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(c) doc b(c) doc bDoc Brown's Chemistry  Revising Advanced Level Organic Chemistry

Revision Notes Part - AROMATIC COMPOUNDS What are they?

Notes on the Molecular Structure and Nomenclature-naming of Aromatic Compounds - Arenes and substituted products

How do you name aromatic compounds e.g. arenes - aromatic hydrocarbons. Examples of displayed formula of aromatic compounds, graphic formula, molecular formula, skeletal formula, structural formula of aromatic hydrocarbons (arenes like benzene and methylbenzene), aromatic halides, phenols, aromatic aldehydes, aromatic ketones, aromatic carboxylic acids and derivatives, primary aromatic amine, secondary aromatic amines, tertiary aromatic amines, aromatic amides, diazonium Salts and azo dyes , nitro-aromatics, aromatic sulphonic/sulfonic acids . Please note that some 'old' names are quoted in () though their use should be avoided if possible, however, many are still used - just put one into Google! However, most names are based on the generic hexagonal 'benzene' ring with modifications to this based on the highest functional group attached to the ring (i.e. a ring substituent replacing a hydrogen)

KEYWORDS: fast track to nomenclature examples of ... Aromatic hydrocarbons (Arenes, including multiple ring compounds) ... Halo-Aromatics ... Phenols ... Aromatic aldehydes and aromatic ketones ... aromatic carboxylic acids and Derivatives ... Aromatic amines (prim/sec/tert) ... Aromatic amides ... Diazonium Salts and azo dyes ... Nitro-Aromatics ... Aromatic sulphonic/sulfonic Acids  

(c) doc b Type in name QUIZ * Functional Group Tests * Summary of FUNCTIONAL GROUPS

See ALKANES page for comparison with aliphatic and alicyclic compounds

The shapes and bond angles of simple molecules - section on bond angles in organic molecules

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Aromatic/aryl Hydrocarbons (Arenes)

benzene molecular formula C6H6, skeletal formula (c) doc b, structural/displayed formula (c) doc b

ALL bond angles 120o, symmetrical hexagonal ring, planar molecule

 

methylbenzene (toluene) C7H8, C6H5CH3(c) doc b(c) doc b(c) doc b

 

ethylbenzeneC8H10(c) doc b(c) doc b and propylbenzene(c) doc b(c) doc b

 

phenylethene ('styrene'), C8H8,(c) doc b is usually named as a derivative of ethene, even though it is also technically an aromatic compound,

the C6H5- aromatic ring grouping is called a phenyl group when quoted as a substituent  prefix.

So phenylethene is named as a derivative of ethene.

 

1,2-dimethylbenzene, C8H10, (c) doc b(c) doc b, 1,3-dimethylbenzene(c) doc b(c) doc b, 1,4-dimethylbenzene(c) doc b(c) doc b

(once called xylenes, ortho, meta and para-xylene)

 

1-ethyl-2-methylbenzene, C9H12,(c) doc b(c) doc b and the two other positional structural isomers

1-ethyl-3-methylbenzene, C9H12,(c) doc b(c) doc b and

TOP and all LINKS1-ethyl-4-methylbenzene, C10H14,,(c) doc b(c) doc b

 

1,2-diethylbenzene, C10H14, (c) doc b, 1,3-dipropylbenzene (c) doc b

 

1-methyl-2-propylbenzene, C10H14, (c) doc band the two other positional structural isomers

1-methyl-3-propylbenzene, C10H14,(c) doc band

1-methyl-4-propylbenzene, C10H14,(c) doc b

 

1-ethyl-4-propylbenzene, C11H16, (c) doc b

4-ethyl-1,2-dimethylbenzene, C10H14, (c) doc b (uses lower numbers than 1-...-3,4-...)

Examples of fused ring systems of arene hydrocarbon compounds - polynuclear aromatic ring compounds

Two aromatic rings that share a pair of carbon atoms are said to be fused. These molecules behave exactly like benzene in their chemistry i.e. they display the typical aromaticity of the single ring arenes above in undergoing electrophilic substitution reactions.

naphthalene , C10H8  consists of two fused aromatic rings

 

anthracene or  C14H10  consists of three fused rings.

Naphthalene and anthracene are two of the simplest examples of what are called polynuclear aromatic compounds or in this specific case polycyclic aromatic hydrocarbons (PAHs), also known as polycyclic arenes or polyaromatic hydrocarbons. They are molecules which consist of at least two 'benzene' rings 'fused' together. Incidentally the ultimate molecule with this carbon based aromatic ring system is graphene (essentially an individual layer of graphite!)

