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What is the molecular formula of benzene

C6H6

What is kekule drawing and systematic name of benzene and how did kekule predict benzene structure

In benzene bond angle around each carbon is 120 degrees and molecule if planar- kekules structure predicts this as around each C there are 3 sets of electrons which all repel so get as far apart as possible giving planar triangle shape with bond angles 120 degrees

Problems proposed with kerkules structure

• if it was correct benzene would react like an alkene and decolourise aqueous bromine, but there is no reaction

• If correct benzene should have 2 different carbon-carbon bond lengths because double bonds are shorter than single but all 6 are equal in length in between expected lengths

• If correct there would be 2 different isomers of 1,2-dimethylbenzene but there’s only one

• Change in enthalpy calculations using bind enthalpies and assuming kekule structure came out wrong and benzene does not give out as much energy as it should so it’s more stable

How does the hydrogenation show benzene is more stable than the kekule structure said it should be

If benzene has kekule structure with 3 c double bonds c the change in enthalpy for its hydrogenation would be 360KJ/moo 3X that for the hydrogenation of 1 C double bond C but its gives our a lot less energy then expected so benzene is more stable than the kekule structure which is confirmed as more severe conditions needed for hydrogenation:

• higher temp and pressure and a special finely divided form of the nickel catalyst

What is the true structure of benzene

Each C uses 3 of its 4 outer electrons to bond:

• single bonds are all sigma bins and form the sigma framework

• This leaves one outer electron on every C each in a p- orbital at 90 degrees to the planar sigma framework.

• Instead of overlapping in pairs giving 3 separate pi bonds

• All 6 p- orbitals overlap sideways so the 6 electrons, one from each carbon, are spread out evenly so they are shared by all 6 carbon atoms in the ring

• This spreading out of electrons is called delocalisation.

• The 6 delocalised electrons form a delocalised donut shaped pi cloud of negative charge above and below the ring of carbons and is represented as

Why is the actual delocalised structure of benzene more stable than the kekule structure

As electrons repel so the more they spread out the more stable.

When benzene reacts it gives out less energy than the kekule structure would because of benzenes extra stability due to the delocalisation

How do we draw benzene rungs fused together

Not represented with circles in rings as they are not separate delocalised systems so drawn as alternating double and single bonds though are really extended delocalised systems

When is the benzene ring classed as phenyl

What do you call a benzene ring with aldehyde attached and draw it

Draw butyl benzoate and phenyl propanoate

D is butyl benzoate

E is phenyl propanoate

What sort of reacting species are attracted to delocalised electrons in benzene attracted

The regions of negative charge attract electrophiles

Definition of elcetrophile

A positive ion or s+ part of a molecules which reacts by accepting a pair of electrons forming a covalent bond

What other family of organic molecules react with electrophiles and is the reaction easier or harder with arenes

Alkenes do elctrophillic addition

Reaction is harder with arenes and quicker with alkenes because the negative charge in pi cloud of an alkene is more concentrated whereas in an arene it is more spread out due to delocalisation. Reacting with benzene ring involves disrupting the stable delocalised ring and the activation energy is higher so arenes need a more powerful electrophiles and more severe reaction conditions

Why do arenes not do electrophillic addition

As the stable delocalised structure would be lost

What type of reaction do arenes undergo

Electrophillic substitution where the stable delocalised ring is retained. These reactions are relatively slow as the first step in the reaction has to disrupt the delocalisation which requires large amount and often a catalyst is needed to create an electrophile powerful enough to attack a benzene ring. Electrophiles used will need a full positive charge and delta plus is not powerful enough.

Chlorination of benzene

In chlorination of benzene why do we use AlCl3 catalyst and why is it covalent

Covalent because Al3+ ions are small and highly charged and they pull back electrons from cl- ions. In AlCl3 Al still needs 2 more electrons for noble gas configuration so AlCl3 is excellent electron pair acceptor.

AlCl3 takes an electron pair from the cl2 molecules and the cl-cl bond breaks by heterolytic fission giving Cl+ ion which is a powerful electrophile which attacks the benzene ring

Equation of AlCl3 catalyst being regenerated

Bromination of benzene

Nitration of benzene

What electrophile needed for the nitration of benzene and what is it made from

What is HNO3 acting as in this reaction

What is the overall equation to make the electrophile NO2 +

How does the NO2 + electrophile attack benzene and what happens if the temp gets too high

Sulphonation of benzene

How does benzenesulphonic acid react in alkali

in friedel crafts alkylation what is alkylation and what type of molecule or species is needed as an electrophile

• Alkylation = putting an alkyl group (carbon chain) onto the ring

• The molecule/species as an electrophile = carbocation (carbon chain with C+ where we want it to attach to benzene ring) which is made from the s+ C of haloalkane using AlCl3 catalyst.

Reangents and conditions for Friedel- Crafts alkylation

The required chloroalkane with its Cl where the ring will go, AlCl3 catalyst, anhydrous, reflux

Balanced equation of friedel crafts alkylation

What does the AlCl3 catalyst do in friedel crafts alkylation alkylation

CHCl3