I Cam R.elate
PcXar IV€oleeu]es and" Sreaeli
&bCIut €t
EhEa?k
NH3, in some
Polar molecuies have a potent smell. The smell of ammonia,
gas,
CHu, in-gas stoves
methane
window cleaners is quite strong. In contrast, the
though the C-H bonds are all polar. So, why can't you
does not have a sm.i'l
"v"n
smell CHn gas?
1{hat d+es
pto-larit,v har-e tr> dc lv'it}r sineli?
To answer this question, you will examine
@
Polarity of Molecules
S
NonPolar Molecules
@
PolaritY and Smell
ToPie
& Polarity of Molecules
ExpBoring tl'ae
.
i.,jj
as a;li
If t;1e bondsbetween atoms of a molecule are po}ar, what abodt the molecule
its
overall
to
examine
.;
whole unit? One way to tell whether u *ol".ui. is polar is
shapg
here has an irregular
shape. Notice that each of the three molecules shown
a
molecule polar'
makes
asymmetry
$
or sorne sort of asymmetry. This
E+
H
5+
6-
g_N:6-
'o'
HH
5+ 5+
H
6+
Water, H2O
Ammonia, NH3
'
Hydrogen fluoride, HF
,@ i*.
Ammonia, water, and hYdrogen
2.10
fluoride are all Polar. In these
molecules there is a partial negative
5+
charge on the nitrogen, oxYgen, and
are
atoms
fluorine atoms because these
2. t0
cHro
more electronegative than hydrogen
..-3 / -//
Lr n
atoms. The hydrogen atoms have partial
I
positive charges. To figure out whichDirection of
end of a molecule has a partial negative
charge and which end has apartial
ancl the overall
posiiive charge check the electronegativities of the atoms
The numbers
the molecule. Consider formaldehyde, CHrO' shown here'
the atoms are their electronegativities'
232
Unit
2 Smells
Section
lll
Molecules in Act-ion
The oxygen end of the formaldehyde molecule has a partial negative charge
because the oxygen atom is the most electronegative atom in the formaldehyde
molecule. Similarly, the hydrogen atoms have partiai positive charges because
=#Ed*RSru&tg&a?AL
e#cshegeYa#&i Chlorotrifluoromethone.
olso known cs Freon, wes
developed ecrly in ihe
?Oth cenlury cnd used
widely as o refrigeroni.
However, use of Frecn and
other chlorofluorocorbons
{CFCs} wos significantly
reduced in the lote
l?80s due to their effecis
on the ozone loyer of
ihe oimosphere.
H
they are the least electronegative atoms in the molecule.
Hx*rnple E
i
Phosphine,PH,
Is phosphine, PH,
polar? What is the direction of the dipole?
SolutiorT
A Lewis dot structure of PH, shows four pairs of electrons, three
bonding pairs and one lone pair. These four pairs of electrons are
arranged around the P atom in a tetrahedral shape.
]E
H
H-P:
H
Because the molecule is asymmetrical, it
is polar. You can find the electronegativity
ffi
ffi
ffi
t
i
values for phosphorus and hydrogen in the
electronegativity scale in Lesson 7: Thinking
(Electro) Negatively. The electronegativity
value for P is 2.19, and for H it is 2.10. Because
the electronegativity of the phosphorus atom is
greater, the P atom attracts the electrons more
strongly and has a partial negative charge. The
hydrogen atoms have partial positive charges.
The o...erail dipole is shown.
E+ z.to
"l .,, .-..=-.=*-_-,-=_-
I
#
Nonpolar Molecules
Diatomic molecules with two identical atoms are nonpolar. The electrons between
the two identical atoms are shared equally. However, these are not the only kind
of nonpolar molecules. Some molecules are symmetrical. The symmetry in these
molecules can make them nonpolar even though they have polar bonds within
them. The polarities of the individual polar bonds balance each other out due to
the shape of the molecule.
6-
E+
D-
:6:i:6:
d
5+
H
u*'-f
:'i:
16-
-H
l6+..
E+
H
6-:F-C-F:El"
:F:
5+
d
:
il
il
"#*
coz
CF+
CH+
.:1tr
Nonpolar molecules
Lesson
lB
I Can
Relate
233
@ Polarity and Smell
€*&$sus&gR
es$€s8e?N*N
Corbon ieirachloride, CClo'
is o letrohedrol, nonPolcr
molecule. But it is said
to hove o sweet smell'
This molecule wos widelY
used cs o drY-cleaning
solvenl becouse it is verY
oood ot dissolving oiher
ionpolor comPounds,
such os fots ond oils' lt
wos clso widelY used os
o
o fire extinguisher cnd
pesticide for rodents until
ii wos discovered to be
highlY toxic ond bonned
from consumer Products
in
.1970.
