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

Introduction to naming aromatic compounds

For two substituent groups, ether the same or different, there are three possible positional isomers.

The alkyl substituent part of the name is usually the prefix of the aromatic compound's name.

The simplest aromatic hydrocarbon is benzene, 

The structures of benzene on the left and right are presented in 'old fashioned' Kekule style.

However, benzene is NOT a triene, so this style ignores the delocalised rings of pi electrons, and this style will not be used for most of the rest of the page.

Unfortunately, the Kekule style is widely used, though in some cases it is justified for multiple benzene ring systems.

benzene molecular formula C₆H₆, skeletal formula , structural/displayed formula

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

The empirical formula for benzene is CH

methylbenzene (toluene) C₇H₈, C₆H₅CH₃ , the simplest molecule where an alkyl groups has replaced a hydrogen atom in the benzene ring.

Both the molecular and empirical formula of methylbenzene are C₇H₈

 

ethylbenzene C₈H₁₀ 

 propylbenzene, C₉H₁₂     and isomeric 2-phenylpropane,1-methylethyl)benzene

phenylethene ('styrene'), C₈H₈, 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, , 1,3-dimethylbenzene ,

and 1,4-dimethylbenzene , three positional structural isomers of  C₈H₁₀ (once called ortho, meta, para-xylene)

1-ethyl-2-methylbenzene, C₉H₁₂, , and the two other positional structural isomers

1-ethyl-3-methylbenzene, C₉H₁₂, , and

1-ethyl-4-methylbenzene, C₉H₁₂, ,

1,2-diethylbenzene, C₁₀H₁₄, ,   1,3-diethylbenzene,   1,4-diethylbenzene

1,2-dipropylbenzene    , 1,3-dipropylbenzene  ,

and 1,4-dipropylbenzene, 

1-methyl-2-propylbenzene, C₁₀H₁₄, and the two other positional structural isomers

1-methyl-3-propylbenzene, C₁₀H₁₄, and 1-methyl-4-propylbenzene, C₁₀H₁₄,

1-ethyl-4-propylbenzene, C₁₁H₁₆,

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

TOP OF PAGE and sub-index

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.

It is usual to display these aromatic fused ring molecules in the Kekule skeletal formula style.

naphthalene   (one of three possible resonance structures), 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 representations 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.

In fact, there are equivalent to five C=C bonds in naphthalene, contributing to the overlap in forming the pi electron system - delocalisation. The hexagons are not perfect, there are four slightly different bond lengths in naphthalene 0.143, 0.136, 0.142 and 0.141.

All of these bond lengths are less than a single C-C bond (0.154 nm) and greater than double C=C bond (0.134 nm), which supports the merging of the single and double bonds into a delocalised system similar to benzene i.e. naphthalene (and anthracene) are true aromatic compounds.

A The three resonance structure of naphthalene (See section 7.2 for the resonance structures of benzene)

Evidence suggests that structure (I) is more dominant than (II) and (III), and consists of two overlapping 'benzene' rings, each with the equivalent of three double bonds - when the structure of naphthalene is written out in 'Kekule' style.

B A monosubstituted benzene compound (with substituent X) does not need a ring number to denote its position.

A second substituent in the benzene ring must be denoted by a number 2, 3 or 4 in the name to show its position.

C Three examples of naming monosubstituted naphthalene compounds:

However a number is required (1 or 2) for a monosubstituted naphthalene compound - two possible positions.

e.g. 1-nitronaphthalene,  1-bromonaphthalene  and  naphalene-2-sulfonic acid.

 

OLYMPICENE molecular structure, shape and bond angles

Olympicene (caught my imagination in 2012!), C₁₉H₁₂, (M_r = 240, bpt 512^oC), 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 ~120^o. 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 109^o and 120^o (probably around 115^o?). 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?

Halo-arenes - aryl/aromatic 'halides'

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

The halogen atom substituent part of the name is usually the prefix of the aromatic compound's name.

chlorobenzene, C₆H₅Cl, and

1,2 or 1,3 or 1,4-dichlorobenzene, C₆H₄Cl₂, , ,

 

C₇H₇Cl, , (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

1-chloro-2-methylbenzene, 1-chloro-3-methylbenzene and 1-chloro-4-methylbenzene, (old names o-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, dichloro-2,3-methylbenzene, dichloro-2,4-methylbenzene, dichloro-2,5-methylbenzene

or 1,2-dichloro-3-methylbenzene,  2,4-dichloro-1-methylbenzene  and  2,5-dichloro-1-methylbenzene

dichloro-2,6-methylbenzene,   dichloro-3,4-methylbenzene,   dichloro-3,5-methylbenzene,

or 1,3-dichloro-2-methylbenzene,  1,2-dichloro-4-methylbenzene  and  3,5-dichloro-1-methylbenzene (1,3-dichloro-5-methylbenzene)

C₈H₉Cl, 1-chloro-2,4-dimethylbenzene , 1-chloro-2,3-dimethylbenzene,

1-chloro-3,4-dimethylbenzene (4-chloro-1,2-dimethylbenzene), 2-chloro-1,3-dimethylbenzene

1-chloro-2,4-dimethylbenzene, 1-chloro-3,5-dimethylbenzene

and 2-chloro-1,4-dimethylbenzene

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 - don't confuse the two.

