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Reactions of Benzene

Reactions of Benzene

Have you ever wondered what happens when trinitrotoluene, more commonly known as TNT, explodes? Detonation is triggered by a wave of pressure which causes a decomposition reaction. This produces large amounts of gas in an extremely exothermic reaction; the combination of rapidly expanding gas and heat is what makes TNT so deadly. TNT has a low melting point of only 80 °C, meaning it can be used in liquid form, and is insoluble in water. These properties enable it to be used in a variety of situations, from mining endeavours to military tasks in wet environments.

The decomposition of TNT produces a variety of products: the gases nitrogen, hydrogen, and carbin monoxide, as well as solid carbon. The hydrogen and carbon atoms come from TNT's benzene ring. The nitrogen and oxygen atoms come from three nitrate groups attached to the benzene ring. TNT also contains a methyl group, as shown below.

Reactions of Benzene TNT structure StudySmarterThe structure of TNT. It contains three nitrate groups and one methyl group attached to a benzene ring. StudySmarter Originals

But how do we get from a planar benzene molecule to such a highly branched structure? To understand this, we need to look at the reactions of benzene.

  • This article is about the reactions of benzene in organic chemistry.
  • First of all, we’ll recap the structure of benzene.
  • After that, we'll look at why benzene reacts in electrophilic substitution reactions.
  • We'll explore specific examples of electrophilic substitution reactions, including nitration, chlorination, Friedel-Crafts acylation, and Friedel-Crafts alkylation.
  • We’ll then look at other reactions of benzene, such as combustion and hydrogenation.
  • Finally, we'll consider reactions of benzene derivatives, including the reduction of nitrobenzene and the oxidation of methylbenzene.

What is benzene?

Before we go any further, let’s first remind ourselves about benzene.

Benzene is an aromatic hydrocarbon, also known as an arene, with the molecular formula C6H6.

Each carbon atom within benzene is bonded to two other carbon atoms and one hydrogen atom, forming a cyclical ring. Each carbon atom also has a spare valence electron. We find these electrons in an area formed by overlapping pi orbitals above and below the benzene ring. The electrons can move freely within this area - we say that they are delocalised.

Reactions of Benzene pi orbitals electron delocalisation StudySmarterOverlapping pi orbitals in benzene form a ring of delocalisation. commons.wikimedia.org

If you want a more in-depth explanation of the structure and bonding within benzene, check out Aromatic Chemistry and Benzene Structure.

Electrophilic substitution reactions of benzene

As we now know, benzene contains delocalised pi electrons found in a ring. The ring of delocalisation is relatively strong and stable because it distributes the negative charge of the electrons across a wider area. This means that it takes a lot of energy to disrupt the delocalisation. As a result, benzene doesn’t readily take part in any reactions that involve breaking the ring, such as addition reactions. But on the other hand, it does take part in substitution reactions. To be more precise, these tend to be electrophilic substitutions.

Electrophilic substitution reactions are reactions in which one atom, group of atoms or functional group is replaced by another on a molecule. The reaction is triggered by an electrophile, which is an electron-pair acceptor.

Electrophiles are electron-deficient species, meaning that they have a vacant electron orbital and a positive or partially positive charge on one of their atoms. They are particularly attracted to benzene's ring of delocalisation because of its high electron density. When electrophiles attack benzene, they trigger a substitution reaction. These reactions involve getting rid of some of the hydrogen atoms attached to the carbon ring and replacing them with other, more useful groups of atoms such as nitrate groups or chlorine atoms. But note that we don't disrupt the benzene ring by adding or taking away any of the delocalised electrons - this would simply require too much energy.

Electrophilic substitution reactions of benzene include:

  • Nitration.

  • Chlorination. We'll also look at bromination.

  • Friedel-Crafts reactions.

Each reaction has three steps:

  1. The electrophile is generated.

  2. The electrophile reacts with benzene.

  3. The catalyst is regenerated.

We'll look at each of the reactions in turn.

