Lecture5: 123.101

Unit One Part 5:intermolecular forces

we have looked at the bonds in molecules,

now turn our attention to the bonds / forces between molecules

Unit OnePart5Properties of molecules (pg56-58)Intermolecular forces (pg59-66)Solubility (pg66-68)

Gecko feet

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

Gecko feet


how do geckos walk on walls? to understand

this cool phenomena we have to understand the

attraction between molecules...

OH

2-methylpropan-2-oltert-butanol

C4H10Omp 26°C

O

ethoxyethanediethyl ether

C4H10Omp –116°C

lets start by looking at these two simple

molecules...they are structural isomers...

OH

2-methylpropan-2-oltert-butanol

C4H10Omp 26°C

O

ethoxyethanediethyl ether

C4H10Omp –116°C

same atoms...but very different

properties...why?

why are the physical characteristics so different?

is it the bonds in the molecule?

yes...C–O–C versus C–O–H

...but...

...or is it something more?

pentaneC5H12

bp 36.2˚C

2,2-dimethylpropane

C5H12bp 9.6˚C

2-methylbutaneC5H12

bp 28˚Cthree more

isomers...this time no change in

functional groups...

pentaneC5H12

bp 36.2˚C

2,2-dimethylpropane

C5H12bp 9.6˚C

2-methylbutaneC5H12

bp 28˚C

...but they still have different physical

properties!

similar bonds, but very different properties

need to understand forces between

molecules

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


...and the electrons (of course)


before we can look at the forces we

need to define a few terms...

δ+ δ–H Cl

Bond dipoles(separation of charge)

H Clδ+ δ–

H Cl=

we now know that electrons are not

shared evenly between atoms...

δ+ δ–H Cl


H Clδ+ δ–

H Cl= ...they are attracted towards the most

electronegative atom and the bond is said

to be polarised

δ+ δ–H Cl


H Clδ+ δ–

H Cl=

the bond dipole refers to the

difference in charge on each atom and their separation

H2.1Li1.0

Be1.5

B2.0

C2.5

N3.0

O3.5

F4.0

Na0.9

Mg1.2

Al1.5

Si1.8

P2.1

S2.5

Cl3.0

K0.8

Ca1.0

Br2.8

Rb0.8

Sr1.0

I2.5

Bond Type ENdifference Examples Calculation

ionic > 1.7 NaCl 3.0(Cl) - 0.9(Na) = 2.1

polar covalent 0.5 – 1.7 CH3O–HH–Cl

3.5(O) - 2.1(H) = 1.43.0(Cl) - 2.1(H) = 0.9

covalent 0 – 0.4 CH3–HH–H

2.5(C) - 2.1(H) = 0.42.1(H) - 2.1(H) = 0.0

Pg35

Pauli scale of electronegativities allows us to predict

bond polarity...

electro-positive

electro-negative

1 18

H 2 13 14 15 16 17 He

Li Be B C N O F Ne

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

Fr Ra Ac

9F

87Fr

We do not have to rememberall the values, just the general trend (and eventually the effect of different

functional groups)

ClC

HHH

Molecular dipoles(sum of bond dipoles)

polar molecules

if we add all the individual bond

dipoles together we get the...

dipolemoment

ClC

HHH


polar molecules

...the molecular dipole or dipole

moment

dipolemoment

OH

H

ClC

HHH


polar molecules

dipolemoment...compounds with

a dipole moment are said to be polar

molecules


non-polar m o l e c u l e s

ClC

ClClCl

no dipole moment

if the bond dipolescancel each other out (thats why shape is important), the molecule will have no dipole

moment and isnon-polar

Inductive effects( long range effects)

electron-withdrawing group

H3CH2C

Cl orH3C

H2CCl

>> orH3C

H2CClδ+δ–δ+

a functional group that attracts electrons is an electron withdrawing

group (EWG)