Both naphthalene and anthracene are correctly shown in Kekule style skeletal formula here (principal IUPAC accepted style), though some textbooks consider drawing molecules like naphthalene in the following style ...

and are considered as incorrect because, although both rings involve delocalised pi electron systems, and, they are both merged into a continuous delocalised pi electron system, this style of diagram implies two separate delocalised ring systems, which is incorrect. However, some undergraduate textbooks and high school textbooks use the 'multi-inner circle' style of skeletal formula. BUT such a style, also implies for the 'inner' vertical C-C bonds (as drawn above between rings), that they are equivalent to a double bond (bond order 2), but each C-C bond in the rings has an average bond order of 1.5 in the delocalised aromatic ring system. Although beyond the scope of this site, anthracene (naphthalene?) does show some ready addition reactions characteristic of a non-aromatic diene!

Olympicene 2OLYMPICENE molecular structure, shape and bond angles

Olympicene 1Olympicene (caught my imagination in 2012!), C19H12, (Mr = 240, bpt 512oC), was one of the latest arrivals of a polynuclear aromatic hydrocarbon compound from synthetic organic chemistry in 2011. It was conceived and synthesised by a British research group for the London 2012 Olympic Games. The Olympicene molecule consists of five 'fused' hexagonal rings of carbon atoms (the official Olympic symbol consists of five 'interlocking rings' - can't show due to copyright law!).

Consisting of five shared-fused hexagonal carbon atom rings, Olympicene is a 'flat' or planar molecule (if you ignore the H atoms of the saturated -CH2- which will 'stick out' above/below the planarity of the molecule). In Olympicene, four of the hexagonal carbon atom rings are truly aromatic rings with C-C-C and C-C-H bond angles of ~120o. However, due to valence bond limitations, (in left-hand representation), one of the carbon atoms of the bottom-right ring, must be saturated with two hydrogen atoms (shown in diagram). This means in the delocalised system of '42/3' rings there are 18 delocalised pi electrons NOT 19 in Olympicene. In the 2nd isomer shown (on the right), the saturated carbon atom is at the top of the top middle ring. This means these cases the H-C-H bond angle of the saturated carbon will be between 109o and 120o (probably around 115o?). What is shown in the above diagram are two of the many possible isomeric 'Kekule' skeletal formula structures. You can write more structures of Olympicene in a similar manner eg put the saturated carbon in one of three positions (a sort of 1, 2 or 3 position) in the top right hexagonal carbon ring, or on the bottom carbon atom of one of the lower rings - try this exercise out and let me know how you get on! am I right, that there are six structural isomers? I'm not exactly sure what isomers were actually produced in the synthesis of Olympicene? and I'm not sure if there is a dynamic equilibrium between two or more of the isomers, though going from one isomer to another involves a proton migration from a strong C-H bond?

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Halo-arenes - aryl/aromatic 'halides'

or aryl halide, aromatic halogen compounds, the halogen is directly attached to the benzene ring.

chlorobenzene, C6H5Cl, (c) doc b and

1,2 or 1,3 or 1,4-dichlorobenzene, C6H4Cl2, (c) doc b,(c) doc b,(c) doc b

 

C7H7Cl, (c) doc b(c) doc b chloromethylbenzene, or (chloromethyl)benzene (benzyl chloride). It is not a true aryl halogen compound, the halogen atom is in a non-aromatic side chain, so it is a primary aliphatic halogenoalkane.

 

(c) doc b(c) doc b(c) doc b Three positional structural isomers of C7H7Cl

chloro-2-methylbenzene, chloro-3-methylbenzene and chloro-4-methylbenzene,or 1-chloro-2-methylbenzene, 1-chloro-3-methylbenzene and 1-chloro-4-methylbenzene (old names 0-chlorotoluene, m-chlorotoluene and p-chlorotoluene). These are true aryl halides with the halogen attached directly to the benzene ring and they are isomeric with (chloromethyl)benzene above.

 

C7H6Cl2, 2,3-dichloromethylbenzene(c) doc b and

C8H9Cl, 1-chloro-2,4-dimethylbenzene(c) doc b

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Phenols - aryl/aromatic hydroxy compounds

If the OH group (hydroxy) is directly attached to a benzene ring, the molecule is classified as a 'phenol'.

If not, the molecule is classified as an aliphatic alcohol.

