inter molecular forces - chemistry from examville.com
TRANSCRIPT
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Unit One Part 5:intermolecular forces
we have looked at the
bonds in molecules,now turn our attention
to the bonds / forces
between molecules
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Unit One
Part5
Properties ofmolecules (pg56-58)Intermolecular forces (pg59-66)Solubility(pg66-68)
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Gecko feet
Autumn, K., et al. 2002. Evidence for van der Waals adhesion in gecko setae. Proc. Natl. Acad. Sci. USA 99, 12252-12256
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Gecko feet
Autumn, K., et al. 2002. Evidence for van der Waals adhesion in gecko setae. Proc. Natl. Acad. Sci. USA 99, 12252-12256
how do geckos walk on
walls? to understand
this cool phenomena we
have to understand the
attraction between
molecules...
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OH
2-methylpropan-2-oltert-butanol
C4H10O
mp 26C
O
ethoxyethanediethyl ether
C4H10O
mp 116C
lets start by looking at
these two simple
molecules...they are
structural isomers...
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OH
2-methylpropan-2-oltert-butanol
C4H10O
mp 26C
O
ethoxyethanediethyl ether
C4H10O
mp 116C
same atoms...but
very different
properties...why?
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whyare the physicalcharacteristics so different?
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is it the bondsin the molecule?
yes...
COC versus COH
...but...
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...or is itsomething more?
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pentane
C5H12bp 36.2C
2,2-dimethyl
propaneC5H12bp 9.6C
2-methylbutane
C5H12bp 28Cthree more
isomers...this time
no change in
functional groups...
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pentane
C5H12bp 36.2C
2,2-dimethyl
propaneC5H12bp 9.6C
2-methylbutane
C5H12bp 28C
...but they still have
different physical
properties!
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similar bonds, but verydifferent properties
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need to understandforces betweenmolecules
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...of course, this is controlled bythe bonds in the molecules...
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...of course, this is controlled bythe bonds in the molecules...
...and the electrons(of course)
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+
Bond dipoles(separation of charge)
H Cl
+ H Cl
=
we now know that
electrons are not
shared evenlybetween atoms...
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+
Bond dipoles(separation of charge)
H Cl
+ H Cl
=...they are attracted
towards the most
electronegative atom
and the bond is saidto be polarised
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+
Bond dipoles(separation of charge)
H Cl
+ H Cl
=
the bond dipolerefers to the
difference in chargeon each atom and
their separation
M l l di l
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Cl
CH
H
H
Molecular dipoles
(sum of bond dipoles)
polarmolecules
if we add all the
individual bonddipoles together we
get the...
M l l di l
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dipolemoment
Cl
CH
H
H
Molecular dipoles
(sum of bond dipoles)
polarmolecules
...the moleculardipole or dipole
moment
M l l di l
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dipolemoment
O
H
H
Cl
CH
H
H
Molecular dipoles
(sum of bond dipoles)
polarmolecules
dipole
moment...compounds witha dipole momentaresaid to be polar
molecules
M l l di l
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Molecular dipoles
(sum of bond dipoles)
non-polarm o l e c u l e s
Cl
C
ClClCl
no dipole
moment
if the bond dipolescancel each other out (thats
why shape is important), the
molecule will have no dipolemoment and is
non-polar
I d ti ff t
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Inductive effects( long range ef fects)
electron-withdrawing group
H3C
H2C
Clor
H3C
H2C
Cl>
>or
H3CH2C
Cl+
+
a functional group that
attracts electrons is an
electron withdrawinggroup (EWG)
I d ti ff t
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Inductive effects( long range ef fects)
electron-withdrawing group
H3C
H2C
Clor
H3C
H2C
Cl>
>or
H3CH2C
Cl+
+
first it causes a bonddipole in its own
bonds
I d ti ff t
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Inductive effects( long range ef fects)
electron-withdrawing group
H3C
H2C
Clor
H3C
H2C
Cl>
>or
H3CH2C
Cl+
+
...and this dipole
induces a bond dipolein bonds next to it
I d ti ff t
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Inductive effects( long range ef fects)
electron-withdrawing group
H3C
H2C
Clor
H3C
H2C
Cl>
>or
H3CH2C
Cl+
+
the further fromthe functional
group the smaller
this polarisation
I d ti ff t
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Inductive effects( long range ef fects)
electron-donating group
H3C C>
a group that
pushes electrons
away (alkyl) is an
electron donatinggroup
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...and the inductiveeffect in action...