H3CH2C

Cl orH3C

H2CCl

>> orH3C

H2CClδ+δ–δ+

first it causes a bond dipole in its own

bonds



H3CH2C

Cl orH3C

H2CCl

>> orH3C

H2CClδ+δ–δ+

...and this dipole induces a bond dipole

in bonds next to it



H3CH2C

Cl orH3C

H2CCl

>> orH3C

H2CClδ+δ–δ+

the further from the functional

group the smaller this polarisation


electron-donating group

H3C C>

a group that pushes electrons away (alkyl) is an electron donating

group

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

h i g hpKa

l o wpKa

H3C OH

OOH

HOH

HH

H3C O

O

Acidity

we measure acidity by how

readily a compound looses

H+

h i g hpKa

l o wpKa

H3C OH

OOH

HOH

HH

H3C O

O

Aciditythe more stable the anion the

more readily the compound looses

H+

h i g hpKa

l o wpKa

H3C OH

OOH

HOH

HH

H3C O

O

Acidity

the more stable the anion, the more acidic the compound and the reaction shifts to the

right...

h i g hpKa

l o wpKa

H3C OH

OOH

HOH

HH

H3C O

O

Acidity

this is measured by pKa...the lower the pKa

the more acidic the compound...more about

this later in semester

h i g hpKa

l o wpKa

H3C OH

OOH

HOH

HH

H3C O

O

Acidity

electron withdrawing groups help stabilise

negative charges

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

remember is...

high pKa

R OH

O

wants H+

molecule is basic, this means it wants the

proton H+ or the anion is unstable

low pKa

losses H+

R O

OH

molecule with a low pKa will loose a proton H+

readily to go from HA to H+ and A– or...

low pKa

negative charge stable

R O

OH

...the molecule can stabilise an anion (negative charge)

H3C OH

O

OH

OO

HO

OH

O

ClCl

Cl

Cl

ClCl

pKa = 4.75 pKa = 2.85 pKa = 1.48 pKa = 0.70

< «

CO2H

CO2HCO2H

Cl

ClCl

pKa = 2.85 pKa = 4.05 pKa = 4.50

H3C OH

OOH

HOH

HH

H3C O

O

Acidity

H3C OH

OOH

HOH

HH

H3C O

O

H3C OH

O

OH

OO

HO

OH

O

ClCl

Cl

Cl

ClCl

pKa = 4.75 pKa = 2.85 pKa = 1.48 pKa = 0.70

< «

CO2H

CO2HCO2H

Cl

ClCl

pKa = 2.85 pKa = 4.05 pKa = 4.50

Acidity

wants H+

H3C OH

O

OH

OO

HO

OH

O

ClCl

Cl

Cl

ClCl

pKa = 4.75 pKa = 2.85 pKa = 1.48 pKa = 0.70

< «

CO2H

CO2HCO2H

Cl

ClCl

pKa = 2.85 pKa = 4.05 pKa = 4.50

H3C OH

OOH

HOH

HH

H3C O

O

Aciditythe more electron

withdrawing groups the more stable the negative charge and so the pKa is lower and the compound

is more acidic

CO2H

CO2HCO2H

Cl

ClCl

pKa = 2.85 pKa = 4.05 pKa = 4.50

H3C OH

O

OH

OO

HO

OH

O

ClCl

Cl

Cl

ClCl

pKa = 4.75 pKa = 2.85 pKa = 1.48 pKa = 0.70

< «

H3C OH

OOH

HOH

HH

H3C O

O

Aciditylosses H+

CO2H

CO2HCO2H

Cl

ClCl

pKa = 2.85 pKa = 4.05 pKa = 4.50

H3C OH

O

OH

OO

HO

OH

O

ClCl

Cl

Cl

ClCl

pKa = 4.75 pKa = 2.85 pKa = 1.48 pKa = 0.70

< «

H3C OH

OOH

HOH

HH

H3C O

O

AcidityR O

O

stable

H3C OH

O

OH

OO

HO

OH

O

ClCl

Cl

Cl

ClCl

pKa = 4.75 pKa = 2.85 pKa = 1.48 pKa = 0.70

< «

CO2H

CO2HCO2H

Cl

ClCl

pKa = 2.85 pKa = 4.05 pKa = 4.50

H3C OH

OOH

HOH

HH

H3C O

O

Acidity

the further away the electron withdrawing

group from the negative charge the smaller the effect

you do not needto learn these values!!

so what intermolecular forces are there?

how does all this effect intermolecular

forces??

