AROMATIC COMPOUNDS

AROMATIC COMPOUNDSAromatic compounds are benzene and compound that resemble benzene in chemical behavior.

STRUCTURE OF BENZENEBenzene is the parent substance of aromatic hydrocarbons. It was first isolated by Michael Faraday in 1825, Its correct molecular formula, C6H6, was established a few years later. Properties of benzene was a very difficult problem for chemists in the mid-19th century

In 1826, August Kekule proposed that carbon atoms in a benzene molecule are arranged in a six-membered ring with one hydrogen atom bonded to each carbon atom and with three carbon-carbon double bonds. He further suggested that the double bonds are in rapid oscillation within the molecule. Therefore he proposed that the structure of benzene could be represented in this fashion.

Or simply

Kekules formulas have one serious shortcoming: they represent benzene and related substances as highly unsaturated compounds. Yet benzene does not react like a typical alkeneModern theory accounts for structure of the benzene molecule in this way: the orbital hybridization of the carbon atom is sp2. a planar hexagonal ring is formed by the overlapping of two sp2 orbitals on each of six carbon atom. The other sp2 orbital on each carbon atom overlaps an s orbital of hydrogen atom, bonding the carbon atom, overlap each other and form doughnut-shaped pi electron clouds above an below the plane ring.

Neither of two structures proposed by kekule therefore represents benzene. The real benzene molecule is in fact a hybrid of the two kekule structures and is commonly represented by a hexagon with a circle. The circle represent the pi cloud of electron.

Or

NOMENCLATURE OF AROMATIC COMPOUNDSMonosubstituted benzenesA substituted benzene is by replacing one or more hydrogen atoms of benzene by another atom or group of atoms.Named by adding the name of substituted group as a prefix to the word benzene.Cl | CH2CH3

NO2Chlorobenzene ethylbenzene nitrobenzene

Certain monosubstituted benzenes have special names. These are used as parent names for further substituted compounds so they should be learned.CH3 OH COOH CHO NH2 | | | | |

Toluene phenol benzoic acid benzaldehyde anlineThe C6H5- group is known as the phenyl group, and the name phenyl is used to name compounds that can not easily be named as benzene derivatives.CH3 - CH2- | - CHCHClCH2CH3 diphenylmethane 3-chloro-2-phenylpentane

Disubstituted benzenesWhen two substituted groups replace two hydrogen atoms in a benzene molecule, three different isomeric compounds are possible. The prefix ortho-(o), meta-(m-), and para-(p-) are used to name disubstituted benzens in one nomenclature system. Ortho designates 1,2 disubstitution, meta reprsents 1,3 disubtitution, and para designates 1,4 disubtitution.Consider the dichlorobenzens, C6H4Cl2 Cl Cl Cl | | Cl | Cl Clo-dichlorobenzene m-dichlorobenzene p-dichlorobenzene(1,2-dichlorobenzene) (1,3-dichlorobenzene) (1,4-dicholorobenzene)

The dimethylbenzenes have special name xyleneCH3 CH3 CH3 | CH3 | |

CH3 CH3O-xylene m-xylenep-xyleneWhen the two susbstituents are different and neither is part of a compound with a special name. The names of the two substituent are given in alphabetical order, followed by the word benzene. NO2 Cl |

CH2CH3O-bromocholorobenzene m-ethylnitrobenzene

3. Polysubstituted benzenesWhen there are more than two substituted on beneze ring, the carbon atoms in the ring are numbered starting of the substituted groups.Numbering may be either clockwise or counterclockwise but musst be done in the direction that gives the lowest posible numbers to the substituted group. BrBr OHBr

Cl

Br 1,2,4-tribromobenzene 5-bromo-2-chlorophenol

4. Polycyclic aromatic compoundsPolycyclic or fused aromatic ring system structure consist of two or more rings which two carbon atoms are common to two rings. Three of the most common hydrocarbons : Napthalene (C10H8), anthracene (C14.H10), and phenanthrene (C14H10). One hydrogen is attached to each carbon atom except at the carbons that are common to two rings.

