501. Diethyl ether does not release hydrogen gas when treated with sodium metal under ordinary conditions mainly because it:
ⓐ. contains no carbon atoms
ⓑ. is strongly acidic
ⓒ. contains a carbonyl group
ⓓ. has no \(\mathrm{O-H}\) bond
Correct Answer: has no \(\mathrm{O-H}\) bond
Explanation: Sodium releases hydrogen from alcohols and phenols by replacing the hydrogen attached to oxygen. In an ether, oxygen is bonded to two carbon groups. There is no hydroxyl hydrogen available for this acid-metal reaction. The carbon-hydrogen bonds are not replaced in the same manner under ordinary test conditions. Ether therefore does not show the characteristic hydrogen evolution observed with an alcohol.
502. An ether can behave as a Lewis base because its oxygen atom:
ⓐ. contains an empty valence shell
ⓑ. releases a proton readily
ⓒ. donates an oxygen lone pair
ⓓ. carries a permanent positive charge
Correct Answer: donates an oxygen lone pair
Explanation: A Lewis base donates an electron pair. Ether oxygen possesses two lone pairs that are not involved in its two carbon-oxygen sigma bonds. One of these lone pairs can coordinate to an electron-deficient species or metal centre. Ether can therefore form donor-acceptor complexes without breaking its carbon-oxygen framework. This behaviour explains several important solvent applications of ethers.
503. Dry ether is commonly used as a solvent for Grignard reagents because:
ⓐ. ether oxygen coordinates to magnesium
ⓑ. ether supplies hydroxyl protons to the Grignard reagent
ⓒ. ether oxidises the Grignard reagent into an acid
ⓓ. ether converts magnesium into sodium
Correct Answer: ether oxygen coordinates to magnesium
Explanation: The magnesium centre in a Grignard reagent can accept electron density. Ether oxygen donates a lone pair and coordinates to magnesium. This interaction helps solvate and stabilise the organomagnesium species in solution. The ether must remain chemically compatible and free from reactive hydroxyl groups. Its Lewis-base character is therefore central to its role as the solvent.
504. Why must the ether used for preparing a Grignard reagent be dry?
ⓐ. Water converts ether into an alkene
ⓑ. Water makes magnesium completely nonmetallic
ⓒ. Water supplies protons that destroy the Grignard reagent
ⓓ. Water changes the alkyl halide into an aromatic compound
Correct Answer: Water supplies protons that destroy the Grignard reagent
Explanation: A Grignard reagent behaves as a very strong base as well as a carbon nucleophile. Water contains protonic oxygen-hydrogen bonds. The Grignard carbon removes a proton from water and is converted into a hydrocarbon. The useful organomagnesium reagent is therefore consumed before it can react with the intended substrate. Dry apparatus and anhydrous ether are required to exclude this destructive proton source.
505. A reaction requires a solvent that can coordinate to a metal centre but should not donate an acidic hydroxyl proton. Which solvent is most suitable among the following?
ⓐ. water
ⓑ. dry ether
ⓒ. ethanol
ⓓ. ethanoic acid
Correct Answer: dry ether
Explanation: Diethyl ether contains oxygen lone pairs and can coordinate to a metal centre. It has no oxygen-hydrogen bond and therefore cannot donate a hydroxyl proton. Water, ethanol, and ethanoic acid are all protic substances. They can react with strongly basic or organometallic reagents. Dry diethyl ether combines donor ability with the absence of an acidic hydroxyl group.
506. A bottle of a lower ether is placed beside an open flame. The principal immediate safety concern is that the ether:
ⓐ. is nonvolatile and accumulates only as a solid
ⓑ. forms flammable vapour that can ignite
ⓒ. releases carbon dioxide before ignition
ⓓ. neutralises oxygen in the room
Correct Answer: forms flammable vapour that can ignite
Explanation: Lower ethers commonly have low boiling points and high vapour pressures. They can therefore produce substantial amounts of vapour even at ordinary laboratory temperatures. Ether vapours mix readily with air and ignite easily. A flame or spark may ignite vapour that has travelled away from the liquid container. Ether handling consequently requires good ventilation and the exclusion of ignition sources.
