1. Unit One Part 5:
intermolecular forces
we have looked at the
bonds in molecules,
now turn our attention
to the bonds / forces
between molecules

2. 5
Unit One
Part of molecules (pg56-58)
Properties
Intermolecular forces (pg59-66)
Solubility (pg66-68)

3. Gecko feet
Autumn, K., et al. 2002. Evidence for van der Waals adhesion in gecko setae. Proc. Natl. Acad. Sci. USA 99, 12252-12256

4. Gecko feet
how do geckos walk on
walls? to understand
this cool phenomena we
have to understand the
attraction between
molecules...
Autumn, K., et al. 2002. Evidence for van der Waals adhesion in gecko setae. Proc. Natl. Acad. Sci. USA 99, 12252-12256

5. O
H
O
2-methylpropan-2-ol ethoxyethane
tert-butanol diethyl ether
C4H10O C4H10O
mp 26°C mp by looking at
–116°C
lets start
these two simple
molecules...they are
structural isomers...

6. O
H
O
2-methylpropan-2-ol ethoxyethane
tert-butanol diethyl ether
C4H10O C4H10O
mp 26°C mp –116°C
same atoms...but
very different
properties...why?

7. why are the physical
characteristics so different?

8. is it the bonds
in the molecule?
yes...
C–O–C versus C–O–H
...but...

9. ...or is it
something more?

10. pentane 2-methylbutane 2,2-dimethyl
C5H12 C5H12 propane
bp 36.2˚C bp 28˚C C5H12
three more bp 9.6˚C
isomers...this time
no change in
functional groups...

11. pentane 2-methylbutane 2,2-dimethyl
C5H12 C5H12 propane
bp 36.2˚C bp 28˚C C5H12
bp 9.6˚C
...but they still have
different physical
properties!

12. similar bonds, but very
different properties

13. need to understand
forces between
molecules

14. ...of course, this is controlled by
the bonds in the molecules...

15. ...and the electrons
(of course)
...of course, this is controlled by
the bonds in the molecules...

16. before we can look
at the forces we
need to define a few
terms...
...of course, this is controlled by
the bonds in the molecules...

17. Bond dipoles
(separation of charge)
H Cl δ+ δ–
we now know that
electrons are not
shared evenly
between atoms...
δ+ δ– =
H Cl H Cl

18. Bond dipoles
(separation of charge)
H Cl δ+ δ–
δ+ δ– = ...they are attracted
towards the most
H Cl H Cl
electronegative atom
and the bond is said
to be polarised

19. Bond dipoles
(separation of charge)
H Cl δ+ δ–
the bond dipole
refers to the
difference in charge
on each atom and
their separation
δ+ δ– =
H Cl H Cl

20. H
2.1
Li Be B C N O F
1.0 1.5 2.0 2.5 3.0 3.5 4.0
Na Mg Al Si P S Cl
Pauli scale of 0.9 1.2 1.5 1.8 2.1 2.5 3.0
electronegativities K Ca Br
allows us to predict0.8 1.0 2.8
bond polarity...
Rb Sr I
0.8 1.0 2.5
EN
Bond Type difference Examples Calculation
ionic > 1.7 NaCl 3.0(Cl) - 0.9(Na) = 2.1
CH3O–H 3.5(O) - 2.1(H) = 1.4
polar covalent 0.5 – 1.7
H–Cl 3.0(Cl) - 2.1(H) = 0.9
covalent 0 – 0.4
CH3–H 2.5(C) - 2.1(H) = 0.4 Pg
H–H 2.1(H) - 2.1(H) = 0.0
35

21. electro-
negative
1 18
H 2 13 14 15 16 17 He
9
Li Be B C N O F Ne
F
Na Mg 3 4 5 6 7 8 9 10 11 12 Al Si P S Cl Ar
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe
Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn
87 We do not have to remember
Fr Ra Ac
Fr
all the values, just the general trend
(and eventually the effect of different
functional groups)
electro-
positive

