Page 37 - FPGEE Medicinal and Organic Chemistry Q&A Book

Krisman

462. (d) The basicity of any given compound Electron donating groups such as -CH ,
3
can be determined by its ability to accept pro- -OCH , and -NH release electrons and make
3 2
tons or donate electrons. When comparing phenoxy oxygen highly negative. This result in
aniline with aliphatic amine, the latter one is a firm holding of the phenoxy hydrogen by the
more basic than the former one. This is because phenoxy oxygen, and hence results in a weaker
a lone pair of electrons on nitrogen atoms is acid.
delocalized into the benzene ring by resonance The relative acid-strength: p-
in aniline, whereas in aliphatic amine this pair Nitrophenol > p-Chlorophenol > Phenol > p-
of electrons is fully localized on the nitrogen Cresol.
atom. As we have discussed previously, any
group or atom that increases electron density 465. (b) Capsaicin is the active component of
on nitrogen atoms will make the compound chili peppers, which are plants belonging to the
more basic, whereas any group or atom that genus Capsicum. It is an irritant for mammals,
reduces electron density on nitrogen atoms will including humans, and produces a sensation of
make the compound less basic. burning in any tissue with which it comes into
In the case of o-nitroaniline, the elec- contact. Capsaicin and several related
tron withdrawing -NO group reduces the elec- compounds are called capsaicinoids. It is also
2
tron density on nitrogen atoms makes the com- used in topical ointments to relieve arthritis pain.
pound less basic compared to aniline, whereas
in the case of o-toluidine, the electron releasing H O
-CH group enhances the electron density on
3 NH (CH ) CH=CH-CH (CH )
3 2
2 4
nitrogen atoms makes the compound more ba- H C O CH C
3 2
sic than aniline. Arrangement in order of
Capsaicin O
decreasing basicity: ethyl amine > o-toluidine
> aniline > o-nitroaniline.
466. (d) Phenol, also known as carbolic acid,
463. (d) Aromatic amine (P) is a weaker base has antiseptic properties. It is also the active
compared to nonaromatic cyclic amine (Q) due ingredient in some oral anesthetics such as
to delocalization of a lone pair of electrons of Chloraseptic spray. It is also used in the
nitrogen in the aromatic ring. The option R production of certain pharmaceutical drugs as
would be the least basic due to the presence of starting material. It is also used in the prepara-
the electron withdrawing group -Cl. The option tion of cosmetics including sunscreens, hair
(S) would be the most basic among the given dyes, and skin lightening preparations.
choices since a lone pair of electrons are local-
ized on the nitrogen atom and not into the aro- 467. (b) A hydrogen bond results from a dipole-
matic ring; hence, it acts as a primary aliphatic dipole force with a hydrogen atom bonded to
amine (-CH -NH ). nitrogen, oxygen or fluorine, thus the name
2 2
"hydrogen bond," which must not be confused
464. (d) Electron withdrawing group substitu- with a covalent bond to hydrogen. The energy
tion such as -NO , -X, -CHO, -COOH make of a hydrogen bond (typically 5 to 30 kJ/mole)
2
compounds more acidic. These groups withdraw is comparable to that of weak covalent bonds
electrons from the aromatic ring, which in turn (155 kJ/mol), and a typical covalent bond is only
pulls electrons from the phenoxy oxygen. This 20 times stronger than an intermolecular hydro-
makes phenoxy hydrogen easily available and gen bond. These bonds can occur between mol-
hence results in a stronger acid. ecules (intermolecularly), or within different

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