}Br$ formed by adding HBr to 2-pentyne are:
- (a) 1 and 2
- (b) 2 and 4
- (c) 4 and 2
- (d) 2 and 1
Correct Answer: (b)
Explanation: Adding HBr to 2-pentyne yields 2 structural isomers (2-bromo-2-pentene and 3-bromo-2-pentene). Since each can exist as cis- and trans- forms, there are 4 configurational isomers total.
17. $CH_{3}CH_{2}CH_{2}Br \xrightarrow{\text{aq. NaOH}} X \xrightarrow{Al_{2}O_{3}/123 K} Y \xrightarrow{HOCl} Z$. Z is:
- (a) mixture of $CH_{3}CH(Cl)CH_{2}OH$ and $CH_{3}CH(OH)CH_{2}Cl$
- (b) $CH_{3}CH(OH)CH_{2}Cl$
- (c) $CH_{3}CH(Cl)CH_{2}OH$
- (d) $CH_{3}CH(Cl)CH_{2}Cl$
Correct Answer: (b)
Explanation: Propan-1-ol (X) dehydrates to propene (Y). Propene reacts with HOCl such that the hydroxyl group attaches to the more substituted carbon, yielding 1-chloropropan-2-ol.
18. Alkyl halides react with dialkyl copper reagents to give:
- (a) alkenes
- (b) alkyl copper halides
- (c) alkanes
- (d) alkenyl halides
Correct Answer: (c)
Explanation: In the Corey-House synthesis, an alkyl halide reacts with a lithium dialkyl cuprate ($R_{2}CuLi$) to form a higher alkane ($R-R'$).
19. $C_{3}H_{6}Cl_{2}(A)$ gives $C_{3}H_{6}O(B)$ with alkali. B oxidises to $C_{3}H_{6}O_{2}$. C ($C_{3}H_{4}$) from A gives D ($C_{3}H_{6}O$) with $Hg^{2+}/H_{2}SO_{4}$. D gives iodoform test. A is:
- (a) $CH_{3}CH_{2}CHCl_{2}$
- (b) $CH_{3}CCl_{2}CH_{3}$
- (c) $CH_{2}ClCH_{2}CH_{2}Cl$
- (d) $CH_{3}CHClCH_{2}Cl$
Correct Answer: (a)
Explanation: Compound A must be 1,1-dichloropropane. With alkali, it forms propanal (B). Treatment of A with alcoholic KOH yields propyne (C), which undergoes hydration to form acetone (D). Acetone gives the iodoform test.
20. Which of the following statements is true?
- (a) Allyl chloride is more reactive than vinyl chloride.
- (b) Vinyl chloride is as reactive as allyl chloride.
- (c) Vinyl chloride is more reactive than allyl chloride.
- (d) Both are more reactive than chlorobenzene.
Correct Answer: (a)
Explanation: The allyl carbocation is resonance-stabilized, making allyl chloride more reactive. In vinyl chloride, the C-Cl bond has partial double bond character due to resonance, making it less reactive.
21. In elimination reactions of alkyl halides, reactivity of halogens follows the order:
- (a) $I > Br > Cl$
- (b) $Cl > Br > I$
- (c) $Br > Cl > I$
- (d) none of these
Correct Answer: (a)
Explanation: Reactivity increases with bond length and decreasing bond strength: C-I > C-Br > C-Cl.
22. Ethylene on treatment with chlorine gives:
- (a) ethylene dichloride
- (b) ethylene chlorohydrin
- (c) $CH_{4}$
- (d) $C_{2}H_{6}$
Correct Answer: (a)
Explanation: Chlorine adds across the double bond of ethylene to form ethylene dichloride (1,2-dichloroethane).
23. The C-Mg bond in $CH_{3}CH_{2}MgBr$ is:
- (a) ionic
- (b) non-polar covalent
- (c) polar covalent
- (d) hydrogen
Correct Answer: (c)
Explanation: The carbon-magnesium bond in Grignard reagents is covalent but highly polar due to the electronegativity difference.
24. $S_{N}1$ reaction is favoured by:
- (a) non-polar solvents
- (b) more alkyl groups on the carbon attached to halogen
- (c) small groups on the carbon attached to halogen
- (d) none of these
Correct Answer: (b)
Explanation: $S_{N}1$ reactions are favoured by factors that stabilize the intermediate carbocation, such as increasing alkyl substitution (inductive and hyperconjugation effects).
