lecture4: 123.101
DESCRIPTION
Lecture 4 of "chemistry and living systems" 123.101TRANSCRIPT
Unit One Parts 3 & 4:molecular bonding
Unit OneParts3&4
Bond strengthBond polarisationResonance Pages
34 & 46
Unit OneParts3&4
Bond strengthBond polarisationResonance Pages
34 & 46
...today we continue to make our simple
model more complex!
how strong are bonds?
...and we’re talking about covalent
bonds...the important ones for organic chemists
C C
C C
C C
C C
C C
C C> >
bond strength
836kJ mol–1
610kJ mol–1
347kJ mol–1 Pg
40
the values aren’t important...only the concept / pattern
C C
C C
C C
C C
C C
C C> >
bond strength
836kJ mol–1
610kJ mol–1
347kJ mol–1 Pg
40
obviously, it takes more energy to break
an alkyne apart...breaking
three bonds not one
C C
C C
C C
C C
C C
C C> >
bond strength
836kJ mol–1
610kJ mol–1
347kJ mol–1 Pg
40
the differences are getting smaller...nearly twice as
much energy needed to break 2 bonds but much less
needed to break the third
but...
C C C C
C C C C
bond strength
Pg40
a single σ bond is much stronger than a
single π bond (head-to-head results in better
overlap)
C C C C
C C C C
bond strength
Pg40
...this is the reason alkenes are functional groups but alkanes are
not!
what about bond lengths?
are bond length and bond strength
related?
C C
C C
C C
←120→
←134pm→
←154pm→
bond strength
Pg40
shorter the bond the stronger it normally is...
bond strength
C F
C Cl
C Br
←138→
←178pm→
←193pm→C Br
C F
C Cl
Pg40
shorter the bond the stronger it normally is...better overlap of
atoms / orbitals
how do we explain?
C C←134pm→
610 kJ mol–1C O←122pm→
736 kJ mol–1
similar size and bond lengths but big difference in
energy; why?
C O
77 pm 73 pm
similar size so good orbital
overlap...
bond polarisation
so far, our picture of bonds has said
electrons are shared evenly between two
atoms...
bond polarisation...as always, we teach you a simple model
and then say “reality is more complex!” So lets
take a step back...
is HCl covalent or ionic?
two kinds of bond...which one is
it?
H+ Cl–H Cl
H Cl H Cl
Polar covalent bond
Pg34
electrons shared evenly in a covalent
bond...or...
H+ Cl–H Cl
H Cl H Cl
Polar covalent bond
Pg34
one electron lost from H and given to Cl (an ionic bond)
H Cl
H Cl H ClH Clδ+ δ–
Polar covalent bond
Pg34
...or somewhere in the middle...
H Cl
H Cl H ClH Clδ+ δ–
Polar covalent bond
Pg34
a covalent bond but with the electrons
predominantly on one atom (ionic character)
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
electrons rarely shared evenly in a covalent bond...
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
...electrons will be closer to the more electronegative atom...given by the Pauli
scale above (bigger number more electronegative)
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
...difference in value indicates the
nature of the bond...
you do not needto learn these values!!
polarisation explains carbonyl bond strength (& reactivity)
C C
C Oδ+ δ–C Oδ+ δ–
so, carbonylstronger bond than alkene because it has ionic bond
character (electronicattraction between the
two atoms)
polarisation explains carbonyl bond strength (& reactivity)
C C
C Oδ+ δ–C Oδ+ δ–
but carbonyl also more reactive because the δ+ charge attracts
electrons
Helectrons move
Helectrons move
organic chemistry is all about the
movement electrons
HHreactions are the movement of electrons
H H H H
reactions are the movement of electrons
H H H H
HHreactions are the
movement of electrons...
polarisation explains reactivity
I HO
HO
Iδ+ δ–
HO
δ+ δ–
...so if we can predict where the electrons start and where they finish (want to go)...
polarisation explains reactivity
IHO
HO
HO
δ+δ–δ+
δ–I...then we can
predict reactions
H3C I CH3OH
H3C
O
OMeCH3
MgBr
δ+δ+
δ+ δ+δ+ δ–
δ–
δ–
δ–δ–
polarisation explains reactivity of molecules
δ– means more electrons or partial
negative charge
H3C I CH3OH
H3C
O
OMeCH3
MgBr
δ+δ+
δ+ δ+δ+ δ–
δ–
δ–
δ–δ–
polarisation explains reactivity of molecules
δ+ means lack of electrons or partial
positive charge
H3C I CH3OH
H3C
O
OMeCH3
MgBr
δ+δ+
δ+ δ+δ+ δ–
δ–
δ–
δ–δ–
polarisation explains reactivity of molecules
δ+ (or slightly positive) part of a molecule will be
attacked by...