Electrophilic Substitution of Benzene is actually a pretty complex topic. For example, did you know that different substituents tend to replace different hydrogen atoms in the benzene ring, depending on their identity? This explains why the nitrate groups in TNT have such a particular arrangement. To find out more about the electrophilic substitution of benzene, including the general mechanism, check out the article linked above. In that article, you'll also be able to practice applying the mechanism to specific examples, such as bromination and nitration.

Nitration of benzene

Do you remember TNT from the start of the article? It has three nitrate groups and one methyl group attached to a benzene ring. We nitrate benzene in an example of an electrophilic substitution reaction. Nitrated arenes are important industrially as they are the first step in synthesising aromatic amines, used in products like dyes.

We'll explore how you make aromatic amines later on in the article.

Benzene is nitrated using the nitronium ion, NO2+, which acts as our electrophile. It is generated by mixing concentrated sulfuric and nitric acids (H2SO4 and HNO3). Sulfuric acid is a stronger acid than nitric acid, so nitric acid is forced to act as a base - it accepts a proton given up by sulfuric acid. The reaction forms H2NO3+ and the bisulfate ion (HSO4-); H2NO3+ then breaks down into water and a nitronium ion. The overall equation is shown below:

$$ H_2SO_4+HNO_3\rightarrow NO_2^++HSO_4^-+H_2O $$

Not sure what bases are? Take a quick look at Acids and Bases for more information.

The nitronium ion is an electrophile - an electron pair acceptor with a vacant electron orbital and a positive or partially positive charge. The nitronium ion reacts with benzene because is attracted to benzene’s ring of delocalisation, an area of high electron density. It replaces one of the benzene ring's hydrogen atoms. The reaction involves heating benzene at 50 °C with concentrated sulfuric and nitric acids, using reflux to prevent any volatile components from escaping. This produces nitrobenzene (C6H5NO2) and a hydrogen ion (H+). The hydrogen ion then reacts with the bisulfate ion generated earlier to reform sulfuric acid. This means that sulfuric acid is just a catalyst.

Here's the overall equation:

$$ C_6H_6+HNO_3\rightarrow C_6H_5NO_2+H_2O $$

So how do we get from one nitrate group to three, as seen in TNT? Well, if you heat the reaction up to even higher temperatures, you increase the chance of further nitration reactions happening. Another hydrogen atom is ‘kicked out’ and replaced with a nitrate group. If we count the carbon atom with the original nitrate group as carbon 1, the second nitrate group tends to be directed towards carbon 3 or 5. This is because nitrate groups are electron-withdrawing. For example, nitration reactions produce a lot of 1,3-dinitrobenzene but you won’t find much 1,2-dinitrobenzene!

Reactions of Benzene 1,3-dinitrobenzene StudySmarter1,3-dinitrobenzene. StudySmarter Originals

You might have also noticed the methyl group in TNT. A benzene ring with a methyl group attached is commonly known as toluene, and it reacts a lot faster than a benzene ring without any methyl groups. In fact, if you want to prevent further nitration reactions happening, you have to keep the temperature below 30 °C. Methyl groups are electron releasing and direct any nitrate groups towards positions 2, 4 and 6 in the benzene ring. Just watch out - if you manage to substitute three nitrate groups into the molecule, you’ll have TNT on your hands!

This is just one example of the directing effects of different substituents in electrophilic substitution reactions of benzene. As we mentioned earlier, you'll out more about this topic, as well as the mechanism for not only the nitration of benzene but also all the other electrophilic substitution reactions that you need to know about, over at Electrophilic Substitution of Benzene.

Chlorination of benzene

We can also swap hydrogen atoms on a benzene ring with chlorine atoms, using aluminium chloride (AlCl3) as a catalyst. This is another type of electrophilic substitution reaction and takes place at room temperature.

