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A. The carbon-halogen bond is polar, with the carbon bearing a partial positive charge
B. They are completely non-polar
C. The halogen has very low electronegativity
D. They lack any lone pairs
Answer: A. The carbon-halogen bond is polar, with the carbon bearing a partial positive charge
Explanation: The higher electronegativity of the halogen polarizes the C-X bond, making carbon electrophilic and susceptible to nucleophilic attack.
2. SN1 reactions proceed through the formation of a(n):
A. Carbocation intermediate
B. Concerted transition state without intermediate
C. Free radical intermediate
D. Direct one-step substitution
Answer: A. Carbocation intermediate
Explanation: In SN1 mechanism, the leaving group departs first to form a carbocation intermediate, followed by nucleophilic attack.
3. Tertiary alkyl halides predominantly react via the:
A. SN1 mechanism
B. SN2 mechanism
C. Elimination only, never substitution
D. Addition mechanism
Answer: A. SN1 mechanism
Explanation: Steric hindrance disfavors SN2 attack on tertiary carbons, while the stability of the resulting tertiary carbocation favors SN1.
4. Which of the following alkyl halides shows the highest reactivity towards the SN2 mechanism?
A. CH3Br
B. (CH3)3CBr
C. (CH3)2CHBr
D. CH3CH2CH(CH3)Br
Answer: A. CH3Br
Explanation: Methyl halides have the least steric hindrance around the carbon, allowing the fastest backside nucleophilic attack in SN2 reactions.
5. A Grignard reagent is prepared by reacting a haloalkane with:
A. Magnesium metal in dry ether
B. Sodium metal in wet ether
C. Zinc metal in water
D. Copper metal in ethanol
Answer: A. Magnesium metal in dry ether
Explanation: Alkyl or aryl halides react with magnesium turnings in dry ether under anhydrous conditions to form Grignard reagents (RMgX).
6. Chlorobenzene is less reactive than an alkyl halide towards nucleophilic substitution mainly because:
A. Resonance gives the C-Cl bond partial double bond character, strengthening it
B. Chlorine has an unusually high electronegativity
C. It has no lone pairs on chlorine
D. Of steric hindrance alone
Answer: A. Resonance gives the C-Cl bond partial double bond character, strengthening it
Explanation: Delocalization of a chlorine lone pair into the aromatic ring increases the C-Cl bond order and shortens/strengthens it, reducing reactivity.
7. Freons (chlorofluorocarbons) are mainly used as:
A. Refrigerants and aerosol propellants
B. Insecticides
C. Solvents for polymerization
D. Anesthetics
Answer: A. Refrigerants and aerosol propellants
Explanation: Freons such as CCl2F2 have historically been widely used as refrigerants and in aerosol sprays.
8. DDT is chemically known as:
A. Dichlorodiphenyltrichloroethane
B. Dichlorodifluoromethane
C. Trichloroethylene
D. Tetrachloroethane
Answer: A. Dichlorodiphenyltrichloroethane
Explanation: DDT (dichlorodiphenyltrichloroethane) is a well-known organochlorine insecticide.
9. Treatment of haloalkanes with alcoholic KOH predominantly gives:
A. Alkenes, through an elimination reaction
B. Alcohols
C. Ethers
D. Alkanes
Answer: A. Alkenes, through an elimination reaction
Explanation: Alcoholic KOH acts as a base and favors β-elimination (dehydrohalogenation) to form alkenes.
10. Racemization is most commonly observed in reactions proceeding through the:
A. SN1 mechanism
B. SN2 mechanism
C. Simple addition reactions
D. Free radical halogenation only
Answer: A. SN1 mechanism
Explanation: The planar carbocation intermediate in SN1 reactions can be attacked from either face, giving a racemic mixture of products.
11. Which type of halide is generally most reactive towards the SN1 mechanism?
A. Tertiary halide
B. Primary halide
C. Methyl halide
D. Vinyl halide
Answer: A. Tertiary halide
Explanation: Tertiary halides form the most stable carbocation intermediates, favoring the SN1 pathway.
12. The iodoform test is used to detect the presence of:
A. A CH3CO- group, or a CH3CH(OH)- group that can be oxidized to it
B. A -COOH group
C. A -OH group in any alcohol
D. A -CHO group only
Answer: A. A CH3CO- group, or a CH3CH(OH)- group that can be oxidized to it
Explanation: Compounds containing a methyl ketone group (or oxidizable to one) give a positive iodoform test, forming a yellow precipitate of CHI3.
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