module 6 - organic chemistry & analysis

What are the characteristics of benzene?

• colourless, sweet smelling highly flammable liquid

• found in naturally crude oil, component of petrol & found in cigarette smoke

• classified as a carcinogen

What type of hydrocarbon is benzene?

An aromatic hydrocarbon or an arene.

What is the basic structure of benzene?

Hexagonal ring of 6 carbon atoms with each carbon bonded to one hydrogen & 2 other carbons.

What is the molecular formula of benzene?

C₆H₆

Why was it a struggle for scientists to make a correct model of benzene from its molecular formula & the experimental evidence initially?

The molecular formula C₆H₆ suggested a structure containing many double bonds or triple bonds.

Double bond containing compounds were known to be very reactive, however benzene appeared unreactive.

What is the Kekule model of benzene?

Benzene is a six membered ring of carbon atoms joined by alternate single & double bonds.

What are the main pieces of evidence used to disprove Kekule’s model of benzene?

• The lack of reactivity of benzene

• The length of carbon-carbon bonds in benzene

• Hydrogenation enthalpies

Why is the lack of reactivity of benzene evidence to disprove Kekule’s model?

If benzene contained the alternate C=C bonds it should decolourise bromine in an electrophilic addition reaction however:

• Benzene doesn’t undergo electrophilic addition reactions

• Benzene doesn’t decolourise bromine under normal conditions

This has led scientists to believe benzene doesn’t have any C=C bonds.

Why is the lengths of the carbon-carbon bonds in benzene evidence to disprove Kekule’s model?

The use of X-Ray diffraction allows scientists to measure bond lengths in a molecule. When benzene was examined in 1929 it was found all the bonds in benzene had a length of 0.139nm which is between the length of a single bond and a double bond.

Why are the hydrogenation enthalpies of benzene evidence to disprove Kekule’s model?

If Kekule’s model was correct it could be given a name containing cyclohexene which means we can compare it’s hydrogenation enthalpies to cyclohexene.

It would be expected to have an enthalpy change of hydrogenation that is 3x that of cyclohexene - which has an enthalpy change of -120 - so one of -360.

However the actual enthalpy change is -208 meaning the actual benzene model is more stable than the Kekule one.

What did the contrasting evidence against Kekule’s model for benzene lead scientists to propose?

The delocalised model of benzene.

What is the general structure of benzene in the delocalised model?

Benzene is a planar, cyclic, hexagonal hydrocarbon containing six carbons and six hydrogens

How does the delocalised model of benzene work?

• Each carbon uses 3 of its available 4 electrons in bonding to 2 other carbons and one hydrogen

• Each carbon has one electron in a p-orbital at right angles to the plane of the bonded carbons & hydrogens

• Adjacent p-orbital electrons overlap sideways in both directions - above and below the plane of the carbons to form a ring of electron density

• This overlapping creates a system of pi bonds which spread over all 6 of the carbons in the ring structure

• The 6 electrons in the pi bonds are said to be delocalised

What are the different substituent groups of benzene that use a prefix?

Short alkyl chains - methyl, ethyl etc

Halogens - fluoro, chloro , bromo, idodo

Nitrogen - nitro

When is a group bound to benzene a substituent?

When the group contains less then 7 carbons or isn’t a carbon chain with an alkyl group e.g. an alcohol group.

The benzene ring is considered to be the parent chain.

When is benzene not considered a parent chain if a group is bound onto it?

When a benzene ring is attached to an alkyl chain with a functional group or to an alkyl chain with 7 or more carbon atoms - in these cases benzene is considered the substituent.

Instead of benzene the prefix phenyl is used in the name.

What are some exceptions to the rule for naming benzene etc that need to be remembered?

• Benzoic acid - bezenecarboxylic acid

• Phenylamine

• Benzaldehyde - benzenecarbaldehyde

How do you name a benzene compound with multiple groups?

Same idea with a carbon chain where the substituent groups are listed in alphabetical order using the smallest numbers possible.

What is the typical reaction of benzene to form a derivative?

Electrophilic substitution where one of the hydrogens is substituted with another group.

What are the different electrophilic substitution reactions that benzene undergoes?

• Nitration - to form a nitrobenzene

• Halogenation - to form a “halo”benzene

• Alkylation - to form an “alkyl”benzene

• Acylation - to form something like phenylethanone

What reactants & conditions are needed for a nitration of benzene reaction to occur?

• Benzene (duh)

• HNO₃

• Catalyst - H₂SO₄

• Temperature below 50 degrees Celsius

What is the group that needs to bind to benzene in a nitration reaction?

NO₂⁺

How is NO₂⁺ formed in a nitration of benzene from the original reactants & conditions?

It is formed in situ -

H₂SO₄ + HNO₃ —> HSO₄^- + H₂NO₃⁺

• This happens because the nitric acid is a weaker acid than the sulfuric acid so acts as a base in this reaction

The H₂NO₃⁺ is very unstable to splits apart to form H₂O & NO₂⁺ giving the NO₂⁺ electrophile required for the reaction.

• H₂NO₃⁺ —-> H₂O + NO₂⁺

Why is a temperature below 50 degrees celsius important to the nitration of benzene?

If it was above this could lead to multiple nitrations which is especially a problem in the nitration of methylbenzene as multiple nitrations can form Trinitrotoluene aka TNT.