Aluminium chloride reacts with chlorine to form a positive chlorine cation (Cl+) and a negative aluminium tetrachloride ion (AlCl4-). Here's the equation:

$$ Cl_2+AlCl_3\rightarrow Cl^++AlCl_4^- $$

The chlorine cation acts as our electrophile. It reacts with benzene, forming chlorobenzene (C6H5Cl) and a hydrogen ion. Like in the nitration reaction, the hydrogen ion reacts with the aluminium tetrachloride ion produced earlier to reform our catalyst, aluminium chloride. This also produces hydrochloric acid (HCl).

Here's the overall equation:

$$ C_6H_6+Cl_2\rightarrow C_6H_5Cl+HCl $$

We can brominate benzene in a similar way. Simply swap chlorine gas for bromine (Br2), and use the catalyst aluminium bromide (AlBr3) instead of aluminium chloride. Alternatively, you can use iron(III) chloride or iron(III) bromide (FeCl3 or FeBr3) for these two respective reactions.

Friedel-Crafts reactions of benzene

Friedel-Crafts reactions were invented in 1877 by the chemists Charles Friedel and James Crafts, of French and American origin respectively. They are a means of attaching different substituents to an aromatic benzene ring.

Friedel-Crafts reactions include:

  • Friedel-Crafts acylation. We'll look at this reaction with both acyl chlorides and acid anhydrides.
  • Friedel-Crafts alkylation.

Friedel-Crafts acylation of benzene

You might know from Acylation that acylation reactions involve adding the acyl group, -RCO-, to another molecule. Benzene is acylated by heating an acid derivative, such as an acyl chloride (RCOCl) or acid anhydride (RCOOCOR), with aluminium chloride at 60 °C. The reaction takes place in anhydrous conditions under reflux. We'll focus on acylation using an acyl chloride.

Our acyl chloride first reacts with aluminium chloride, our catalyst, to generate the electrophile (RCO+) and a negative aluminium tetrachloride ion (AlCl4-). Note that in the equation below, R represents the acyl chloride's alkyl group:

$$ AlCl_3+RCOCl\rightarrow RCO^++AlCl_4^- $$

The electrophile reacts with benzene to form a ketone with a benzene ring attached (C6H5COR), and a hydrogen ion (H+). We name the ketone using the prefix phenyl-. Like before, the hydrogen ion is used to regenerate the catalyst, which also produces hydrogen chloride (HCl).

Remember - this reaction takes place in anhydrous conditions, meaning HCl is hydrogen chloride. If the reaction was instead in solution, we would call this species hydrochloric acid.

Overall, we end up with the following reaction:

$$ C_6H_6+RCOCl\rightarrow C_6H_5COR+HCl $$

Write an equation for the reaction between ethanoyl chloride and benzene in the presence of aluminium chloride. Name the organic product formed.

Ethanoyl chloride is an acyl chloride with the formula CH3COCl. It therefore reacts with benzene to produce a ketone with a benzene ring attached, and hydrochloric acid. Ethanoyl chloride has two carbon atoms, and so our ketone produced also has two carbon atoms.. Hence, the organic product is named phenylethanone.

$$ C_6H_6+CH_3COCl\rightarrow C_6H_5COCH_3+HCl $$

Friedel-Crafts acylation using acid anhydrides

In much the same way as acyl chlorides, acid anhydrides react with benzene to produce a ketone with a benzene ring attached. This reaction is almost exactly the same as all the other ones we've explored today - making it a lot easier to remember!

Once again, we use aluminium chloride as a catalyst to produce a positive electrophile cation (RCO+). But this time we produce a different negative ion (AlCl3OOCR-):

$$ AlCl_3+RCOOCOR\rightarrow RCO^++AlCl_3OOCR^- $$

The positive cation electrophile reacts with benzene to produce a ketone and a hydrogen ion. Again, the hydrogen ion regenerates the catalyst. However, this time the regeneration produces a carboxylic acid (RCOOH) instead of hydrochloric acid.

