(18) session 18 arenes (aromatic hydrocarbons) (1)

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Session 18

Organic Chemistry, UNAM School of Medicine1

Hydrocarbons:Hydrocarbons:Hydrocarbons:Hydrocarbons:

Arenes (Aromatic Hydrocarbons)

Dr L.H.A. Prins (Ph.D.)

Dept. of Pharmacy

UNAM

Learning Outcomes

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� By the end of this session, the student should be able to:

�Understand the importance of arenes

�Differentiate arenes from other hydrocarbons

�Describe the structure of arenes

�Determine if a compound is aromatic or not

�Name arenes according to IUPAC system

�Describe the physical properties of arenes

Organic Chemistry, UNAM School of Medicine

3/27/2012

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Importance??Importance??Importance??Importance??

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� Several medicines are aromatic compounds:


Salicylic acid

Included in Willow bark.Used for centuries to treat pain & fever.


4 Organic Chemistry, UNAM School of Medicine

Acetylsalicylic acid (aspirin)Disprin®, etc.

A French chemist, Charles Frederic Gerhardt, was the first to prepare acetylsalicylic acid in 1853.

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IbuprofenBrufen®, etc.

Aspirin & Ibuprofen = Non-steroidal Anti-Inflammatory Drugs (NSAID’s).

Discovered by Andrew Dunlop & other UK colleagues. Patented in 1961.

ArenesArenesArenesArenes/Aromatic hydrocarbons/Aromatic hydrocarbons/Aromatic hydrocarbons/Aromatic hydrocarbons

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� Arenes are cyclic planar hydrocarbons characterised by alternating single & double bonds between carbons. It refers to benzene & its structural relatives.

� Arenes – contain delocalised π e-

� Delocalised e- = Do not belong to a single atom nor are they confined to a bond between 2 atoms

= Shared by 3 or more atoms


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ArenesArenesArenesArenes/Aromatic hydrocarbons/Aromatic hydrocarbons/Aromatic hydrocarbons/Aromatic hydrocarbons

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� Localised e- = Belong to a single atom or are confined to a bond between 2 atoms


:

Lone pair e- belong to N e- pair confined to bond between 2 atoms

Structure of benzeneStructure of benzeneStructure of benzeneStructure of benzene

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� Benzene molecular formula: C6H6

� Early assumptions of benzene structure - a cyclic 6-membered ring

with alternating & = bonds

� Analytical techniques show – all 6 C-C bonds in benzene have same

length (1.39 Å)� Shorter than C-C bond (1.54 Å)

� Longer than C-C = bond (1.33 Å)

� Therefore: Benzene does not actually have alternating & =

bonds!


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� Each C of benzene is sp2 hybridised & have three sp2 orbitals

that lie in a plane with angles of 120° (trigonal planar molecular

geometry) (Session 8)

� Remaining p orbital is perpendicular to the plane (90° to plane)


��


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� Each C of benzene uses two sp2 orbitals to bond to two other C

atoms to form σ bonds � The third sp2 orbital of each benzene C overlaps the s orbital of a H

� Remaining unhybridised p orbital of each benzene C overlaps the

other unhybridised p orbitals on both adjacent/neighbouring C’s

� Result: Continuous “doughnut-shaped” e- cloud above & below the

plane of benzene ring

� This is called “conjugation in a benzene ring”


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(a) Each C of benzene has a p orbital

(b) Overlap of the p orbitals forms a cloud of e- above & below the plane of benzene

ring

(c) Electrostatic potential map for benzene shows that all the C–C bonds have same e-

density



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� NB!!!! -The 2 “doughnut-shaped” rings above & below plane of the molecule represent one molecular orbital/bonding orbital

� The 2 delocalised e- - found anywhere within those rings.

� The other 4 delocalised e- - live in 2 similar (but not identical) molecular orbitals/bonding orbitals

� Previous diagram shows 1 of the molecular orbitals containing 2 of the delocalised e-

� Never describe structure as p orbitals on C’s overlapping sideways to produce a delocalised π bond (πbond – only accommodate 2 e-)

� Instead – describe as a "π system" or delocalised e- system (containing 6e-)


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� Therefore:The 6 π e- in benzene – shared by all 6 C atoms

: Not localised on a single atom or in a bond between 2 C atoms

:They are delocalised, i.e. roam freely within “doughnut-shaped” clouds that lie above & below the Cring

� Correct benzene line-bond structure should actually be:



14

� Disadvantages of using dashed lines or circle representation:

� Does not indicate amount of π e- in molecule

� Not useful when drawing reaction mechanisms

� In fused rings – actually incorrect to use dashed lines/circles


- Suggests that Naphthalene has 12 π e-

- Actually only has 10 π e-

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� Although π e- are delocalised in benzene, the structure

containing = bonds (localised e-) is preferred for benzene

representation


Determining Determining Determining Determining aromaticityaromaticityaromaticityaromaticity

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� Not all cyclic molecules with alternating & = bonds are necessarily aromatic!!!

