Inter Molecular Forces - Chemistry from Examville.com

8/9/2019 Inter Molecular Forces - Chemistry from Examville.com

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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


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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+


H Cl

+ H Cl

=...they are attracted

towards the most

electronegative atom

and the bond is saidto be polarised


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+


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


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


polarmolecules

dipole

moment...compounds witha dipole momentaresaid to be polar

molecules

M l l di l


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Molecular 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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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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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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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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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


36/106

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


37/106

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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H

O

H

Na

H

O

H

Cl+

--

+--

+

+

interaction of

an ion (chloride)


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H

O

H

Na

H

O

H

Cl+

--

+--

+

+

with a

compound with

a permanentdipole (water)


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H

O

H

Na

H

O

H

Cl+

--

+--

+

+

relatively

strong...hence

salt dissolves


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H

O

H

Na

H

O

H

Cl+

--

+--

+

+note that water

interacts with

both anion and

cation...really

helps


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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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OH3C

CH3 O CH3

CH3

+

+H3C C

H2

H2

C CH3H3C

CH2

H2C

CH3

propanone


butane


...but very different

boiling points...


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OH3C

CH3 O CH3

CH3

+

+H3C C

H2

H2

C CH3H3C

CH2

H2C

CH3

propanone


butane


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...



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+

momentarydipole

...chance allows the

electrons of one molecule

to bunch at one end

causing an imbalance of

electrons...



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+

momentarydipole...a disturbance in the

force...or setting up a

momentary dipole...



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+

momentarydipole

+

induceddipole

...as electrons dont like each

other this new bunch repulseelectrons in a near by molecule

and set up an induceddipole...



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+

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


...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


88/106h d hili

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


91/106h d h bi

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


100/106

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


102/106

g

hexane

OHO

HO

OH

OH

OH

...each OH group is polar

and thus will not mix with

hexane...

sugar - hydrophilic


103/106

g y p

hexane hydrophobic

OHO

HO

OH

OH

OH

sugar


104/106

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

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