matriculation chemistry aromatic compound

8/12/2019 Matriculation Chemistry Aromatic Compound

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The origin of August Kekulé’s view of benzene structure:

“There I sat and wrote mytetbook! but things did notgo well" my mind wasoccu#iedwith other matters! I turnedthe

chair towards the fire#laceandbegan to doze$



“%nce again the atoms dancedbefore my eyes$ This timesmaller grou#s modestly

remained in the background$&y mental eye! shar#ened byre#eated a##aritions of similar kind! now distinguished larger

units of various sha#es$'

“(ong rows! fre)uently *oined more densely" everything inmotion! twisting and turninglike snakes$ And behold! whatwas that+ %ne of the snakescaught hold of its own tail andmockingly whirled around my

eyes$



“, awoke! as if by lightening"

this time! too! I s#ent the rest ofthe night working out theconse)uences of thishy#othesis'



“(et us learn to dream! gentlemen! then #erha#s we shall find the truth'

A-.-/T K0K-(1



0dison! 0instein and many others have used thesubconscious mind to give them the insight and the

“know2how' to bring about their great achievements$

August Kekulé



“In variably! my device works as I imagined it should$

In twenty years there has not been

single ece#tion$' 3IK%(A T0/(A



“Imagination is more im#ortant than knowledge$'

A(405T 0I3/T0I3



A5%&ATI6 6%&7%-38/

Introduction toaromatic 6om#ounds

,9$,

Arenes aromatic com#ounds:

the word aromatic has nothing to do with odour $



Kekulé structure

resonance structure

aromatic

ortho meta #ara

;riedel26raft acylationelectro#hilic substitution

activating anddeactivating grou#

ortho2#ara and meta director

oidation of

alkylbenzene

substitution of

toluene

K0<=%58/



,>?@ &ichael ;araday4ritishB

Isolated a #ure com#ound of

boiling #oint >Co6

0m#irical formula 6D

3amed “bicarburet of hydrogen'

8I/6%E05< %; 403F030



,>G9 0ilhard &itscherlich .ermanB

7re#ared benzene from benzoic acid6HD@6%?D 6a% 6HDH 6a6%G

heat

&olar mass J>! molecular formula 6HDH

named “benzin'

benzene benzin benzoin

ben*oin;renchB

gum benzoin (uban awiArabicB



,>HH ;riedrich August Kekulé .ermanB

7ro#osed a cyclic structure for benzene$



A5%&ATI6 6%&7%-38/

In earlier time! com#ounds are called aromatic because of their #leasant odours$

Today! we use the word aromatic to refer to

benzene and its structural relatives$

4enzene has strong

#leasant odour $



6

66

66

6

D

D

D

D

D

D

or

K0K-(1’/ /T5-6T-50

Kekule was the first to formulate areasonable re#resentation of benzene

The Kekule structure suggests alternating doubleand single carbon2carbon bonds



The si π electrons com#letely delocalized around the ring



All si 6 atoms and si p orbitals are e)uivalent

The circle re#resents the siπ electrons! distributed over

the si atoms of the ringN



TD0 65IT05IA ;%5 A5%&ATI6IT<

9 structural criteria must be satisfied forcom#ound to be aromatic

6 o m # l e t e l y

c o n * u g a t e d

6 y c l i c 7 l a

nar

6 o n t a i n # a r t i c u l a r n u m b e r o f ∏ e l e c t r o n o b e y s D Ü c k e l ’ s 5 u l e




%-/% -35 5-6789

:en;ene

,$ Aromatic com#ounds with a single ring

[4n+2] ∏ = [4(1 + 2]∏

= # ∏ electrons

aromatic

:en;ene is aromatic !ecause<

contains #∏ electrons

(obeys HÜc'els ule

cyclic, planar and has dou!le

!ond in the ring




?$ Aromatic com#ounds with more than one ring

na#hthalene

4n+2= 4(2+ 2 10 ∏ electrons

romatic

3"o !en;ene rings joined together $orms naphthalene

%-/%



A5%&ATI6 6%&7%-38/

3omenclature of benzene and its derivatives,9$?





