Aromatic Chemistry (25) Flashcards
What is benzene?
A colourless, sweet smelling highly flammable liquid
Where is benzene found?
Crude oil naturally but also in petrol and cigarette smoke
What is the chemical structure of benzene?
It is a hexagonal ring of 6 carbon atoms and each carbon is attached to 2 carbons and 1 hydrogen
Which chemical group is benzene in?
It is an aromatic compound and is in the arenes which are aromatic alkenes
What is the molecular formula of benzene?
C6H6
What is the Kekule model?
Kekule suggested that benzene is a six carbon compound with the carbon atoms joined between single and double bonds
Give three reasons why the Kekule model is not correct
- The lack of benzenes reactivity
- The lengths of the carbon-carbon bonds
- Hydrogenation enthalpies
What is the reactivity of Benzene like?
If Kekule’s model was correct, then you would expect benzene to decolourise bromine water because of its double bonds. However, benzene doesn’t undergo electrophilic addition and it doesn’t decolourise bromine water which suggests it doesn’t have double bonds
What is weird about the length of the carbon-carbon bonds in benzene?
Using a technique called x-ray diffraction, you can measure the length of bonds. When benzene was examined, all of it’s carbon-carbon bonds were 0.14nm which is between the lengths of single ( 0.15nm) and double (0.13nm) bonds
What is weird about the hydrogenation enthalpies in benzene?
According to Kekule benzene could also be called cyclohexa-1,3,5-triene. When cyclohexene is hydrogenated one double bond reacts with hydrogen and the enthalpy change is -120kj. You would therefore assume that the enthalpy change of hydrogenation for benzene would be -360kjmol. When tested, the enthalpy change of benzene was actually -208 kjmol which means benzene is actually more stable as less energy is produced than expected.
What is the delocalised model of Benzene?
- Six carbon atoms and six hydrogen atoms all on the same plane in a cyclic hexagonal hydrocarbon
- Each carbon uses three of its four electrons to bond to carbons and hydrogens
- Each carbon has one electron in a p-orbital at right angles to the carbon atoms
- The adjacent p-orbitals overlap to form a ring of electron density above and below the carbon atoms
- This overlapping ring creates a system of pi-bonds above and below the hydrocarbon plane which contain delocalised electron
How do you name benzene compounds with one substituent group?
The benzene is considered the parent chain and you would put the alkyl, halogens or nitro groups as prefixes.
What happens to the name if there are alkyl groups with more than 7 carbons OR a alkyl chain with a functional group?
Benzene is now considered to be the substituent and you use the prefix of phenyl- instead of benzene
What are the 3 exceptions to this rule?
- Benzoic acid for benzene with carboxylic acid
- Phenylamine for benzene with amine
- Benzaldehyde for benzene with aldehyde
How do you name benzene compounds with two substituent groups?
You have to number the carbon ring so it is the smallest number combination possible and arrange the groups into alphabetical order
What reactions do benzene undergo the most?
Electrophilic substitution is what benzenes undergo where one of their hydrogen atoms are replaced by another group of atoms
What is the nitration of benzene?
This is where benzene reacts slowly with nitric acid to make nitrobenzene. One of the Hs on the benzene is replaced by the nitro group and water is also formed as a product
What conditions are needed for the nitration of benzene?
- It needs to be heated at 50 degrees
- It also needs a sulphuric acid catalyst
Why does it need to be kept at 50 degrees?
Because otherwise further substitution will occur to make a dinitrobenzene
What is the electrophile for the nitration of benzene?
NO2+ which is called a nitronium ion
Explain the mechanism for the nitration of benzene?
- The nitric acid reacts with the sulphuric acid to make a nitronium ion which is the electrophile
- The electrophile is then attracted to the delocalised electron density ring in benzene
- It accepts two electrons from the benzene ring and forms a dative covalent bond
- The benzene ring is now unstable as there are too many things joined to it and not enough electrons
- It stabilizes when the hydrogen donates its electrons to the electron ring and becomes a H+ ion
- The benzene ring is now stable
What is the halogenation of benzene?
When a halogen is added to benzene to create a halogen benzene
What is needed for the halogenation of benzene?
A halogen carrier is needed which is a catalyst that is required in order for halogens to react with benzene
What does in situ mean?
It means it can be formed in the reaction vessel
Give examples of halogen carriers
- AlCl3
- FeCl3
- AlBr3
- FeBr3
What are the conditions for the bromination of benzene?
- Room temperature and pressure
- Needs a halogen carrier (AlBr3)
- Reflux
What kind of reaction is the bromination of benzene?
A electrophillic substitution
What is the electrophile in the bromination of benzene?