This difference is illustrated below with molecules containing a benzene ring (Phenols ROH, R=aryl only)

(c) doc b(c) doc b(c) doc bphenol.

Phenols form salts with strong bases e.g. the alkali sodium hydroxide gives ....

sodium phenoxide (old name sodium phenate?)

 

  C6H5OCl, (c) doc b2-chlorophenol (o-chlorophenol)

C7H8O, (c) doc b3-methylphenol (m-methylphenol, meta-cresol)

 

 

C6H7NO, (c) doc b 2-aminophenol, (c) doc b 4-aminophenol and (c) doc b2-nitrophenol

 

C7H6O3, (c) doc b3-hydroxybenzoic acid and (c) doc b2-hydroxybenzoic acid

 

TOP and all LINKSC7H6OCl2,(c) doc b2,5-dichloro-4-methylphenol

 

C6H6O2, or 1,4-dihydroxybenzene (benzene-1,4-diol, 1,4-benzenediol, 'quinol')

which is oxidised to

C6H4O2, or 2,5-cyclohexadiene-1,4-dione (cycohexa-2,5-diene-1,4-dione, 'p-quinone')

 

If the OH is not attached to a benzene ring you get an aliphatic alcohol which is isomeric with a phenols or an ether.

C7H8O,(c) doc bphenylmethanol (old name 'benzyl alcohol') is a primary aliphatic alcohol

which is isomeric with methylphenols and the ether,(c) doc b methoxybenzene (anisole).

 

C8H10O,(c) doc b1-phenylethanol, which is  isomeric with the ether

C8H10O,(c) doc b ethoxybenzene (phenetole) and the ethylphenols or dimethylphenols.

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Aromatic aldehydes and ketones

True aromatic aldehydes, R-CHO, have the aldehyde group -CHO directly attached to the ring e.g.

C7H6O, (c) doc bor(c) doc bbenzaldehyde, and

 C7H6O2, (c) doc b2-hydroxybenzaldehyde

 

Ketones, R2C=O, are often 'mixed' aliphatic-aromatic in the sense that one R group is alkyl and the other is aryl e.g.

1-phenylethanone (acetophenone, methyl phenyl ketone), C8H8O,  (c) doc b, and C8H8O2, (c) doc b 

Diphenylmethanone (diphenyl ketone), C13H10O, ,(c) doc b is a completely aromatic ketone.

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Aromatic acids and their derivatives

Note: quite a few of the 'old' names are still used and accepted

C7H6O2, (c) doc b, (c) doc b, (c) doc b, (c) doc b, benenecarboxylic acid, benzoic acid

 

C7H5OCl, (c) doc b, (c) doc b, (c) doc b, (c) doc b, benzenecarbonyl chloride, benzoyl chloride

 

C7H7ON, (c) doc b, (c) doc b , (c) doc b,(c) doc b , benzenecarboxamide, benzamide

 

C8H8O2, (c) doc b, (c) doc b, (c) doc b, (c) doc b, methyl benzenecarboxylate, methyl benzoate

 

TOP and all LINKSC8H8O3, (c) doc bmethyl 2-hydroxybenzoate

 

C9H10O2, (c) doc b, (c) doc b, (c) doc b, ethyl benzenecarboxylate, ethyl benzoate

 

 C10H12O2, (c) doc b, (c) doc b, (c) doc b, propyl benzenecarboxylate, propyl benzoate

 

  C7H5O3, (c) doc b(c) doc b(c) doc b2 3 or 4-hydroxybenzenecarboxylic acid, 2 3 or 4-hydroxybenzoic acid

 

C7H5ClO2, (c) doc b2-chlorobenzoic acid

 

C8H6O4, (c) doc b(c) doc b(c) doc bbenzene-1,2-dicarboxylic acid (or 1,3 or 1,4)

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Primary AROMATIC amines

(primary, 2 H's and only one R group attached to the N of the amine group, R-NH2 where R = alkyl or aryl)

The amino (prefix) or amine (suffix) group is directly attached to the aromatic benzene ring

e.g. C6H7N, the simplest is C6H5NH2, phenylamine (c) doc b

 

C6H6NCl, 2- or 3- or 4-chlorophenylamine (c) doc b(c) doc b(c) doc b

 

C6H8N2, 1,3-diaminobenzene,  (c) doc b,  3-aminobenzoic acid,  (c) doc b

 

C7H9N, 2-methylphenylamine, methyl-2-phenylamine, 1-amino-2-methylbenzene (c) doc b or 2-aminomethylbenzene

This is isomeric with benzylamine (c) doc b which is a primary aliphatic amine because the amine group is not directly attached to the ring.