3A&I3. 403F030 805IEATIE0/

&any organic molecules contain a benzene ring with one or more substituents$

&any common name arerecognized by the I-7A6system

6DG

6ommon: tolueneI-7A6: methylbenzene

0OA&7(0:



4enzene is the #arent name and the substituent

is indicated by a #refi$

;

fluorobenzene

6l

chlorobenzene

4r

bromobenzene

3%?

nitrobenzene

6D?6DG

ethylbenzene

&%3% /-4/TIT-T08 403F030



I-7A6 rules allow some common names to be retained$

6DG

toluene

%D

#henol

3D?

aniline

6%%D

benzoic acid

6D%

benzaldehyde



5elative #osition of subsituents are indicated by #refies ortho! meta! and para o L! m L! and p LB

or by the use of number $

4r

4r , ?

,!?Ldibromobenzeneor

o Ldibromobenzene

4r

4r

,?

G

,!GLdibromobenzeneor

m Ldibromobenzene

4r

4r

,!9Ldibromobenzeneor

p Ldibromobenzene

,?

G9

8I/-4/TIT-T08 403F030

3"o 9ame 9u!stituents



6%%D3%?

,?

?Lnitrobenzoic acidor

o Lnitrobenzoic acid

,?

G

GLnitrobenzoic acidor

m Lnitrobenzoic acid

6%%D

3%?

9Lnitrobenzoic acidor

p Lnitrobenzoic acid

, ?

G9

6%%D

3%?

/elect one of the substituent that givenew #arent name and numbered as 6,$

8I/-4/TIT-T08 403F030

3"o 8i$$erent 9u!stituents



7osition of substituents must be indicated bynumbers$

4r

4r

4r

, ?

G9

,!?!9Ltribromobenzene

The substituents are listed al#habetically when writing the name$

6l

4r

I?LbromoL,LchloroLGLiodobenzene

TD500 %5 &%50 /-4/TIT-03T/



6 atom bearing the subtituent that define thenew #arent name is numbered as 6,$

%D

3%?

3%?

?!9Ldinitro#henol

D% %D

6%%D,

?

G

9

,?

G

9

@

H

G!@Ldihydroybenzoic acid

4r

6DG

6DG

,

?G9

9LbromoL,!?Ldimethylbenzene



4r

6DG

6DG

,

?G

9

9LbromoL,!?Ldimethylbenzene

o L! m L and p L naming system is usedfor arenes with ? substituents onlyN

9LbromoLo Ldimethylbenzene

correct

14 2-11



If alkyl substituent is larger than the ringmore than H 6B! the com#ound is named as

#henyl2substituted alkane$

6DL6D? L6D? L6D? L6D? L6DG

6D?,

?G 9 @ H J

?L#henylhe#tane

14.2 11

7D03<( .5%-7

4enzene ring as substituent$

7henyl 6HD@ L 7h



A5%&ATI6 6%&7%-38/

6hemical #ro#erties of benzeneand its derivatives

,9$G



14.3-01



403F030

4r ? S 66l9 no addition of 4r ?no decolorizationB

K&n%9 S D?% no oidation

no decolorizationBD? S 3islow addition at hightem#erature and #ressure

-3-/-A( 50A6TI%3/ %; 403F03014.3 01

Alkenes readily undergo additionreaction! benzene does notN

14 3-03



Involves substituents attached to the ring

0lectro#hilic aromatic substitution

4r ?;e4r G

4r

benzene bromobenzene

Involves the benzene ring itself

6DG6D?6D?6DG K&n%9

D?%

6%%D

butylbenzene benzoic acid

50A6TI%3 %; A5030/ 14.3 03



14.3-05



OO?! ;eOG

O 6l 4rB DOhalogenatio

n

D%3%?

D?/%9

3%?

D?%nitratio

n

56l ! Al6lG

5 can rearrangeB

5 D6l

;riedel26raftsAlkylation

566l ! Al6lG

%

6L5%

D6l

;riedel26raftsAcylation

14.3 05

14.3-07



/T07 , ;ormation of arenium ion

D

D

D

D

D

D 0

D

D

D

D

0D

D

D D

D

D

0D

D

D

D

D

D

D

0D

.0305A( &06DA3I/&14.3 07

arenium ion

14.3-08



0

0lectro#hile takes two electrons of siLelectronsπ system to form σ bond$

benzene ring

This interru#ts of cyclic system of π electrons$

4enzene ring arenium ion aromaticB nonaromaticB

The four π electrons delocalized through these

the five 6 atom p orbitalsB

0

D

arenium ion

A503I-& I%314.3 08

14.3-09



/T07 ? (oss of D

D

D

D

D

D

0D

D

D

D

D

0

D

0D

0

H π electrons 9 π electrons

0 L D

H π electrons

/ubstitution reaction allowaromatic si π electrons to

be regenerated$

14.3 09

14.3-10



Kekulé structures are more a##ro#riate for writing

mechanisms$ ;or sim#licity! however! we can show the mechanism

in the following way:

