Module 6 Section 1: Aromatic Compounds and Carbonyls Flashcards
General structure of benzene
Benzene has the formula C6H6
Has a cyclic structure with the 6 carbon atom joined together in a ring
Planar (flat) molecule as it has a ring of carbon atoms with hydrogen sticking out on a flat plane
How can benzene be represented in a diagram
Can be represented using the kekulé model or the delocalised model
What does Kekulé’s structure look like
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Friedrich Kekulé proposed that benzene was made up of a planar (flat) ring of carbon atoms with alternating single and double bonds between them
In Kekulé’s model each carbon atom is also bonded to one hydrogen atom
How was Kekulé’s model slightly altered
Later adapted to show that the benzene molecule was constantly flipping between two forms (isomers) by switching the double and single bonds
Problems with Kekulé’s model of benzene
If this model was correct there should be 3 bonds with the length of a C-C bond (0.154pm) and 3 bonds with the length of a C=C (0.134pm)
However, X-ray diffraction studies have shown that all the carbon-carbon bonds in benzene have the same length of 140pm
Meaning that they are between the length of a single bond and a double bond
This means that Kekulé’s structure can’t be completely right
What does the delocalised model look like
The delocalised model says that the p-orbitals of all 6 carbon atoms overlap to create a π-system
The π-system is made up of two ring shaped clouds of electrons - one above and one below the plane of the 6 carbon atoms
All the bonds in the ring are the same length because all the bonds are the same
The electrons in the rings are delocalised (don’t belong to specific carbon atom)
Represented as a circle inside the ring of carbons rather than as a double or single bonds
How do alkenes normally react with H2
Reacting alkenes with hydrogen gas makes two hydrogen atoms add across the double bond, this is hydrogenation
How do enthalpies of hydrogenation show that Kekulé’s model may be flawed
Cyclohexene has 1 double bond
It’s enthalpy change of hydrogenation in -120kJ mol-1
If benzene had 3 double bonds (in the Kekulé structure) the enthalpy change of hydrogenation would be expected to be 3x-120=-360kJmol-1
The experimental enthalpy of hydrogenation of benzene is -208kJmol-1 which is far less exothermic than expected
This means that more energy is needed to break the bonds in benzene than in Kekulé’s structure
This difference shows that benzene is more stable than the Kekulé structure would be
Benzene’s resistance to reaction gives more evidence for it being more stable than the Kekulé structure suggests
This stability is thought to be due to the delocalised ring of electrons
How to name an aromatic compound with more than one functional group that are all the same compared to if they are different
Pick any group to start from and count round the way that gives the smallest numbers
Start from whichever functional group gives the molecule its suffix (e.g. the OH group for a phenol) and continue counting round the way that gives the smallest numbers
With two prefixes (e.g. methyl and chloro) order them alphabetically
Examples of aromatic compounds that use ‘benzene’ in the name
Chlorobenzene
Nitrobenzene
1,3-dimethylbenzene
Examples of aromatic compounds that use ‘phenyl’ in the name
Phenol
Phenylamine
When is benzene a substituent
When the carbon chain or functional group bonded to benzene has 7 or more carbon atoms
The prefix phenyl is used
E.g. 2-phenyloctane, phenylethanone
Common exceptions with naming aromatic compounds
Phenylamine (1 NH2 bonded)
Benzoic acid/benzenecarboxylic acid (COOH bonded)
Benzaldehyde (CHO bonded)
OMP rules
A group is ortho- if it is on the carbon next to the first carbon
A group is meta- if it is on the carbon 2 away from the first carbon
A group is para is if it is on the carbon 3 away from the first carbon (directly opposite on the ring)
How does electrophilic addition reaction fault the Kekulé structure
You would expect benzene to react similarly with bromine as alkenes in electrophilic addition to turn bromine water colourless
With benzene you need to heat up the molecule and UV light where it is still difficult to facilitate the reaction.
Why can’t benzene undergo electrophilic addition and why does this show evidence for the delocalised model
The pi system in benzene has spread out delocalised electrons which makes the benzene ring very stable
This means that it is unwilling to undergo addition reactions as it would destroy the stable ring
This supports the delocalised model as alkenes have a double bond with high electron density which attract electrophiles but the negative charge is spread out in benzene
This means that benzene prefers to react by electrophilic substitution
What do electrophilic substitution reaction result in
Result in a hydrogen atom being substituted by an electrophile
Two steps of electrophilic substitution
Addition of an electrophile to form a positively intermediate
The loss of H+ from the carbon atom attached to the electrophile
This reforms the delocalised ring
What are halogen carriers
Halogen carriers are catalysts which make atoms into stronger electrophiles
An electrophile needs to have a strong positive charge to attack the benzene ring
Most compounds aren’t polarised enough
How do halogen carrier make stronger electrophiles
A halogen carrier accepts a lone pair of electrons from a halogen atom on an electrophile
As the lone pair of electrons is pulled away, the polarisation in the molecule increases and a carbocation can sometimes form
This makes the electrophile stronger
Examples of halogen carriers
Aluminium halides
Iron halides
Iron
Halogenation mechanism with halogen carrier
Benzene will react with halogens (e.g. Br2) in the presence of an aluminium chloride catalyst, AlCl3
The catalyst polarises the halogen, allowing one of the halogen atoms to act as an electrophile
During the reaction, a halogen atom is substituted in place of a H atom (called halogenation)
What are Friedel-Crafts reactions used for
Useful for forming C-C bonds in organic synthesis
Carried out by refluxing benzene with a halogen carrier and either a haloalkane or an acyl chloride
Overall reaction for friedel-craft alkylation
Mechanism for friedel-craft alkylation
This puts any alkyl group onto a benzene ring using a haloalkane and a halogen carrier
Overall reaction for friedel-crafts acylation
Full mechanism for friedel-crafts acylation including generation of catalyst LOOK AT SPEC, DONT NEED TO KNOW?