Here's the overall equation:

$$ C_6H_6+RCOOCOR\rightarrow C_6H_5COR+RCOOH $$

Friedel-Crafts alkylation of benzene

Finally, let's consider Friedel-Crafts alkylation of benzene. If acylation reactions involve adding an acyl group to a molecule, then alkylation reactions involve adding an alkyl group to a molecule. We do this by reacting benzene with a halogenoalkane, typically a chloroalkane (RCl), in the presence of an aluminium chloride catalyst.

The chloroalkane first reacts with aluminium chloride to produce a positive carbocation (R+) and a negative ammonium chloride ion:

$$ RCl+AlCl_3\rightarrow R^++AlCl_4^- $$

The positive carbocation is an electrophile, and attacks benzene to produce an alkylarene (C6H5R) and a hydrogen ion. The hydrogen ion regenerates the catalyst and also releases hydrochloric acid.

Overall, we get the following reaction:

$$ C_6H_6+RCl\rightarrow C_6H_5R+HCl $$

Write an equation for the reaction between chloromethane and benzene in the presence of aluminium chloride. Name the organic product formed.

Chloromethane is a halogenoalakane and so reacts with benzene to produce an alkylarene and hydrochloric acid. As the name suggests, chloromethane consists of a methane molecule with one chlorine atom instead of a hydrogen atom, and so our final organic product will be a methyl group attached to a benzene ring. This molecule is named methylbenzene.

$$ C_6H_6+CH_3Cl\rightarrow C_6H_5CH_3+HCl $$

Sulfonation of benzene

We can introduce the sulfonic acid group, SO3, to benzene in another electrophilic substitution reaction. This is done, for example, by heating benzene with concentrated sulfuric acid under reflux. It forms white crystals of benzenesulfonic acid.

Summary of benzene electrophilic substitution reactions

Phew - you made it through all the electrophilic substitution reactions! Here's a handy table to help summarise the new material.

Name of reactionReactantCatalystProducts
NitrationHNO3H2SO4C6H5NO2, H2O
ChlorinationCl2AlCl3C6H5Cl, HCl
Friedel-Crafts acylation with acyl chlorideRCOClAlCl3C6H5COR, HCl
Friedel-Crafts alkylationRXAlCl3C6H5R, HCl

Other reactions of benzene

Although electrophilic substitution reactions are the most common type of reaction involving benzene, aromatic compounds do take part in other reactions. You don’t need to know the mechanisms for these reactions. Some examples include:

Combustion

Benzene burns just like any other hydrocarbon to produce carbon dioxide and water in a combustion reaction.

Try writing an equation for the complete combustion of benzene. You should get the following:

$$ C_6H_6+7.5 O_2\rightarrow 6CO_2+3H_2O $$

However, because of its high proportion of carbon, benzene often combusts incompletely. This produces a lot of carbon in the form of soot.

Hydrogenation

As the name suggests, hydrogenation involves adding hydrogen to a molecule. Hydrogenating benzene creates a cyclic alkane, cyclohexane. However, the reaction has a high activation energy as it involves breaking benzene’s stable ring of delocalised electrons. It uses hydrogen gas (H2), a nickel catalyst and high temperatures and pressures.

For example, hydrogenating methylbenzene (C6H5CH3) produces methylcyclohexane (C6H11CH3), as shown below:

$$ C_6H_5CH_3+2.5H_2\rightarrow C_6H_{11}CH_3 $$

Reactions of Benzene hydrogenation methylbenzene StudySmarterThe hydrogenation of methylbenzene. StudySmarter Originals

Reactions of benzene derivatives

Last of all, let's consider some reactions of benzene derivatives. These include:

  • Oxidation of an alkylarene to produce benzoic acid.
  • Reduction of nitrobenzene to produce a phenylamine.

Oxidation of alkylarenes

Producing carboxylic acids usually involves the Oxidation of Alcohols. But to produce benzoic acid (C6H5COOH), which is a molecule containing the carboxyl group (-COOH) attached to a benzene ring, we can simply oxidise the side chain of an alkylarene. This involves refluxing an alkylarene with alkaline potassium manganate(VII) (KMnO4) followed by sulfuric acid (H2SO4). The same reaction can be done with almost any alkylarene and always produces benzoic acid, plus water. There is just one catch - the carbon atom directly bonded to the benzene ring must also be joined to a hydrogen atom.

