Topic 18 - Organic Chemistry 3 Flashcards
What are arenes?
compounds containing a benzene ring
eg. phenol, nitrobenzene, etc
What is the molecular formula of benzene?
C6H6
What is the structure of benzene according to the Kekulé model?
6 carbon atoms in a ring with alternating single and double carbon-carbon bonds
What were the issues with the Kekulé model of benzene?
- x-ray diffraction shows that benzene only has 1 carbon-carbon bond length but with the Kekulé model, you would expect two
- IR spectrum has fewer peaks than expected for the Kekulé model
- benzene does not undego electrophilic addition reactions, which you would expect it to if it had C=C bonds
- enthalpy change of hydrogenation is much lower than expected for the Kekulé model (benzene is more stable)
What is the structure of benzene according to the delocalised model?
- 6 carbon atoms in a ring where each carbon atom forms a sigma bond with its hydrogen and each neighbouring carbon and the remaining p-orbital forms a ring of delocalised electrons
- p orbitals overlap above and below the molecule, forming a delocalised pi-electron system
benzene + oxygen →
carbon dioxide + water
benzene + bromine →
AlCl3/FeBr3 catalyst present
bromobenzene
What are the conditions for the halogenation of benzene?
- dark
- AlCl3/FeBr3 catalyst (halogen carrier)
benzene + nitric acid →
sulfuric acid catalyst present
nitrobenzene + water
What is in a nitrating mixture?
conc nitric acid and conc sulfuric acid
What are the conditions for the nitration of benzene?
- conc sulfuric acid catalyst
- 55°C
Why should the nitration of benzene be at 55°C?
to avoid multiple substitution of benzene
What is the purpose of using a nitrating mixture for the nitration of benzene?
it produces a stronger electrophile
-nitronium ion
What reaction happens in the nitrating mixture?
HNO3 + H2SO4 → NO2+ + HSO4- + 2H2O
What reaction happens to reform the catalyst after the nitration of benzene?
H+ + HSO4- → H2SO4
benzene + halogenoalkane →
AlCl3 catalyst
alkyl benzene + hydrogen halide
What conditions are needed for Friedel-Crafts alkylation (benzene and halogenoalkane)?
- aluminium chloride catalyst
- heat under reflux
benzene + acyl chloride →
phenyl ketone + hydrogen chloride
What conditions are needed for Friedel-Crafts acylation (benzene and acylchloride)?
- aluminium chloride catalyst
- heat under reflux
What is the molecular formula of phenol?
C6H6OH
What are the properties of phenol?
- slightly acidic
- not soluble
- solid at room temp
- more reactive than benzene
Why is phenol more reactive than benzene?
-phenol’s oxygen activates the benzene ring
-oxygen’s lone pair of e- are incorporated into the delocalised pi-electron system
-this increases electron density in the ring
-it can attract electrophiles more easily
(catalysts/halogen carriers not needed)
phenol + bromine water →
2,4,6 tribromophenol + hydrogen bromide
What are the properties of amines?
- fishy smell
- generally soluble (can H-bond with water)
- have higher mp/bp than respective hydrocarbons
- have basic nature (nitrogen’s lone pair of e- can accept H+)
How does ammonia, primary aliphatic and primary aromatic amine’s basicity compare?
primary aliphatic amines > ammonia > primary aromatic amines
Why are primary aliphatic amines more basic than ammonia and primary aromatic amines?
- alkyl groups are e- pushing groups
- this increases the nitrogen’s electron density
- nitrogen can accept H+ more easily
Why are primary aromatic amines less basic than primary aliphatic amines and ammonia?
- NH2 is an activating group
- nitrogen’s lone pair of e- are incorporated into the delocalised pi-electron system
- nitrogen’s lone pair of e- are less accessible to accept H+
amine + water →
ammonium salt + hydroxide ion
alkali soln
amine + acid →
salt
amine + acyl chloride →
N-substituted amide + hydrogen chloride
primary amine + halogenoalkane →
secondary amine + ammonium halide
secondary amine can go on to react with halogenoalkane to form tertiary amine
amine + copper(II) ions →
complex ions
[Cu(RNH2)4)]4+ (aq)
royal blue soln
How can primary aliphatic amines be prepared?
- halogenoalkane and excess ammonia + reflux
- reducing nitriles using lithium aluminium hydride in dry ether or hydrogen gas with a nickel catalyst
How can primary aromatic amines be produced?
reducing nitrobenzene using tin and conc hydrochloric acid + reflux
How are polyamides produced?
condensation polymerisation
dicarboxylic acid and diamine
What is the general structure of an amino acid?
- amine group (NH2)
- carboxyl group (COOH)
- variable region
Why are amino acids optically active?
they have a chiral centre
except glycine
What is a zwitterion?
when an amino acid’s amine group has been protonated and carboxy group has been deprotonated at the same time (at specific pH)
- neutral (charges cancel)
- solid at room temp (can form giant ionic lattices due to charges)
How can amino acids be separated?
in chromatography
How are dipeptides produced?
condensation reaction between two amino acids
-peptide bond formed
-water produced
(if dipeptides are joined, polypeptide if produced)
How can a carbon chain length be increased?
- using cyanide ions (can add one carbon)
- using Grignard reagents (can add multiple carbons)
How are Grignard reagents prepared?
magnesium and halogenoalkane
in dry ether
How are Grignard reagents used to produce an alcohol?
Grignard reagent is reacted with a carbonyl compound
- aldehyde for primary/secondary alcohol
- ketone for tertiary alcohol
How are Grignard reagents used to produce a carboxylic acid?
Grignard reagent is reacted with carbon dioxide
How are Grignard reagents used to produce an alkane?
Grignard reagent is reacted with water
How can a carbon chain length be decreased?
using triodomethane reactions (iodine in alkali with strong acid)
How is recrystallisation carried out?
- compound is dissolved in minimum volume of hot solvent
- it is then filtered hot and allowed to cool (in ice bath)
- it is then filtered and washed with a small amount of cold solvent
- crystals are dried between filter papers/using a Buchner funnel
What is the purpose of recrystallisation?
to puridy solids
How is drying carried out?
- using a Buchner funnel and Buchner flask (suction filtration)
- patting with filter paper
How is melting/boiling point determination carried out?
- sample is loaded into a capillary tube and placed in apparatus
- temp is slowly increased and sample is observed
- mp range is from when liquid begins to appear to when no solid is left
- mp can be compared to known values
What is the purpose of refluxing?
allows continuous heating so all of the reactants react
What is the purpose of distillation?
separates liquids based on boiling points
How is solvent extraction carried out?
- impure product is dissolved in water/organic solvent and shaken
- two layers (organic and aqueous layers) form in separating funnel and they can be run off
How is purification by washing carried out?
impurity in organic product can be removed by adding another liquid which impurities react with
