6.1 Aromatic compounds, carbonyls and acids Flashcards
Define benzene
A naturally occurring aromatic compound, which is a very stable planar ring structure with delocalised electrons.
Define model
A simplified version that allows us to make predictions and understand observations more easily.
Define substitution reaction
Where a group or atom is exchanged for another group or atom in a chemical reaction
Define benzene derivative
Benzene ring that has undergone a substitution reaction
Define electrophilic substitution
a substitution reaction where an electrophile is attracted to an electron-rich atom or part of a molecule and a new covalent bond is formed by the electrophile accepting an electron pair.
Define reaction mechanism
A model with steps to explain and predict a chemical reaction.
What is a Friedel-Crafts reaction?
A substitution reaction where hydrogen is exchanged for an alkyl or acyl, chain.
What are phenols?
A class or aromatic compounds where a hydroxyl group is directly attached to the aromatic ring.
What is the directing effect?
How a functional group attached directly to an aromatic ring affects which carbon atoms are more likely to undergo substitution.
Define nucelophile.
A species attracted to an electron-deficient part of a molecule, where it donates a pair of electrons to make a new covalent bond.
What did Kekule discover?
When one group was attached to the benzene, only one isomer was ever made
When two groups were added there were always three structural isomers.
Problems with Kekule model
- Benzene is resistant to addition reactions
- Enthalpy of hydrogenation shows that benzene is much more stable then predicted
- All six carbon bonds were the same length
How did Kekule explain the resistance to reaction?
By saying the double and single bonds changed position in a very fast equilibrium.
Explain the delocalised structure of benzene
Each of the six carbon atoms donates one electron from its p-orbital.
Form ring above and below.
Why is benzene very stable?
A lot of energy is needed to disrupt the delocalisation.
What do hydrogen carriers include?
Iron
Iron halides
Aluminium halides
Nitration of benzene describe process
Nitric and sulphuric acid mixed in a flask and held in an ice bath
Benzene added under reflux
Condenser
Keep at 50 degrees
Why is the sulphuric acid needed in nitration of benzene?
To generate the NO2+ electrophile from the nitric acid
Catalyst as it is regenerated
What is a base?
A chemical that will react with an acid
What do you need when you add alkyl groups to benzene?
Strong Lewis acid
What is a Lewis acid?
Accepts a pair of electrons
Functional group of acyl chloride
RCOCl
Use of acyl chloride
Used in friedel crafts reaction as the halogen carrier
Conditions for acylation of benzene
60 degrees 30 mins Reflux AlCl3 catalyst Anhydrous conditions
Testing for carbonyl functional group
Brady’s reagent
Orange
Mix of methanol, sulfur if acid and a solution of 2,4-dinitrophenylhydrazone
What does Brady’s reagent identify?
Ketone or aldehyde but not carboxylic acid or ester
What do you need when you add alkyl groups to benzene?
Strong Lewis acid
What is a Lewis acid?
Accepts a pair of electrons
Use of acyl chloride
Used in friedel crafts reaction as the halogen carrier
What do you use to attach carbonyl groups to aromatic compounds?
Acyl chlorides
Why can we only get single substitution when we add carbonyl groups on benzene?
The carbonyl group removes electron density from the ring
Why is phenol a weak acid?
Because it partially dissociates in water and releases H+
why doesn’t phenol react with weak bases?
Because it is a weak acid
Why is phenol more reactive then benzene
Higher electron density, non bonding electrons join delocalised system
Things that smell like antiseptic
tri bromo and chloro phenol
when is the directing effect more noticable?
when we have a NH2 substituted instead of phenol
When do we get a 3 directing effect?
with NO2 group attached
Why is there a colour change from orange to green during the oxidation of aldehydes?
the oxidation state of the chromium changes
source of H- ions
sodium tetrahydrideborate (III), NaBH4
formula of cyanide ions
CN-
Source of cyanide ions
HCN and NaCN
Why do we acidify the solution before adding CN- to increase chain length?
increases the polarity go C=O group
What is Brady’s reagent made from?
methanol, sulphuric acid and a solution of 2,4 dinitrophenylhydrazine (2,4 - DNP)
What does bradys tell us?
if there is an aldehyde or ketone present
what happens when bradys is added to aldehyde or ketone
orange yellow precipitate
Why do we measure the boiling points of the precipitates?
bigger difference than the substances themselves
how do we make Tollen’s reagents?
Add sodium hydroxide to silver nitrate solution until a brown precipitate is formed
add dilute ammonia drop by drop until it redissolves
What is Tollen’s reagent
A weak oxidising agent
Why is a silver mirror formed?
Silver is oxidised, the Ag+ ions turn into solid silver
How do we make esters using carboxylic acids?
Alcohol and carboxylic acid sulphuric acid and gentle heating if they are large esters we need to heat under reflux reversible not suitable for phenol + derivatives
What are the two main ways of making esters?