507. Assertion: Diethyl ether is generally more volatile than butan-1-ol.
Reason: Diethyl ether cannot form strong intermolecular hydrogen bonds with itself because it lacks an oxygen-hydrogen bond.
ⓐ. Both Assertion and Reason are true, and Reason explains Assertion
ⓑ. Both Assertion and Reason are true, but Reason does not explain Assertion
ⓒ. Assertion is true, but Reason is false
ⓓ. Assertion is false, but Reason is true
Correct Answer: Both Assertion and Reason are true, and Reason explains Assertion
Explanation: Butan-1-ol molecules form intermolecular hydrogen bonds through their hydroxyl groups. Considerable energy is required to separate these associated molecules during boiling. Diethyl ether has oxygen lone pairs but no oxygen-hydrogen bond, so ether molecules cannot donate hydrogen bonds to one another. Its intermolecular attractions are consequently weaker. The lower boiling point and greater volatility follow from the difference stated in the Reason.
508. A graph compares vapour pressure against temperature for diethyl ether and an alcohol of similar molar mass. At the same temperature, the ether curve lies higher. The best interpretation is:
ⓐ. ether molecules form a stronger hydrogen-bonded network
ⓑ. the ether is less volatile than the alcohol
ⓒ. weaker association raises ether vapour pressure
ⓓ. the ether must have a higher boiling point
Correct Answer: weaker association raises ether vapour pressure
Explanation: A higher vapour pressure at the same temperature indicates that a larger fraction of molecules has escaped into the gas phase. Ethers cannot form self-associated hydrogen-bonded networks because they lack oxygen-hydrogen bonds. An alcohol of similar molar mass generally has stronger intermolecular attraction. The ether is therefore more volatile and normally has the lower boiling point. The graph reflects intermolecular-force differences rather than greater ether hydrogen bonding.
509. An organic solvent is required for extracting a neutral compound from water and must later be removed easily. A lower ether may be suitable because it is relatively inert and volatile, but it should be handled:
ⓐ. in a sealed vessel while being heated strongly
ⓑ. with concentrated sodium hydroxide to prevent evaporation
ⓒ. beside a burner so that removal is faster
ⓓ. away from flames and sparks in a well-ventilated area
Correct Answer: away from flames and sparks in a well-ventilated area
Explanation: Relative chemical inertness allows an ether to dissolve many neutral organic compounds without reacting with them. Its volatility also permits convenient removal after extraction. The same high volatility creates a large concentration of flammable vapour above the liquid. Heating near an open flame can therefore cause rapid ignition. Safe use requires controlled evaporation, ventilation, and strict exclusion of ignition sources.
510. The first essential step in acid cleavage of an ether by hydrogen iodide is:
ⓐ. protonation of the ether oxygen
ⓑ. removal of a beta hydrogen by iodide
ⓒ. oxidation of the ether into an ester
ⓓ. formation of a carbon-carbon double bond
Correct Answer: protonation of the ether oxygen
Explanation: Ether oxygen contains lone pairs and behaves as a Lewis base. It accepts a proton from hydrogen iodide and forms a protonated ether, or oxonium ion. Protonation makes one of the carbon-oxygen bonds much easier to break. Iodide can then attack an appropriate carbon atom or assist ionisation, depending on the ether structure. Cleavage does not begin by oxidation or beta elimination.
511. Protonation makes an ether more susceptible to cleavage mainly because:
ⓐ. the oxygen atom becomes a stronger nucleophile
ⓑ. iodide changes into an electrophile
ⓒ. both carbon-oxygen bonds become nonpolar
ⓓ. the fragment leaves as an alcohol
Correct Answer: the fragment leaves as an alcohol
Explanation: Direct displacement from an unprotonated ether would require formation of an alkoxide ion as the leaving group. Alkoxide ions are strong bases and poor leaving groups. Protonation converts the oxygen-containing fragment into a species that can depart as a neutral alcohol. This substantially lowers the energetic difficulty of carbon-oxygen bond cleavage. The activated oxonium ion is therefore the key intermediate in ether cleavage.
