Module 6: Chapter 25 - Aromatic Chemistry Flashcards
What does the kekulé structure of benzene look like?
What is the molecular formula of benzene?
C₆H₆
What is the relative formula mass of benzene?
78
What is an arene?
An aromatic hydrocarbon, a hydrocarbon containing one or more benzene rings
What are the risks associated with benzene?
- It is highly toxic and carcinogenic
- It is colourless and highly volatile
- It can be absorbed through the skin
- It is highly flammable
What are the 3 problems associated with the kekulé structure?
- The lack of reactivity of Benzene
- The length of carbon-carbon bonds
- Hydrogenation enthalpies
What are the 2 models of benzene?
- Kekulé structure
- Delocalised Model
How is the lack of reactivity of benzene evidence disproving the kekule model?
If benzene contains double bonds it should decolourise bromine water in an electrophilic addition reaction. However benzene does not undergo any electrophilic addition reactions and it does not decolourise bromine water under normal conditions
How is the lengths of carbon-carbon bonds evidence disproving the kekule model?
Double bonds are slightly shorter than single bonds, this should make the structure of benzene an irregular hexagon. However, benzene is a regular hexagon with each bond being of equal length
How are hydrogenation enthalpies evidence disproving the kekule model?
If benzene did have the kekule structure, it would be named cyclohexa-1,3,5-triene. Therefore, it would be expected to have an enthalpy change of hydrogenation of 3 times that of cyclohexene (as it contains 3 times as many double bonds). However, the actual enthalpy change is much lower (in magnitude) than that expected
Why was the delocalised model of Benzene produced?
Due to evidence disproving the kekule structure
Describe the structure of the delocalised model of benzene
- Benzene is planar, cyclic, hexagonal hydrocarbon containing 6 carbon atoms and 6 hydrogen atoms
- Each carbon atom uses 3 of its available electrons in bonding to 2 other carbon atoms and 1 hydrogen atom
- Each carbon atom has one electron in the p orbital at right angles to the plane of the bonded carbon and hydrogen atoms
- Adjacent p orbital electrons overlap sideways in both directions above and below the plance of the carbon atoms to form a ring of electron density
- This overlapping of the p-orbitals creates a system of π-bonds which spread over all six of the carbon atoms in the ring structure
- The 6 electrons occupying this system of π-bonds are said to be delocalised
Which groups are substituent to benzene?
- Short alkyl chains (6 carbon atoms or less)
- Halogens
- Nitro groups
How do you name an aromatic compound when it has a substituent group attatched to it?
Add the prefix of the substituent group onto benzene
When is benzene considered to be the substituent?
- When it is connected to an alkyl chain with a functional group
- When it is connected to an alkyl chain 7 or more carbon atoms long
How do you name an aromatic compound when benzene is the substituent?
Instead of using “benzene”, the prefix “phenyl” is used
Name this molecule
ethylbenzene
Name this molecule
Nitrobenzene
Name this molecule
Phenylethanone
Name this molecule
2-phenyloctane
What are 4 common compounds that are special cases in the aromatic nomenclature scheme?
- Benzoic acid / benzenecarboxylic acid (rather than phenyl methanoic acid)
- Phenylamine
- Benzaldehye (rather than phenyl methanal)
- Phenol (rather than hydroxybenzene)
Name this molecule
Benzoic acid
Name this molecule
Phenylamine
Name this molecule
Benzaldehyde
How do you name an aromatic compound if it has more than 1 substituent on a benzene chain?
- The carbon atoms on the ring are numbered 1-6
- Number the carbon atoms so that you use the smallest numbers possible
- List the substituent groups in alphabetical order
i.e: 2-bromomethylbenzene, 1,4-dichlorobenzene, 3-chloromethylbenzene
What is toluene?
Methylbenzene
What is phenol?
Hydroxybenzene
What is important when drawing aromatic compounds?
Sigma bonds coming off the benzene ring must be the same length as the bonds within the benzene ring
Name this molecule
2,4,6-trinitromethylbenzene
(2,4,6-trinitrotoluene aka TNT!)
Name this molecule
2-nitrophenol
Name this molecule
3-hydroxybenzoic acid
Name this molecule
2-ethylbenzoic acid
Name this molecule
benzene-1,2-diol
name this molecule
Phenylpropan-1-one
If the atom directly bonded to the carbon ring is also connected to a less electronegative atom, the substituent is:
IMPORTANT
- Electron-donating
- Activating
- Ortho-para directing
If the atom directly bonded to the carbon ring is also connected to a more electronegative atom, the substituent is:
- Electron-withdrawing
- Deactivating
- Meta directing
Name this molecule
2-phenylphenol
Name this molecule
2-benzyl-4-chlorophenol
What is the benzyl group?