Using carboxylic acids
Using acid anhyrides
How do we make esters from acid anhydrides?
acid anhydrides react with alcohols non reversible reactions react with phenol + derivatives higher yield produces ester and carboxylic acid
How can we reverse esterification?
Using hydrolysis
What happens if we refiux an ester with a hot aqueous acid (acid hydrolysis)
reversible decomposition
acid catalyst
alcohol and carboxylic acid
Describe alkaline hydrolysis
Irreversible decomposition
forms a carboxylate salt and alcohol
Whats alkaline hydrolysis used for ?
saponification
What are acyl chlorides?
Derivative of carboxylic acids where the -OH group is replaced by a -Cl group
Properties of short chained acyl chlorides?
fuming colourless liquids
How do we make acyl chlorides?
carboxylic acid and SOCl where Cl replaces the OH group
Why is it better to use acyl chlorides to make esters?
Higher yield
Bad thing about using acyl chlorides to form esters
HCl fumes
How do we get ester involved with aromatic rings?
Phenol and carboxylic acid won’t react
using with acyl chlorides
violent reactions that produce corrosive fumes
Describe the reaction to form carboxylic acid from acyl chloride
Add water
very exothermic
misty HCl fumes
Describe forming primary amides using acyl chlorides
React with conc ammonia
an ammonium salt is also made
Forming secondary amides using acyl chlorides
React with primary amines
called N substituted amides (secondary amide another name for)
What happens when acyl chlorides react with secondary amines?
N,N amides which have two alkyl groups on the nitrogen
How do you make Tollen’s reagent?
1) Add sodium hydroxide to silver nitrate solution until a brown precipitate forms
2) Add dilute ammonia drop by drop until the precipitate re-dissolves
Why does Tollen’s reagent work?
its a weak oxidising agent and will oxidise aldehydes but not ketones as they cannot be further oxidised
When oxidising, the Ag+ ions are reduced to solid silver
Why are carboxylic acids more soluble than their similar alcohols?
Because they can form 2 hydrogen bonds per molecule
Properties of short chained acyl chlorides
fuming colourless liquids which are very reactive and Cl is replaced by other groups
CH3COOH + SOCl2 = ?
CH3COCl + SO2 + HCl
and then distil
Why are triglycerides soluble in non-polar solvents and not in water?
-There is van der Waals between triglycerides.
- There is van der Waals between triglycerides and (non-polar) solvent.
- Triglycerides cannot hydrogen bond (to water)(enough).
-Because there are not enough suitable sites/oxygen atoms
-Or long hydrocarbon chains do not hydrogen
bond/would interfere with hydrogen bonding in water
Conditions for alkylation of benzene
CH3Cl
AlCl3
Conditions for Friedel Crafts acylation of benzene
- reflux
- 60 degrees
- 30 mins
- anhydrous conditions
- single substitution only
- CH3COCl
- AlCl3
Conditions for nitration of benzene
- conc nitric and sulphuric mixed in ice bath
- benzene added
- reflux condenser set up at 50 degrees
Why is the mixture kept at 50 degrees for nitration of benzene?
to prevent further substitutions
Equation for adding sulphuric acid and nitric acid together in an ice bath
H2SO4 + HNO3 = NO2+ + HSO4- + H2O
What conditions are needed for the halogenation of benzene?
- X2
- AlX3 or Fe or FeX3
How does iron act as a halogen carrier?
Forms a iron halide in situ
Conditions for the reduction of nitrobenzene
- Reflux at 100 degrees with Sn/conc. HCl
- After refluxing for 20 mins, add NaOH to remove excess acid
Why is the 3 directing effect seen on -NO2 groups are attached to the aromatic ring?
the nitro groups withdraw electrons from the pi-system
Which groups do 2- and 4- directing effect?
phenol
NH2
Which groups do 3 directing effect?
NO2
Conditions for adding a nitro group to phenol?
dilute HNO3 for single substitution
conc. HNO3 for triple substitution
What happens when you add NaOH to phenol?
Na+O- salt forms
Why are all of the bonds in benzene the same length?
the pi bonds are delocalised
Acidic hydrolysis of esters
alcohol + COOH
reversible
Alkaline hydrolysis of esters
alcohol and carboxylate salt
not reversible
What do you observe when you make a primary amide from an acyl chloride?
white smoke
some products remain in colourless solution
What do you observe when you make a secondary amide from an acyl chloride?
white solid
What did they expect the enthalpy of hydrogenation of benzene to be and what was it actually?
expected = -360kJmol-1 actual = -208kJmol-1
When is nucleophilic addition?
C=O
When is electrophilic substitution?
Benzene and phenol
Why does CH3COCl produce acidic solutions?
in water it produces a carboxylic acid and HCl
the carboxylic acid dissociates
How to test for phenol?
- add NaOH to form colourless solution
- doesnt effervesce when sodium carbonate or hydrogencarbonate ions are added