This substitutes an acyl group for an H atom on benzene
Just reflux benzene with an acyl chloride instead of chloroalkane
Produces phenyl ketones (or benzaldehyde is R is H)
Mechanism for nitration
Warming benzene with conc nitric acid and conc sulfuric acid creates nitrobenzene
Generating the nitronium ion in nitration of benzene
Sulfuric acid is a catalyst to help make a nitronium ion (NO2+)
This is an electrophile which is regenerated at the end of the reaction mechanism
Formula of phenol
C6H5OH
Why is phenol more reactive than benzene
The OH group means that phenol is more likely to undergo electrophilic substitution than benzene
One of the lone pairs of electrons in a p orbital of the oxygen atom overlaps with the delocalised ring of electrons in the benzene ring
So the lone pair of electrons from the oxygen atom is partially delocalised into the π system
This increases the electron density of the ring, making it more likely to be attacked by electrophiles as the electrophiles are more polarised
How do the reactivity of unsubstituted and substituted benzene rings compare
In an unsubstituted benzene ring, all the carbon atoms are the same so electrophiles can react with any of them
With a substituted benzene ring (e.g. phenol) the functional group can change the electron density at certain carbon atoms, making them more or less likely to react
What are electron-donating groups with examples
OH, NH2, CH3
They have electrons in orbitals that overlap with the delocalised ring and increase its electron density
They increase electron density at carbons 2-, 4- and 6- so electrophiles are most likely to react at these positions
They are ortho, para directing
What are electron-withdrawing groups with examples
E.g. NO2, COOH
These groups don’t have any orbitals that can overlap with the delocalised ring and it’s electronegative, so it withdraws electron density from the ring
They withdraws electron density from 2-,4- and 6- so electrophiles are unlikely to react at these position
This directs the effect of directing electrophilic substitution to the 3- and 5- position
They are meta directing
How to predict the products of electrophilic substitution reactions
Benzene ring is only substituted once, so it doesn’t matter whether you number carbons clockwise or anticlockwise.
Means that 3- and 5- positions are the same
How does phenol react with bromine water
Phenol is more reactive than benzene
Shaking phenol with orange bromine water will decolourise it as it reacts
OH group is electron donating so it directs substitution to carbons 2-,4-,6-
Product is called 2,4,6-tribromophenol - its insoluble in water and precipitates out of the mixture
How does phenol react using dilute nitric acid
Phenol can react with dilute nitric acid to give two isomers of nitrophenol and water
This is much easier than nitrating benzene as OH is an activating group
More likely to get NO2 groups at positions 2 and 4 on the carbon ring
What does phenol react with bases and sodium to form
Phenol is weakly acidic, so will undergo typical acid base reactions
Phenol reacts with sodium hydroxide solution at room temperature in a neutralisation reaction to form sodium phenoxide and water
It doesn’t react with sodium carbonate solution as it is not a strong enough acid
Friedel-Crafts acylation mechanism including generation of electrophile and regeneration of catalyst
Electrophile is the acylium ion
Electrophilic substitution of acid anhydride mechanism including generation of electrophile and regeneration of catalyst
Electrophile is the acylium ion
Features of aldehyde structures
Aldehydes have their carbonyl
group at the end of the carbon chain.
Their names end in -al
Features of ketone structures
Ketones have their carbonyl group in the middle of the carbon chain.
Their names end in -one, and often have a number to show which carbon the carbonyl group is on.
Displayed formula equation for how an alcohol is oxidised to aldehyde and then to carboxylic acid
Displayed formula for reducing an aldehyde to primary alcohol
Displayed formula for reducing a ketone to a secondary alcohol
Mechanism for how a ketone is reduced to an alcohol
Equation for how HCN is weakly acidic when added to water
Mechanism for the creation of a hydroxynitrile from hydrogen cyanide and a ketone
Equation for reduction of Tollens reagent using electrons from aldehyde
What type of acids are carboxylic acids
They are weak acids
They partially dissociate into carboxylate ions and H+ ions
This means that the equilibrium lies to the left because most of the molecules don’t dissociate
Draw a diagram for a carboxylic acid dissolving in water
How do carboxylic acids react with reactive metals, with example
Carboxylic acids react with the more reactive metals in a redox reaction to form a salt and hydrogen gas
The gas fizzes out of the solution
How do carboxylic acids react with carbonates, with example
Carboxylic acids react with carbonates CO3 2- to form a salt, carbon dioxide and water
The gas fizzes out of the solution
How can carboxylic acids be neutralised, with examples
Carboxylic acids are neutralised by other bases to form salts and water
Neutralised by alkalis and metal oxides
How are acyl chlorides made from carboxylic acids, with example
They are made by reacting carboxylic acids with SOCI2 (thionyl chloride)
The -OH group in the acid is replaced by -Cl.
How do acyl chlorides react with water
This is a vigorous reaction with cold water to produce a carboxylic acids
How do acyl chlorides react with alcohol
This is a vigorous reaction at room temperature to produce an ester
How do acyl chlorides react with ammonia
This is a violent reaction at room temperature producing a primary amide
The true products are ethanamide, HCl and ammonium chloride as the HCl reacts with the NH3 reactant
How do acyl chlorides react with amines
This is a violent reaction at room temperature to produce a secondary amide
How to make phenyl ethanoate
Ethanoyl chloride reacts slowly with phenol at room temperature, producing the ester phenyl ethanoate and hydrogen chloride gas