Benzyl is a substituent group which consists of a benzene ring connected to a CH2 group connected to another group. C₆H₅CH₂-
The groups highlighted in blue are the benzyl group:
What is a Phenyl group?
A benzene ring as a substituent
What type of reaction mechanism does benzene undergo?
Electrophilic Aromatic Substitution
What are Activating groups?
Electron donating groups
What are deactivating groups?
Electron withdrawing groups
What direction do deactivating groups direct towards?
Meta (With the exception of halogens)
What direction do activating groups direct towards?
ortho-para
What direction do halogens direct towards?
ortho-para
Despite being a deactivating group, halogens are 2-, 4- (ortho-, para-) directing. This is an exception to the rule as all other deactivating groups are meta directors
If there are multiple groups bonded to the benzene, what determines the direction of the incoming group?
If there are multiple groups bonded to the benzene, then the most activating or the least deactivating group is what directs the incoming group
Is benzene stable or reactive?
Stable
What is the typical electrophilic substitution reaction for benzene?
An electrophile is substituted in for a hydrogen
What are the 2 mechanisms you have to know for benzene reactions?
- Nitration of Benzene
- Halogenation of Benzene
What is the equation for the Nitration of benzene?
Benzene + Nitric acid -> Nitrobenzene + Water
Benzene reacts slowly with nitric acid to form nitrobenzene and water. Therefore, a sulfuric acid catalyst is used and it is heated to 50 degrees celcius
What are the conditions for the nitration of benzene?
- Sulfuric acid catalyst
- 50 degrees celcius
Why must the temperature be maintained at 50 degrees celcius in the nitration of benzene?
- It must be raised to 50 from room temperature in order to increase the rate of reaction
- However, any higher and further substitution may occur, leading to the production of dinitrobenzene
What is the mechanism for the nitration of benzene?
IMPORTANT
What are the uses of nitrobenzene?
It is an important starting material in the preparation of:
* Dyes
* Pharmaceuticals (such as paracetamol)
* Pesticides
What is the nitronium ion?
NO₂⁺
What is the bromonium ion?
Br⁺
What is the reaction for the halogenation of benzene (use bromination as an example)
Benzene reacts with halogen with a halogen carrier catalyst to form halobenzene and hydrogen halide
What are the conditions of halogenation of benzene?
- Halogen carrier catalyst
What is a halogen carrier?
a catalyst which consists of a metal bonded to halogens
What are common halogen carrier catalysts?
- FeBr₃
- AlBr₃
What is the mechanism for halogenation of benzene (use bromination as an example)
Why does the halogenation of benzene require a halogen carrier as a catalyst?
Benzene is too stable to directly react with a non-polar halogen molecule, therefore it must react with the positive ion
What is a Friedel-crafts alkylation reaction?
The substitution of a hydrogen atom in the benzene ring by an alkyl group
What is the reaction for a Friedel-crafts alkylation reaction?
Benzene + haloalkane -> alkylbenzene + hydrogen halide
What is the catalyst in a Friedel-crafts alkylation reaction?
AlCl₃
Aluminium Chloride
What is an acylation benzene reaction?
When benzene reacts with an acyl chloride in the presence of aluminium chloride (AlCl₃) catalyst, an aromatic ketone is formed
What is the reaction for the acylation of benzene?
Benzene + acyl chloride -> phenylketone + hydrogen halide
What are the conditions for the acylation of benzene?
AlCl₃ catalst
Why are alkenes more reactive than arenes?
Alkenes contain an area of localised electron density (within the pi bond), whereas arenes do no have this localised charge as the electrons are delocalised in the benzene ring. This makes alkenes more reactive than arenes
Why can alkenes discolour bromine water but arenes cannot?
The pi bond in the alkene contains localised electrons above and below the plane of the 2 carbon atoms. This produces an area of high electron density. This area of high electron density induces a dipole in the non-polar bromine molecule allowing it to act as an electrophile and allowing an electrophilic addition reaction to occur. However, unlike alkenes benzene does not contain this localised charge (as the pi electrons are delocalised in the ring structure) and therefore cannot induce a dipole in the benzene ring
How can you determine if a group is activating/electron donating or deactivating/electron withdrawing?
Determine if the group will push electron density into the ring or draw it out of the ring by considering the electronegativity of all neighbouring atoms and any lone pairs present
Where is the ortho position?
carbon atoms 2 and 6
Where is the meta position?
Carbon atoms 3 and 5
Where is the para position?