H3C I CH3OH
H3C
O
OMeCH3
MgBr
δ+δ+
δ+ δ+δ+ δ–
δ–
δ–
δ–δ–
polarisation explains reactivity of molecules
...the negative part of a Grignard reagent...in fact we can explain
most chemical reactions by these δ–/+
charges
so far, so good...
i hope!
so let's apply what we know...
draw nitromethaneCH3NO2
C + 3H + O2+N
Pg45
here are the constituent
atoms...
CH
HH N
O
OC + 3H + O2+N
Pg45
this structure obeys the octet
rule
≡ H3C NO
OCH
HH N
O
OC + 3H + O2+N
Pg45
doesn’t look quite right...need
to sort out formal charges
fc = 6-4-½(4)=0
≡ H3C NO
OCH
HH N
O
OC + 3H + O2+N
Pg45
top oxygen has no formal
charge
≡ H3C NO
OCH
HH N
O
OC + 3H + O2+N
fc = 5-0-½(8)=+1
Pg45
nitrogen has a +1 charge
≡ H3C NO
OCH
HH N
O
OC + 3H + O2+N
fc = 6-6-½(2)=-1
Pg45
bottom oxygen has –1
charge
H3C NO
O
≡ H3C NO
OCH
HH N
O
OC + 3H + O2+N
116 pm
130 pmPg45
so structure is this?? (one N=O bond and one
N–O bond)
so in theory it is all very easy...
but reality is a little more complex...
H3C NO
O122 pm
Pg46
turns out both N–O bonds
are identical
H3C NO
O122 pm
Pg46
...they are somewhere in between a N–O bond
and a N=O bond...a structure called a...
resonance h y b r i d
H3C NO
O122 pm
Pg46
H3C NO
OH3C N
O
O
resonance structures
H3C NO
O≡
Pg46
the two extremes are
resonance structures...
H3C NO
OH3C N
O
O
resonance structures
H3C NO
O≡
Pg46
reality is a resonance
hybrid
H3C NO
OH3C N
O
O
resonance structures
H3C NO
O≡
Pg46we can convert
the extremes by pushing electrons
(not atoms)
H3C NO
OH3C N
O
O
resonance structures
H3C NO
O≡
Pg46
lets try and explainthe relationship between resonance structures and
resonance hybrids...
resonance structures
imagine you took one man...lets call him
Peter...as one of your resonance structures
resonance structures
...and one spider as the other resonance structure...and now you combine them...
resonance structures
the resulting cross is no longer either a man or a spider...
resonance structures
...and it certainly isn’t something that flicks back and forth between the two...no instead you have a
hybrid or...
resonance hybrid
© Marvel Comics
HC
CC
H
H
H
H
resonance structuresDO NOT EXISTbut are useful
HC
CC
H
H
H
H
HC
CC
H
H
H
H
resonance structuresDO NOT EXISTbut are useful
HC
CC
H
H
H
H
& easier to draw
HC
CC
H
H
H
H
resonance structuresDO NOT EXISTbut are useful
HC
CC
H
H
H
H
& easier to draw
they are Lewis structures so obey
octet rule so we can draw them...
resonance hybridE X I S T SLewis structure impossible
HC
CC
H
H
H
H
resonance hybridE X I S T SLewis structure impossible
HC
CC
H
H
H
H
...the Lewis structure no longer obeys
octet rule (how many electrons on central
C?)
resonance hybridE X I S T SLewis structure impossible
HC
CC
H
H
H
H
...the circle you draw in the centre of benzene is a resonance hybrid but the double bonds I draw make
its chemistry easier to predict...
only electrons move
only electrons move between resonance structures (and in
reactions)
all atoms are stationary...the atoms remain
stationary
curly arrow
this is used to represent the
movement of two electrons...
curly arrow
it is possibly the most important
‘scribble’ an organic chemist
ever learns...
curly arrow
with this you can bin most text books and just predict reactions instead of learning
them...
curly arrow
words cannot describe how
wonderful I think this little doodle
is!
how do we draw resonance structures?
so now we know what a resonance structure
is...we need to be able to spot them and draw
them...