25. Order of increasing reactivity of C-X bond towards nucleophile in given structures:
- (a) $I < II < IV < III$
- (b) $II < III < IV < I$
- (c) $IV < III < II < I$
- (d) $III < II < IV < I$
Correct Answer: (a)
Explanation: Tertiary alkyl halides (III) are the most reactive toward nucleophiles (via $S_{N}1$). Aryl halides (I) are typically the least reactive due to resonance, but the presence of electron-withdrawing nitro groups (II) increases their reactivity compared to simple aryl halides.
26. $C_{6}H_{5}CH_{2}Br \xrightarrow{1. Mg, Ether, 2. H_{3}O^{+}} X$. Product X is:
- (a) $C_{6}H_{5}CH_{2}OCH_{2}C_{6}H_{5}$
- (b) $C_{6}H_{5}CH_{2}OH$
- (c) $C_{6}H_{5}CH_{3}$
- (d) $C_{6}H_{5}CH_{2}CH_{2}C_{6}H_{5}$
Correct Answer: (c)
Explanation: Benzyl bromide forms a Grignard reagent, which then reacts with hydronium ions to yield toluene.
27. It is not possible to detect chlorine unless sodium extract is prepared in:
- (a) $tert-butyl chloride$
- (b) allyl chloride
- (c) chlorobenzene
- (d) benzyl chloride
Correct Answer: (c)
Explanation: Chlorobenzene is highly resistant to nucleophilic substitution under standard Lassaigne test conditions (except when fused with sodium), making detection difficult compared to aliphatic halides.
28. Major product of p-iodochlorobenzene + NaCN in DMF:
- (a) p-dicyanobenzene
- (b) p-chlorobenzonitrile
- (c) p-iodobenzonitrile
- (d) none of these
Correct Answer: (d)
Explanation: Under standard $S_{N}2$ conditions (CN- in DMF), aliphatic halides are substituted easily. In p-iodochlorobenzene, neither halogen is easily replaced because they are attached to an aryl ring. (Note: If substitution occurred, iodide is usually the better leaving group, but the source notes both are difficult to substitute).
29. The best method to prepare neopentyl chloride is:
- (a) Neopentyl alcohol + $PCl_{5}, \Delta$
- (b) Neopentyl alcohol + $HCl, \Delta$
- (c) Neopentyl alcohol + $SOCl_{2}, \text{pyridine}$
- (d) Neopentane + $Cl_{2}, hv, \Delta$
Correct Answer: (d)
Explanation: Converting neopentyl alcohol with acidic reagents (a, b, c) causes rearrangement of the carbocation. Free radical chlorination of neopentane does not involve carbocations and thus prevents rearrangement, yielding the desired chloride.
30. Wurtz-Fittig reaction involves action of sodium metal on:
- (a) two molecules of alkyl halide
- (b) one molecule of alkyl halide and one of aryl halide
- (c) two molecules of aryl halide
- (d) two molecules of chloroform
Correct Answer: (b)
Explanation: The Wurtz-Fittig reaction couples one alkyl halide and one aryl halide using sodium in dry ether.
31. The correct formula for dichlorodiphenyltrichloroethane (DDT) is:
- (a) (refer to standard structures)
- (b) (refer to standard structures)
- (c) (refer to standard structures)
- (d) (refer to standard structures)
Correct Answer: (a)
Explanation: DDT features two chlorophenyl groups attached to a $CH-CCl_{3}$ unit.
32. Chlorobenzene + aq. $NH_{3} + Cu_{2}O$ at high temp/pressure gives:
- (a) benzamide
- (b) nitrobenzene
- (c) aniline
- (d) chloroaminobenzene
Correct Answer: (c)
Explanation: This reaction uses a copper catalyst to overcome the low reactivity of chlorobenzene, converting it into aniline.
33. Chlorobenzene + sodium in dry ether gives diphenyl. The reaction name is:
- (a) Fittig reaction
- (b) Wurtz-Fittig reaction
- (c) Sandmeyer reaction
- (d) Gatterman reaction
Correct Answer: (a)
Explanation: The coupling of two aryl halides using sodium is called the Fittig reaction.
34. The compound that does not hydrolyse by $S_{N}1$ mechanism is:
- (a) $CH_{2}=CHCH_{2}Cl$
- (b) $C_{6}H_{5}Cl$
- (c) $C_{6}H_{5}CH_{2}Cl$
- (d) $(CH_{3})_{3}C-Cl$
Correct Answer: (b)
Explanation: Aryl halides like chlorobenzene do not undergo $S_{N}1$ hydrolysis under ordinary conditions because the phenyl cation is highly unstable.
