25 Flashcards
Kekule midel of benzene
6 planar ring joined by alternate single and double bonds
- this would result in LOCALISED electron density, allowing it to behave like alkenes
1) however it could not de Lourdes bromine water
2) the bond length predicted was wrong,
3) the enthakpy change of hydrogenation of actual benzene ended up being less than the predicted kekule model (135 hex tri ene), suggesting more stable. Wa Speedo tee to have 3 times of normal cyclohexene , but didn’t
This created delocalised model
What is delocalised model of benzene
Benzene is a planar ring made of 6 carbons. Each carbon uses one electron to bond with hydrogen atoms that sit in the plane in sigma bonds, and 2 electrons to bond with adjacent carbons.
The 4th electorn sits in a p orbital perpendicular to the plane. Adjacent p orbitals overlap sideways to form a ring of electron density above and below the ring of carbons.
This forms a system of pi bonds where the electrons are said to be delocalised
Why does delocalised model explain why brominstion couldn’t happen
In the delocalised model, the electron density is spread out between all the carbons, such that compared to a normal alkene where the electron density is localised, on any two carbons, the electron density will be less, meaning it can induce a weaker dipole and is less susceptible to electrophikic attack or attracting electophilee
Thus in reactions benzene need catalysts etc
How to name aromatic
Alkyl groups less than 7 and halogens and nitro are prefixes
Soon as any functional group or an alkyl group more than 6 joins , phenyl is prefix now
So phenylamine phenyloctane etc
Ben pic acid
Benzaldehyde
If there’s more than one sis the unit, just combine names, so like 2 broom methylbenzene’
Delocalised version more stable?
Electorn density spread out rather than in one place
INDUSTRIAL USES OF AROMATIC COMPOUNDS
pharmaceuticals like paracetamol
Edplosoices like tnt
Also dyes
Nitration
Requires first h2so4 conc and Nitric acid, and 50° , they react and cat,Sydney regenerated
If 70° more happens
Bromiantions
Alkylation
Acylstiom
Required halogen carrier catalyst, room temp
Alklyaltiojnrewuired haloalkane
Acyl chlroides are much more reactive and this happens more, also halogen carrier can form ketones
How to form ketones from benzene
Berne + acyl chlroide in electrophikic substion reaction
Phenols
Solubility
- less soluble than alcohols because of non polar ring, even though hydrogen bonds can be formed
- phenols more acidic than alcohols as it is a weak acid that partially dissociates in water , but weaker than cabroylic acid still. Carbox is the strongest weak acid
As a result carboxylic can react with weak base which is sodium carbonate, and phenol only with sodium hydroxide that’s it
Reactions under bromine
Due to partial delocalisation they happen easier
No h2so4 catalyst needed, weak h k3 and room temp
Similarly no halogen carrier and can happen three times before no more
Reactivity of phenol
Lone pair on oxygen partially delocalised into sustem of pi bonds increasing electonrgativity meaning more susceptible to attack compared to benzene alone
WHAT HAPPENS WHEN PHENOL MIXES WITH BROMINE NOW
IT WILL DECOLOURISE
AND FORM A WHITE PRECIPATET
Directiong and activisitng groups
OH on ohenolamctivtated the benzene ring, which makes it easier to react with things. Here this is why it reacts with bromine. Halogens are deactivating groups however and nitro too, but halogen less. That’s why each halogen makes reactivity less, 3 is cut off. But one nitro enough
In summar, all activating groups are 2, 4 directing (OH , and bromine goes everywhere) and all deactivating groups are 3 directing
However halogens are deactivating, they are 2 , 4
Again
All activating groups 2, 4 (OH and bromine) and all decsticiting is 3
But while halogens deactivating, they also 2 ,4