22. Molecular dipoles
(su m of b on d dipo les)
if we add all the
individual bond
dipoles together we
Cl get the...
H C
H
H
polar
molecules

23. Molecular dipoles
(su m of b on d dipo les)
...the molecular
dipole or dipole
Cl moment
H C
H
H dipole
moment
polar
molecules

24. Molecular dipoles
(su m of b on d dipo les)
Cl
H C
H O
H
H dipole H
moment dipole
...compounds with moment
a dipole moment are
polar
said to be polar
molecules
molecules

25. Molecular dipoles
(su m of b on d dipo les) if the bond dipoles
cancel each other out (thats
why shape is important), the
Cl molecule will have no dipole
moment and is
Cl C Cl non-polar
Cl
no dipole
moment
non-polar
m o l e c u l e s

26. Inductive effects
( lo n g ran ge e ffect s)
a functional group that
attracts electrons is an
electron withdrawing
group (EWG)
H2 H2 H2
C
>C>
or or δ+ C δ–
H3C Cl H 3C Cl H3C δ+ Cl
electron-withdrawing group

27. Inductive effects
( lo n g ran ge e ffect s)
first it causes a bond
dipole in its own
bonds
H2 H2 H2
C
>C>
or or δ+ C δ–
H3C Cl H 3C Cl H3C δ+ Cl
electron-withdrawing group

28. Inductive effects
( lo n g ran ge e ffect s)
...and this dipole
induces a bond dipole
in bonds next to it
H2 H2 H2
C
>C>
or or δ+ C δ–
H3C Cl H 3C Cl H3C δ+ Cl
electron-withdrawing group

29. Inductive effects
( lo n g ran ge e ffect s)
the further from
the functional
group the smaller
this polarisation
H2 H2 H2
C
>C>
or or δ+ C δ–
H3C Cl H 3C Cl H3C δ+ Cl
electron-withdrawing group

30. Inductive effects
( lo n g ran ge e ffect s)
a group that
pushes electrons
away (alkyl) is an
electron donating
group
H 3C > C
electron-donating group

31. ...and the inductive
effect in action...

32. Acidity
O H O H H
H O O
H3C O H H3C O H
we measure
acidity by how
readily a
high low
compound looses
H+
pKa pKa

33. Acidity
the more stable
the anion the
more readily the
compound looses
O H H+ O H H
H O O
H3C O H H3C O H
high low
pKa pKa

34. Acidity
O H O H H
H O O
H3C O H H3C O H
the more stable the
anion, the more acidic
the compound and the
high low
reaction shifts to the
right...
pKa pKa

35. Acidity
O H O H H
H O O
H3C O H H3C O H
this is measured by
pKa...the lower the pKa
the more acidic the
high low
compound...more about
this later in semester
pKa pKa

36. Acidity
O H O H H
H O O
H3C O H H3C O H
electron withdrawing
groups help stabilise
high low
negative charges
pKa pKa

37. unit 3
This topic is covered in
detail in unit 3 but at the
moment all we have to
remember is...

38. high pKa
O
molecule is basic, this
means it wants the
H proton H+ or the anion is
unstable
R O
wants H+

39. low pKa
O
H
R O
molecule with a low pKa
will loose a proton H+
readily to go from HA to
losses H+
H+ and A– or...