H ClH Cl

covalent bond (strong)

govern reactions

intermolecular attraction

(weak)physical properties

forces between molecules are

relatively weak BUT very important!

sorry...

the next slide is awful (in all the

worst senses of the word)

interaction typical energy (kJmol–1)

intramolecular forces

ionic-ionic (ionic bond) 250intramolecular

forces carbon-containing covalent bond 350intramolecular forces

oxygen-hydrogen covalent bond 460

intermolecular forces

hydrogen (H-) bond 20


ion-dipole 15intermolecular forces dipole-dipole 2


London (dispersion) 2

shows just how weak intermolecular forces are by comparison...

transferable skill...

• 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)

don’t get me started on the differences between...

thesethesethese

learn to do presentations

right...it’ll serve you well!

only the first is acceptable to me (but most people can’t see the difference between

1 & 3)

why does NaCl dissolve in water? ...or, now lets look at

the intermolecular forces (at last)!

Ion-dipole forces (15 kJmol-1)

Cl–δ+

δ+

δ+ δ+

δ+

δ+

δ–

δ–

δ–

HO

HNa

HO

H

Clδ+δ--

δ+

δ--δ+

δ+


Cl–δ+

δ+

δ+ δ+

δ+

δ+

δ–

δ–

δ–

HO

HNa

HO

H

Clδ+δ--

δ+

δ--δ+

δ+

interaction of an ion (chloride)


Cl–δ+

δ+

δ+ δ+

δ+

δ+

δ–

δ–

δ–

HO

HNa

HO

H

Clδ+δ--

δ+

δ--δ+

δ+

with a compound with a permanent dipole (water)


Cl–δ+

δ+

δ+ δ+

δ+

δ+

δ–

δ–

δ–

HO

HNa

HO

H

Clδ+δ--

δ+

δ--δ+

δ+

relatively strong...hence salt dissolves


Cl–δ+

δ+

δ+ δ+

δ+

δ+

δ–

δ–

δ–

HO

HNa

HO

H

Clδ+δ--

δ+

δ--δ+

δ+

note that water interacts with

both anion and cation...really

helps


OH

NaHO

HOδ–

δ+δ–

δ+

organic compounds can

do the same (but only O–H

bond)

Dipole-dipole forces (≈ 2 kJmol-1)

δ–δ+ δ–δ+

δ–δ+

δ– δ+

two molecules with a dipole will

interact...weakly

Polar / polar molecules mix

CHCl

ClClδ+ δ–δ–δ+ OC

H3C

H3C

δ+ δ–δ–δ+δ–δ+

δ– δ+

δ+ δ–

δ+δ–

δ+δ–

δ+ δ–δ–δ+

δ–δ+

δ–δ+

δ–δ+δ– δ+

δ+ δ–

this is why two polar molecules (like acetone

& chloroform) will mix...they are attracted

to each other

Polar / non-polar molecules do not mix

δ–δ+HO

H

CH3CH2

H2C

CH2

H2C

H3C

δ–δ+δ– δ+

δ–δ+

δ– δ+

δ–δ+δ–δ+

δ– δ+δ– δ+δ– δ+

but polar molecules won’t mix with non–

polar...polar molecules run an exclusive club

and they won’t let anyone else in...