1L e w i s structure
H3C CO
O
Pg46
...first part is relatively easy (or at
least covered in earlier material!)
2..pushable electrons‘ .’
N , Clone pairs
C Cπ bonds
Pg48
now we need to identify which electrons can be moved or pushed (for
some reason we always talk about pushing
‘curly arrows’
N , Clone pairs
C Cπ bonds
Pg50
the source of electrons
2..pushable electrons‘ .’
2..pushable electrons‘ .’
N , Clone pairs
C Cπ bonds
Pg48
lone pairs of electrons are often
‘pushable’
2..pushable electrons‘ .’
N , Clone pairs
C Cπ bonds
Pg48
as are double (or triple) bonds...
3receptors
positive charges C
electronegative atoms C O
atoms with ‘pushable’ electrons
C
Pg48
the next step is to find a target for the
electrons...somewhere they want to go...
3receptors
positive charges C
electronegative atoms C O
atoms with ‘pushable’ electrons
C
Pg50
where electrons are ‘happy’
3receptors
positive charges C
electronegative atoms C O
atoms with ‘pushable’ electrons
C
Pg48
all of the above will happily accept
electrons so are good receptors
3receptors
positive charges C
electronegative atoms C O
atoms with ‘pushable’ electrons
C
Pg48
this is only a receptor because it can also loss
electrons (remember we do not want more than eight 8 electrons around an atom)
3receptors
positive charges C
electronegative atoms C O
atoms with ‘pushable’ electrons
C
Pg48
NOTE: the donor and acceptor must be
one bond apart (no more no less)
4H3C C
O
OH3C C
O
OH3C C
O
O
resonance f o r m s Pg48
finally, move the electrons and form a
new, valid Lewis structure
4H3C C
O
OH3C C
O
OH3C C
O
O
resonance f o r m s Pg48
here are three resonance structures
for the molecule in steps one (the
ethanoate anion)
4H3C C
O
OH3C C
O
OH3C C
O
O
resonance f o r m s Pg48
remember the resonance hybrid will
be somewhere in between all these as shown on the next
slide...
H3C CO
Oor H3C C
O
O
–1/2
–1/2
delocalisation
C–O 130 pm
all bond lengths are the same...showing that the compound
never has a single C–O bond or a double C=O
bond
delocalisation
C–O 130 pmthe electrons are said
to be delocalised over the three atoms
(O–C–O)
H3C CO
Oor H3C C
O
O
–1/2
–1/2
delocalisation
C–O 130 pmelectrons are happy
when they are delocalised as they are
spread over a larger area...so are further
apart
H3C CO
Oor H3C C
O
O
–1/2
–1/2
H3CC
CCH3
N
H
Ph
H3CC
CCH3
N
H
Ph
examples...
Xwhy is this wrong?
...because it has 10 electrons in valence shell of C, which is
never allowed!
H3CC
CCH3
N
H
Ph
examples...the correct way involvespushing the lone pair of the
nitrogen anion down one bond to make a double C=N bond and then
pushing the electrons off thecarbon (so that it doesn’t have
10 valence electrons)and...
H3CC
CCH3
N
H
Ph
examples...
H3CC
CCH3
N
H
Ph
...moving them to the carbon at the end of the
double bond (we can’t move them two bonds) and
forming this new anion
≡H3C
CC
CH3
N
H
Ph δ–
δ–H3C
CC
CH3
N
H
Ph
examples...
H3CC
CCH3
N
H
Ph
the resonance hybrid shares (delocalises) the
electrons over two bonds or three atoms...
examples...
H3CC
CCH3
NPh
H HH3C
CC
CH3
NPh
H HX
we cannot move this double bond as there is
no electron acceptor (and we can’t have 5 bonds or 10 valence
electrons on C)
example...
