Aromatic Chemistry

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Classes of organic chemicals and where benzene fit

There are two major classes of organic chemicals:

  • Aliphatic: straight or branched chain organic substances aromatic
  • Arene: includes one or more ring of six carbon atoms with delocalised bonding.
  • All of the organic substances we have looked at so far have been aliphatic.
  • Benzene belongs to the aromatic class. 


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In other molecules, the benzene ring can be regarded as a substituent side group on another molecule, like alkyl groups are. The C6H5 - group (the benzene ring) is known as the phenyl group. 

(                                           ( (Benzene ring with OH)              

(                                   (

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Formation of Acylation electrophile

Formation of Acylation electrophile (RCO+)


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Nitration of Benzene Mechanism

Remember: curly arrows = movement of an electron pair


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Naming aromatic molecules

Naming aromatic compounds can be complicated. The simplest molecules are derivatives of benzene and have benzene at the root of the name


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Acylation of Benzene Mechanism

Step 1:


Step 2


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Enthalpy of hydrogenation

Enthalpy of hydrogenation: the enthalpy change when 1 mole of unsaturated compound is saturated by reacting with hydrogen under standard conditions.

( Benzene is 152 kJ/mol more stable than Kekule's suggested benzene,as it only releases -208 kJ/mol on hydrogenation compared to the -360 kJ/mol that it should. This is because of the delocalisation of 6 pi electrons around carbon atoms in its ring structure. 

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Nitration of benzene- General info

  • Change in functional group: benzene nitrobenzene
  • Reagents: conc nitric acid in the presence of concentrated sulphuric acid (catalyst)
  • Mechanism: Electrophilic Substitution
  • Electrophile: NO2+
  • This reaction is done at about 50 degrees. If it occurs any higher, multiple nitrate groups may be substituted

Equations of electrophile formation:
1. H2SO4 + HNO3 -> HSO4- + H2NO3+
2. H2NO3+ -> NO2+ + H2O

Importance of this reaction
Nitration of benzene and other arenes is an important step in synthesising useful compounds e.g. explosive manufacture (like TNT, trinitrotoluene/ 2,4,6- trinitromethylbenzene) and formation of amines from which dyestuffs are manufactured

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Friedel Crafts Acylation- General info

  • Change in functional group: benzene -> phenyl ketone
  • Reagents: acyl chloride. Any acyl chloride can be used RCOCl where R is any alkyl group e.g. –CH3 , -C2H5 . The electrophile is the RCO+, this is called an acylium ion.
  • Catalyst: Anhydrous aluminium chloride (AlCl3). It can do this job because it is deficient of electrons. Draw a dot and cross to check this. The AlCl3 is said to be a Lewis Acid because it is an electron pair acceptor, as AlCl3 accepts an electron pair from the Cl in the ROCl to form a dative bond. 
  • Conditions: heat under reflux (50oC) 
  • Mechanism: Electrophilic Substitution

Equation for Formation of the electrophile:  AlCl3 + CH3COCl -> CH3C+=O (electr.) + AlCl4- 

  • These are important reactions in organic synthesis because they introduce a reactive functional group on to the benzene ring.
  • Used to make aromatic ketones
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Benzene structure

Benzene’s Structure

  • There are two major classes of organic chemicals aliphatic : straight or branched chain organic substances aromatic or arene: includes one or more ring of six carbon atoms with delocalised bonding.
  • The simplest arene is benzene. It has the molecular formula C6H6
  • Its basic structure is six C atoms in a hexagonal ring, with one H atom bonded to each C atom. Each C atom is bonded to two other C atoms and one H atom by single covalent σ-bonds.
  • This leaves one unused electron on each C atom in a p orbital, perpendicular to the plane of the ring. The Six p electrons are delocalised in a ring structure above and below the plane of carbon atoms.
  • The six electrons in the pi bonds are delocalised and spread out over the whole ring. Delocalised means not attached to a particular atom
  • Delocalisation of p electrons makes benzene more stable than the theoretical molecule cyclohexa-1,3,5-triene.
  • Benzene is a planar molecule.The evidence suggests all the C-C bonds are the same and have a length and bond energy between a C-C single and C=C double bond.
  • The H-C-C bond angle is 120o in Benzene 
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Two or more substituents on a benzene

If two or more substituents are present on the benzene ring, their positions must be indicated by the use of numbers. This should be done to give the lowest possible numbers to the substituents. When two or more different substituents are present, they are listed in alphabetical order and di, tri prefixes should be used.


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Toxicity of Benzene

Toxicity of Benzene

  • Benzene is a carcinogen (cancers causing molecule) and is banned for use in schools.
  • Methylbenzene is less toxic and also reacts more readily than benzene as the methyl side group releases electrons into the delocalised system making it more Nitration attractive to electrophiles.
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Benzene: Electrophilic substitution vs Addition

The delocalisation of the pi electrons confers greater stability on the compound and makes them LESS LIKELY to undergo electrophilic ADDITION and therefore more likely to undergo electrophilic substitution. 

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Benzene is more stable than cyclohexa-1,3,5-triene

Delocalisation of p electrons makes benzene more stable than the theoretical molecule cyclohexa-1,3,5-triene (Kekule's Benzene)

                      vs.                   Image result for benzene symbol

There is only one identifiable form of 1,2-dichlorobenzene. This discredits the structure of benzene proposed by Kekule because if he was correct, there should be 2 isomers of 1,2-dichlorobenzene however only one exists.

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Benzene bond length

  • C-C bond length in cyclohexane is 0.15nm
  • C=C bond length in cyclohexene is 0.13nm
  • C-..-C bond length in benzene is 0.14nm

All bond lengths in benzene are the same and are 0.14nm. This implies that there are a mixture of double and single bonds.

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