Naphtalene anthracene phenanthrene

CHEMICAL REACTION OF BENZENE AND ALXYLBENZENESCharacteristically, the reagents the react the aromatic ring of benzene and its derivatives are electrophiles. general formula of Arene ArH, where Ar - aryl group, The electrophilic portion of the reagent replaces one of the hydrogens on the rings. Ar H + E+ Ar E

2. Halogenation of benzeneBromine reacts with benzene in the presence of Febr3 as a catalyst to give bromobenzene. Chlorine reacts similarly in the presence of FeCl3 to give chlorobenzene.C6H6 + Cl2 FeCl3 C6H5NO2 + HCl3. Nitration of benzeneWarning benzene with a mixture of concentrated nitric acid and concentrated sulfuric acid gives nitrobenzene. In this reaction a nitro group (-NO2) replaces a hydrogen atom of benzene.C6H6 + HO - NO2 H2SO4 60c C6H5NO2 + H2O

Friedel-crafts alkylation of benzeneAlkyl halides react with benzene in the presence of AlCl3 to yield alkyl benzenesC6H6 + CH3CH2Br ArCl3 C6H5SO3H + H2O

5. Friedel-crafts alkylation of benzeneAn analogous reaction occurs when acyl halides react with benzene in the presence of AlCl3. The products are acylbenzenes.

C6H6 + CH3COCl AlCl3 C6H5COCH3 + HCl

6. Oxidation of alkylbenzenesAn alkyl side chain on a benzene ring is oxidized on being heated with a mixture of aqueous Na2Cr2O7 and H2SO4. the alkyl groups, regardless of their chain leght, are converted to carboxyl groups; thus, the product is benzoic acid or a substituted derivative of benzoic acid. The other carbon atoms of the side chain are converted to CO2.

- CH3 + (O) - COOH

O2N- - -CH2CH3 + (O) O2N- -COOH + CO2

7. Halogenation of alkylbenzenesSide-chain halogenation of toluene can yield successively the mono-, di-, and tri- halo compounds.CH3 CH2ClCHCl2 CCl3 || || Cl2Cl2 Cl2uv uv uv Toluenebenzyl cholridebenzal chloride benzotichloride

RATE AND ORIENTATION IN ELECTROPHILIC AROMATIC SUBSTITUTIONSubstituents already present on an aromtic ring have two effects:It effects the rate of electrophilic aromatic ring have two effects:It effects the regioselectivity (orietation) of electrophillic aromatic substitution CH3 CF3||

ToluenebenzenetriflouromethylbenzeneMost reactive least reactive18

EFFECT ON RATESUBSTITUENTEFFECT ON ORIENTATIONVery strongly activating-NH2Amanoortho-para directing-NHR Alkylamino-NH2dialkylamino-OHhydroxylStrongly activating-NHCORAcylaminoOrtho-para directing-ORAlkoxylActivating-RAlkylOrtho-para directing

Deactivating-XHalogenOrtho-para directingStrongly deactivating-CHOFormylMeta directing-CORAcyl-COOHCarboxyl-COOREster-COClAcyl choloride-CNCyano-SO3HSulfuric acidVery strongly deactivating-CF3TriflouromethylMeta directing-NO2nitro

MULTIPLE SUBSTITUENT EFFECTSWhen a benzene ring bears two or more substituents, noth its reactivity and orientation of further substitution can, in most cases, be predicted from the cumulative effects of its substituents.CH3 CH3 | O O | COCH3 + CH3COCCH3 AlCl3| |CH3CH3P-xylene 2,5-dimethylacetophenone

In cases inwhiich the directing effects of individual substituents oppose each other, it is the more activating substituent that controls the regioselectively of electrophilic aromatic substitution. Thus, bromination occurs ortho to the N-methylamino group in 4-Chloro-N-methylaniline because this group is a very powerful activating substituent whilt the chlorine is weakly deactivating.NHCH3NHCH3| |Br+ Br2 HAr

Cl Cl 4-chloro-N-methylaniline 2 bromo-4chloro-N-methylaniline

When two positions are comparably activated by alkyl groups, substitution usually occurs at the methyl group in presence to those ortho to the larger tert-butyl group. This is an example of steric effect. CH3 CH3 | | NO2 + HO NO2 H2SO4

| |C(CH3)3 C(CH3)3 P-tert-butyltouluene 4-tert-butyl-2-nitrotoluene

Nitration of m-xylene is directed ortho to one methyl group and para to other. CH3 CH3 | | + HO NO2 H2SO4

CH3 | CH3 NO2 m-xylene2,4-dimethyl-1-nitrobenzene

The ortho position between the two methyl groups is less reactive because it is more sterically hindred.

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AROMATIc COMPOUNDSSUBMITTED BY : SHERLENE A. GESIMSUBMITTED TO: MRS. ELIZABETH VIVAR