512. One mole of dimethyl ether reacts with one mole of hydrogen iodide. The principal products are:
ⓐ. two moles of methyl iodide and water
ⓑ. methane and methanol
ⓒ. methyl iodide and methanol
ⓓ. methanol only
Correct Answer: methyl iodide and methanol
Explanation: The ether oxygen is first protonated by hydrogen iodide. Iodide then attacks one of the equivalent methyl groups through an \(\mathrm{S_N2}\) process. One carbon-oxygen bond breaks, producing methyl iodide and methanol. Because only one equivalent of hydrogen iodide is available, the methanol is not necessarily converted further. The initial equation is \(\mathrm{CH_3OCH_3+HI\rightarrow CH_3I+CH_3OH}\).
513. Methoxyethane reacts with one equivalent of hydrogen iodide. The major products expected from the usual \(\mathrm{S_N2}\) cleavage are:
ⓐ. methyl iodide and ethanol
ⓑ. iodoethane and methanol exclusively
ⓒ. methane and ethanal
ⓓ. ethene and methanol
Correct Answer: methyl iodide and ethanol
Explanation: Protonation first converts methoxyethane into an oxonium ion. Iodide then attacks the less hindered carbon atom. The methyl carbon is less hindered than the primary ethyl carbon and is especially favourable for \(\mathrm{S_N2}\) attack. Cleavage therefore produces methyl iodide and ethanol. The preference is controlled by steric accessibility rather than by random breaking of either carbon-oxygen bond.
514. Excess hot hydrogen iodide reacts with diethyl ether. The final organic product is:
ⓐ. ethanol only
ⓑ. iodoethane
ⓒ. ethanal
ⓓ. ethene only
Correct Answer: iodoethane
Explanation: Initial cleavage of diethyl ether gives one molecule of iodoethane and one molecule of ethanol. In the presence of excess hot hydrogen iodide, the ethanol is protonated and its hydroxyl group is replaced by iodide. A second molecule of iodoethane is consequently formed. The overall reaction is \(\mathrm{C_2H_5OC_2H_5+2HI\rightarrow2C_2H_5I+H_2O}\). Thus both ethyl groups ultimately appear as iodoethane.
515. Assertion: Hydrogen iodide generally cleaves ethers more readily than hydrogen bromide.
Reason: Hydrogen iodide protonates ether oxygen effectively and supplies iodide, which is a stronger nucleophile than bromide in the relevant medium.
ⓐ. Both Assertion and Reason are true, but Reason does not explain Assertion
ⓑ. Both Assertion and Reason are true, and Reason explains Assertion
ⓒ. Assertion is true, but Reason is false
ⓓ. Assertion is false, but Reason is true
Correct Answer: Both Assertion and Reason are true, and Reason explains Assertion
Explanation: Ether cleavage requires both acid activation and attack by a halide ion. Hydrogen iodide is a strong acid, so it readily forms the protonated ether intermediate. Iodide is highly polarisable and attacks carbon efficiently. Hydrogen bromide performs the same functions but usually less effectively. The two properties stated in the Reason account for the greater reactivity of hydrogen iodide.
516. A reaction-coordinate graph compares direct attack of iodide on an unprotonated ether with attack on the corresponding protonated ether. The pathway involving the protonated ether should show:
ⓐ. a higher activation barrier because oxygen carries a positive charge
ⓑ. no transition state
ⓒ. a lower cleavage barrier after bond activation
ⓓ. an identical activation barrier for every ether
Correct Answer: a lower cleavage barrier after bond activation
Explanation: An unprotonated ether would have to release an alkoxide ion during direct cleavage. This is energetically unfavourable because alkoxide is a poor leaving group. Protonation allows the oxygen-containing fragment to depart as a neutral alcohol. The transition state for carbon-oxygen bond breaking is therefore lower in energy. The graph should show a smaller activation barrier for the protonated pathway.
517. A \(14.8\,\mathrm{g}\) sample of diethyl ether is heated with excess hydrogen iodide. What theoretical mass of iodoethane is formed? Use \(M(\mathrm{C_2H_5OC_2H_5})=74\,\mathrm{g\,mol^{-1}}\) and \(M(\mathrm{C_2H_5I})=156\,\mathrm{g\,mol^{-1}}\).