Dipole-dipole & boiling points

OH3C

CH3 O CH3

CH3

δ–δ+

δ–δ+

H3CCH2

H2C

CH3H3C

CH2

H2C

CH3

propanoneacetone

Mol Wt. 58; bp 56°Cpermanent dipole

butane

Mol Wt. 58; bp –0.6°Cno dipole

compare these two molecules...similar size and identical

weight...


OH3C

CH3 O CH3

CH3

δ–δ+

δ–δ+

H3CCH2

H2C

CH3H3C

CH2

H2C

CH3

propanoneacetone


butane


...but very different boiling points...


OH3C

CH3 O CH3

CH3

δ–δ+

δ–δ+

H3CCH2

H2C

CH3H3C

CH2

H2C

CH3

propanoneacetone


butane


this is becauseacetone has dipole–dipole attractions holding molecules

together and butane doesn’t

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

δ–δ–

δ+

δ+

δ+

δ+

HO

HH O

Hhydrogen bond

Hydrogen bonding

δ–δ–

δ+

δ+

δ+

δ+

HO

HH O

Hhydrogen bond

Hydrogen bonding

a special kind of dipole–dipole

interaction...occurs between...

H-bond donor

H Xpolar bond

(X = O, N etc)

H-bond acceptor

Xlone pair on

electronegative atom

H-bonding

water’s abnormal properties

F.W. Starr/Wesleyan Univ.

it is responsible for water being so

wonderfully odd

HO

H

HO

HOH

HO H

H

OH

H

δ–

δ–

δ–

δ–

δ–

δ+

δ+

δ+δ+

δ+ δ+

δ+δ+δ+

δ+

H2O (MW=18): boiling point 100°CH2S .(MW=34): boiling point –60°CCH4 (MW=16): boiling point –162°C

three molecules of similar size and / or shape

HO

H

HO

HOH

HO H

H

OH

H

δ–

δ–

δ–

δ–

δ–

δ+

δ+

δ+δ+

δ+ δ+

δ+δ+δ+

δ+

H2O (MW=18): boiling point 100°CH2S .(MW=34): boiling point –60°CCH4 (MW=16): boiling point –162°C

...yet water has phenomenally high

boiling point...all due to H–bonding! (also

explains ice)

H3CO

H

H

OCH3

H

OH

CH3

OCH3

δ–δ–

δ–

δ–

δ+δ+

δ+δ+

Methanol

1 hydrogen bondboiling point 62°C

methanol only has one O–H bond so can only

form one H-bond so has much lower bp (less

attraction)

H-bondingcarboxylic acids

H3CO

O H

H O

OCH3

δ+

δ+

δ+δ– δ–

δ–δ–

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)

H3CO

O H

H O

OH

HHOH

Hδ+

δ+

δ+

δ–

δ– δ–

δ–

δ–

δ+

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)

Hydrogen bonding vital in biology...

protein secondary structure

NN

O

H R1

N

O

R2 HH

O H

H

ON

R3 H

H

ON

H R4

H

δ+ δ–

δ+δ–

Hydrogen bonding

β-sheetsand α-helix etc are all formed by H-bonding

between amides

©Benny Herudek 3D Hifi - High Fidelity 3d Graphics SolutionsDNA

withoutH-bonding no DNA double

helix!

Hydrogen bonding

NN

N

N

N HH

O

H NN

O

CH3

adenine thymine

Watson-Crick base pairing (or

whatever the biochemists call it)

all molecules can interact...

don’t need a dipole to interact because...

London (van der Waals or dispersion) forces

here we have two molecules...


δ+δ–

momentary dipole

...chance allows the electrons of one molecule

to bunch at one end causing an imbalance of

electrons...


δ+δ–

momentary dipole...a disturbance in the

force...or setting up a momentary dipole...


δ+δ–

momentary dipole

δ+δ–

induced dipole

...as electrons don’t like each other this new bunch repulse

electrons in a near by molecule and set up an induced

dipole...