35. Iodination of alkanes is carried out in the presence of:
- (a) $HNO_{3}$
- (b) $HI$
- (c) $P/I_{2}$
- (d) $H_{2}O$
Correct Answer: (a)
Explanation: Iodination is reversible. An oxidizing agent like $HNO_{3}$ or $HIO_{3}$ is added to destroy the byproduct $HI$ and drive the reaction forward.
36. Which of the following does not form Grignard reagent?
- (a) $CH_{3}F$
- (b) $CH_{3}Cl$
- (c) $CH_{3}Br$
- (d) $CH_{3}I$
Correct Answer: (a)
Explanation: The C-F bond energy is very high, making methyl fluoride insufficiently reactive to form a Grignard reagent with magnesium.
37. Which is a gem-dihalide?
- (a) $CH_{3}CHBr_{2}$
- (b) $CH_{2}BrCH_{2}Br$
- (c) $CH_{3}CHBrCH_{2}Br$
- (d) none of these
Correct Answer: (a)
Explanation: A gem-dihalide has both halogen atoms attached to the same carbon atom, such as in ethylidene dibromide.
38. Reactivity order of primary(I), secondary(II), and tertiary(III) alcohols with halogen acids:
- (a) $I > II > III$
- (b) $III > II > I$
- (c) $II > I > III$
- (d) $I > III > II$
Correct Answer: (b)
Explanation: The reactivity of alcohols with halogen acids depends on the stability of the intermediate carbocation, following the order: $3^{\circ} > 2^{\circ} > 1^{\circ}$.
39. $CH_{3}(CH_{2})_{5}-CH(Br)CH_{3} + OH^{-} \rightarrow HO-CH(CH_{3})(CH_{2})_{5}CH_{3}$. This reaction is:
- (a) $S_{E}1$
- (b) $S_{N}2$
- (c) $S_{N}1$
- (d) $S_{E}2$
Correct Answer: (b)
Explanation: For optically active alkyl halides, the formation of a product with inverted configuration is characteristic of an $S_{N}2$ mechanism.
40. Reagents to produce Freon-12 ($CCl_{2}F_{2}$):
- (a) $C + F_{2} + Cl_{2}$
- (b) $CH_{3}Cl + F_{2}$
- (c) $CCl_{4} + SbF_{3} \xrightarrow{SbCl_{5}}$
- (d) $CCl_{4} + F_{2}$
Correct Answer: (c)
Explanation: Freon-12 is produced by reacting carbon tetrachloride with antimony trifluoride in the presence of an antimony pentachloride catalyst.
41. IUPAC name for diethylbromomethane:
- (a) 1-Bromo-1,1-diethylmethane
- (b) 3-Bromopentane
- (c) 1-Bromo-1-ethylpropane
- (d) 1-Bromopentane
Correct Answer: (b)
Explanation: Diethylbromomethane is $CH_{3}CH_{2}-CH(Br)-CH_{2}CH_{3}$, which systematically is 3-bromopentane.
42. Reagent for: $n-butane \rightarrow n-butyl chloride + sec-butyl chloride$:
- (a) $Cl_{2}/UV \text{ light}$
- (b) $NaCl + H_{2}SO_{4}$
- (c) $Cl_{2} \text{ in dark}$
- (d) $Cl_{2} + Fe \text{ in dark}$
Correct Answer: (a)
Explanation: Free radical chlorination of alkanes requires UV light (or high temperature) to initiate the reaction.
43. $C_{2}H_{5}Br + KCN_{(aq)} \rightarrow C_{2}H_{5}CN + KBr$. This is:
- (a) elimination
- (b) nucleophilic substitution
- (c) electrophilic substitution
- (d) redox change
Correct Answer: (b)
Explanation: This is a nucleophilic substitution reaction where the bromide ion is replaced by the cyanide ($CN^{-}$) nucleophile.
44. Tertiary alkyl halides are practically inert to $S_{N}2$ because of:
- (a) insolubility
- (b) instability
- (c) inductive effect
- (d) steric hindrance
Correct Answer: (d)
Explanation: $S_{N}2$ reactions involve a backside attack by the nucleophile. In tertiary halides, the presence of three bulky alkyl groups creates significant steric hindrance, blocking the nucleophile.
45. Compound added to prevent chloroform from forming phosgene gas is:
- (a) $CH_{3}COOH$
- (b) $CH_{3}OH$
- (c) $CH_{3}COCH_{3}$
- (d) $C_{2}H_{5}OH$
Correct Answer: (d)
Explanation: Chloroform oxidises in air to form poisonous phosgene ($COCl_{2}$). Adding 1% ethanol converts any phosgene into non-toxic diethyl carbonate.