molecular bonding part ii - organic chemistry from examville.com

8/9/2019 Molecular Bonding Part II - Organic Chemistry from Examville.com

1/101

Unit One Parts 3 & 4:molecular bonding


2/101

Unit One

Parts3&4

BondstrengthBond polarisation

ResonancePages

34 & 46


3/101

Unit One

Parts3&4

BondstrengthBond polarisation

ResonancePages

34 & 46

...today we continue

to make our simple

model more

complex!


4/101

how strong arebonds?

...and were talking

about covalent

bonds...the

important ones for

organic chemists


5/101


6/101

C C

C C

C C

C C

C C

C C

bondstrength

836kJ mol1

610kJ mol1

347kJ mol1

Pg

40

obviously, it takes

more energy to break

an alkyne

apart...breaking

three bonds not one


7/101

C C

C C

C C

C C

C C

C C

bondstrength

836kJ mol1

610kJ mol1

347kJ mol1

Pg

40

the differences are gettingsmaller...nearly twice as

much energy needed to break

2 bonds but much less

needed to break the third


8/101

but...


9/101

C C C C

C C C C

bondstrength

a single bond is

much stronger than a

single bond (head-to-

head results in better

overlap)


10/101

C C C C

C C C C

bondstrength

...this is the reason

alkenes are functional

groups but alkanes are

not!


11/101

what about bondlengths?

are bond length

and bond strength

related?


12/101

C C

C C

C C

120

134pm

154pm

bondstrength

Pg

40

shorter the bond

the stronger it

normally is...

P


13/101

bondstrength

C F

C Cl

C Br

138

178pm

193pmC Br

C F

C Cl

Pg40

shorter the bond the

stronger it normally

is...better overlap of

atoms / orbitals


14/101

how dowe explain?

C C134pm

610 kJ mol1C O122pm

736 kJ mol1

similar size and

bond lengths but

big difference in

energy; why?


15/101

C O

77 pm 73 pm

similar size so

good orbital

overlap...


16/101

bond polarisation

so far, our picture of

bonds has said

electrons are shared

evenly between two

atoms...


17/101

bond polarisation

...as always, we teach

you a simple model

and then say reality is

more complex! So letstake a step back...


18/101

is HCl covalent orionic?

two kinds of

bond...which one is

it?


19/101

ClH Cl

H Cl H Cl

Polarcovalent bond

Pg

34

electrons shared

evenly in a covalent

bond...or...


20/101

ClH Cl

H Cl H Cl

Polarcovalent bond

Pg

34

one electron lost

from H and given to

Cl (an ionic bond)


21/101

H Cl

H Cl H ClH Cl+

Polarcovalent bond

Pg

34

...or somewhere in

the middle...


22/101

H Cl

H Cl H ClH Cl+

Polarcovalent bond

Pg

34

a covalent bond but

with the electrons

predominantly on one

atom (ionic character)


23/101

Bond TypeEN

differenceExamples Calculation

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

polar covalent 0.5 1.7CH3OH

HCl

3.5(O) - 2.1(H) = 1.4

3.0(Cl) - 2.1(H) = 0.9

covalent 0 0.4CH3H

HH

2.5(C) - 2.1(H) = 0.4

2.1(H) - 2.1(H) = 0.0

electrons rarely

shared evenly in a

covalent bond...


24/101

Bond TypeEN


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


HCl

3.5(O) - 2.1(H) = 1.4

3.0(Cl) - 2.1(H) = 0.9

covalent 0 0.4CH3H

HH

2.5(C) - 2.1(H) = 0.4

2.1(H) - 2.1(H) = 0.0

...electrons will be closer to

the more electronegative

atom...given by the Pauli

scale above (bigger number

more electronegative)


25/101

Bond TypeEN


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


HCl

3.5(O) - 2.1(H) = 1.4

3.0(Cl) - 2.1(H) = 0.9

covalent 0 0.4CH3H

HH

2.5(C) - 2.1(H) = 0.4

2.1(H) - 2.1(H) = 0.0

...difference in

value indicates the

nature of the

bond...


26/101

you do not need

to learn these values!! l i i


27/101

H3C ICH3O

H

H3C

O

OMeCH3

MgBr

+ +

+ ++

polarisationexplains reactivity

of molecules

means more

electrons or partial

negative charge

l i i


28/101

H3C ICH3O

H

H3C

O

OMeCH3

MgBr

+ +

+ ++


of molecules

+ means lack of

electrons or partial

positive charge

l i ti


29/101

H3C ICH3O

H

H3C

O

OMeCH3

MgBr

+ +

+ ++


of molecules

+ (or slightly

positive) part of a

molecule will be

attacked by...

l i ti


30/101

H3C ICH3O

H

H3C

O

OMeCH3

MgBr

+ +

+ ++


of molecules...the negative part of aGrignard reagent...infact we can explain

most chemical

reactions by these /+

charges

l i ti


31/101

polarisationexplains carbonyl

bond strength (& reactivity)

C C

C O+

C O+

so, carbonyl

stronger bond than alkene

because it has ionic bond

character (electronic

attraction between thetwo atoms)

l i ti


32/101

polarisationexplains carbonyl

bond strength (& reactivity)

C C

C O+

C O+

but carbonyl alsomore reactive because

the + charge attracts

electrons


33/101

so far, so good...

i hope!