δ+δ–

momentary dipole

δ+δ–

induced dipole

attraction

this causes dipole–dipole attraction (momentarily)...but it

will soon stop as the electrons are always on the move

Larger surface area

Bigger the force the larger the molecule,

the bigger the surface area and the more electrons

involved...

Larger surface area

Bigger the force

...this means a bigger momentary dipole can be

formed...

Larger surface area

Bigger the force and thus greater attraction

HC

HH

H CC

CC

CCH

H H

H H

H H

H H

H HH

H H

methaneCH4 (MW=16)

mp –182°C; bp –164°Cgas at rt

hexaneC6H14 (MW=86)

mp –95°C; bp 69°Cliquid at rt

Largersurface area

CC

CC

CCH

H H

H H

H H

H H

H H

H H

CC

CC

CC

HH

HH

HH

HH

HHC

HH

CC

CC

CCC

H H

H H

H H

H H

H HH

H H

H

H

H

H

eicosaneC20H42 (MW=282)

mp 36°C; bp 343°Csolid at rt Bigger

the force ...this goes

someway to explaining why bigger molecules have higher boiling point

(bp)...but other factorsalso involved

Largersurface area

Biggerthe force

pentanebp 36°C

2,2-dimethylpropanebp 9.5°C

and at last explains the difference between isomers

Gecko feet


...and it is van der Waals forces that are

responsible for Gecko’s ‘sticky’ feet!

solvation

interaction of molecules & solvent

solubleO

H

HO

H

OH

Hpropanol

water

propanol & water mix as they can interact

by H-bonds...

alcohol (OH) makes propanol

hydrophilic

means water loving in Latin

(I think)

insoluble

hexane

water

OHH

OHH

OH

HOH

Hno interaction

between molecules so hexane floats on

water

insoluble

hexane

water

OHH

OHH

OH

HOH

H

polar molecules (water) do not mix

with non-polar molecules (hexane)

non-polar grease makes hexane

hydrophobicmeans water

fearing (hating)

polardissolves

polar...conversely, non-polar

compounds dissolve other non-polar

compounds

hydrophilic

dissolveshydrophilicdoesn’t really

need explanation...

hydrophobic

dissolveshydrophobic

✔propanoic acid

water

OH

HO

H

OH

HOH

OH

H-bonding allows molecules to interact, thus mixing...hydrophobic ethyl chain too small to effect

interaction

✔hydrophilic

water

OH

HO

H

OH

HOH

OH

✔OH

HO

H

OH

HOH

OH

butanoic acid

wateraddition of one

more carbon to chain does not make much difference...butanoic acid still soluble in

water

✔/✘hexanoic acid

water

OH

HO

H

OH

HO

but a pentyl chain is pushing our luck...non-

polar chain starts to effect solubility and only a little

will dissolve

✘decanoic acid

water

OH

HO

H

OH

HO

get to a point where the blob of grease

controls the properties and overcomes the H-bond interactions...

✘hydrophobic

water

OH

HO

H

OH

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

✘sugar

hexane

OHO

HOOH

OH

OH

we can have the reverse...a very polar

molecule will not dissolve / mix with a

non-polar solvent

✘sugar

hexane

OHO

HOOH

OH

OH

...each OH group is polar and thus will not mix with

hexane...

✘sugar - hydrophilic

hexane - hydrophobic

OHO

HOOH

OH

OH

very little interactions between two molecules so

they do not mix

✔sugar

water

OHO

HOOH

OH

OH

...obvious I hope (sugar dissolves in

coffee!)...due to all the OH groups H-bonding

to water

✔sugar

water

OHO

OO

O

O

HO

H

H

HO

H

OH

H

H

H

HOH

OH

H

H

HO

H

OH

H

lots of hydrogen bonding

doesn’t this look good!

especially after all this chemistry

what have we learnt?

all about intermolecular

forces

Lecture5: 123.101

Economy & Finance

Transcript of Lecture5: 123.101