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Secondary AROMATIC amines

(secondary, one H and two R groups attached to the N of the amine group, R2NH where R = alkyl or aryl)

C7H9N, N-methylphenylamine(c) doc b, and C8H11N, N-ethylphenylamine(c) doc b

 

and C10H11N, diphenylamine(c) doc b

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Aromatic TERTIARY AMINES

(tertiary, no H and three R groups attached to the N of the amine group, R3N where R = alkyl or aryl)

N,N-dimethylphenylamine, C8H11N,  (c) doc b, N,N-diethylphenylamineC10H15N, (c) doc b

 

and C18H15N, triphenylamine (c) doc b

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Aromatic PRIMARY ACID AMIDES

(primary, no alkyl/aryl R group and 2H's on the N of amide group)

The simplest primary aromatic acid amide is

benzamide or benzenecarboxamide, C7H7NO,(c) doc b, (c) doc b,(c) doc b

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Aromatic SECONDARY ACID AMIDES

(secondary, 1 alkyl/aryl R group and 1H on the N of amide group)

N-phenylethanamide, C8H9NO, (c) doc b (this is really a phenyl derivative of an aliphatic amide)

N-methylbenzamide, C8H9NO,  (c) doc b, and

N-phenylbenzamide, C13H11NO,    (c) doc b (both true secondary aromatic amide)

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Aromatic POLYAMIDES

Are secondary amides formed in a condensation reaction between a carboxylic acid and an amine.

Water is eliminated between the two 'monomers' to give the secondary, polyamide (polymer)  linkage ...

-COOH + H2N- ==> -CO-NH- + H2O (c) doc b

 

KEVLAR is an aromatic polyamide formed from benzene-1,4-dicarboxylic acid and 1,4-diaminobenzene

(c) doc b

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Aromatic TERTIARY ACID AMIDES

Tertiary amides would have no H and 2 aryl/alkyl groups on N of amide group.

N,N-dimethylbenzamide, C9H11NO,  (c) doc b

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DIAZONIUM SALTS and AZO DYES:

Diazonium salts are formed when primary aromatic amines reaction with nitrous acid

e.g. C6H5NH2(aq) + HNO2(aq) + H+(aq) ==> C6H5N2+(aq) + 2H2O(l)

The diazonium cation has a nitrogen-nitrogen triple bond system directly attached to the benzene ring e.g.

(1) (c) doc bfrom phenylamine

(2) (c) doc bfrom 4-methylphenylamine

(3) (c) doc bfrom 2-aminobenzoic acid.

 

In alkaline solution these diazonium salts couple with phenols and aromatic amines to form azo dyes.

These dyes have benzene rings linked with an azo -N=N- bond system e.g.

reacting (1) with phenol gives (c) doc b

reacting (1) with phenylamine gives (c) doc b

reacting (2) with phenol gives(c) doc b 

reacting (2) with phenylamine gives (c) doc b

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NITRO-AROMATICS

These have the nitro -NO2 group directly attached to the ring. On reduction they form primary aromatic amines.

nitrobenzene, C6H5NO2, (c) doc band , C6H4N2O4,   1,3-dinitrobenzene, (c) doc b

 

3 isomers of C6H4NO2Cl, 1-chloro-2-nitrobenzene (c) doc b, 1-chloro-3-nitrobenzene (c) doc b, 1-chloro-4-nitrobenzene(c) doc b 

 

C7H7NO2,  methyl-2-nitrobenzene, (c) doc bor 1-methyl-2-nitrobenzene (o-nitrotoluene, ortho nitrotoluene)

and the two other positional structural isomers

methyl-3-nitrobenzene, or 1-methyl-3-nitrobenzene (m-nitrotoluene, meta nitrotoluene)

and

methyl-4-nitrobenzene, or 1-methyl-4-nitrobenzene (p-nitrotoluene, para nitrotoluene)

 

and a substituted aromatic carboxylic acid, C7H5NO3, 3-nitrobenzoic acid, (c) doc b

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Aromatic Sulphonic Acids (sulfonic acids)

These molecules have a strongly mono-basic acidic group -SO2OH directly attached to the benzene ring e.g.

benzenesulphonic acid, C6H6SO3, C6H5SO3H, C6H5SO2OH,(c) doc b(c) doc b (or benzenesulphonic acid)

2-, 3- or 4-methylbenzenesulfonic acid, C7H8SO3, CH3C6H5SO2OH,(c) doc b, (c) doc b,(c) doc b 

(or 2-, 3- or 4-methylbenzenesulphonic acid)


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WHAT NEXT?