34/101

so let's applywhatwe know...


35/101

draw nitromethaneCH3NO2


36/101

C + 3H + O2+N

Pg

45

here are the

constituent

atoms...


37/101

C

H

H

H N

O

O

C + 3H + O2+N

Pg

45

this structure

obeys the octet

rule


38/101

H3C NO

O

C

H

H

H N

O

O

C + 3H + O2+N

Pg

45

doesnt look

quite right...needto sort out

formal charges


39/101

fc = 6-4-(4)=0

H3C NO

O

C

H

H

H N

O

O

C + 3H + O2+N

Pg

45

top oxygenhas no formal

charge


40/101

H3C NO

O

C

H

H

H N

O

O

C + 3H + O2+N

fc = 5-0-(8)=+1

Pg

45

nitrogen has a+1 charge


41/101

H3C NO

O

C

H

H

H N

O

O

C + 3H + O2+N

fc = 6-6-(2)=-1

Pg

45

bottomoxygen has 1

charge


42/101

H3

C N

O

O

H3C NO

O

C

H

H

H N

O

O

C + 3H + O2+N

116 pm

130 pm

Pg

45

so structure isthis?? (one N=O

bond and one

NO bond)


43/101

so in theoryit is allvery easy...


44/101

but realityis a littlemore complex...

O


45/101

H3C N

O

O122 pm

turns out bothNO bonds

are identical

O


46/101

H3C N

O

O122 pm

...they are somewhere

in between a NO bond

and a N=O bond...a

structure called a...

O


47/101

resonanceh y b r i d

H3C N

O

O122 pm

the two


48/101

H3C N

O

O

H3C N

O

O

resonancestructures

H3C N

O

O

the two

extremes are

resonance

structures...

lit i


49/101

H3C N

O

O

H3C N

O

O

resonancestructures

H3C N

O

O

reality is a

resonance

hybrid


50/101

H3C N

O

O

H3C N

O

O

resonancestructures

H3C N

O

O

we can convert

the extremes by

pushing electrons

(not atoms)


51/101

H3C N

O

O

H3C N

O

O

resonancestructures

H3C N

O

O

lets try and explain

the relationship between

resonance structures and

resonance hybrids...


52/101

resonance

structures

imagine you took one

man...lets call him

Peter...as one of yourresonance structures


53/101

resonance

structures

...and one spider asthe other resonance

structure...and now

you combine them...


54/101

resonance

structures

the resulting cross is

no longer either a

man or a spider...


55/101

resonance

structures

...and it certainly

isnt something that

flicks back and forth

between the two...no

instead you have a

hybrid or...


56/101

resonance

hybrid Marvel Comics


57/101

HC

CC

H

H

H

H

resonance structures

DO NOT EXIST

but are useful

HC

CC

H

H

H

H


58/101

HC

CC

H

H

H

H


DO NOT EXIST

but are useful

HC

CC

H

H

H

H

&easier

to draw


59/101

HC

CC

H

H

H

H


DO NOT EXIST

but 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 hybrid


60/101

resonance hybrid

E X I S T SLewis structure impossible

HC

CC

H

H

H

H

resonance hybrid


61/101

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 hybrid


62/101

resonance hybridE X I S T SLewis structure impossible

HC

CC

H

H

H

H

...the circle you draw inthe centre of benzene is a

resonance hybrid but the

double bonds I draw make

its chemistry easier to

predict...


63/101

only electronsmoveonly electrons move

between resonance

structures (and in

reactions)


64/101

all atoms arestationary...the atoms remain

stationary

this is used to

t th


65/101

curlyarrow

represent themovement of two

electrons...


66/101

curlyarrow

it is possibly the

most important

scribble an

organic chemistever learns...

with this you can bin

most text books and


67/101

curlyarrow

most text books and

just predict reactions

instead of learning

them...


68/101

curlyarrow

words cannot

describe how

wonderful I think

this little doodleis!


69/101

how do we drawresonance structures?

so now we know what a

resonance structure

is...we need to be able to

spot them and draw

them...

...first part is

relatively easy (or at


70/101

1L e w i sstructureH3C C

O

O

Pg

46

relatively easy (or at

least covered in

earlier material!)


