Chapter 25 - Aromatic Chemistry Flashcards
How was benzene discovered?
Benzene, C6H6, was first discovered in 1825 by the English scientist Michael Faraday, who isolated and identified it in an oily residue from the gas that was used for street lighting.
What is Benzene classified as?
a. Benzene is classed as an aromatic hydrocarbon or arene.
b. A benzene molecule consists of a hexagonal ring of six carbon atoms, with each carbon atom joined to two other carbon atoms and to one hydrogen atom
What is the systematic name given to the Kekule structure?
The systematic name given would be cyclohexa-1,3,5-triene.
What evidences disproved the kekule model?
a. Bond length (0.139nm) intermediate between the length of a C-C (long, 0.153nm) and C=C (short, 0.134nm).
b. delta H hydrogenation less eothermic than expected when compared to delta H hydrogenation for cyclohexene.
c. Only reacts with Br2 at high temperature or in presence of a halogen carrier catalyst / resistant to electrophilic attack.
What are the main features of the delocalised model of benzene?
a. Benzene is a planar, cyclic, hexagonal hydrocarbon containing six carbon atoms and six hydrogen atoms.
b. Each carbon atom uses three of its available four electrons in bonding to two other carbon atoms and to one hydrogen atom.
c. Each carbon atom has one electron in a p-orbital at right angles to the plane of bonded carbon and hydrogen atoms.
d. Adjacent p-orbital electrons overlap sideways, in both directions, above and below the plane of the carbon atoms to form a ring of electron density.
e. The overlapping of the p-orbitals creates a system of pi-bonds which spread over all six of the carbon atoms in the ring structure.
f. The six electrons occupying this system of pi-bonds are said to be delocalised.
How are aromatic compounds with one substituent group named?
a. In monosubstituted aromatic compounds, the benzene ring is considered the parent chain. Alkyl groups (CH3, C2H5), halogens (F, Cl, Br, I), and nitro (NO2), groups are all considered the prefixes to benzene.
i. i.e. ethylbenzene, chlorobenzene and nitrobenzene.
b. When a benzene ring is attached to an alkyl chain with a functional group or to an alkyl chain with seven or more carbon atoms, benzene is considered to be a substituent. Instead of benzene, the prefix phenyl is used in the name.
In monosubstituted aromatic compounds, what are the exceptions?
a. Benzoic acid (benzenecarboxylic acid), phenyl amine, and benzaldehyde (benzenecarbaldehyde).
b. Benzoic acid is a benzene attached to a COOH group.
c. Phenylamine is a NH2 group attached to a benzene.
d. Benzaldehyde is a benzene attached to a CHO group.
How are aromatic compounds with more than one substituent group named?
a. Some molecules may contain more than one substituent group on the benzene ring, for example, disubstituted compounds have two substituent groups.
b. The ring is now numbered, just like a carbon chain, starting with one of the substituent groups. The substituent groups are listed in alphabetical order using the smallest numbers possible.
What is the reactivity of benzene and its derivatives?
a. Benzene and its derivatives undergo substitution reactions in which a hydrogen atom on the benzene ring is replaced by another atom or group of atoms.
b. Benzene typically reacts with electrophiles and most of the reactions of benzene proceed by electrophilic substitution.
What is the nitration of benzene?
a. Benzene reacts slowly with nitric acid to form nitrobenzene.
b. The reaction is catalysed by sulfuric acid and heated to 50 degrees to obtain a good rate of reaction. A water bath is used to maintain the steady temperature.
c. In nitration, one of the hydrogen atoms on the benzene ring is replaced by a nitro, -NO2, group.
d. So, benzene reacts with nitric acid at 50 degrees with sulfuric acid to form nitrobenzene and water.
What happens if the temperature of the reaction rises above 50 degrees?
Further substitution reactions may occur leading to the production of dinitrobenzene. i.e. increasing the temperature to 70 degrees may result in a disubstituted benzene, if excess nitric acid is used.
What is nitrobenzene important for?
a. Nitrobenzene is an important starting material in the preparation of dyes, pharmaceuticals, and pesticides.
b. It can be used as a starting material in the preparation of paracetamol.
What is the reaction mechanism for the nitration of benzene, what type of reaction is it and how is the catalyst regenerated?
It is an electrophilic substitution reaction.