/T07 ,

0

0

D

arenium ion

/T07 ? 0

D

0

D

14.3-11



4r

4r ?no reactiondecolorization not observedB

4r ?;e4r G

bromobenzene

D4r

DA(%.03ATI%3

5eactants: benzene and halogens 6l? or 4r ?B$

6onditions:

(ewis acid such as ;e6lG and ;e4r G

14.2-13



/T07 , ;ormation of 4r

4r L4r ;e4r G 4r R4rL;e4r G L

4r ;e4r 9 L

com#le

4r

/T07 ? ;ormation of arenium ion

/T07 G (oss of D

4r

D4r

&06DA3I/&

;e4r 9 L

;e4r G

4r D

4r

D



12.5-49



/T07 , ;ormation of nitronium ion 3%?B

D?% 3%?

&06DA3I/&

D

. .

. .

D2%23%? D2%/%GD D2%23%? D/%9 L..

nitronium ion



3%?


/T07 G (oss of D

3%?

D?/%9

D/%9 L

3%?

D

3%?

D



12.5-52



6D6DGB?


/T07 G (oss of D

6D6DG

B?

D6l

Al6l9 L

6D6DGB?

D

6D6DGB?

D

%TD05 ;A6T/ A4%-T 12.5-55



5earrangement can occur! es#ecially when

,o haloalkanes are used$

0OA&7(0:

6DG6D?6D?6D?6l

6D?6D?6D?6DG

butylbenzene

6D6D?6DG

6DG

sec Lbutylbenzene

H@

G@

5earrangement:

6DGR6D?R6DR6D?

D

,o carbocation

6DGR6D?R6DR6DG

?o carbocation

Al6lG

%TD05 ;A6T/ A4%-T;5I080(L65A;T A(K<(ATI%3

;5I080( 65A;T A6<(ATI%312.5-56



0OA&7(0:

6DG6R6l%

acetyl chloride

Al6lG66DG

%

aceto#henone

D6l

;5I080(L65A;T A6<(ATI%3

5eactants: benzene and acid chloride$

6onditions:

6atalyst: (ewis acid such as Al6lG$

7roduct: ketone$

A6<( .5%-712.5-57



0OA&7(0:

6DG6R

%

acyl grou#

benzoyl grou#

L6R

%

56R%

acetyl grou#

A6<( .5%-7

Acylation transfer anacyl grou# to benzene

&06DA3I/&12.5-59



/T07 , ;ormation of acylium ion

acylium ion

5L6L6l Al6lG 5L6L6lLAl6lG

:%: :%:

5L6 P % 5L6Q% Al6l9 L

&06DA3I/&

:

:



5L6 P%


/T07 G (oss of D

D6l6P%

5

6%

D

5

6%

D

5 Al6l9 L

/-4/TIT-03T 0;;06T12.5-61



4ut! what would ha##en if we were to carry out a reactionon aromatic ring that already has a substituent+

/-4/TIT-03T 0;;06T

0;;06T %3 50A6TIEIT<12.5-62



Activating grou#s:

/ubstituents that activate the ring!making it more reactive than benzene$

3%? 6l D %D

relative rateof nitration

0OA&7(0:

H ,C L> C$CGG , ,CCC

reactivity

0;;06T %3 50A6TIEIT<

8eactivating grou#s:

/ubstituents that deactivate the ring!making it less reactive than benzene$

0;;06T %3 %5I03TATI%312.5-63



0OA&7(0:

6DG

D3%G

D?/%9

6DG

3%?

6DG

3%?

6DG

3%?

@MB GJ

9

ortho2#ara director

%rthoL#ara directors

0;;06T %3 %5I03TATI%3

&eta directors

Tend to direct the incoming grou# intoortho and #ara #ositions$

Tend to direct the incoming grou# intometa #osition$

6(A//I;I6ATI%3 %; /-4/TIT-03T/12.5-64



6(A//I;I6ATI%3 %; /-4/TIT-03T/

ortho L ! para L directingactivators

ortho L ! para L directingdeactivators

meta L directing deactivators

%5TD%L ! 7A5AL 12.5-65



!8I506TI3. A6TIEAT%5/

R3D?

