Aromatic Chemistry Flashcards
what is the difference between aliphatic and aromatic?
compounds that do not contain a benzene ring are described as aliphatic whilst those that do contain a benzene ring are described as aromatic
what was august kelule’s benzene model?
he suggested benzene was a 6 membered hydrocarbon ring with alternating double bonds
what were the problems with the kekule model?
1-alot of energy is needed to disrupt the delocalised system so benzene will not readily undergo addition
2-the delocalisation of electrons gives benzene extra stability worth 152 kJ mol ^-1 compared to having 3 distinct double bonds (-120 X 3= -360 but its instead -208 kJmol^-1)
3-the electrons are evenly shared, so all bond lengths are the same
what is the problem with the expected reactivity of benzene according to kekule ad the actual reactivity?
benzene undergoes electrophilic substitution reactions, rather than the electrophilic addition reactions that cyclohexa-1,3,5-triene would be expected to undergo
what does the problem with hydrogenation of benzene mean?
- benzene has gained an extra 152 kJmol^-1 of stabilisation energy due to the stabilisation that the delocalised structure provides.
- this makes it difficult to break the delocalised system, which is why benzene undergoes substitution rather than addition reactions
what is the problem with the modelling benzene according to the Kekule model?
- using x-ray diffraction, the bond length can be measured and compared to what we expect.
- all bonds in benzene are the same length (between a single and double bond 0.140nm)
- kekue predicts 2 diff lengths
- benzene has a planar cyclic structure consisting of a ring of carbon atoms, each with a single H atom attached that sticks out into a flat plane
- its now believed that benzene has a delocalised electrons system, arising due to the overlap of one p-orbital from each carbon atom. Delocalised electrons occupy these orbitals
- the ring has 2 regions of electron density, one above and one below the plane of the ring
how do you name benzenes and other aromatic compound? what are some common groups that are substituted into benzene?
common groups= chloro (Cl) nitro (NO2) ethyl (CH3CH2) bromo (Br)
prefixbenzene - alphabetical order
-if benzne isnt the main functional group of the molecule, the prefix changes
phenylMainFunctionalGroup
what happens in nitration of benzene if the temperature is too high? what are nitrated benzene rings used for?
- if the temp is toohigh, nitration will occur at multiple positions
- nitrated benzene rings are a useful intermediate in the production of explosives and industrial dyes.
- as part of the process of synthesising these products, nitrobenzene must be reduced with a tin catalyst and HCl to form phenylamine
what is a Friedel craft reaction? whats the acyl chloride that’s used?
a type of electrophilic substitution reaction
- acyl chlorides can be used t attach carbonyl groups to benzene rings
- the acyl chloride used = ethanoyl chloride
what is the halogen carrier used to convert acyl chloride into an electrophile?
AlCl3
how many times does a acylation of benzene reaction occur?
the carbonyl group withdraws electron density from the benzene ring, which makes it less susceptible to electrophilic attack. This means that only a single substitution occurs
describe the structure of Phenol
- has a hydroxyl group (OH) directly attached to a benzene ring
- the OH group must be directly attached to the ring, else the molecule would be called a benzyl alcohol
- Phenol does not behave like a typical alcohol- it cant act as a nucleophile
- its a weak acid because it partially dissociates in water
- the acidic nature of phenol is highlighted through its reactivity with a strong base, such as potassium hydroxide, to form a salt
explain the reactivity of Phenol
- phenol is more reactive than benzene due to having a greater electron density within the ring
- despite oxygen’s high electronegativity, the oxygen atom can donate its lone pair of electrons into the benzene ring
- the electrons are now incorporated into the delocalised system, increasing the electron density
- this extra electron density makes phenol more susceptible to attack from electrophiles. Is also more able to induce a dipole in non-polar molecules
what happens when a bromine molecule approaches a phenol molecule?
- the electron density in phenol repels electrons in the bromine-bromine bond in a similar manner to an alkene
- this pushes electrons away from phenol, polarising the molecule
- this explains why phenols undergo direct halogenation and will decolourise bromine water, whereas benzene will not
