Doc 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
Aromatic/aryl Hydrocarbons (Arenes)
benzene molecular formula C6H6, skeletal formula , structural/displayed formula
methylbenzene (toluene) C7H8, C6H5CH3
ethylbenzeneC8H10 and propylbenzene
phenylethene ('styrene'), C8H8, is named as a derivative of ethene,
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, , 1,3-dimethylbenzene, 1,4-dimethylbenzene
(once called xylenes, ortho, meta and para-xylene)
1-ethyl-2-methylbenzene, C9H12, and the two other positional structural isomers
1-ethyl-3-methylbenzene, C9H12, and
1,2-diethylbenzene, C10H14, , 1,3-dipropylbenzene
1-methyl-2-propylbenzene, C10H14, and the two other positional structural isomers
4-ethyl-1,2-dimethylbenzene, C10H14, (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 molecular structure, shape and bond angles
Olympicene (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!).
Halo-arenes - aryl/aromatic 'halides'
or aryl halide, aromatic halogen compounds, the halogen is directly attached to the benzene ring.
chlorobenzene, C6H5Cl, and
1,2 or 1,3 or 1,4-dichlorobenzene, C6H4Cl2, ,,
C7H7Cl, 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.
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 and
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)
Phenols form salts with strong bases e.g. the alkali sodium hydroxide gives ....
sodium phenoxide (old name sodium phenate?)
C6H5OCl, 2-chlorophenol (o-chlorophenol)
C7H8O, 3-methylphenol (m-methylphenol, meta-cresol)
C6H7NO, 2-aminophenol, 4-aminophenol and 2-nitrophenol
C7H6O3, 3-hydroxybenzoic acid and 2-hydroxybenzoic acid
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,phenylmethanol (old name 'benzyl alcohol') is a primary aliphatic alcohol
which is isomeric with methylphenols and the ether, methoxybenzene (anisole).
C8H10O,1-phenylethanol, which is isomeric with the ether
C8H10O, ethoxybenzene (phenetole) and the ethylphenols or dimethylphenols.
Aromatic aldehydes and ketones
True aromatic aldehydes, R-CHO, have the aldehyde group -CHO directly attached to the ring e.g.
C7H6O, orbenzaldehyde, and
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, , and C8H8O2,
Diphenylmethanone (diphenyl ketone), C13H10O, , is a completely aromatic ketone.
Aromatic acids and their derivatives
Note: quite a few of the 'old' names are still used and accepted
C7H6O2, , , , , benenecarboxylic acid, benzoic acid
C7H5OCl, , , , , benzenecarbonyl chloride, benzoyl chloride
C7H7ON, , , , , benzenecarboxamide, benzamide
C8H8O2, , , , , methyl benzenecarboxylate, methyl benzoate
C9H10O2, , , , ethyl benzenecarboxylate, ethyl benzoate
C10H12O2, , , , propyl benzenecarboxylate, propyl benzoate
C7H5O3, 2 3 or 4-hydroxybenzenecarboxylic acid, 2 3 or 4-hydroxybenzoic acid
C7H5ClO2, 2-chlorobenzoic acid
C8H6O4, benzene-1,2-dicarboxylic acid (or 1,3 or 1,4)
(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
C6H6NCl, 2- or 3- or 4-chlorophenylamine
C6H8N2, 1,3-diaminobenzene, , 3-aminobenzoic acid,
C7H9N, 2-methylphenylamine, methyl-2-phenylamine, 1-amino-2-methylbenzene or 2-aminomethylbenzene
This is isomeric with benzylamine which is a primary aliphatic amine because the amine group is not directly attached to the ring.
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, and C8H11N, N-ethylphenylamine
and C10H11N, diphenylamine
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, , N,N-diethylphenylamineC10H15N,
and C18H15N, triphenylamine
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,, ,
Aromatic SECONDARY ACID AMIDES
(secondary, 1 alkyl/aryl R group and 1H on the N of amide group)
N-phenylethanamide, C8H9NO, (this is really a phenyl derivative of an aliphatic amide)
N-methylbenzamide, C8H9NO, , and
N-phenylbenzamide, C13H11NO, (both true secondary aromatic amide)
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
KEVLAR is an aromatic polyamide formed from benzene-1,4-dicarboxylic acid and 1,4-diaminobenzene
Aromatic TERTIARY ACID AMIDES
Tertiary amides would have no H and 2 aryl/alkyl groups on N of amide group.
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) from phenylamine
(2) from 4-methylphenylamine
(3) from 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
reacting (1) with phenylamine gives
reacting (2) with phenol gives
reacting (2) with phenylamine gives
These have the nitro -NO2 group directly attached to the ring. On reduction they form primary aromatic amines.
nitrobenzene, C6H5NO2, and , C6H4N2O4, 1,3-dinitrobenzene,
3 isomers of C6H4NO2Cl, 1-chloro-2-nitrobenzene , 1-chloro-3-nitrobenzene , 1-chloro-4-nitrobenzene
C7H7NO2, methyl-2-nitrobenzene, or 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)
methyl-4-nitrobenzene, or 1-methyl-4-nitrobenzene (p-nitrotoluene, para nitrotoluene)
and a substituted aromatic carboxylic acid, C7H5NO3, 3-nitrobenzoic acid,
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, (or benzenesulphonic acid)
2-, 3- or 4-methylbenzenesulfonic acid, C7H8SO3, CH3C6H5SO2OH,, ,
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
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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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