A idit
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h i g h
pK
a
l o w
pK
a
H3C OH
O
O
H
H
O
H
HH
H3C O
O
Acidity
we measure acidityby how readily a
compound losses
H+
A idit
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h i g h
pK
a
l o w
pK
a
H3C OH
O
O
H
H
O
H
HH
H3C O
O
Aciditythe more stable
the anion the
more readily the
compound lossesH+
A idit
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h i g h
pK
a
l o w
pK
a
H3C OH
O
O
H
H
O
H
HH
H3C O
O
Acidity
the more stable theanion, the more acidic
the compound and the
reaction shifts to the
right...
A idit
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h i g h
pK
a
l o w
pK
a
H3C OH
O
O
H
H
O
H
HH
H3C O
O
Acidity
this is measured bypKa...the lower the pKa
the more acidic the
compound...more about
this later in semester
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Acidit
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H3C OH
O
OH
O
OH
O
OH
O
ClCl
Cl
Cl
ClCl
pKa = 4.75 pKa = 2.85 pKa = 1.48 pKa = 0.70
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H3C OH
O
OH
O
OH
O
OH
O
ClCl
Cl
Cl
ClCl
pKa = 4.75 pKa = 2.85 pKa = 1.48 pKa = 0.70
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H3C OH
O
OH
O
OH
O
OH
O
ClCl
Cl
Cl
ClCl
pKa = 4.75 pKa = 2.85 pKa = 1.48 pKa = 0.70
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you do not need
to learn these values!!
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so what intermolecularforces are there?
how does all this
effect intermolecular
forces??
l t b d
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covalent bond(strong)
govern reactions
intermolecularattraction
(weak)
physical properties
forces between
molecules are
relatively weak BUT
very important!
sorry
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sorry...
the next slide is
awful (in all the
worst senses of the
word)
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transferable skill...
T bl l f i
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Tables are rarely of any use in a
presentation...
Lots of text on a PowerPoint (orKeynote) slide is not only really dull
but looks crap and is second onlyto the use of...
...bullet points in making you (and
me) a little sleepy So stick to pictures (and a lot of
preparation) dont get me started on the
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don t get me started on thedifferences between...
thesethese
these
learn to do
presentations
right...itll serve
you well!
only the first isacceptable to me (but
most people cant see
the difference between
1 & 3)
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why does NaCldissolve in water? ...or, now lets look atthe intermolecular
forces (at last)!
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Ion-dipoleforces (15 kJmol-1)
H
O
H
Na
H
O
H
Cl+
--
+--
+
+
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Ion-dipoleforces (15 kJmol-1)
H
O
H
Na
H
O
H
Cl+
--
+--
+
+
interaction of
an ion (chloride)
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Ion-dipoleforces (15 kJmol-1)
H
O
H
Na
H
O
H
Cl+
--
+--
+
+
with a
compound with
a permanentdipole (water)
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Ion-dipoleforces (15 kJmol-1)
H
O
H
Na
H
O
H
Cl+
--
+--
+
+
relatively
strong...hence
salt dissolves
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Ion-dipoleforces (15 kJmol-1)
H
O
H
Na
H
O
H
Cl+
--
+--
+
+note that water
interacts with
both anion and
cation...really
helps
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Ion-dipoleforces (15 kJmol-1)
O
H
Na
H
O
HO+
+
organic
compounds can
do the same
(but only OH
bond)
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Dipole-dipoleforces ( 2 kJmol-1)
+ +
+
+
two molecules
with a dipole willinteract...
weakly
P l / l
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Polar / polarmolecules mix
+ +
+ +
+
+
+
+
+
+
+
+
+
+ +
+
this is why two polarmolecules (like acetone
& chloroform) willmix...they are attractedto each other
Polar / non polar
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Polar / non-polarmolecules do not mix
+
+
+
+
+
++
+
+
+
but polar molecules
wont mix with non
polar...polar moleculesrun an exclusive club
and they wont let
anyone else in...