40. low pKa
O ...the molecule can
stabilise an anion
(negative charge)
H
R O
negative charge stable

41. Acidity
O H O H H
H O O
H3C O H H3C O H
O O
O O
Cl H Cl H
H Cl H O « O
H3C O < O Cl
Cl Cl
pKa = 4.75 pKa = 2.85 pKa = 1.48 pKa = 0.70
CO2H Cl
Cl CO2H
CO2H
Cl
pKa = 2.85 pKa = 4.05 pKa = 4.50

42. Acidity
O H wants H
O+ H H
H O O
H3C O H H3C O H
O O
O O
Cl H Cl H
H Cl H O « O
H3C O < O Cl
Cl Cl
pKa = 4.75 pKa = 2.85 pKa = 1.48 pKa = 0.70
CO2H Cl
Cl CO2H
CO2H
Cl
pKa = 2.85 pKa = 4.05 pKa = 4.50

43. Acidity
the more electron
O H
withdrawing groups the O H H
more stable the negative
O O
H charge and so the pKa is
H3C O H
lower and the compound
H3C O H
is more acidic
O O
O O
Cl H Cl H
H Cl H O « O
H3C O < O Cl
Cl Cl
pKa = 4.75 pKa = 2.85 pKa = 1.48 pKa = 0.70
CO2H Cl
Cl CO2H
CO2H
Cl
pKa = 2.85 pKa = 4.05 pKa = 4.50

44. Acidity
losses H H+ H
O O H
H O O
H3C O H H3C O H
O O
O O
Cl H Cl H
H Cl H O « O
H3C O < O Cl
Cl Cl
pKa = 4.75 pKa = 2.85 pKa = 1.48 pKa = 0.70
CO2H Cl
Cl CO2H
CO2H
Cl
pKa = 2.85 pKa = 4.05 pKa = 4.50

45. Acidity
O
R O
O H stable O H H
H O O
H3C O H H3C O H
O O
O O
Cl H Cl H
H Cl H O « O
H3C O < O Cl
Cl Cl
pKa = 4.75 pKa = 2.85 pKa = 1.48 pKa = 0.70
CO2H Cl
Cl CO2H
CO2H
Cl
pKa = 2.85 pKa = 4.05 pKa = 4.50

46. Acidity
O H O H H
H O O
H3C O H H3C O H
the further away the
electron withdrawing
group from the
O O
O Onegative charge the
smaller the effect
Cl H Cl H
H Cl H O « O
H3C O < O Cl
Cl Cl
pKa = 4.75 pKa = 2.85 pKa = 1.48 pKa = 0.70
CO2H Cl
Cl CO2H
CO2H
Cl
pKa = 2.85 pKa = 4.05 pKa = 4.50

47. you do not need
!
to learn these values!

48. how does all this
effect intermolecular
forces??
so what intermolecular
forces are there?

49. covalent bond
(strong)
govern reactions
H Cl H Cl
forces between
molecules are
relatively weak BUT
very important!
intermolecular
attraction
(weak)
physical properties

50. sorry...
the next slide is
awful (in all the
worst senses of the
word)

51. typical energy
interaction (kJmol–1)
ionic-ionic (ionic bond) 250
intramolecular carbon-containing covalent bond 350
forces
oxygen-hydrogen covalent bond 460
hydrogen (H-) bond 20
intermolecular ion-dipole 15
forces
dipole-dipole 2
London (dispersion) 2
shows just how weak intermolecular
forces are by comparison...

52. transferable skill...

53. • Tables are rarely of any use in a
presentation...
• Lots of text on a PowerPoint (or
Keynote) slide is not only really dull
but looks crap and is second only
to the use of...
• ...bullet points in making you (and
me) a little sleepy
• So stick to pictures (and a lot of
preparation)

54. don’t get me started on the
differences between...
only the first is
acceptable to me (but
most people can’t see
these
these
the difference between
1 & 3)
learn to do
presentations
right...it’ll serve
these
you well!

55. why does NaCl
dissolve in water? ...or, now lets look at
the intermolecular
forces (at last)!