Carbon atom 4
How can directing effects be used in organic synthesis?
The directing effect of substituent groups can be used when planning an organic synthesis, When carrying out more than one electrophilic substitution reaction on an aromatic compound, you may have to consider the order in which the reactions are carried out to ensure the correct substitution pattern and that the required product is prepared
Are alcohols or phenols more soluble in water and why?
Alcohols are more soluble than phenols. This is because phenols have the presence of the non-polar benzene ring causing it to be less soluble in the polar solvent water
Is phenol or alcohols more acidic?
Phenol is more reactive than alcohols as it partially dissociates in solution. It is classified as a weak acid
Why is phenol classified as a weak acid?
Phenol partially dissociates in solution to form a proton and the phenoxide ion
What is the equation for phenol partially dissociating?
Phenol ⇌ Phenoxide ion + Proton
What is the phenoxide ion?
How can you distinguish between alcohols, phenols, and carboxylic acids?
Phenols are more acidic than alcohols but less acidic than carboxylic acids, this can be used to distinguish them:
* Alcohols do not react with strong bases (sodium hydroxide) or weak bases (sodium carbonate)
* Phenols react with strong bases (sodium hydroxide) but do not react with weak bases (sodium carbonate)
* Carboxylic acids react with strong bases (sodium hydroxide) and weak bases (sodium carbonate)
What is an example of a strong base?
Sodium hydroxide
What is an example of a weak base?
Sodium carbonate
What is the reaction of phenol with sodium hydroxide?
Phenol + Sodium Hydroxide -> Sodium Phenoxide + Water
What is the reaction for the bromination of phenol?
Phenol + 3Br₂ -> 2,4,6-tribromophenol + 3 HBr
What are the observations of the addition of phenol to bromine water?
- Bromine water is decolourised
- A white precipitate is formed
What are the conditions for the bromination of phenol compared to benzene?
It reacts at room temperature and does not require a halogen carrier catalyst
What is the reaction for the nitration of phenol?
Phenol reacts readily with nitric acid at room temperature to form a mixture of 2-nitrophenol and 4-nitrophenol
What are the conditions for the nitration of phenol compared to benzene?
Phenol reacts with much milder conditions, it takes place at room temperature and does not require a sulfuric acid catalyst
Compare and explain the difference in reactivity of phenol and benzene
Phenol reacts much more readily with bromine and nitric acid than benzene does. This increased reactivity is caused by the lone pair of electrons from the oxygen p-orbital of the -OH group being donated into the pi-system. therefore it is acting as an activating group causing the benzene ring to be more reactive
Why does an activating group make the benzene ring more reactive?
The activating group pushes electron density into the benzene ring. This increased electron density attracts electrophiles more readily and it is therefore more susceptible to attack from electrophiles.
How do activating and deactivating groups affect the reactivity of the benzene ring?
- Activating groups - increase reactivity
- Deactivating groups - decrease reactivity
Why can phenol react with bromine without a catalyst?
The oxygen atom in the hydroxy group pushes electron density into the benzene ring. This increased electron density allows the phenol ring to polarise bromine molecules and therefore no halogen carrier catalyst is required
Describe the direction and type of “-COOH” as a directing group
- Deactivating (electron withdrawing group)
- 3 (meta) directing
Describe the direction and type of “-CN” as a directing group
- Deactivating (electron withdrawing group)
- 3 (meta) directing
Describe the direction and type of “-OR” as a directing group
- Activating (Electron Donating Group)
- 2,4 (ortho, para directing)
Describe the direction and type of “-NHCOR” as a directing group
- Activating (Electron Donating Group)
- 2,4 (ortho, para directing)
Describe the direction and type of “-NH₂” as a directing group
- Activating (Electron Donating Group)
- 2,4 (ortho, para directing)
Describe the direction and type of “-NO₂” as a directing group
- Deactivating (electron withdrawing group)
- 3 (meta) directing
Explain the Electrophilic Aromatic substitution mechanism with benzene
What is the difference between “Phenol” and “Phenols”?
- Phenol is a specific molecule, it consists of a single hydroxyl group bonded directly to a benzene ring and no other substituents
- Phenols are a family of molecules, they are any molecules which contain a hydroxyl group bonded directly to the benzene ring
Which of these molecules is not a phenol?
The fourth molecule is not a phenol as the hydroxyl group is not bonded directly to the benzene ring. It is instead an aromatic alcohol
What role does sulfuric acid play in the nitration of benzene?
It protonates the nitric acid, which leads to the loss of water and the formation of a nitronium ion
Name this molecule:
3-methylcyclohexene
Methyl group is on carbon 3
