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
How to nitrate benzene
This requires conc nitric acid and conc sulfuric acid catalyst
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
What type of directing group are halogens on the benzene
Halogens on a benzene ring are the only deactivating group that is ortho/para directing
What makes a molecule a strong activating group
They have a lone pair of electrons to donate into ring
(Excludes halogens)
What makes a molecule a electron withdrawing group
Have a positive charge on the atom directly attached to the benzene ring
How to identify aldehydes and ketones
Aldehydes and ketones are carbonyl compounds - they contain the carbonyl functional group C=O
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.
How to create aldehydes and carboxylic acids from alcohols
Acidified potassium dichromate(VI) are used to mildly oxidise alcohols
K2Cr2O7/H2SO4
Displayed formula equation for how an alcohol is oxidised to aldehyde and then to carboxylic acid
How to manipulate the conditions to control how far the alcohol is oxidised
If you gently heat a primary alcohol with acidified potassium dichromate(VI) in a distillation apparatus, then you will form the aldehyde.
If you reflux a primary alcohol (or an aldehyde) with acidified potassium dichromate(VI), then you’ll oxidise the aldehyde further and form a carboxylic acid.
Ketones are made by oxidising secondary alcohols with acidified potassium dichromate(VI)
They can’t be oxidised any further
Displayed formula for reducing an aldehyde to primary alcohol
Displayed formula for reducing a ketone to a secondary alcohol
What reducing agent is usually used to turn a ketone or aldehyde back to an alcohol
NaBH4 (sodium tetrahydriborate(III) or sodium borohydride)
This is dissolved in water with methanol
Process for how a ketone is reduced to an alcohol
The reducing agent, e.g. NaBH4, supplies hydride ions, H-.
H- has a lone pair of electrons, so it’s a nucleophile and can attack the δ+ carbon on the carbonyl group of an aldehyde or ketone
This removes the double bond between oxygen and carbon leaving a single C-O bond and the oxygen has a lone pair
Water is then added
The lone pair on O is donated to δ+ H on H2O creating an OH group in the carbon compound and an OH- left on the water
Mechanism for how a ketone is reduced to an alcohol
What do carbonyl compounds react with hydrogen cyanide to form
Produces hydroxynitrile (molecules with a CN and OH group)
It’s a nucleophilic addition reaction
Process for the creation of a hydroxynitrile from hydrogen cyanide and a ketone
Hydrogen cyanide is a weak acid _ it partially dissociates in water to form H+ and CN- ions.
The CN- ion attacks the slightly positive carbon atom and donates a pair of electrons to it.
Both electrons from the double bond transfer to the oxygen.
H+ (from hydrogen cyanide or water) bonds to the oxygen to form the hydroxyl group (OH).
Hydroxynitrile created
Equation for how HCN is weakly acidic when added to water
Mechanism for the creation of a hydroxynitrile from hydrogen cyanide and a ketone
Implications of using HCN to turn carbonyls into hydroxynitriles
Hydrogen cyanide is a highly toxic gas.
When this reaction is done in the laboratory, a solution of acidified sodium cyanide is used instead, to reduce the risk.
Even so, the reaction should be done in a fume cupboard
How to test for carbonyls
When 2,4-dinitrophenylhydrazine (2,4-DNP or Brady’s reagent) is dissolved in methanol and concentrated sulfuric acid
Reacts with carbonyl groups to form a bright orange precipitate.
This only happens with C=O groups, not with ones like COOH, so it only tests for aldehydes and ketones.
How to specifically identify the carbonyl compound using Brady’s reagent
The orange precipitate from using Brady’s reagent is a derivative of the carbonyl compound.
Each different carbonyl compound produces a crystalline derivative with a different melting point.
So if you measure the melting point of the crystals and compare it against the known melting points of the derivatives, you can identify the carbonyl compound.
How to distinguish between an aldehyde and ketone using reagents
Tollens reagent test lets you distinguish between an aldehyde and a ketone.
It uses the fact that an aldehyde can be easily oxidised to a carboxylic acid, but a ketone can’t.
How does Tollens reagent allow you to test for an aldehyde
Tollens’ reagent is a colourless solution of silver nitrate dissolved in aqueous ammonia.
When heated together in a test tube, the aldehyde is oxidised and the silver ions in the Tollens’ reagent are reduced to silver causing a silver mirror to form
The test tube should be heated in a beaker of hot water, rather than directly over a flame.
Equation for reduction of Tollens reagent using electrons from aldehyde
What can Tollens reagent also be called
Tollens’ reagent can also be called ammoniacal silver nitrate
How to know when an alcohol has been oxidised
The orange potassium dichromate(VI) will be oxidised to Cr3+ ions turning green
How to identify carboxylic acids
Carboxylic acids contain the carboxyl functional group -COOH.
How to name carboxylic acids
To name them, you find and name the longest alkane chain, take off the ‘e’ and add ‘-oic acid’.
The carboxyl group is always at the end of the molecule.
It’s more important than other functional groups (when it comes to naming) so all the other functional groups in the molecule are numbered starting from this carbon
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
Are carboxylic acids soluble in water
Carboxylic acids are polar molecules, since electrons are drawn towards the O atoms.
Hydrogen bonds are able to form between the highly polarised δ+ H atom and δ- O atoms on other molecules.
This makes small carboxylic acids very soluble in water — they form hydrogen bonds with the water molecules
Draw a diagram for a carboxylic acid dissolving in water
What types of carboxylic acids are soluble
Smaller carboxylic acids as there are less non polar R groups
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
What are the salts from carboxylic acids called
Carboxylates
Their names end in -oate
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 to identify acyl chlorides
Have functional group COCl
General formula is Cn H2n-1 OCl
Names end with -oyl chloride
Carbon atoms are numbered from the end with the acyl functional group
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
What type of reaction is the reaction when Cl in the acyl chloride is substituted with an oxygen or nitrogen group
This is a nucleophilic addition-elimination reaction
Why use acyl chlorides to react with phenol to make an ester instead of a carboxylic acid
You can normally make esters by reacting an alcohol with a carboxylic acid.
Phenols react very slowly with carboxylic acids, so it’s faster to use an acyl chloride, such as ethanoyl chloride.
How to make phenyl ethanoate
Ethanoyl chloride reacts slowly with phenol at room temperature, producing the ester phenyl ethanoate and hydrogen chloride gas
Reactants to reduce any benzene ring
Tin
Conc HCl
What is the aldehyde turned into after reacting with tollens reagent
A carboxylic acid
Why do carboxylic acids tend to have higher boiling points than alcohols
Carboxylic acids can form hydrogen bonds between molecules as well as alcohols with the hydroxyl groups
Can also form permanent dipole-dipole interactions between the carbonyl groups unlike alcohols
Both molecules have London forces