56. H δ+ Cl Hδ+
δ--
O Na O δ--
H δ+ Hδ+
δ–
δ+ δ+
δ– δ+
Cl–
δ+ δ+
δ+ δ–
Ion-dipole
forces (15 kJmol-1)

57. H δ+ Cl Hδ+
δ--
O Na O δ--
H δ+ Hδ+
δ–
δ+ δ+
interaction of
an ion (chloride)
δ– δ+
Cl–
δ+ δ+
δ+ δ–
Ion-dipole
forces (15 kJmol-1)

58. H δ+ Cl Hδ+
δ--
O Na O δ--
H δ+ Hδ+
δ–
δ+ δ+
δ– δ+
Cl–
δ+ with a
δ+
compound with
a permanent
δ+ δ–
dipole (water)
Ion-dipole
forces (15 kJmol-1)

59. H δ+ Cl Hδ+
δ--
O Na O δ--
H δ+ Hδ+
δ–
δ+ δ+
δ– δ+
Cl–
δ+ δ+
δ+ δ–
relatively
Ion-dipole
strong...hence
salt dissolves
forces (15 kJmol-1)

60. H δ+ Cl Hδ+
δ--
O Na O δ--
H δ+ Hδ+
δ–
δ+note that water
δ+
interacts with
both anion and
cation...really
δ– δ+ helps
Cl–
δ+ δ+
δ+ δ–
Ion-dipole
forces (15 kJmol-1)

61. δ– HO Hδ+
O Na O δ–
H δ+
organic
compounds can
do the same
(but only O–H
bond)
Ion-dipole
forces (15 kJmol-1)

62. δ+ δ– δ+ δ–
two molecules
with a dipole will
interact...
δ+ δ–
weakly
δ– δ+
Dipole-dipole
forces (≈ 2 kJmol-1)

63. Polarmix polar
molecules
/
H3C Cl
δ+ C O δ– δ+ H C Cl δ–
H3C Cl
this is why two polar
molecules (like acetone
& chloroform) will δ+ δ–
δ+ δ– δ+ δ–
mix...they are attracted δ– δ+
–δ +δ δ– δ+
to each other δ+ δ–
δ+ δ– δ+ δ–
δ+ δ– δ+ δ– δ+ δ–
δ+ δ– δ– δ+

64. Polardo/not mix
molecules
non-polar
H H2 H2
δ+ O δ– C C CH3
H3C C C
H H2 H2
but polar molecules
won’t mix with non–
polar...polar molecules
run an exclusive club
δ+ δ– δ+ δ–
δ+δ– and they won’t let
δ– δ+
δ– δ+ anyone else in...
δ– δ+
δ– δ+
δ– δ+
δ+ δ–

65. δ– H2
H3C δ+ O CH3 H3C C
H2 C CH3
H3C C H2
CH3 O δ+ CH3 C CH3
δ– H2
propanone butane
acetone
Mol Wt. 58; bp 56°C Mol Wt. 58; bp –0.6°C
permanent dipole no dipole
compare these two
molecules...similar
Dipole-dipole
size and identical
weight...
& boiling points

66. δ– H2
H3C δ+ O CH3 H3C C
H2 C CH3
H3C C H2
CH3 O δ+ CH3 C CH3
δ– H2
propanone butane
acetone
Mol Wt. 58; bp 56°C Mol Wt. 58; bp –0.6°C
permanent dipole no dipole
Dipole-dipole
& boiling points ...but very different
boiling points...

67. δ– H2
H3C δ+ O CH3 H3C C
H2 C CH3
H3C C H2
CH3 O δ+ CH3 C CH3
δ– H2
propanone butane
acetone
Mol Wt. 58; bp 56°C Mol Wt. 58; bp –0.6°C
permanent dipole no dipole
this is because
acetone has dipole–
dipole attractions
holding molecules
Dipole-dipole
together and butane
doesn’t
& boiling points

68. ...one intermolecular
force to rule them all...