Here's the equation for the oxidation of the simplest alkylarene, methylbenzene (C6H5CH3):

$$ C_6H_5CH_3+3[O]\rightarrow C_6H_5COOH+H_2O $$

Reactions of benzene oxidation of methylbenzene StudySmarterThe oxidation of methylbenzene. StudySmarter Originals

With more complicated alkylarenes such as propylbenzene (C6H5CH2CH2CH3) or 1-methylethylbenzene (C6H5CH(CH3)CH3), it suddenly becomes a lot trickier to write a balanced chemical equation for this reaction. However, all of these oxidation reactions produce the same organic molecule - no matter the structure of the original alkyl side chain. As long as the carbon directly bonded to the benzene ring is also joined to a hydrogen atom, the alkylarene will always oxidise into benzoic acid.

Reduction of nitrobenzene

Earlier in the article, we explored how we create nitrobenzene by nitrating benzene. We can replace the nitro group (-NO2) in nitrobenzene with an amine group (-NH2) in a reduction reaction, forming phenylamine (C6H5NH2). We first heat nitrobenzene under reflux with tin (Sn) and concentrated hydrochloric acid (HCl), then add sodium hydroxide (NaOH).

Here's the equation:

$$ C_6H_5NO_2+6[H]\rightarrow C_6H_5NH_2+2H_2O $$

Reactions of Benzene reduction of nitrobenzene StudySmarterThe reduction of nitrobenzene. StudySmarter Originals

Reactions of Benzene - Key takeaways

  • Benzene is an aromatic compound with the formula C6H6. It contains a ring of delocalised electrons.
  • Benzene’s ring of delocalisation is very strong, meaning it doesn’t take part in addition reactions. Most of the reactions of benzene are instead electrophilic substitution reactions.
  • Electrophilic substitution reactions of benzene include:
    • Nitration, producing nitrobenzene.
    • Chlorination, producing chlorobenzene.
    • Acylation, producing an aromatic ketone.
    • Alkylation, producing an alkyarene.
  • Benzene can also take part in combustion and hydrogenation reactions.
  • Certain benzene derivatives can be both oxidised and reduced to produce benzoic acid and phenylamine respectively.

Frequently Asked Questions about Reactions of Benzene

Bromination of benzene is a type of electrophilic substitution reaction.

Benzene normally undergoes electrophilic substitution reactions. This is because addition reactions would involve disrupting its stable ring of delocalised electrons.

Addition reactions of benzene are difficult because they would involve disrupting benzene's stable ring of delocalised electrons. This takes a lot of energy. 

Nitration of benzene is a substitution reaction because a hydrogen atom from benzene is swapped for a nitrate group. 

No, benzene doesn't normally give elimination reactions.

Final Reactions of Benzene Quiz

Question

Name the organic family benzene belongs to.

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Answer

Arenes

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Question

Describe the structure and bonding within benzene.


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Answer

  • Benzene has 6 carbon and 6 hydrogen atoms.
  • Each carbon atom forms bonds with two other carbon atoms and one hydrogen atom using electrons in sigma orbitals.
  • The remaining electron from each carbon atom is found in a pi orbital. 
  • All these pi orbitals overlap to form a ring of delocalisation above and below the benzene ring.

Show question

Question

Define electrophile.


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Answer

An electron pair acceptor with a positive or partial positive charge and a vacant electron orbital.

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Question

Explain what happens in a substitution reaction.


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Answer

One atom or group of atoms is replaced by another.

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Question

Explain why benzene does not readily take part in addition reactions.


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Answer

It would involve breaking benzene’s strong and stable ring of delocalised electrons, which requires a lot of energy.

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Question

What sort of reaction is the nitration of benzene?


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Answer

Electrophilic substitution

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Question

Which of the following conditions are required for the nitration of benzene?