H3C CO
O Hethanoic acid
H3C CO
CH3propanone
H3CH2C
O Hethanol
124pm129pm 146pm
122pm
the bonds in ethanoic acid are not what we would
predict compared to other simple molecules...(C=O
longer & C–O shorter)...why?
example...
H3C CO
OH3C C
O
O H H
δ–
δ+≡H3C C
O
O H
H3C CO
O Hethanoic acid
H3C CO
CH3propanone
H3CH2C
O Hethanol
124pm129pm 146pm
122pm
...reason is that lone pair of electrons are pushable and
the C=O is a good receptor...
example...
H3C CO
OH3C C
O
O H H
δ–
δ+≡H3C C
O
O H
H3C CO
O Hethanoic acid
H3C CO
CH3propanone
H3CH2C
O Hethanol
124pm129pm 146pm
122pm...which allows a new resonance structure that
can contribute to the resonance hybrid and gives
the C–OH bond double bond character so causes it
to shrink...
example...
H3C CO
O H
H3C CO
O H5 bonds!
H3C CO
O H2 molecules
H3C CO
O H2 molecules
X we cannot start from the lone pair on the carbonyl
example...
H3C CO
O H
H3C CO
O H5 bonds!
H3C CO
O H2 molecules
H3C CO
O H2 molecules
Xcarbon can never have 5
bonds (or 10 valence electrons)
example...
H3C CO
O H
H3C CO
O H5 bonds!
H3C CO
O H2 molecules
H3C CO
O H2 molecules
Xas soon as we split the
molecule in two we have performed a reaction and not looking at resonance
anymore.
NH
O
HONH2
CO2H HN NH2
NH
NH
O
HONH2
CO2H HN NH2
NH
example...
kytotorphinpain regulation
CC
CCC
CH
H
HH
H
HCC
CCC
CH
H
HH
H
HCC
CCC
CH
H
HH
H
H≡
example...
why is phenol acidic?
or why is phenol a separate functional group
and not an alcohol?
why is phenol acidic?
for a group to be acidic it must be able to give away
H+ (a proton)
CC
CCC
CO
H
HH
H
H
delocalisation......if phenol losses H+ then we are left
with O–...is this stable (will it readily form)?
CC
CCC
CO
H
HH
H
HCC
CCC
CO
H
HH
H
H
delocalisation......we can move the lone pair to form C=O as we
can push the electrons of C=C...we have spread the electrons over three atoms
so they are happy...
CC
CCC
CO
H
HH
H
HCC
CCC
CO
H
HH
H
HCC
CCC
CO
H
HH
H
HCC
CCC
CO
H
HH
H
H
delocalisation...
turns out we can form many other resonance structures so the
electrons are delocalised over 7 atoms...they are really jolly. So
anion stable so loss of H+ easy so phenol is acidic
CC
CCC
CO
H
HH
H
H
δ–
δ–δ–
δ–
H3C CH3
H3C CH3CH3
CH3 CH3
CH3 CH3
H3C
H3C CH3
H3C CH3CH3
CH3 CH3
CH3 CH3
H3C
conjugationany double bonds separated by a one
single bond can delocalise
H3C CH3
H3C CH3CH3
CH3 CH3
CH3 CH3
H3C
H3C CH3
H3C CH3CH3
CH3 CH3
CH3 CH3
H3C
conjugationsuch double bonds
are said to be conjugated
H3C CH3
H3C CH3CH3
CH3 CH3
CH3 CH3
H3C
H3C CH3
H3C CH3CH3
CH3 CH3
CH3 CH3
H3C
conjugationconjugation
leads to coloured compounds...this is carotene from (you guessed it)
carrots
NN
OSO2
HO
SO2O
Na
NN
OSO2
HO
SO2O
Na
NN
OSO2
HO
SO2O
Na
NN
OSO2
HO
SO2O
Na
Cl food green 4 E142
NN
OSO2
HO
SO2O
Na
NN
OSO2
HO
SO2O
Na
NN
OSO2
HO
SO2O
Na
NN
OSO2
HO
SO2O
Na
Cl food green 4 E142
hopefully, you can see that if we have alternating double and single bonds
we can form multiple resonance structures or delocalise the electrons
what have....we learnt?
•e l e c t r o n s where they are
•b o n d s& their strength
•resonance Image created by Cary Sandvig of SGI
©the bbp@flickr
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