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Chapter 1 : Benzene, Subtituted Benzene and Aromaticity

Tata nama benzene, benzen tersubstitusiSifat Fisis Hidrokarbon AromatikIkatan dalam benzenPersyaratan aromatisitas

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Aromatic Compoundsn Aromatic was used to described some fragrant

compounds in early 19th century n Not correct: later they are grouped by chemical

behavior (unsaturated compounds that undergo substitution rather than addition)

n Current: distinguished from aliphatic compounds by electronic configuration

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Why this Chapter?

n Reactivity of substituted aromatic compounds is tied (terikat/tergantung) to their structure

n Aromatic compounds provide a sensitive probe (pemeriksaan) for studying relationship between structure and reactivity

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Molecular Orbital* p electron cloud delocalized all over the ring

* the resonance picture this helps to explain lack of reactivity of benzene

* great stability (substitution not addition )

Benzene Structure

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The bonds between carbon atoms in benzene are in between these two lengths at 0.139 nm.

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BenzeneStructural Formulas and Short-hand (cepat) Symbols

n The Lewis structure and the Kekulé symbol both indicate that the carbons in the ring have alternating double and single bonds.

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1. Sources and Names of Aromatic Hydrocarbonsn From high temperature distillation of coal tar (aspal

cair)n Heating petroleum at high temperature and pressure

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Naming Aromatic Compounds

n Many common names (toluene = methylbenzene; aniline = aminobenzene)

n Monosubstituted benzenes systematic names as hydrocarbons with –benzenen C6H5Br = bromobenzenen C6H5NO2 = nitrobenzene, and C6H5CH2CH2CH3 is

propylbenzene

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The Phenyl Group

n When a benzene ring is a substituent, the termphenyl is used (for C6H5

)n You may also see “Ph” or “f” in place of “C6H5”

n “Benzyl” refers to “C6H5CH2”

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Disubstituted Benzenesn Relative positions on a benzene ring

n ortho- (o) on adjacent carbons (1,2)n meta- (m) separated by one carbon (1,3)n para- (p) separated by two carbons (1,4)

n Describes reaction patterns (“occurs at the para position”)

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(Z)-2-Phenyl-2-butene

4-(3-Methoxyphenyl)-2-butanone

1-Phenyl-1-pentanone

O OH3CO

Ph

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Naming Benzenes With More Than Two Substituentsn Choose numbers to get lowest possible valuesn List substituents alphabetically with hyphenated

numbersn Common names, such as “toluene” can serve as root

name (as in TNT)

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2. Structure and Stability of Benzene: Molecular Orbital Theoryn Benzene reacts slowly with Br2 to give

bromobenzene (where Br replaces H)n This is substitution rather than the rapid addition

reaction common to compounds with C=C, suggesting that in benzene there is a higher barrier

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Heats of Hydrogenation as Indicators of Stabilityn The addition of H2 to C=C normally gives off about 118 kJ/mol – 3

double bonds would give off 356kJ/mol n Two conjugated double bonds in cyclohexadiene add 2 H2 to give

off 230 kJ/moln Benzene has 3 unsaturation sites but gives off only 206 kJ/mol on

reacting with 3 H2 moleculesn Therefore it has about 150 kJ more “stability” than an isolated set of

three double bonds

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Benzene’s Unusual Structuren All its C-C bonds are the same length: 139 pm — between

single (154 pm) and double (134 pm) bondsn Electron density in all six C-C bonds is identicaln Structure is planar, hexagonaln C–C–C bond angles 120°n Each C is sp2 and has a p orbital perpendicular (tegak

lurus) to the plane of the six-membered ring

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Molecular Orbital Description of Benzenen The 6 p-orbitals combine to give

n Three bonding orbitals with 6 p electrons,n Three antibonding with no electrons

n Orbitals with the same energy are degenerate

Y 1: zero nodesY 2 and Y 3: one node

Y 4 and Y 5: two nodesY 6: three node

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Physical Properties of BenzenePhysical Properties of Benzene