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Answer

50 °C

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Question

State the conditions required for Friedel-Crafts acylation of benzene.


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Answer

60 °C, anhydrous, reflux, aluminium chloride catalyst

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Question

Name the organic molecule formed when methylbenzene is fully hydrogenated.


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Answer

Methylcyclohexane

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Question

Name the reactants, catalyst, and products of Friedel-Crafts alkylation of benzene.

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Answer

  • Reactants: Benzene, halogenoalkane.
  • Catalyst: Aluminium chloride.
  • Products: Alkylbenzene, hydrochloric acid

Show question

Question

Which of the following reactions of benzene require an aluminium chloride catalyst?

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Answer

Nitration

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Question

How do you reduce nitrobenzene, and what does this reaction produce?

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Answer

  • Reflux with tin (Sn) and concentrated hydrochloric acid (HCl), then add sodium hydroxide (NaOH). 
  • Produces phenylamine and water.

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Question

How do you oxidise alkylbenzenes, and what does this reaction produce?

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Answer

  • Reflux with alkaline potassium manganate(VII) (KMnO4) followed by sulfuric acid (H2SO4).
  • Produces benzoic acid and water.

Show question

Question

Which of the following statements is true?

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Answer

Alkylbenzenes can be oxidised if the carbon atom bonded to the benzene ring is also joined to a hydrogen atom. This reaction always produces benzoic acid.

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Question

What are electrophilic substitution reactions?

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Answer

Electrophilic substitution reactions are reactions in which one atom, group of atoms or functional group is replaced by another on a molecule. The reaction is triggered by an electrophile, which is an electron-pair acceptor with an empty electron orbital and a positive or partially-positive charge.

Show question

Question

Which of the following are benzene electrophilic substitution reactions?

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Answer

Friedel-Crafts acylation

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Question

True or false? Benzene electrophilic substitution reactions all follow the same general mechanism.

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Answer

True

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Question

True or false? Electrophiles are electron pair donators.

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Answer

False

Show question

Question

Give the catalyst for the nitration of benzene.

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Answer

Concentrated sulphuric acid

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Question

Give the overall equation for the nitration of benzene.

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Answer

H2SO4 + HNO3  NO2+ + HSO4- + H2O 

Show question

Question

What are the three steps of a benzene electrophilic substitution reaction?

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Answer

  1. The electrophile is generated.
  2. The electrophile reacts with benzene using the general benzene electrophilic substitution mechanism.
  3. The catalyst is regenerated.

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Question

Which of the following are possible combinations of a reagent and catalyst for the halogenation of benzene?

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Answer

A halogen and aluminium halide.

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Question

Write the general equation for the halogenation of benzene.

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Answer

C6H+ X2  C6H5X + HX

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Question

Write the general equation for the acylation of benzene.

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Answer

C6H+ RCOCl  C6H5COR + HCl 

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Question

Write the general equation for the alkylation of benzene.

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Answer

C6H+ RCl  C6H5R + HCl

Show question

Question

Which of the following electrophilic substitution reactions of benzene produce HCl?

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Answer

Chlorination

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Question

Methylbenzene can take part in two types of chlorination reaction: electrophilic substitution involving its benzene ring, and free radical substitution involving its methyl group. Give the conditions for each type of reaction.

Show answer

Answer

Electrophilic substitution: Room temperature, chlorine gas, aluminium chloride catalyst.


Free radical substitution: chlorine gas, UV light.

Show question

Question

Which of the following statements are true?

Show answer

Answer

Electron-withdrawing groups direct electrophilic attack towards carbon atoms 3 and 5.

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Question

How do directing effects influence benzene substitution reactions?

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Answer

Certain functional groups attached to the benzene ring affect further substitution reactions and encourage electrophiles to attack certain carbon atoms.

Show question

Question

Which of the following groups are electron-donating?

Show answer

Answer

-R

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Question

Which of the following groups are electron-withdrawing?

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Answer

-NO2

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