●●

R5

R3D6%5●●

R%5●●

●●

R%D

●●

●●

R3D5●●

R35?

●●

Increasing activation

.eneralstructure:R5 or RF

●●

Alkyl grou#s or havenonbonded electron #air on the atom bonded to

benzene ring

12.5-66



0OA&7(0:

6D?6DG

4r ?

;e4r G

6D?6DG

4r

6D?6DG

4r G>B

ethylbenzene H?B

%5TD%L ! 7A5AL 12.5-67



!8I506TI3. 80A6TIEAT%5/

R;●●

●●

●●

R6l●●

●●

●●

R4r ●●

●●

●●

R I●●

●●

●●

.eneral structure

RO halogensB

●●

●●

●●

12.5-68



0OA&7(0:

6l

D3%G

D?/%9

6l

3%?

6l

3%?

G@Bchlorobenzene

H9B

&0TA 8I506TI3. A6TIEAT%5/12.5-69



&0TAL 8I506TI3. A6TIEAT%5/

R6D%

Increasing deactivation

.eneralstructure:R< or LB

R6%5

R6%%5

R6%%DR63

R/%GD

R3%?

R35G

Dave a full or #artial#ositive charge on the

atom bonded tobenzene ring

0OA&7(0

12.5-69



0OA&7(0:

3%?

D3%G

D?/%9

nitrobenzene

3%?

3%?MGB

I38-6TIE0 0;;06T12.5-70



8ue to:

I38-6TIE0 0;;06T

0lectronegativity of the atoms in thesubstituent$

7olarisability of the substituent$

0OA&7(0:

6l

6lR electronLwithdrawing

6DG

6DGR electronLdonating

12.5-71



Activating .rou#s

5elease electrons to the ring /tabilise arenium ion

;orm faster

6DG

8eactivating .rou#s

=ithdraw electrons from the ring

8estabilise arenium ion

;orm slower

3%?

6l

3%?

0OA&7(0:12.5-72



6;G

trifluoromethylBbenzene benzene

6DG

toluene increasing rate of nitration

6;G withdraws e2! arenium ion less stable

ring less reactive

6DG releases e2! arenium ion more stable

ring more reactive

6;G

3%?

3%?6DG

3%?



0OA&7(0: 12.5-76



6DG

toluene

K&n%9

heat

6%%D

6DG%?3

p Lnitrotoluene

3a?6r ?%J

heat6%%D%?3

p Lnitrobenzoic acid

6D6DGB?6DG 6%%DD%%6

K&n%9

heat

benzoic acid

iso#ro#yl toluene tere#ththalic acid



DA(%.03ATI%3 %; T%(-03012.5-78



Take #lace at high tem#erature or in the#resence of uv light$

&echanism: freeLradical substitution

6l or 4r re#laces D atom of alkyl grou#

6DG

6l?

heat orlight

6D?6l

0OA&7(0:

6l?

heat orlight

6D6l?6l?

heat orlight

66lG

toluene benzyl chloride

dichloromethylBbenzene

trichloromethylBbenzene

DA(%.03ATI%3 %; T%(-030

ree radical su!stitution reaction

DA(%.03ATI%3 %; T%(-030



DA(%.03ATI%3 %; T%(-030

%lectrophilic aromatic su!stitution reaction

6DG

toluene

4r ?

;e4r G

6DG

4r ?

6DG

4r ?

6D?● 12.5-79



4enzylic radicals are even more stable than Go radicalsN

benzylicradical

6A56I3%.03I6 0;;06T 12.5-34



&any aromatic com#ounds are carcinorgenic and toic$

6DG

toluenebenzene

6A56I3%.03I6 0;;06T

0am#le: benzene! benzoUaV#yrene$

12.5-35



At one time! benzene was widely used as solvent$

/tudies revealed benzene is carcinorgenic can cause cancer B$

5e#laced by toluene$

12.5-36



4enzoUaV#yrene is found in cigarette smoke! automobile ehaust! and the fumes from charcoal grills$

benzoUaV#yrene

=hen ingested or inhaled! it oidised to carcinogenic #roducts$

12.5-21



12.5-20



4enzoic acid! the sim#lest organic acid! #revent the

growth of many organism$

12.5-22



widely used as a food #reservative$

12.5-19



;resh wild berries



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