What is the overall reaction of the nitration of bezene?

What is the mechanism for electrophilic substitution reactions of benzene?

• The electrophile E+ is attracts pair of electrons in the delocalised p-orbital electron structure forming a bond

• An intermediate is formed where the delocalised structure is disrupted and both the hydrogen and E are bound onto the benzene

• This intermediate ion has a partially delocalised structure but away from the carbons close to/bound to H & E

  • The ion has a positive charge because the E+ was positive

• The hydrogen will then bind to a nucleophile in the reaction mixture - e.g. O: - this doesn’t need to be represented

• This means the 2 electrons in the dative hydrogen bond with benzene are no longer in the bond and so they move down to plug the gap in the delocalised electrons restoring the delocalised ring of electrons

What are the reactants & conditions required for the halogenation of benzene?

• Benzene (duh)

• The halogen e.g. Cl₂

• A corresponding iron or aluminium compound catalyst e.g. FeCl₃ or AlCl₃

  • This can also be formed in situ from a reaction between the metal and halogen

What is the electrophile in a halogenation reaction of benzene?

Cl⁺ or Br⁺ etc

How is the Cl⁺ or Br⁺ etc formed in the halogenation of benzene?

AlCl₃ + Cl₂ —> AlCl₄^- + Cl⁺

What is the overall reaction for the halogenation of benzene?

How is the catalyst reformed in the halogenation of benzene?

AlCl₄^- + Cl₂ —> AlCl₃ + HCl

• The H is from the benzene

What can the catalyst used in halogenation & alkylation of benzene reactions also be called?

A halogen carrier.

What reactants and conditions are needed for the alkylation of benzene?

• Benzene (duh)

• An haloalkane - CH₃Cl

• An catalyst - AlCl₃

What is the electrophile required for the alkylation of benzene?

A carbocation - CH₃⁺

How is an electrophile generated from the reactants in an alkylation of benzene reaction?

AlCl₃ + CH₃Cl —> CH₃⁺ + AlCl₄^-

What is the overall reaction for the alkylation of benzene?

How is the catalyst in an alkylation of benzene reaction reformed?

H⁺ + AlCl₄^- —> HCl + AlCl₃

What is an acyl group?

A carbon double bonded to oxygen and bonded to a hydrogen with one bond left unbonded which can have a specific group attached to it e.g. chlorine.

What are the reactants and condition needed for an acylation of benzene reaction?

• Benzene (duh)

• An acyl chloride - CH₃COCl is the first member of the series

• A catalyst - AlCl₃

What is the electrophile required in the acylation of benzene?

CH₃COCl + AlCl₃ —→ AlCl₄^- + CH₃CO⁺

What is the overall reaction for the acylation of benzene?

How is the catalyst used in an acylation of benzene reaction reformed?

AlCl₄^- + H⁺ —> AlCl₃ + HCl

What are the organic products formed from the electrophilic substitution reactions that benzene undergoes?

• Nitration of benzene - nitrobenzene

• Halogenation of benzene - “halo”benzene

• Alkylation of benzene - methyl/ethylbenzene

• Acylation of benzene - an aromatic ketone e.g. phenylethanone

How are alkylation & acylation reactions of benzene different to nitration and halogenation reactions?

In alkylation & acylation reactions of benzene a carbon chain is added so there is a new carbon bond is formed increasing the number of carbon atoms in the compound.

What is an alkene vs an arene?

An arene is the equivalent aromatic compound of an alkene e.g. cyclohexene and benzene.

Alkenes undergo electrophilic addition reactions (to break the double bond) whereas arenes undergo electrophilic substitution reactions (there are no double bonds to break and if a C-C bond was broken it would disrupt the delocalised structure)

Why is cyclohexene able to react with bromine without a catalyst while benzene cannot?

• Benzene has delocalised pi electrons spread above and below the plane of the carbon ring

• Cyclohexene has its electrons localised in single C=C bonds

• This means that the electron density between any 2 carbons in benzene is less than the electron density between one C=C bond in cyclohexene

• Halogen molecules are non-polar so need to be polarised by electron density to form a delta positive halogen

• Cyclohexene can do this without a catalyst due the higher electron density that benzene doesn’t have

• This is why benzene needs a catalyst or halogen carrier

What do ortho, para and meta mean in substitution reactions of bezene?

They are positions where another substituent group can bind onto relative to the ‘first’ substituent X.

What does it mean for a benzene with a substituent to be activated or deactivated?

A further substituent added onto a benzene (with one already) can activate or deactivate the ring - the rate of reaction is sped up or slowed down.

The activation or deactivation also limits which positions the further substituent can take.

What does an activated benzene ring look like?

The second substituent group can form bonds at ortho and para so at carbons 2,6, & 4.

What does a deactivated benzene ring look like?

The second substituent group can form bonds at meta so at carbons 3 & 5.

Why is there a faster rate of reaction when benzene is activated?

There is a higher electron density donated to the delocalised benzene ring.

What are some activating substituents?

NH₂, OH, OCH₃, CH₃

Why do activating substituents activate benzene rings?

They all have lone pairs of electrons in their structure which can overlap with p orbitals to join in/donate electron density to the delocalised structure.

CH₃ doesn’t have any lone pairs but still acts as an activating substituent because it has 9 total electrons - still less than the other ones but more so than just hydrogen.