F324 - Rings, Acids And Amines Flashcards
What does aromatic mean?
“Fragrance”
Any compound with a benzene ring is aromatic.
What is an arene?
Aromatic hydrocarbons containing one or more benzene ring.
Define empirical formula.
Simplest whole number ratio of atoms of each element present in a compound.
Define molecular formula.
The number if atoms of each element present in a molecule.
Describe the kukele structure of benzene.
Alternate single and double bonds.
Cyclohexa-1,3,5-triene.
Describe the problems with the kukele model of benzene.
Low reactivity:
Kukeles model has C=C bonds but does not react like alkenes.
It should also react with bromine water and decolourise it which it doesn’t.
Bond lengths:
Bond lengths were revealed to be all the same length, which is kukeles model they would not be.
Hydrogenation of benzene:
Kukule has 3 C=C bonds, so should have an enthalpy change of hydrogenation 3 times that of a single double bond, but it was much less.
More stable.
How did kukele respond to the problems given to his structure?
He suggested benzene has two forms, differing by the positions of the double bonds.
He suggested these two forms were in rapid equilibrium, and hence bromine could not react with the double bonds.
What are delocalised electrons?
Electrons shared between more than two atoms.
Describe the delocalised model of benzene.
Cyclic hydrocarbon with 6 carbon and hydrogen atoms.
Each carbon is bonded to two carbon and one hydrogen atoms.
The shape at each carbon atom is trigonal planar with an angle of 120.
The bonds to carbon and hydrogen are sigma bonds, this leaves a fourth outer shell electron in a 2p orbital above and below the plane of carbon atoms.
The electron in a p orbital of a carbon atom overlaps with the electrons adjacent to it, this results in a ring of electron density above and below the plane of carbon atoms.
The overlap produced a system of pi bonds which spreads over all six carbon atoms.
What reaction does benzene most readily undertake?
Substitution with electrophiles , not addition.
What is an addition reaction?
When a reactant is added to an unsaturated molecule to make a saturated molecule.
What is a substitution reaction?
When an atom or group of atoms is replaced with a different atom or group of atoms.
What is an electrophile?
An atom that is attracted to an electron rich centre, where it accepts a pair of electrons to form a new covalent bond.
Describe the nitration of benzene.
Nitrobenzene formed.
One hydrogen atom is replaced by a nitro group (NO2).
50*c and a H2SO4 catalyst (conc sulphuric acid)
Benzene + conc nitric acid —> nitrobenzene + water.
Formation of NO2+
HNO3 + H2SO4 —> NO2+ + HSO4- + H2O
NO2 then substituted with H+
H+ + HSO4- —> H2SO4
Catalyst.
Describe the nitration of methylbenzene.
Faster than benzene, same reactants and catalysts.
Can lead to the formation of TNT - 2,4,6-trinitromethylbenzene.
Describe the halogenation of benzene.
Needs a halogen carrier for a catalyst. Eg FeCl3, FeBr3, AlCl3, AlBr3.
Room temp.
Benzene + Cl2 —> chlorobenzene + HCl
Benzene + Br2 —> Bromobenzene + HBr
Perpetration of Br+
Br2 + FeBr3 —> Br+ + FeBr4-
H+ + FeBr4- —> FeBr3 + HBr
Catalyst
Describe the uses of chlorobenzene and Bromobenzene.
Chlorobenzene:
Solvent.
Production of pesticides.
Bromobenzene:
Preparation of pharmaceuticals.
What is a substitution reaction?
When an atom or group of atoms is replaced with a different atom or group of atoms.
Why is Electrophilic substitution?
A type of substitution reaction in which an electrophile is attracted to an electron rich centre or atom, where it accepts a pair of electrons to form a new covalent bond.
What is a reaction mechanism?
A series of steps that together make up the overall reaction.
What does the curly arrow represent?
A symbol used in reaction mechanisms to show the movement of an electron pair in the breaking or formation of covalent bonds.
Why does Electrophilic substitution occur in benzene?
The electron dense ring attracts an electrophile, and the electrophile accepts a pair of the pi electrons from the delocalised ring to form a covalent bond.
An intermediate forms that contains both the electrophile and the hydrogen atom that is being substituted.
Intermediate is very unstable.
The intermediate rapidly loses the hydrogen as a H+ ion and the delocalised ring of electrons reforms as stability is restored.
Draw Electrophilic substitution mechanism.
Draw nitration mechanism.
Draw halogenation mechanism.
Yeah.
Why does Br2 react with cyclohexane?
Cyclohexane + bromine —> 1,2-dibromocyclohexane
The pi bond has localised electrons above and below 2 carbon atoms, this has a region of high density.
When bromine approaches the electrons in the pi bond repel the electrons in a Br-Br bond and induced a dipole.
One end of Br2 becomes slightly positive an the other slightly negative, Br2 is now polar.
The pi electron pair is attracted to the positive bromine atom, causing the double bond to break.
A new bond forms between them, forming a positive carbocation.
The Br bond breaks by heterolytic fission forming a Br-.
The Br- is attracted to the carbocation forming a covalent bond.
Mechanism for bromine and cyclohexane.
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Why does benzene not react with bromine?
Compare this to cyclohexane.
Requires a halogen carrier, such as iron.
Benzene has delocalised pi electrons over all it’s carbon atoms.
Alkenes have pi electrons over only 2 carbons.
Benzene has a lower pi electron density than alkenes.
There is insufficient pi electron density to cause enough polarisation in non polar molecules that approach.
Describe the reaction of phenol and sodium hydroxide.
C6H5OH + NaOH —> C6H5O-Na+ + H2O
Forms sodium phenoxide and water.
What happens to phenol in water?
Dissociates.
C6H5O- + H+