A bromonium ion Br+
Describe the bromination of benzene?
- The Br2 reacts with the halogen carrier to form a bromonium ion which is the electrophile
- The bromonium ion approaches the benzene and accepts a pair of electrons from its electron density ring
- It forms an unstable intermediate
- The hydrogen donates a pair of electrons to the electron density ring and becomes a H+
- Bromobenzene is formed with a H+ ion that reacts with the FeBr4- to reform the halogen carrier
What is an alkylation reaction of benzene?
This is when a hydrogen is substituted by an alkyl group which can add a carbon chain to a benzene ring to increase the number of carbon atoms. You need to react benzene with a haloalkane
What conditions are needed for the alkylation of benzene?
- You need a halogen carrier which is AlCl3
- Room temperature
What is the electrophile in the alkylation of benzene?
A carbocation
What is an acylation reaction of benzene?
This is when benzene reacts with an acyl chloride
( Cl-C=0) to form an aromatic ketone
What are the conditions for an acylation reaction?
- Reflux
- 50 degrees
Explain why cycloalkenes can decolourise bromine water but benzene cannot
Alkenes decolourise bromine in an electrophilic addition reaction by turning it into a cyclohaloalkane. It does this by adding the bromine across the double bond and the Br2 in the bromine water becomes an electrophile. This happens when the non-polar bromine approaches the localised electrons in the pi-bond of the cycloalkene and a dipole becomes induced because the electron repel. An electrophilic addition is now carried out. This cannot happen in benzene as its ring of electron density is delocalised which means it is more stable than cycloalkenes and the electron density around each C-C bond is a lot less than in C=C bonds of cycloalkenes. This means there is insufficient electron density around each C-C bond to be able to induce a dipole in Br2 so it cannot react with it unless a halogen carrier is present
What are phenols?
Phenols are aromatic compound with a hydroxyl group attached
What is the general formula for a phenol?
C6H5OH
What happens if the hydroxyl group is not directly bonded to the benzene ring?
If the hydroxyl group is present on an alkyl chain attached to the benzene, it will not be a phenol, it will be an alcohol
What is the solubilty of phenols like compared to alcohols?
Phenols are not as soluble in water as alcohols because of the non-polar benzene ring which decreases the solubility
What are benzenes described as when they dissolve in water and why?
They are described as weak acids because when they dissolve in water, they partially dissociate into phenoxide ions and protons. The ability to produce protons makes it a weak acid
Compare the acidities of alcohols, phenols and carboxylic acids
Phenols are more acidic than alcohols but less acidic than carboxylic acids.
- Alcohols cannot react with sodium hydroxide which I a strong base
- Both phenols and carboxylic acids can react with strong bases like sodium hydroxide
- Only carboxylic acids can react with weak bases like sodium carbonate
This is found by looking at the acid dissociation constant, which increases the more acid it gets
Describe the reaction between phenol and sodium hydroxide
Phenol will undergo a neutralisation reaction with a strong base like sodium hydroxide to form a salt called sodium phenoxide and water
What are the reactions of phenol like compared to benzene?
Phenols also undergo electrophilic substitution, however, their conditions can be milder because they react more readily due to the hydroxyl group
What is the Bromination of phenol
This is when phenol reacts with an aqueous solution of bromine (bromine water) to form a white precipitate of 2,4,6-tribromophenol and HBr
What are the conditions for the bromination of phenol?
- No halogen carrier required
- Room temperature
What is the Nitration of phenol
Phenol reacts readily with dilute nitric acid to form a mixture of products which are 2-nitrophenol and 4-nitrophenol along with water
What are the conditions of the nitration of phenol?
- No concentrated sulphuric acid
- Room temperature
Compare the reactivity of alcohols and phenols
Phenols can react more easily with both bromine and nitric acid. It doesn’t need halogen carriers or concentrated acids. This is because the oxygen from the OH group donates a pair of electrons from its P orbital into the benzene ring of electron density which will increase the electron density. This means electrophiles will be attracted more than to benzene and it is more susceptible to attack. This is also why bromine doesn’t need a halogen carrier, as the electron density in phenol is enough to induce a dipole in the bromine
What group activates the benzene and what does this mean?
-NH2 (amine) activates the benzene ring so that it reacts more readily with electrophiles and it directs the second substituents to positions 2 and 4
What group activates the benzene and what does this mean?
- NO2 (nitrogen dioxide) deactivates the benzene ring so that it reacts less readily with an electrophile and will need a hydrogen carrier and high temperatures. It will also direct second substituents to position 3
What position is ortho?
position 2 (activating group)
What position is para?
position 4 (activating group)
What position is meta?
position 3 ( deactivating group)
