Unit 14 Aim B: Reactions and Properties of Aromatic Compounds Assignment (DISTINCTION)
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Unit 14 - Applications of Organic Chemistry
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PEARSON (PEARSON)
This is my distinction grade assignment for unit 11 aim B on the reactions and properties of aromatic compounds benzene, phenol and toluene/methylbenzene. All criteria were met and I was awarded distinction.
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Emily Bullas - Reactions and Properties of Aromatic Compounds
Structure of Benzene
Benzene is an aromatic compound made up of carbon and hydrogen atoms. Each carbon atom is
bonded to two carbon atoms and a hydrogen atom. Carbon usually makes 4 bonds, but in this
structure, it does not have enough unpaired electrons to form a fourth bond. Because each carbon
forms only 3 bonds, only 3 of their orbitals need to be hybridised before forming bonds. They
hybridise the 2s electron and two 2p electrons, but the third 2p electron is not changed. The orbitals
formed are called sp² orbitals as they are made of an s orbital and two p orbitals. The hybridised
orbitals are arranged at 120° so that they are as far away from each other as possible, with the
unhybridized p orbital at a right angle to them (1) The electrons are delocalised and form a ring
above and below the carbon atoms.
The structure of benzene was proven Xray crystallography in 1928. This experiment proved that each
bond in the structure is the same length, due to hybridisation and electron delocalisation. This
disproved the previous model, the Kelule model, which suggested that there was two versions of
benzene, single bonded and double bonded, which alternate in a resonance structure. However,
single bonds have a length of 0.154nm and double bonds are 0.133nm long, whereas benzene’s
bonds are 0,139nm long (2).
Chemical Properties of Benzene, Phenol and Methylbenzene
Benzene is generally quite reactive due to its unsaturation, however, is more likely to undergo
substitution reactions than addition reactions since addition would reduce stability as delocalisation
is broken. In substitution reactions, one or more hydrogen atom is replaced, and the delocalised
electrons remain as they are. Methylbenzene’s reaction is divided into two sections, the reactions of
methyl group and the reactions of the benzene ring. The two elements both impact on the others
reactivity, with the methyl group reacting differently when attached to a benzene ring and the ring
reacting differently when it has a methyl group attached. The methyl group has a tendency to repel
electrons, towards the ring, which creates a small permanent dipole, making it more reactive than
benzene (3). Phenol also has a tendency to undergo substitution reactions, even more so than
benzene, due to the oxygen’s lone pair of electrons being donated into the molecule, which
increases the electron density around the ring. This increased density allows phenol to polarise and
attract electrophiles more easily than benzene and methylbenzene (11).
Friedel-Craft Acylation of Phenol and Methylbenzene
Acylation reactions involve the substitution of an acyl group in place of a hydrogen on the aromatic
ring, forming a new carbon-carbon bond. This reaction requires a Lewis acid catalyst such as
aluminium chloride and an acid chloride.
The acyl group most commonly used is CH₃CO-, the ethanoyl group. The most reactive substance
which contains an acyl group is acyl chloride, which have the general formula RCOCl. Ethanoyl
chloride is reacted with benzene to produce phenylethanone, seen in figure 1 below. The conditions
for this reaction are a temperature of around 60°C and a catalyst of aluminium chloride. The acyl
group substitutes into the 2 and 4 positions, but almost all in the 4 position. (7)
, Figure 1: Friedel-Craft Acylation of Methylbenzene with Ethanoyl Chloride
Under the same conditions, phenols react in a way that means the acyl group is substituted in place
of the hydrogen in the hydroxyl group, not a hydrogen in the aromatic ring. This is known as O-
acylation and produces a phenolic ester. This means that, since the acyl group is not substituted into
the ring and no carbon-carbon bond is formed, phenol does not undergo Friedel-Craft Acylation (8).
The O-acylation reaction which it undergoes is shown in figure 2 below.
Figure 2: O-acylation of Phenol
Halogenation of Phenol and Methylbenzene
Phenols contain a hydroxyl group with a highly activating effect, which allows them to undergo
halogenation, even without the presence of Lewis acids. When they are treated with bromine,
monobromophenols are formed. The conditions required for this reaction are low temperatures and
the presence of a low polarity solvent, such as CHCl₃ (5). Figure 3 below shows the equation for this
reaction.
Figure 3: Halogenation of Phenol Using Bromine
Methylbenzene can have two possible halogenation reactions with bromine and chlorine depending
on the conditions of the reaction. The halogen group can substitute into either the ring or the
methyl group. At room temperature, in the presence of aluminium chloride/bromide or iron, and in
the absence of UV light, the chloride/bromide will be substituted into the ring. The halogen group
will substitute into the ring in the position either directly next to or opposite the methyl group – this
is position 2 and 4 respectively, assuming the methyl group is in the 1 position. Figure 4 below shows
the two possible reaction equations for the reaction between methylbenzene and chlorine,
producing 2-chloromethylbenzene and 4-chloromethylbenzene. In reality, a mixture of both of these
products would be formed (6).
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