71/101

2..pushableelectrons

N , Clone pairs

C C bonds

Pg

48

now we need to identify

which electrons can be

moved or pushed (for

some reason we always

talk about pushing

curly arrows

lone pairs ofelectrons are often

h bl


72/101


N , Clone pairs

C C bonds

Pg

48

pushable


73/101


N , Clone pairs

C C bonds

Pg

48

as are double (or

triple) bonds...

positive charges C


74/101

receptors

positive charges C

electronegativeatoms C O

atoms withpushable electrons

C

Pg

48

the next step is to find

a target for the

electrons...somewhere

they want to go...

positive charges C


75/101

receptors

positive charges C



C

Pg

48

all of the above will

happily accept

electrons so are good

receptors

positive charges C


76/101

receptors

positive charges C



C

Pg

48

this is only a receptor

because it can also loss

electrons (remember we do

not want more than eight 8

electrons around an atom)

positive charges C


77/101

receptors

positive charges C



C

Pg

48

NOTE: the donor and

acceptor must be

one bond apart (no

more no less)


78/101

4

H3C C

O

O

H3C C

O

O

H3C C

O

O

resonance

f o r m s

Pg

48

finally, move the

electrons and form a

new, valid Lewis

structure


79/101

4

H3C C

O

O

H3C C

O

O

H3C C

O

O

resonance

f o r m s

Pg

48

here are threeresonance structures

for the molecule in

steps one (the

ethanoate anion)


80/101

4

H3C C

O

O

H3C C

O

O

H3C C

O

O

resonance

f o r m s

Pg

48

remember theresonance hybrid will

be somewhere in

between all these as

shown on the next

slide...

delocalisation


81/101

delocalisation

H3C C

O

O

or H3C C

O

O

1/2

1/2

CO130 pm

all bond lengths are

the same...showing

that the compoundnever has a single CO

bond or a double C=O

bond

delocalisation


82/101

delocalisation

H3C C

O

O

or H3C C

O

O

1/2

1/2

CO130 pm

the electrons are said

to be delocalised

over the three atoms

(OCO)

delocalisation


83/101

delocalisation

H3C C

O

O

or H3C C

O

O

1/2

1/2

CO130 pm

electrons are happywhen they are

delocalised as they are

spread over a larger

area...so are further

apart

examples


84/101

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!

examples

the correct way involves


85/101

H3CC

CCH3

N

H

Ph

examples...pushing the lone pair of thenitrogen anion down one bond to

make a double C=N bond and then

pushing the electrons off the

carbon (so that it doesnt have10 valence electrons)

and...

examples


86/101

H3CC

CCH3

N

H

Ph

examples...

H3CC

CCH3

N

H

Ph

...moving them to the

carbon at the end of the

double bond (we cant move

them two bonds) and

forming this new anion

examples


87/101

H3C

CC

CH3

N

H

Ph

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


88/101

examples...

H3CC

CCH3

NPh

H H

H3CC

CCH3

NPh

H HX

we cannot move this

double bond as there is

no electron acceptor(and we cant have 5

bonds or 10 valence

electrons on C)

example


89/101

example...

H3C C

O

O Hethanoic acid

H3C C

O

CH3propanone

H3C

H2C

O H

ethanol

124pm

129pm 146pm

122pm

the bonds in ethanoic acid

are not what we would

predict compared to other

simple molecules...(C=O

longer & CO shorter)...why?

example


90/101

example...

H3C C

O

O

H3C C

O

O H H

+H3C C OO H

H3C C

O

O Hethanoic acid

H3C C

O

CH3propanone

H3C

H2C

O H

ethanol

124pm

129pm 146pm

122pm

...reason is that lone pair of

electrons are pushable and

the C=O is a good

receptor...


91/101

example


92/101

C

CC

C

CC

H

H

H

H

H

HC

CC

C

CC

H

H

H

H

H

HC

CC

C

CC

H

H

H

H

H

H

example...


93/101

why is phenol

acidic? or why is phenol aseparate functional group

and not an alcohol?


94/101

why is phenol

acidic? for a group to be acidic itmust be able to give away

H+ (a proton)

delocalisation


95/101

C

CCC

CC

O

H

H

H

H

H

delocalisation......if phenol losses

H+ then we are leftwith O...is this

stable (will it

readily form)?

delocalisation


96/101

C

CCC

CC

O

H

H

H

H

H

C

CCC

CC

O

H

H

H

H

H

delocalisation......we can move the lone

pair to form C=O as wecan push the electrons of

C=C...we have spread the

electrons over three atoms

so they are happy...

delocalisation


97/101

C

CC

C

CC

O

H

H

H

H

HC

CCC

CC

O

H

H

H

H

HC

CCC

CC

O

H

H

H

H

H

C

CCC

CC

O

H

H

H

H

H

delocalisation...

turns out we can form many other

resonance structures so the

electrons are delocalised over 7

atoms...they are really jolly. Soanion stable so loss of H+ easy so

phenol is acidic

C

CC

C

CC

O

H

H

H

H

H


98/101

H3C CH3

H3C CH3

CH3

CH3 CH3

CH3 CH3

H3C

H3C CH3

H3C CH3CH3

CH3 CH3

CH3 CH3

H3C

conjugation

any double bondsseparated by a one

single bond can

delocalise


99/101


100/101

H3C CH3

H3C CH3

CH3

CH3 CH3

CH3 CH3

H3C

H3C CH3

H3C CH3CH3

CH3 CH3

CH3 CH3

H3C

conjugation

conjugation

leads to colouredcompounds...this

is carotene from

(you guessed it)

carrots

what havelearnt?


101/101

....welearnt?

e l e c t r o n swhere they are

b o n d s

molecular bonding part ii - organic chemistry from examville.com

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