CEM 351 Quiz #1
Wednesday, 8 September, 2003
This quiz covers the first 2 chapters in Jones’s “Organic Chemistry” 2nd Ed.
1) (5 pts). Both boron trifluoride (BF3) and nitrogen trifluoride (NF3) contain a central atom bonded to three
fluorine atoms. One of these molecules is polar while the other is not. Identify which is which, and explain.
F
F
B
F
N
+
+
F
To lower the energy of its
lone electron pair, N is sp3
hybridized (pyramidal). The
bond dipoles add to yield a
F
net dipole along molecule's 3F
fold axis. So NF3 is POLAR
F
+
+
B +
+
With only an electron sextet, B is sp2 hybridized F
(planar) with an empty p
orbital. Bond dipoles
cancel; no net dipole, so
BF3 is NONPOLAR
F
2) (5 pts). Arrange the following five valence orbitals of, say, carbon in order of decreasing (highest to
lowest) energy: s, p, sp, sp2, sp3.
Since p is highest and s is lowest in the valence atomic orbital energy levels, the energy just falls in order
of increasing s contribution:
p > sp3 > sp2 > sp > s
3) (5 pts). Use the partially completed Newman projections below to draw the three staggered
conformations of1,2-dichloroethane. One of these is expected to be less polar than the other two; identify
which is which, and explain.
Cl
Cl
Cl
H
H
H
H
carbon 2
Cl
H
H
H
Cl
H
H
H
H
Cl
carbon 1
H
In the middle structure, the C-Cl bond dipoles point in opposite directions and hence cancel each other,
so it is nonpolar. In the other two structures the dipoles point partially in the same directions, adding up
to a net molecular dipole.
4)
(5 pts).
Draw all five possible isomers for an alkane with the formula C6H14.
hexane
2-methylpentane
2,2-dimethylbutane
3-methylpentane
2,3-dimethylbutane
1