KEYWORDS: fast track to nomenclature examples of ... Hydrocarbons (Arenes) ... Halo-Aromatics ... Phenols ... Aldehydes and Ketones ... Carboxylic Acids and Derivatives ... Amines (prim/sec/tert) ... Amides ... Diazonium Salts and azo dyes ... Nitro-Aromatics ... Sulphonic Acids  

(c) doc b Type in name QUIZ * Functional Group Tests * Summary of FUNCTIONAL GROUPS

See ALKANES page for comparison with aliphatic and alicyclic compounds

The shapes and bond angles of simple molecules - section on bond angles in organic molecules


A level Revision notes for GCE Advanced Subsidiary Level AS Advanced Level A2 IB Revise AQA GCE Chemistry OCR GCE Chemistry Edexcel GCE Chemistry Salters Chemistry CIE Chemistry, WJEC GCE AS A2 Chemistry, CCEA/CEA GCE AS A2 Chemistry revising courses for pre-university students (equal to US grade 11 and grade 12 and AP Honours/honors level courses)

formula keywords: how to name naming nomenclature empirical molecular formula graphic formula displayed formula skeletal formula structural isomers isomerism Hydrocarbons (Arenes) ... Halo-Aromatics ... Phenols ... Aldehydes and Ketones ... Carboxylic Acids and Derivatives ... primary prim secondary sec tertiary tert Amines (prim/sec/tert) ... Amides ... Diazonium Salts and azo dyes ... Nitro-Aromatics ... Sulphonic Acids   C6H6 C7H8 C6H5CH3 C8H10 CH3C6H4CH3 C6H5CH2CH3 C8H18 C6H5CH=CH2 C9H12 C10H14 C11H16 C6H5Cl C6H4Cl2 ClC6H4Cl C7H7Cl C6H5CH2Cl CH3C6H4Cl C7H6Cl2 C8H9Cl C6H6O C6H5OH C6H5OCl C6H5ClO ClC6H4OH C7H8O CH3C6H4OH HOC6H4NH2 NH2C6H4OH H2NC6H4OH C6H7ON C6H7NO C7H6O3 HOC6H4COOH C7H6OCl2 C7H6Cl2O C6H6O2 HOC6H4OH C6H4O2 C6H5CH2OH C6H5OCH3 C8H10O C6H5OCH2CH3 C6H5OCH2CH7H6O2 C6H5COOH C7H6O C6H5CHO C7H6O2 HOC6H4CHO C8H8O C6H5COOCH3 C8H8O2 C13H10O C7H6O2 C6H5COOH C7H5OCl C7H5ClO C68H11N C6H5NHCH2CH3 C6H5COCl C7H7ON C7H7NO C6H5CONH2 C8H8O2 C6H5COOCH3 C8H8O3 C9H10O2 C6H5COOCH2CH3 C9H10O2 C10H12O2 C6H5COOCH2CH2CH3 C7H6O3 HOC6H4COOH C7H5ClO2 C7H5O2Cl ClC6H4COOH C8H6O4 HOOCC6H6COOH C6H7N C6H5NH2 C6H6NCl C6H6ClN C6H8N2 H2NC6H4NH2 NH2C6H4NH2 C7H9N CH3C6H4NH2 C6H5CH2NH2 C6H5NHCH3 C8H11N C6H5NHCH2CH3 C10H11N C6H5NHC6H5 C9H11N C10H15N C18H15N C7H7NO C7H7ON C6H5CONH2 C8H9NO C8H9ON CH3CONHC6H5 C6H5CONHCH3 C13H11NO C13H11ON C6H5CONHC6H5 C9H11NO C9H11ON C6H5NO2 C6H5ON2 C6H4N2O4 C6H4O4N2 C6H4NO2Cl C6H4O2NCl C6H4O2ClN C6H4ClNO2 C6H4ClO2N C6H4NClO2 ClC6H4NO2 O2NC6H4Cl NO2C6H4Cl C7H7NO2 C7H7O2N CH3C6H4NO2 O2NC6H4CH3 NO2C6H4CH3 C7H5NO3 O2NC6H4COOH NO2C6H4COOH C6H6O3S C6H6SO3 C6H5SO3H C6H5SO2OH C7H8SO3 CH3C6H4SO2OH C10H8 C14H10 C19H12

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