\
Smalimoleculesthatarepolar|ravelsmell.Smallmoleculesthatarenonpolardo
t" i" *itt the smell of small moiecules?
not have a sme1l.
I
t
I
W#l;;;"larity
I
f:--'*-'---':--
ii
t
t
I
I
!
i
I
HCl, 'i
i
6H'Fl- i
CHF3,NH:,
vrrr3,r!!5j,
ii
nr,, ng::
\*. -. -,*.
I
i
i
ii
Exceptions
i
.1
",**}*-**-"";*--';**;'-*i=*-*-*;:-@=***-=
clr' Br, ccln are
HF,
rr1'rrv" i
I F,,-' lbrrthaveasmell'
] nonPotar
H2s, cH3F'
I
!
I
!
polurr Have i Nonpolar:-'-"^,,
'"'i*"ll ,l,not have a smelt
i
i.
have
* -.*. *--..--
i
*
N,, o,, co2,
cHa,
cFu
iuroisPolarbutdoesnot
I
"_*. i
I
-t_
Ilavs4Druwtr.
haveasmell.
._-*-
_
I
*"*-*}
sruLr^ro-:ted
ur@L distinsuish
Lrtrr that
properties
Polar molecules have proPer
to other polar molecules. One
are al
and
potu, .not..ules dissolve in water
jissolvein the mucous membrane of
*or".rrr",
well_accepted theory t;;;;;i;
largely of water'
sites. The mucus is composed
I"*par
rhe nose and then
ll:Ti,?3,ffif *Tffi::."d;
I
I
1
I
,J
"r".rri"
Receptor Sites in the Nose
Receptorsitesinthenosecontainlarge.polarproteinmolecules.Scientistsbelieve
sites after they enter
u,*u.,.d1o th. polar receptor
that smal1 polar mol"cir"i' ur"
like a magnet' with
as *ort irrg somewliat
the nose. you could trrirrt orporarity
ih. polu, pu-rt of a receptor site' Nonpolar
i"
tii.r.rig;
*or".or"'
polar
small
so they may
the same extei as polar molecules'
to
attractecl
not
are
molecules
not be detected bY the nose'
Ammonia, NHr, dissolves in
they are both Polar liquids
water because
;;;*G;l
Chlorotrifluoromethane' CCIF3'
CF4
and Tetrafluoromethane'
r
n-,--
Comparechlorotrifluoromethanewithtetrafluoromethane.Explainwhyone
the other does not'
;#.tffiffiJ,"*[t
"'a
Solution
B
oth molecule s have a tetr
l"i:til:fi:ffiJ]li'
ahe dr
r""'
;5i.."1. ;t', *i'"rt it "o"por"'
'-F
234
Unit
2
Smells
Section
lll
Molecules in Action
ai
shap
e''
t
f::,lYiT:*Til1ilr:so ;rl
",
bondi are distributed svmmetricallv
Therefore' it does not smell'
Chlorotrifluoromethane is not as symmetrical. This is because one of the F atoms
is replaced by a Cl atom. The molecule is polar because F and Cl have different
electronegativities. Therefore, it smells.
3.98
aa
)
a
7a
a
r.YJ.
3.
t6
NATURE
eSNf.N&e?t&N
Tetrafluoromethane
Mcny onimols, such os
elephonts, seem lo be
oble to smell woler ot
greoi distonces.
Chlorotrifluoromethane
is
symmetrical and nonpolar
is
asymmetrical and polar
Lesson Surmetrary
What dces polarityhave to do with smell?
Small asymmetrical molecules with polar bonds are polar. Molecules that are
symmetrical in every way are nonpolar. Theories suggest that the polarity of
a molecule may help it to llstick" in a receptor site in the nose. aaditionaily,
polar substances are more soluable in water. This may allow polar substances to
dissolve in the mucous membrane of the nose, whereas nonpolar substances are
simply exhaled undissolved and undetected.
RCISES
Questions
How can you determine
if a molecule
is polar?
Describe one theory of why small nonpolar molecules do not have a smell,
easan and Apply
For each of the molecules listed, draw a Lewis dot structure and indicate the.shape of each molecule. Decide whether these substances smell. Explain your
reasoning for each.
a.
HrSe b. H,
c. Ar
For each of the polar molecules
d. HOF e. CHCIF, f.
CH2O
in Exercise 3, draw the dipole.
Water is an exception to our rule about small molecules.
It is a polar
,molecule, yet you don't smell it. What do you think is going on?
y:y think it might be useful if you could smell the air? Explain your
9q
thinking.
thane-, CHn, gas leaks can be very dangerous and can
be explosive, Why
you think dimethyl sulfide, C2H6S, is added to natural gu,
thut is used in
and buiidings?
Lesson
lB I Can Relate 235