The 'phen' part of the name comes from 'phenyl' group C₆H₅, and the 'ol' comes from the OH functional group, but these are NOT aliphatic alcohols - they share some chemistry, but there are significant difference with phenols due to the presence of the benzene ring.

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

, , phenol.

This forms the parent molecule for many of the names of phenols.

The extra substituents (other than OH) as part of the name, are usually the prefix of the phenol's name.

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

sodium phenoxide (old name sodium phenate)

  C₆H₅OCl, 2-chlorophenol (o-chlorophenol),   3-chlorophenol (m-chlorophenol),

and 4-chlorophenol (p-chlorophenol)

C₇H₈O, 2-methylphenol (o-methylphenol, ortho-cresol)

  3-methylphenol (m-methylphenol, meta-cresol), 4-methylphenol (p-methylphenol, para-cresol)

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

C₇H₈O, phenylmethanol (old name 'benzyl alcohol') is a primary aliphatic alcohol which is isomeric with methylphenols (shown above) and the ether, methoxybenzene (anisole).

C₆H₇NO,  2-aminophenol, 3-aminophenol, 4-aminophenol

 2-nitrophenol, 3-nitrophenol, 4-nitrophenol

These molecules can also be called: 2-hydroxynitrobenzene, 3-hydroxynitrobenzene and 4-hydroxynitrobenzene

The above six molecules have two functional groups: phenol and primary aromatic amine and phenol and aromatic-nitro functional groups.

C₇H₆O₃, 3-hydroxybenzoic acid and 2-hydroxybenzoic acid

The carboxylic group is higher ranking than the hydroxy group, so the molecule is named after the parent carboxylic acid.

C₇H₆OCl₂, 2,5-dichloro-4-methylphenol

 

C₆H₆O₂, or 1,4-dihydroxybenzene (benzene-1,4-diol, 1,4-benzenediol, 'quinol') which is easily oxidised to C₆H₄O₂, or 2,5-cyclohexadiene-1,4-dione (cycohexa-2,5-diene-1,4-dione, 'p-quinone')

C₈H₁₀O,1-phenylethanol, a secondary aliphatic alcohol, which is  isomeric with the ether

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

Examples of aromatic fused ring 'phenols', these two are examples of 'naphthols':

1-naphthol and  2-naphthol based on the arene naphthalene

(also called naphthalen-1-ol  and  naphthalen-2-ol)

The 'aldehyde' part of the name is usually the suffix in an aromatic aldehyde's name (as in aliphatic aldehydes).

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

C₇H₆O, or benzaldehyde, and

 C₇H₆O₂, 2-hydroxybenzaldehyde

  Lots more aldehyde structures

The 'ketone' part of the name, one, is usually the suffix in an aromatic ketone's name (as in aliphatic ketones).

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

1-phenylethanone (acetophenone, methyl phenyl ketone), C₈H₈O,   

Ethanone is a fictitious ketone, but the aromatic ring on the 1st carbon of the aliphatic 'eth' part, 'converts' the molecule to a true ketone.

The IUPAC preferred name is 1-phenylethan-1-one.

 

and C₈H₈O₂,  the IUPAC preferred name is 1-(2-hydroxyphenyl)ethanone

  1-(2/3/4-methylphenyl)ethanone (2/3/4-methylacetophenone)

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

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

The name ends in ...carboxylic acid or ...oic acid, in salts and esters the suffix is ...oate

The derivative names are similar for aliphatic carboxylic acids and their derivatives e.g.

acid chlorides -COCl, are named ...oyl chloride,  amides -CONH₂, are named ...amide

and esters are named in the same way e.g. alkyl ...oate

C₇H₆O₂, , , , , benenecarboxylic acid, benzoic acid, (salicylic acid)

sodium benzoate  (the sodium salt of benzoic acid), benzoate ion

C₈H₈O₂, , , , , methyl benzenecarboxylate, methyl benzoate

 

C₈H₈O₃, or   , methyl 2-hydroxybenzoate

(methyl salicylate, 'oil of wintergreen' has mild pain killing properties)

  C₇H₅O₃,  ,  , 2-, 3- or 4-hydroxybenzenecarboxylic acid, 2-, 3- or 4-hydroxybenzoic acid

2-hydroxybenzoic acid is commonly called by its historic trivial name of salicylic acid, aspirin connection!

