Aromatics Directing Effects 1+2 Flashcards
What type of compound is benzene?
An planar, aromatic compound
What makes benzene stable?
The delocalised electrons within the benzene ring.
The resonance forms of benzene are of equal energy true or false?
True
Define Electrophile?
A species which attracts electrons towards itself
Define Nucleophile?
A lone electron pair donor.
Define Oxidation?
The removal of electrons from a molecule (OIL)
Define Reduction?
The addition of electrons to a molecule (RIG)
Define Bases
Electron Rich species (Can be negatively charged)
Define Acids
Electron Deficient species (Can be positively charged)
What is a negative charged species?
contains electrons (nucleophiles/bases)
What is a positive charged species?
devoid of electrons (electrophiles/acids)
What are arrows used for?
A full bladed arrow represents the movement of 2 electrons
What are dots used for?
one dot represents 1 valence electron (e.g. radicals)
2 dots represents 2 valence electrons (e.g. nucleophiles)
Why do the resonance forms of benzene have equal energies?
because of the uninterrupted cyclic cloud of p electrons above and below the plane of benzene (a planar molecule) and there is an odd number of pairs of p electrons.
What is Huckel’s Rule?
For a planar cyclic compound to be aromatic, it must contain and uninterrupted p cloud with (4n+2) p electrons. Where n is any whole number.
What is the aromaticity of benzene?
pi system
What is aromaticity?
a term used to describe a cyclic, planar molecule with a ring of resonance bonds that exhibits more stability than other geometric or connective arrangements with the same set of atoms.
What structure is preferred for drawing mechanisms using the benzene ring?
Kekule’s structure.
Why does benzene not react with electrophiles like alkenes do?
Because it is less reactive than alkenes due to it’s aromatic stabilisation.
What mechanism is used to brominate benzene?
Electrophilic Substitution
What is required to brominate benzene?
An acid catalyst e.g. AlCl3
How does the presence of an acid catalyst allow for benzene to do an electrophilic substitution reaction?
The acid catalyst makes Br2 more electrophilic (look at slide 12 for mechanism)
What is a wheland intermediate?
The carbocation formed after the addition of an electrophile to the benzene ring.
How is the wheland intermediate stabilised?
By the resonance forms
Wha drives re-aromatisation?
Aromatic stabilisation
What is used to analyse the wheland intermediate?
NMR
What two substances are required for the nutrition of benzene?
And draw the mechanism
Nitric acid and sulfuric acid (the catalyst)
What three substances are needed for the sulfonylation of benzene to form crystalline sodium benzene sulfonate?
And draw the mechanism!
SO3
Excess H2SO4
NaCl
What is required for alkylation and acylation reactions?
An acid catalyst (mainly Lewis acids)
What is the difference in reactivity of the starting material and product after undergoing the Friedel-Crafts alkylation reaction?
The product is more reactive than the starting material
What are the three types of precursors that can be employed in the Friedel-Crafts alkylation reaction?
- alkyl halide
- aliphatic alcohols
- alkenes
What is used as the catalyst for reacting benzene with methyl or ethyl halide?
FeCl3
What is used as the catalyst to react alkyl halides, from propyl onwards, with benzene
AlCl3 (a Lewis acid)
Why is there a mixture of products when reacting benzene with alkyl halides with 3 or more carbons?
Because the alkyl halide and AlCl3 react to form varying carbocations (primary and secondary carbocations).
Why is there more of the products formed by the secondary carbocation than the primary carbocation when reacting alkyl halides with benzene?
Because the secondary carbocation is more stable than the primary carbocation.
What happens to the stability and energy as you go from methyl to primary to secondary and then to tertiary alkyl carbocations?
Stability increases but energy decreases
How do we get a monoalkyl product?
Use excess benzene
What is required in the alkylation process with alcohols?
alcohol, strong acid (e.g. H2SO4) and this is used to produce the carbocation to react with benzene.
How does alkylation with olefins work?
Alkenes react with benzene in the presence of a suitable catalyst (e.g. HF, BF3) to produce alkylbenzenes.
How does friedel-crafts acylation work?
Acid chlorides (acyl halides) and acid anhydrides can both react with benzene in the presence of molar amounts of Lewis acids to produce ketones.
What product is formed from Friedel-Crafts acylation reaction?
Aryl ketone which is more basic than the starting product (acyl halide)
Why is water and acid added at the end of the Friedel-Crafts acylation reaction?
To hydrolyse any Lewis acid remaining hence liberating the ketone.
What is the reactive intermediate, formed from the reaction of acyl chloride with AlCl3, of the Friedel-Craft acylation mechanism known as?
Acylium ion
Which mechanism happens faster acylation or alkylation?
Acylation
What is electronegativity?
A measure of an elements ability to pull a pair of bonding electrons towards itself.
What scale is used to measure electronegativity?
Pauling’s scale
What causes a bond to be polar?
Where there’s a difference in the electronegativitiies of the elements bonded together.
What is a dipole moment?
The permanent electric moment of a molecule
What is the negative inductive effect?
When a species pulls electrons towards itself
What is the positive inductive effect?
When a species pushes electrons away from itself.
What happens to the inductive effect as the distance increases?
The effect falls off (weakens)
Why is the meta derivative of benzoic acid stronger than the para isomer (derivative)
Because of resonance. It is a result of conjugation of a lone pair of electrons with the pi-system of the ring.
What do the resonance structures show us?
The regions of increased/decreased electron density.
What can stop resonance occurring?
Bulky substituents
What is hyperconjugation?
A mechanism of both electron release and withdrawal. (happens via C-H and C-C bonds).
Alkyl substituents donate electron density’s via two mechanisms, what are they?
positive inductive effect and hyperconjugation
What is the effect of the positive inductive effect and hyperconjugation on the alkyl substituted benzene?
They make it more susceptible to electrophilic attack than benzene, making it more reactive than the starting material.
Three electronic effects that operate when looking at substituted benzene derivatives?
Inductive effect
Resonance Effect
Hyperconjugative Effect
Three compounds needed for the bromination of a phenol?
Br2, Ethanol, H2o
What directing effect does -OH have on the benzene ring?
Ortho-Para directing (look at slide 46 of lecture for more info)
When looking at HNMR to analyse directing effect, what does the proton shift indicate?
If the HNMR value decreases, then it shows an increase in electron density at those positions.
When a benzene ring has an O substituent, where does the reacting elements attach to?
At the ortho and para sites. (look at slide 48 for more info)
Effect of nitrogen substituent on the reactivity of benzene in comparison to the oxygen substituent?
Makes benzene even more reactive than the oxygen substituent did. (shown by the lower HNMR values at the para and ortho positions with nitrogen than with oxygen-slide 51)
How can we reduce the effect of the nitrogen substituent on the benzene ring?
By reducing the electron donating power of the nitrogen substituent e.g. pull electrons away from the ring, making it a poorer nucleophile, hence less reactive.
Effect of a methyl substituent on the reactivity of benzene?
It increases the reactivity of benzene at the ortho and para positions but not as much as the oxygen and nitrogen substituents do.
How does methyl increase the reactivity of benzene?
It is a weak conjugative electron donor hence increases electron density in the benzene ring.
(The C-H sigma bond of the methyl overlaps with the pi-system of benzene donating its electron density to benzene’s pi-system. The electrons flow between the orbitals.)
What intermediate is formed by the substitution of an electrophile on a benzene ring?
A wheland intermediate
What is the most favourable resonance form of the benzene with a methyl substituent?
The one with the positive charge at the carbon bonded to the methyl group that is pushing electrons towards it.
