Organic Chemistry Multiple Choice Questions on “Reactions of Ethers”.
1. An ether is more volatile than an alcohol having the same molecular formula. What is the reason for this difference?
a) dipolar character of ethers
b) alcohols having resonance structures
c) inter-molecular hydrogen bonding in ethers
d) inter-molecular hydrogen bonding in alcohols
Answer: d
Clarification: The ether are less volatile than alcohol of same molecular mass because in alcohols there is intermolecular H-bonding which makes it less volatile and High B.P. But in case of ether there is no intermolecular H-Bonding which makes it more volatile and less B.P.
2. Ethers may be used as solvents because they react only with which of the following reactants?
a) Acids
b) Bases
c) Oxidising agent
d) Reducing agents
Answer: a
Clarification: Ethers resist the attack of nucleophiles and bases. However, they are very good solvents in many organic reactions due to their ability to solvate cations by donating the electron pair from oxygen atom. Ethers are generally less reactive and react only with acids.
3. What are the products when ethyl isopropyl ether is cleaved with concentrated HI?
a) ethanol and 2-iodo-2-methylpropane
b) ethanol and 2-methylpropane
c) iodoethane and isopropyl alcohol
d) iodoethane and 2-methylpropane
Answer: c
Clarification: Ether + HX → Alkyl halide + Alcohol, Halide attacks at less hindered site to produce alkyl halide. So, Ethyl isopropyl ether + HI/Heat/Water → Isopropyl alcohol + Iodoethane.
4. What are the products when tert-butyl ethyl ether is cleaved with concentrated HI?
a) iodoethane and tert-butanol
b) iodoethane and 2-iodo-2-methylpropane
c) ethanol and 2-iodo-2-methylpropane
d) ethanol and tert-butanol
Answer: c
Clarification: Ether + HX → Alkyl halide + Alcohol, Halide attacks at less hindered site to produce alkyl halide. Clearly the SN2 is not in play here, as the tertiary carbons are much too hindered for a backside attack. However, tertiary carbocations are relatively stable – and “ionization” (i.e. loss of a leaving group) leaves us with an alcohol (R-OH) and a tertiary carbocation, which can then be attacked by iodide ion to give R-I. Again, if excess HI is present then that alcohol will be converted into an alcohol. We’ll have more about that to say in a few posts actually.
5. Which pair of products would result from the acid cleavage of phenyl propyl ether with excess concentrated HBr at an elevated temperature?
a) Phenol and 1-propanol
b) Bromobenzene and 1-propanol
c) Bromobenzene and 1-bromopropane
d) Phenol and 1-bromopropane
Answer: d
Clarification: Ether + HBr —–> Alkyl bromide + Alcohol. Halide attacks at less hindered site to produce alkyl halide.
Ethyl phenyl ether + HBr/Heat/Water —-> Phenol + Bromoethane.
6. Select the ether among following that yields methanol as one of the products on reaction with cold hydroiodic acid.
a) 1-Methoxybutane
b) 1-Methoxy-2-methylpropane
c) 2-Methoxy-2-methylpropane
d) Methoxybenzene
Answer: c
Clarification: When one group is methyl and the other alkyl group is a tertiary group, the halide formed is a tertiary group, the halide formed is a tertiary halide. It is because the attack by I- takes place at that carbon of alkyl group, which has a greater electron pushing inductive effect and a lower electron density.
7. The boiling point of ethyl alcohol is much higher than that of dimethyl ether though both have the same molecular weight. Why?
a) Ether is insoluble in water
b) Methyl groups are attached to oxygen in ether
c) Dipole moment of ethyl alcohol is less
d) Ethyl alcohol shows hydrogen bonding
Answer: d
Clarification: Alcohols have much higher boiling points than the comparable alkanes or the related ethers because molecules of alcohols can interact through hydrogen bonding while those of alkanes and ethers cannot. Thus, Ethanol has a higher boiling point (78℃) than dimethyl ether (−25℃) though they have the same molecular weight.
8. Which pair of products would result from the acid cleavage of tert-butyl propyl ether with excess concentrated HBr at an elevated temperature?
a) tert-butyl bromide and propyl alcohol
b) tert-butyl bromide and propyl bromide
c) tert-butyl alcohol and propyl bromide
d) 2-methyl-2-butene and propyl bromide
Answer: b
Clarification: Ether + HX → Alkyl halide + Alcohol, Halide attacks at less hindered site to produce alkyl halide. Clearly the SN2 is not in play here, as the tertiary carbons are much too hindered for a backside attack. However, tertiary carbocations are relatively stable – and “ionization” (i.e. loss of a leaving group) leaves us with an alcohol (R-OH) and a tertiary carbocation, which can then be attacked by iodide ion to give R-I. Again, if excess HI is present then that alcohol will be converted into an alcohol. We’ll have more about that to say in a few posts actually.
9. Which of the following is the name reaction of the given reaction?
a) Wollf’s rearrangement
b) Favorskii rearrangement
c) Pinacol rearrangement
d) Claisen rearrangement
Answer: d
Clarification: The Claisen rearrangement is an organic reaction where an allyl vinyl ether is converted into an unsaturated carbonyl compound with the input of heat at 200-250oC or a Lewis acid. This reaction belongs to a class of reactions termed “sigmatropic rearrangements” and it is a concerted process where bonds are forming and breaking at the same time.
10. Which of the following will be obtained by keeping ether in contact with air for a long time?
a) C2H5−O−CH (CH3)−O−OH
b) C2H5−OCH2−OH
c) C2H5−O−C2H5OH
d) CH3−O CH (CH3)−O−OH
Answer: a
Clarification: When stored in the presence of air or oxygen, ethers tend to form explosive peroxides, such as diethyl ether peroxide. The reaction is accelerated by light, metal catalysts, and aldehydes.
R−O−R’ + O2/light → C2H5−O−CH(CH3)−O−OH.