C₇H₅ClO₂ ,   ,  , 2-, 3- and 4-chlorobenzoic acid  

C₇H₅NO₃ ,  ,  ,  2-, 3- and 4-nitrobenzoic acid  

C₇H₇NO₂ ,  ,  ,    2-, 3- and 4-aminobenzoic acid  

C₈H₈O₂,  2-methylbenzoic acid ,  3-methylbenzoic acid , 4-methybenzoic acid

C₈H₆O₄,  ,  , benzene-1,2-dicarboxylic acid (or  -1,3-  or  -1,4-)

also 1,2-benzenedicarboxylic acid, 1,3-benzenedicarboxylic acid  and  1,4-benzenedicarboxylic acid

(old names: phthalic acid, isophthalic acid and terephthalic acid)

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 (technically it is aminobenzene, but this name is rarely used.

However, when the amino group is a 2nd substituent, the prefix amino can be used OR in suffix phenylamine.

I'm afraid both namings' are widely used, so just get used to them - the examples below show how both are uses!

C₆H₆NCl, 2- or 3- or 4-chlorophenylamine  ,  ,   (primary aromatic amines)

(Here the suffix phenylamine is used, because the amine group is considered a higher ranking group than chloro)

C₆H₈N₂, 1,3-diaminobenzene,  and 3-aminobenzoic acid,   

(Here the prefix amino is used, and the carboxylic acid is considered a higher ranking group than amine/amino.

C₇H₉N, 2-methylphenylamine, methyl-2-phenylamine, 1-amino-2-methylbenzene  

This is isomeric with benzylamine    which is a primary aliphatic amine because the amine group is not directly attached to the ring.

and isomeric C₆H₅-NH-CH₃ is a secondary amine.

(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

(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,

The quaternary ammonium salts would have the formulae [C₆H₅NH(CH₃)₂]⁺  and  [C₆H₅NH(CH₂CH₃)₂]⁺

and C₁₈H₁₅N, triphenylamine , this can be converted to a quaternary ammonium salt (C₆H₅)₄N]⁺ 

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

The simplest primary aromatic acid amide derived from an aromatic carboxylic acid is

benzamide or benzenecarboxamide, C₇H₇NO, , ,

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

N-phenylethanamide, C₈H₉NO, (this is really a phenyl derivative of the aliphatic amide, ethanamide)

N-methylbenzamide, C₈H₉NO,  , and

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

Are secondary amides formed in a condensation polymerisation 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 , linkage

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

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

N,N-dimethylbenzamide, C₉H₁₁NO,  

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

e.g. C₆H₅NH₂(aq) + HNO_2(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

Nitro-aromatic compounds 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 which can also named more simply as: chloro-2-nitrobenzene, chloro-3-nitrobenzene and 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) and

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

The explosive TNT has the structure   , the acronym comes from its historic-trivial name of 2,4,6-trinitotolune, toluene was the old name for methylbenzene.

A more systematic name for TNT is 1-methyl-2,4,6-trinitrobenzene.

Finally a substituted aromatic carboxylic acid, C₇H₅NO₃, 3-nitrobenzoic acid,

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

benzenesulfonic acid, C₆H₆SO₃, C₆H₅SO₃H, C₆H₅SO₂OH, , (benzenesulfonic acid)

2-, 3- or 4-methylbenzenesulfonic acid, C₇H₈SO₃, CH₃C₆H₅SO₂OH,, , 

(or methyl-2/3/4-benzenesulfonic acid)

4-aminobenzenesulfonic acid

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 ... Sulfonic Acids  

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

formula keywords: how to name naming nomenclature empirical molecular formula graphic formula displayed formula skeletal formula structural isomers isomerism for aromatics aromatic compounds 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 ... Sulfonic 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 These detailed notes on the structure and naming of aromatic compounds include the general formula of aromatic compound molecules, empirical formula of aromatic compound molecules, structural formula of aromatic compound molecules, skeletal formula of aromatic compound molecules, displayed formula of aromatic compound molecules, shapes of aromatic compound molecules, isomers of aromatic compound molecules IUPAC rules for aromatic nomenclature. Students should be able to draw structural formula of aromatic, displayed and skeletal formulas for aromatic organic compounds apply IUPAC rules for nomenclature to name aromatic acid organic compounds including chains and rings and be able to apply IUPAC rules for nomenclature to draw the structural, displayed or skeletal structure of aromatic organic compounds from the aromatic IUPAC name from the homologous series of aromatics

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