Exam III-Free Response Questions Flashcards
(8 cards)
Which unique characteristic of alcohol helps to increase its boiling temperature?
Alcohols form hydrogen bonds. A positively polarized -OH hydrogen atom from one molecule is attracted to a lone pair of electron on a negatively polarized oxygen atom of another molecule. The intermolecular attractions are present in solution but not in the gas phase, thus elevating the BP of the solution.
Explain why 4-nitrophenol is more acidic than 4-aminophenol with suitable resonance contributors.
-In 4‑nitrophenol, the nitro group withdraws electron density from the ring via strong –I and –R effects, stabilizing the negative charge on the oxygen in the phenoxide ion.
-In 4‑aminophenol, the amino group donates electron density (via +R effect) into the ring, destabilizing the negative charge on the oxygen.
Thus, the greater stabilization of the anion by the electron-withdrawing nitro group explains why 4‑nitrophenol is more acidic than 4‑aminophenol.
The boiling point of ethanethiol (36 °C) is less than half of that of ethanol (78 °C) — Why?
Thiols boil at lower temperatures than alcohols because hydrogen bonds cant form between thiol molecules but can form between alcohol molecules. The more hydrogen bonds, the higher the boiling point.
What is “Williamson Ether Synthesis”? Write down the limitation(s) of this synthesis.
SN2 reaction of a prmary alkyl halide (substrate) and an alkoxide (nucleophile) to yield ethers.
-Limitations: Alkyl halide must be primary and the alkoxide ion can be derived from primary, secondary or tertiary alcohols.
Amides are less basic than the corresponding amines — Justify.
In the amines, the nitrogen have a lone pair of electrons that can be given up. In an amide, the lone pair of the nitrogen is close to an oxygen, and as a result gets involved with resonance and is no longer able to be given up.
What is “Claisen rearrangement”?
In the Claisen rearrangement of an allyl vinyl ether undergo a sigma tropic rearrangement which form a carbonyl and an alkene group on heating.
Arylamines and heterocyclic aromatic amines are considerably less basic than alkylamines — Justify.
- The N lone pair electron in arlyamines is delocalized by interaction with the aromatic ring pi electron system - it is therefore less able to accept H+ than alkylamines -arylamines and heteroaromatic (heterocyclic aromatic) amines are considerably less basic than alkyamines (conjugate acid pKa = 5 or less)