How can you undergo halogenation of benzene reactions?
a. Halogens do not react with benzene unless a catalyst called a halogen carrier is present.
b. Common halogen carriers include AlCl3, FeCl3, AlBr3, and FeBr3, which can be generated in situ (in the reaction vessel) from the metal and the halogen.
How can benzene be brominated?
a. At room temperature and pressure and in the presence of a halogen carrier, benzene reacts with bromine in an electrophilic substitution reaction. In bromination, one of the hydrogen atoms on the benzene ring is replaced by a bromine atom.
b. The electrophile in the substitution reaction is Br+ (bromonium ion), which is generated when the halogen carrier catalyst reacts with bromine.
How can benzene be chlorinated?
Chlorine will react with benzene in the same way as bromine and following the same mechanism. The halogen carrier used is FeCl3, AlCl3, or iron metal and chlorine, which react to make FeCl3.
What are alkylation reactions?
a. The alkylation of benzene is the substitution of a hydrogen atom in the benzene ring by an alkyl group. The reaction is carried out by reacting benzene with a haloalkane in the presence of AlCl3 if chlorine is the halogen in the haloalkane used, which acts as a halogen carrier catalyst, generating the electrophile.
b. Alkylation increases the number of carbon atoms in a compound by forming carbon-carbon bonds.
c. The reaction is sometimes called a Friedel-Crafts alkylation after the two chemists who first carrier out the reaction.
What are acylation reactions?
a. When benzene reacts with an acyl chloride in the presence of an AlCl3 catalyst, an aromatic ketone is formed. This is called an acylation reaction and is another example of an electrophilic substitution reaction. The reaction forms carbon-carbon bonds and is again useful in organic synthesis.
b. Ethanoyl chloride, CH3COCL, is the first member of the acyl chloride homologous series. Phenylethanone is produced in the reaction between benzene and ethanoyl chloride and is used in the perfume industry.
What is the difference in reactivity between alkenes and arenes?
a. Alkenes decolourise bromine by an electrophilic addition reaction. i.e. the reaction between cyclohexene and bromine.
b. In this reaction, the pi-bond in the alkene contains the localised electrons above and below the plane of the two carbon atoms in the double bond. This produces an area of high electron density. The localised electrons in the pi-bond induce a dipole in the non-polar bromine molecule making one bromine atom of the bromine molecule slightly positive and the other slightly negative. The slightly positive bromine atom enables the bromine molecule to act like an electrophile.
c. Unlike alkenes benzene do not react with bromine unless a halogen carrier catalyst is present. This is because benzene has delocalised pi-electrons spread above and below the plane of the carbon atoms in the ring structure. The electron density around any two carbon atoms in the benzene ring is less than that in a C=C double bond in an alkene.
d. When a non-polar molecule such as bromine approaches the benzene ring there is insufficient pi-electron density around any two carbon atoms to polarise the bromine molecule. This prevents any reaction taking place.
What is a phenol?
a. Phenols are a type of organic chemical containing a hydroxyl, -OH, functional group directly bonded to an aromatic ring.
b. The simplest member of the phenols, C6H5OH, has the same name as the group – phenol.
What are phenols used in?
a. Phenols are used in many everyday antiseptics.
b. It is also an important chemical used in the production of disinfectants, detergents, plastics, paints and aspirin.
How was phenol originally manufactured?
a. Phenol used to be made from benzene, C6H6, using sulfuric acid and sodium hydroxide in a multi-stage process.
i. C6H6 + H2SO4 + 2NaOH ==> C6H5OH + Na2SO3 + 2H20.
How is phenol currently manufactured?
a. The majority of phenol is manufactured from benzene and propene, C3H6, in a multistep reaction.
i. C6H6 + C3H6 + O2 ==> C6H5OH + CH3COCH3
b. This reaction has an 86% yield of phenol from benzene. The other product is propanone, CH3COCH3 (acetone).
c. Although, acetone is useful, it is in less demand than phenol, and companies find it difficult to make use of all the acetone produced.
Which method are research chemists investigating to manufacture phenol in the future?
a. Research chemists are investigating manufacturing phenol using benzene and nitrogen (I) oxide, N20 (also called nitrous oxide).
i. C6H6 + N2O ==> C6H5OH + N2.
b. This reaction has a 95% yield of phenol from benzene.
c. Nitrous oxide is a gaseous waste product from the production of nylon and cannot be allowed to escape into the atmosphere, as it is a greenhouse gas.