69. δ+
δ– δ+ δ–
δ+ δ+
Hydrogen H
O H O
bonding
H H
hydrogen bond

70. δ+
δ– δ+ δ–
δ+ δ+
Hydrogen
a special kind of
H dipole–dipole
interaction...occurs
O between...
H O
bonding
H H
hydrogen bond

71. H-bond H X
polar bond
dono r (X = O, N etc)
X
H-bond lone pair on
acceptor electronegative
atom

72. H-bonding it is responsible for
water being so
wonderfully odd
water’s
abnormal
properties
F.W. Starr/Wesleyan Univ.

73. δ– H δ+
O
δ+H H δ+
δ–
O H O δ–
δ+ H δ+ Hδ+
δ–
δ+ O δ– O H δ+
H H δ+ H δ+
three molecules
of similar size
H2O (MW=18): boiling point 100°C
and / or shape
H2S .(MW=34): boiling point –60°C
CH4 (MW=16): boiling point –162°C

74. ...yet water has
phenomenally high δ– H δ+
boiling point...all due O
to H–bonding! (also
explains ice) δ+H H δ+
δ–
O H O δ–
δ+ H δ+ Hδ+
δ–
δ+ O δ– O H δ+
H H δ+ H δ+
H2O (MW=18): boiling point 100°C
H2S .(MW=34): boiling point –60°C
CH4 (MW=16): boiling point –162°C

75. Methanol methanol only has one
O–H bond so can only
form one H-bond so has
much lower bp (less
attraction)
CH3
δ+ Oδ–δ+ δ–
H H O
δ– CH3
O δ+
H3C H H δ+
O δ–
CH3
1 hydrogen bond
boiling point 62°C

76. H-bonding
carboxylic acids carboxylic acids are also
capable of forming H–
bonds between OH (H-bond
donor) and C=O (polarised
so lone pair is H-bond
acceptor)
δ– δ+ δ–
O H O
δ+
H3C CH3
O H O
δ– δ+ δ–

77. H-bonding
carboxylic acids
- solubility
...H-bonding allows some
acids to dissolve in water as
good attraction (hence we
can have vinegar (shown) δ+ H O δ–
and glacial acetic acid (very
different) δ–O H
H3C H
δ+ δ– δ+ δ–
H O H O
δ– H
O H δ+

78. Hydrogen bonding
vital in biology...

79. protein
secondary
structure

80. H O R2 H H
N N
N
O H R1 Hδ+ O δ–
δ– O δ+ H H R4
N
N
R3 H O H
Hydrogen
β-sheets
and α-helix etc are all
formed by H-bonding
between amides
bonding

81. without
H-bonding no
DNA double
helix!
©Benny Herudek 3D Hifi - High Fidelity 3d Graphics Solutions
DNA

82. H
N N H O CH3
N N H N
N N
O
adenine thymine
Hydrogen
Watson-Crick base
pairing (or
whatever the
biochemists call it)
bonding

83. all molecules can
interact...
don’t need a dipole
to interact
because...

84. London (van der Waals
or dispersion) forces
here we have two
molecules...

85. London (van der Waals
or dispersion) forces
δ– δ+
...chance allows the
electrons of one molecule
to bunch at one end
causing an imbalance of
electrons...
momentary
dipole

86. London (van der Waals
or dispersion) forces
δ– δ+
momentary
...a disturbance in the dipole
force...or setting up a
momentary dipole...

87. London (van der Waals
or dispersion) forces
δ– δ+ δ– δ+
...as electrons don’t like each
other this new bunch repulse
electrons in a near by molecule
induced momentary
and set up an induced
dipole...
dipole dipole

88. London (van der Waals
or dispersion) forces this causes dipole–dipole
attraction (momentarily)...but it
will soon stop as the electrons are
always on the move
δ– δ+ δ– δ+
attraction
induced momentary
dipole dipole

89. Larger
surface area
the larger the molecule,
the bigger the surface area
and the more electrons
involved...
Bigger
the force