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Dipole-dipole& boiling points
OH3C
CH3 O CH3
CH3
+
+H3C C
H2
H2
C CH3H3C
CH2
H2C
CH3
propanone
acetoneMol Wt. 58; bp 56Cpermanent dipole
butane
Mol Wt. 58; bp 0.6Cno dipole
compare these two
molecules...similarsize and identical
weight...
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Dipole-dipole& boiling points
OH3C
CH3 O CH3
CH3
+
+H3C C
H2
H2
C CH3H3C
CH2
H2C
CH3
propanone
acetoneMol Wt. 58; bp 56Cpermanent dipole
butane
Mol Wt. 58; bp 0.6Cno dipole
...but very different
boiling points...
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Dipole-dipole& boiling points
OH3C
CH3 O CH3
CH3
+
+H3C C
H2
H2
C CH3H3C
CH2
H2C
CH3
propanone
acetoneMol Wt. 58; bp 56Cpermanent dipole
butane
Mol Wt. 58; bp 0.6Cno dipole
this is because
acetone has dipoledipole attractions
holding moleculestogether and butane
doesnt
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...one intermolecularforce to rule them all...
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HO
H
H O
H
hydrogen bond
Hydrogen
bonding
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HO
H
H O
H
hydrogen bond
Hydrogen
bonding
a special kind of
dipoledipoleinteraction...occurs
between...
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H-bonddonorH X
polar bond(X = O, N etc)
H-bondacceptor
X
lone pair onelectronegative
atom
H b di
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H-bonding
watersabnormal
propertiesF.W. Starr/Wesleyan Univ.
it is responsible for
water being so
wonderfully odd
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H
O
H
H
O
H
OH
H
O H
H
OH
H
+
+
++
+ +
++++
H2O(MW=18): boiling point100CH2S.(MW=34): boiling point60CCH4(MW=16): boiling point162C
three molecules
of similar size
and / or shape
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H
O
H
H
O
H
OH
H
O H
H
OH
H
+
+
++
+ +
++++
H2O(MW=18): boiling point100CH2S.(MW=34): boiling point60CCH4(MW=16): boiling point162C
...yet water has
phenomenally high
boiling point...all due
to Hbonding! (also
explains ice)
Methanol
methanol only has one
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H3CO
H
H
O
CH3
H
O
H
CH3
O
CH3
+ +
++
Methanol
1 hydrogen bond
boiling point 62C
y
OH bond so can only
form one H-bond so hasmuch lower bp (less
attraction)
H-bonding
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H bondingcarboxylic acids
H3C
O
O H
H O
O
CH3
+
+
+
carboxylic acids are also
capable of forming H
bonds between OH (H-bonddonor) and C=O (polarised
so lone pair is H-bondacceptor)
H-bonding
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H3C
O
O H
H O
O
H
HHO
H
H+
+
+
+
H bondingcarboxylic acids
- solubility...H-bonding allows some
acids to dissolve in water as
good attraction (hence we
can have vinegar (shown)and glacial acetic acid (very
different)
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Hydrogen bondingvital in biology...
protein
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proteinsecondary
structure
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N
N
O
H R1
N
O
R2 HH
O H
H
O
N
R3 H
H
O
N
H R4
H
+ +
Hydrogen
bonding
-sheets
and -helixetc
are allformed by H-bonding
between amides
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Benny Herudek 3D Hifi - High Fidelity 3d Graphics Solutions
without
H-bonding noDNA double
helix!
