This set of Organic Chemistry Multiple Choice Questions & Answers (MCQs) focuses on “Alkyl Halides”.
1. Which C-X bond has the highest bond energy per mole?
a) C-Br
b) C-Cl
c) C-F
d) C-I
View Answer
Explanation: Bond energies depend on many factors: electron affinities, sizes of atoms involved in the bond, differences in their electronegativity, and the overall structure of the molecule. There is a general trend in that the shorter the bond length, the higher the bond energy.
2. Which alkyl halide has the highest reactivity for a particular alkyl group?
a) R-F
b) R-Cl
c) R-I
d) R-Br
View Answer
Explanation: Reactivity order for the alkyl halides towards Sn2 reaction is R-I>R-Br>R-Cl>R-F. This can be explained by which halogen atom is a better leaving group compared to the other.
3. When ethyl chloride reacts with nascent hydrogen, what is the formed product?
a) Methane
b) Propane
c) Butane
d) Ethane
View Answer
Explanation: When an alkyl halide is treated with zinc and hydrochloric acid, it is reduced to the respective alkane. The nascent hydrogen formed by the reaction between zinc and HCl acid reduces ethyl chloride to ethane.
4. Which alkyl halide out of the following may follow both SN1 and SN2 mechanism?
a) CH3-X
b) (CH3)2CH-X
c) (CH3)3C-X
d) (CH3)3C-CH2-X
View Answer
Explanation: (CH3)2CH-X follows both SN1 and SN2 mechanism because the second CH3 of (CH3)2CH-X further blocks a nucleophile (such as: OH) in backside SN2 attack, but it increases the stability of the carbocation resulting from SN1 ionization. As a result, SN1 and SN2 mechanisms are sometimes competitive for (CH3)2CH-X.
5. When two moles of ethyl chloride react with two moles of sodium in the presence of ether what will be formed?
a) 2 moles of ethane
b) 1 moles of ethane
c) 2 moles of butane
d) 1 moles of butane
View Answer
Explanation: Wurtz reaction is method of preparation of higher alkanes from lower alkyl halides. This is coupling reaction. In this reaction alkyl halides are reacted with sodium metal in presence of dry ether and higher alkanes with even number of carbon atoms only are formed, by this method.
6. Which of the following halide can give best SN2 reaction?
a) Primary alkyl halide
b) Tertiary alkyl halide
c) Secondary alkyl halide
d) All can give SN2 reaction at same rate
View Answer
Explanation: A 1° alkyl halide has only one alkyl group, so it is relatively unstable. It is unlikely to form a 1° carbocation in an SN1 reaction. Instead, it will take the lower-energy SN2 path as 1° alkyl is sterically unhindered. More the steric hinderance the substrate becomes less susceptible to SN2 attack.
7. Why alkyl halides are considered to be very reactive compounds towards nucleophile?
a) they have an electrophilic carbon & a bad leaving group
b) they have a nucleophilic carbon & a good leaving group
c) they have an electrophilic carbon
d) they have an electrophilic carbon & a good leaving group
View Answer
Explanation: Alkyl halides are considered to be very reactive compounds towards nucleophile because they have an electrophilic carbon & a good leaving group as we go down the periodic table, halides that are larger in size will also be able to distribute their charge over a larger volume, making them less reactive (less basic). This is why fluoride is a much poorer leaving group than any of the other halides. So, alkyl halides are good for nucleophilic substitution reactions.
8. In primary alkyl halides, carbon attached to the halogen atom is further attached to how many carbon atoms?
a) 1
b) 2
c) 3
d) 4
View Answer
Explanation: As we can see below, carbon attached to the halogen atom is further attached to one carbon atom.
Example: CH3-X → Methyl halide
CH3-CH2-X → Ethyl halide
CH3-CH2-CH2-X → n-Propyl halide.
9. Which of the following is not the method of preparation of alkyl halide?
a) Darzen’s method
b) Halogenation of alkene
c) Addition of HX on alkenes
d) Hydration of alkene
View Answer
Explanation: Hydration of alkene is electrophilic addition of H2O to alkenes which forms alcohol not alkyl halides.
10. Which of the following reactant gives the best method of preparation of alkyl halides when reacts with alcohol?
a) Zn/HCl
b) PCl5
c) SOCl2/ Pyridine
d) PCl3
View Answer
Explanation: The best method of preparation of alkyl halides is a reaction of alcohol with SOCl2/ Pyridine because by-products formed in the reaction are SO2 and HCl which are in gaseous form and escape into the atmosphere leaving behind pure alkyl chlorides.
CH3CH2−OH + SOCl2 → CH3CH2−Cl + SO2↑ + HCl↑
Sanfoundry Global Education & Learning Series – Organic Chemistry.
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