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nn MPMPbenzenebenzene = = 55..55°°C, C, BPBPbenzenebenzene==8080..11°°CC

nn nonnon--polar Moleculepolar Molecule

nn insoluble in water, soluble in organic insoluble in water, soluble in organic solventsolvent

nn less dense than waterless dense than water

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3. Aromaticity and the Hückel 4n+2 Rule

n heat of hydrogenation 150 kJ/mol less negative than a cyclic triene

n Planar hexagon: bond angles are 120°, carbon–carbon bond lengths 139 pm

n Undergoes substitution rather than electrophilic addition

n Huckel’s rule, based on calculations – a planar cyclic molecule with alternating (pertukaran) double and single bonds has aromatic stability if it has 4n+ 2 p electrons (n is 0,1,2,3,4)

n For n=1: 4n+2 = 6; benzene is stable and the electrons are delocalized

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AromaticityExample Example 11: Benzene: Benzene

n cyclic üü

n planar üü

n conjugated üü

n 6 p electrons üü

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Aromaticity

Other Examples?

cyclic üü

planar üü

conjugated üü6p electrons XX

cyclic üü

planar üü

conjugated üü6p electrons XX

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AromaticityAromaticity

nn cyclooctatetraene cyclooctatetraene is is nonnonaromaticaromatic

nn It is It is notnot planarplanar

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4. Aromatic Ions

n The 4n + 2 rule applies to ions as well as neutral speciesn Both the cyclopentadienyl anion and the cycloheptatrienyl

cation are aromaticn The key feature (ciri) of both is that they contain 6 p

electrons in a ring of continuous p orbitals

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Aromaticity of the Cyclopentadienyl Anion

n 1,3-Cyclopentadiene contains conjugated double bonds joined by a CH2 that blocks delocalization

n Removal of H+ at the CH2produces a cyclic 6-electron system, which is stable

n Removal of H- or H• generates nonaromatic 4 and 5 electron systems

n Relatively acidic (pKa = 16) because the anion is stable

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Cycloheptatrienen Cycloheptatriene has 3 conjugated double bonds

joined by a CH2

n Removal of “H-” leaves the cationn The cation has 6 electrons and is aromatic

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5. Aromatic Heterocycles: Pyridine and Pyrrolen Heterocyclic compounds contain elements other

than carbon in a ring, such as N,S,O,Pn Aromatic compounds can have elements other than

carbon in the ringn There are many heterocyclic aromatic compounds

and many are very commonn Cyclic compounds that contain only carbon are

called carbocycles (not homocycles)n Nomenclature is specialized

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Pyridinen A six-membered heterocycle with a nitrogen atom in its ringn p electron structure resembles (menyerupai) benzene (6 electrons)n The nitrogen lone pair electrons are not part of the aromatic system

(perpendicular orbital)n Pyridine is a relatively weak base compared to normal amines but

protonation does not affect aromaticity

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Pyrrolen A five-membered heterocycle with one nitrogenn p electron system similar to that of cyclopentadienyl anionn Four sp2-hybridized carbons with 4 p orbitals perpendicular to the

ring and 4 pi electronsn Nitrogen atom is sp2-hybridized, and lone pair of electrons

occupies a p orbital (6 p electrons)n Since lone pair electrons are in the aromatic ring, protonation

destroys aromaticity, making pyrrole a very weak base

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6. Why 4n +2?n When electrons fill the various molecular orbitals, it

takes two electrons (one pair) to fill the lowest-lying orbital and four electrons (two pairs) to fill each of nsucceeding energy levels

n This is a total of 4n + 2

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Polycyclic Aromatic Compoundsn Aromatic compounds can have rings that share a set

of carbon atoms (fused rings)n Compounds from fused benzene or aromatic

heterocycle rings are themselves aromatic

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Naphthalene Orbitals

n Three resonance forms and delocalized electrons

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Thank You

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