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Hydrogen
bonding
N
N
N
N
N H
H
O
H N
N
O
CH3
adenine thymine
Watson-Crick base
pairing (orwhatever the
biochemists call it)
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all molecules caninteract...
dont need a dipole
to interact
because...
London (van der Waals
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London (van der Waalsor dispersion) forces
here we have two
molecules...
London (van der Waals
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London (van der Waalsor dispersion) forces
+
momentarydipole
...chance allows the
electrons of one molecule
to bunch at one end
causing an imbalance of
electrons...
London (van der Waals
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London (van der Waalsor dispersion) forces
+
momentarydipole...a disturbance in the
force...or setting up a
momentary dipole...
London (van der Waals
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London (van der Waalsor dispersion) forces
+
momentarydipole
+
induceddipole
...as electrons dont like each
other this new bunch repulseelectrons in a near by molecule
and set up an induceddipole...
London (van der Waals
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London (van der Waalsor dispersion) forces
+
momentarydipole
+
induceddipole
this causes dipoledipole
attraction (momentarily)...but itwill soon stop as the electrons are
always on the move
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Larger
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Largersurface area
Biggerthe force
...this means a bigger
momentary dipole can beformed...
Larger
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Largersurface area
Biggerthe force
and thus greater attraction
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Larger
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Largersurface area
Biggerthe force
pentanebp 36C
2,2-dimethylpropanebp 9.5C
and at last explains the
difference between isomers
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Gecko feet
Autumn, K., et al. 2002. Evidence for van der Waals adhesion in gecko setae. Proc. Natl. Acad. Sci. USA 99, 12252-12256
...and it is van der
Waals forces that areresponsible for Geckos
sticky feet!
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propanol
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soluble
OH
H
O
H
O
H
H
propanol
water
propanol & water mix
as they can interact
by H-bonds...
alcohol (OH) makes propanol
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means water
loving in Latin
(I think)
hexane
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insoluble
hexane
water
OHH
OHH
O
H
H
O
H
Hno interaction
between molecules
so hexane floats on
water
hexane
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insoluble
hexane
water
OHH
OHH
O
H
H
O
H
H
polar molecules(water) do not mix
with non-polar
molecules (hexane)
non-polar grease makes hexane
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means water
fearing
(hating)
polar
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polar
dissolves
l
...conversely, non-polar
compounds dissolveother non-polar
compounds
hydrophilic
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hydrophilic
dissolves
hydrophilicdoesnt really
need
explanation...
hydrophobic
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hydrophobic
dissolves
hydrophobic propanoic acid
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p op o c c d
water
O H
HOH
OH
HOH
OH
H-bonding allows
molecules to interact, thus
mixing...hydrophobic ethyl
chain too small to effect
interaction
hydrophilic
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y p
water
O H
HOH
OH
HOH
OH
butanoic acid
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O HH O HOHHO
HO
H
water
addition of one
more carbon to chain
does not make much
difference...butanoic
acid still soluble in
water
hexanoic acid
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/water
O
H
HOH
O
H
HO
but a pentyl chain is
pushing our luck...non-
polar chain starts to effect
solubility and only a little
will dissolve
decanoic acid
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water
OH
HOH
O
H
HO
get to a point where
the blob of grease
controls the properties
and overcomes the H-
bond interactions...
hydrophobic
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y p
water
OH
HOH
O
H
HO
...to give us a compound
that will not mix with water
as too much of it is non-
polar. So decanoic acid is
hydrophobic
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sugar
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g
hexane
OHO
HO
OH
OH
OH
...each OH group is polar
and thus will not mix with
hexane...
sugar - hydrophilic
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g y p
hexane hydrophobic
OHO
HO
OH
OH
OH
sugar
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g
water
OHO
HO
OH
OH
OH
...obvious I hope
(sugar dissolves in
coffee!)...due to all the
OH groups H-bonding
to water
doesnt this look good!
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g
especially after all this chemistry what have
learnt?
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welearnt?
all aboutintermolecular