ⓐ. \(62.4\,\mathrm{g}\)
ⓑ. \(31.2\,\mathrm{g}\)
ⓒ. \(15.6\,\mathrm{g}\)
ⓓ. \(78.0\,\mathrm{g}\)
Correct Answer: \(62.4\,\mathrm{g}\)
Explanation: \( \textbf{Amount of diethyl ether:} \)
\[
n(\mathrm{ether})=\frac{14.8\,\mathrm{g}}{74\,\mathrm{g\,mol^{-1}}}
\]
\[
n(\mathrm{ether})=0.200\,\mathrm{mol}
\]
\( \textbf{Overall reaction with excess }\mathrm{HI}\textbf{:} \)
\[
\mathrm{C_2H_5OC_2H_5+2HI\rightarrow2C_2H_5I+H_2O}
\]
One mole of ether gives two moles of iodoethane.
\[
n(\mathrm{C_2H_5I})=2\times0.200=0.400\,\mathrm{mol}
\]
\( \textbf{Mass of iodoethane:} \)
\[
m=nM
\]
\[
m=0.400\,\mathrm{mol}\times156\,\mathrm{g\,mol^{-1}}
\]
\[
m=62.4\,\mathrm{g}
\]
The large product mass reflects incorporation of two iodine atoms per ether molecule.
518. Hydrochloric acid is much less effective than hydrogen iodide for ordinary ether cleavage mainly because:
ⓐ. hydrochloric acid cannot protonate oxygen under any condition
ⓑ. chlorine has no electrons
ⓒ. chloride is a weaker nucleophile in this medium
ⓓ. hydrogen chloride contains no hydrogen atom
Correct Answer: chloride is a weaker nucleophile in this medium
Explanation: Hydrochloric acid can protonate ether oxygen under sufficiently acidic conditions. The difficulty arises mainly in the subsequent carbon-oxygen bond-breaking step. Chloride is less effective than iodide at attacking the activated carbon centre in the strongly protic medium. Iodide is larger and more polarisable, making it a better nucleophile under these conditions. Hydrogen chloride therefore gives much slower or negligible cleavage compared with hydrogen iodide.
519. One equivalent of hydrogen bromide reacts with symmetrical dipropyl ether. The initial organic products are:
ⓐ. propene and propanone
ⓑ. two molecules of 1-bromopropane
ⓒ. propanal and propan-1-ol
ⓓ. 1-bromopropane and propan-1-ol
Correct Answer: 1-bromopropane and propan-1-ol
Explanation: The two propyl groups in dipropyl ether are equivalent. Protonation activates the ether oxygen, and bromide attacks one primary propyl carbon. Cleavage forms one molecule of 1-bromopropane and one molecule of propan-1-ol. With only one equivalent of hydrogen bromide, the alcohol need not be converted completely into a second alkyl bromide. Symmetry removes any question of which side of the ether is attacked.
520. The overall equation for complete cleavage of dimethyl ether by excess hot hydrogen iodide is:
ⓐ. \(\mathrm{CH_3OCH_3+2HI\rightarrow2CH_3I+H_2O}\)
ⓑ. \(\mathrm{CH_3OCH_3+HI\rightarrow2CH_3OH}\)
ⓒ. \(\mathrm{CH_3OCH_3+I_2\rightarrow2CH_3I+O_2}\)
ⓓ. \(\mathrm{CH_3OCH_3+2HI\rightarrow C_2H_6+I_2+H_2O}\)
Correct Answer: \(\mathrm{CH_3OCH_3+2HI\rightarrow2CH_3I+H_2O}\)
Explanation: The first molecule of hydrogen iodide cleaves dimethyl ether into methyl iodide and methanol. A second molecule of hydrogen iodide converts methanol into another molecule of methyl iodide. The oxygen atom ultimately appears in water. Both methyl groups remain intact and become methyl iodide. The balanced equation therefore requires two moles of hydrogen iodide per mole of ether.