� For a compound to be aromatic, it must be:

� Cyclic & planar + have an uninterrupted cloud of e- (each p orbital must overlap with p orbitals on either side of it)

� π e- cloud must contain an odd number of pairs of π e- (Hückel’s

Rule) (1, 3, 5, 7, 9, etc., pairs of π e-)


Is the following structure aromatic??

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Nomenclature of substituted benzenesNomenclature of substituted benzenesNomenclature of substituted benzenesNomenclature of substituted benzenes

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� Monosubstituted benzenes - systematically named in the same manner as other hydrocarbons, with -benzene as the parent name/suffix

� Examples:

� Alkyl-substituted benzenes� If alkyl substituent is ≤ than benzene ring (6 or fewer C’s) - named as

alkyl-substituted benzene� If alkyl substituent is > than benzene ring (7 or more C’s) - named as

phenyl-substituted alkane




Phenyl Benzyl

R = Carbon chain

• Examples:

2-phenylheptane

1

23

45

67

hexylbenzene

12

34

56

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� Disubstituted benzenes - named using one of the prefixes ortho- (o), meta- (m) or para- (p)� ortho-disubstituted benzene: 2 substituents in a 1,2 relationship on the ring

� meta-disubstituted benzene: 2 substituents in a 1,3 relationship on the ring

� para-disubstituted benzene: 2 substituents in a 1,4 relationship on the ring




• Examples:

12

3

4

1 1

o-dimethylbenzene(1,2-dimethylbenzene)

m-dimethylbenzene(1,3-dimethylbenzene)

p-dimethylbenzene(1,4-dimethylbenzene)

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� Polysubstituted benzenes - benzenes with more than 2 substituents are named by choosing a point of attachment as C 1 & numbering the substituents on the ring so that the 2nd substituent has lowest possible #

� If ambiguity still exists, number so that the 3rd or 4th substituent has lowest possible #, until a point of difference is found




• Some common benzene names:

Methylbenzene(Toluene)

NH2

Aminobenzene(Aniline)

OH

Hydroxybenzene(Phenol)

Benzenecarboxylicacid

(Benzoic acid)

Benzenesulphonicacid

Benzonitrile

Common names = Can be used as parent names during nomenclature.

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Physical properties of Physical properties of Physical properties of Physical properties of arenesarenesarenesarenes

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� Boiling points:� In benzene - only attractions between neighbouring molecules are

Van der Waals dispersion forces (force between 2 instantaneously induced dipoles)

� No permanent dipole on benzene molecule� Benzene boils at 80°C - Higher than other hydrocarbons of similar

molecular size (example: pentane & hexane) Presumably due to ease with which temporary dipoles can be set up involving the

delocalised e-



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� Melting points:

� Benzene - melts at 5.5°C, Methylbenzene – melts at -95°C

�Molecules must pack efficiently in the solid if they are to make best use of their intermolecular forces

� Benzene is a tidy, symmetrical molecule & packs very efficiently

� Methyl group sticking out in methylbenzene tends to disrupt the closeness of the packing

� If molecules aren't as closely packed, the intermolecular forces don't work as well & melting point falls


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� Water solubility:

� Arenes - insoluble in H2O

� Benzene - quite large compared with a H2O molecule

� For benzene to dissolve - have to break lots of existing H bondsbetween H2O molecules

- have to break the quite strong Van der Waals dispersion forces between benzene molecules

� Not energetically favourable


Quiz: Arenes

Review of General Chemistry, UNAM School of Medicine26

� Are the following compounds aromatic or not:

1. H

� Are the following compounds ortho-, meta- or para- substituted:

3. h

� Give the systematic (IUPAC) name for the following:

5. h

2.

4. h

6.

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Thank you

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END


Review of General Chemistry, UNAM School of Medicine28

� Quiz answers:

1. Aromatic (planar, cyclic & 9 pairs of π e-)

2. Non-Aromatic (planar, cyclic & 4 pairs of π e-)

3. Meta (1,3 relationship)

4. Ortho (1,2 relationship)

5. 2-bromo-1-chloro-4-fluorobenzene(NOT o-bromo-p-fluoro-chlorobenzene or any other combination!! o, m, p: Only used for disubstituted benzenes!!)

6. 4-bromo-aminobenzene/p-bromo-aminobenzene(4-bromoaniline/p-bromoaniline)

(18) session 18 arenes (aromatic hydrocarbons) (1)

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