90. Larger
surface area
...this means a bigger
momentary dipole can be
formed...
Bigger
the force

91. Larger
surface area
and thus greater attraction Bigger
the force

92. Larger
surface area H H H H H H H
H C C C
H C H C C C H
H H H H H H H
methane hexane
CH4 (MW=16) C6H14 (MW=86)
mp –182°C; bp –164°C mp –95°C; bp 69°C
gas at rt liquid at rt
H H H H H H H H H H H H H H H H H H H H
H C C C C C C C C C C
C C C C C C C C C C H
H H H H H H H H H H H H H H H H H H H H
eicosane
C20H42 (MW=282)
mp 36°C; bp 343°C
Bigger
...this goes solid at rt
someway to explaining why bigger
molecules have higher boiling point
(bp)...but other factors
also involved
the force

93. Larger
surface area
pentane 2,2-dimethylpropane
bp 36°C bp 9.5°C
and at last explains the
difference between isomers
Bigger
the force

94. Gecko feet
...and it is van der
Waals forces that are
responsible for Gecko’s
‘sticky’ feet!
Autumn, K., et al. 2002. Evidence for van der Waals adhesion in gecko setae. Proc. Natl. Acad. Sci. USA 99, 12252-12256

95. solvation
interaction of molecules & solvent

96. propanol
H
H O
O
propanol & water mix H
as they can interact
by H-bonds... H H
O
water
soluble

97. alcohol (OH) makes propanol
means water
loving in Latin
(I think)
hydrophilic

98. hexane
H H
O
H H
no interaction
O
between molecules O
so hexane floats on
waterH H
O
H H
water
insoluble

99. hexane
H H
O
H H
O O
H H
polar molecules
O (water) do not mix
H H with non-polar
water
molecules (hexane)
insoluble

100. non-polar grease makes hexane
means water
fearing
(hating)
hydrophobic

101. polar
dissolves
...conversely, non-polar
compounds dissolve
polar
other non-polar
compounds

102. hydrophilic
dissolves
doesn’t really
need
explanation...
hydrophilic

103. hydrophobic
dissolves
hydrophobic

104. propanoic acid
✔
H H
O O H
H O
O
H
H H
O
H-bonding allows
molecules to interact, thus
mixing...hydrophobic ethyl
water
chain too small to effect
interaction

105. hydrophilic
✔
H H
O O H
H O
O
H
H H
O
water

106. butanoic acid
✔
H H
O O H
H O
O
H
H H
O
addition of one
more carbon to chain
does not make much
difference...butanoic
water
acid still soluble in
water

107. hexanoic acid
/
✔✘
O H
H O
O
H
H H
O
but a pentyl chain is
pushing our luck...non-
polar chain starts to effect
water
solubility and only a little
will dissolve

108. ✘
decanoic acid
O H
H O
O
H
H H
O
get to a point where
the blob of grease
controls the properties
and overcomes the H-
water bond interactions...

109. ✘
hydrophobic
O H
H O
O
H
H H
O
...to give us a compound
that will not mix with water
as too much of it is non-
water
polar. So decanoic acid is
hydrophobic

110. ✘
sugar
O OH
HO
HO OH
OH
we can have the
reverse...a very polar
molecule will not
dissolve / mix with a
non-polar solvent
hexane

111. ✘
sugar
O OH
HO
HO OH
OH
...each OH group is polar
and thus will not mix with
hexane
hexane...

112. ✘
sugar - hydrophilic
O OH
HO
HO OH
OH very little interactions
between two molecules so
they do not mix
hexane - hydrophobic

113. ✔
sugar
O OH
HO
HO OH
OH
...obvious I hope
(sugar dissolves in
coffee!)...due to all the
OH groups H-bonding
water to water

114. ✔
sugar
H H
O O
H H O
H H
O O H
HO H O
H H H
O O lots of hydrogen bonding
H O H
H H O
O H O
H
H
water

115. doesn’t this look good!
especially after all this chemistry

116. what have
we learnt?
all about
intermolecular
forces

117. read
part 6
©paulbence@flickr