Organic Chemistry III - topic 18

What is the most common type of reaction of benzene?

substitution (of a H for a different functional group)

What happens to the 4th electron in the p orbital of each C atom in benzene?

It delocalises to form delocalised pi system above and below the hexagon, forming rings of delocalised electron density above and below the hexagon plane.

Describe how the orbitals from carbon atoms interact to form the bonds in a benzene ring

• head on overlap between orbitals to form a sigma bond

• side on overlap between p orbitals to form pi bond

• electrons are delocalised around the ring

what was different than expected about Kekule’s idea of benzene?

• All the bonds are at the same length - proved by X ray diffraction data

• the enthalpy of hydrogenation of benzene is less exothermic than expected, much more stable than expected - proved by thermochemical data, which provide evidence for benzene’s structure and stability

• 1 proton environment, proved by magnetic resonance spectroscopy, kekule would give 2 environments

• infrared data provide evidence for benzene’s structure

Why is benzene stable?

The electrons in the p orbital of C are delocalised to form an electron cloud in the delocalised pi electron system above and below the plane.

why is benzene attacked by electrophiles?

it has high electron density above and below the ring due to delocalised electrons

what is seen when benzene is combusted? Why?

smoky flames due to soot from unburnt carbon because of the high Carbon:Hydrogen ratio

General electrophilic substitution of benzene

with bromine, FeBr3 as catalyst, forms bromobenzene + HBr

with chlorine, FeCl3 as catalyst, form chlorobenzene + HCl

which ion is used to nitrate benzene? How is it generated? (how to turn benzene to nitrobenzene)

what is the catalyst, how is it regenerated?

Nitration reaction: electrophilic substitution where a hydrogen atom is replaced by a nitro group

NO₂ +

H₂SO₄ + HNO₃ → H₂NO₃⁺ + HSO₄^-

H₂NO₃⁺ → H₂O +NO₂⁺

H₂SO₄ + HNO₃ → HSO₄ ^- + NO₂⁺+H₂O

catalyst: conc.H₂SO₄, 55 degree temperature - CANNOT be hotter to prevent further substitution reactions

HSO₄^- + H⁺→ H₂SO₄, H⁺ from the benzene ring

How do substituents with alkyl groups affect further substitution?

Alkyl group has inductive effect, they release electrons into the delocalised electron ring, increasing electron density and making further substitution reactions more likely

Direct substituents to 2,4,6 positions

How do substituents with NH2 affect further substitution?

NH2 has negative inductive effect. Remove electrons from the delocalised electron ring, decreasing the electron density and making further substitution reactions less likely

Direct substituents to 3,5 positions

What type of catalyst is used for a Friedel-Crafts reaction? Alkylation?

A halogen carrier, e.g. AlCl₃

Write an equation to form an electrophile to be used to acylate benzene with AlCl₃ and RCOCl, How is the catalyst reformed?

Acylation:

AlCl₃ + RCOCl → AlCl₄^- + RCO⁺, which can attack benzene

AlCl₄^- + H⁺ → AlCl₃ + HCl

Draw the mechanism for the acylation of benzene from RCO⁺

how to form cyclohexane from benzene?

add hydrogen in the presence of a nickel catalyst

write an equation between phenol and bromine water

draw the mechanism of reaction between phenol and nitric acid

forms two nitrophenol and water, basically plus onto phenol but loses hydrogen (at 2,4 )

why is phenol more reactive than benzene?

the lone pair of electrons on the -OH group becomes delocalised with the delocalised pi electron system, increasing electron density in the ring. This makes phenol more nucleophilic, makes the ring more susceptible to electrophilic attack

why is the reaction of phenol and bromine more reactive than benzene with bromine?

in a phenol the lone pair of electrons on the p orbital on the oxygen is delocalised into the delocalised pi electron system. This increases electron density and increases the attractiveness to the electrophiles.

why can phenols react with sodium hydroxide but not sodium carbonate?

phenols are very weakly acidic even weaker acids than carboxylic acids, Only carboxylic acids will react with sodium carbonate as phenol is not strong enough to react.

why are amines so reactive?

The lone pair of electrons on the Nitrogen due to the polar N-H bond

Solubility of phenylamine? Why?

Not very soluble, due to the non-polarity of the benzene ring C6H5 cannot form hydrogen bonds

Draw a mechanism for the basic action of an amine with water

product formed: ammonium ion

write the equation for the acidic action of an amine with HCl

primary amine, ammonia, aromatic amines, which is the strongest base

primary aliphatic amines are the strongest base as the alkyl groups have inductive effect and push electrons towards the nitrogen atom, increases electron density making electrons more available for the proton, so accept a proton more easily

Draw a mechanism for nucleophilic substitution of NH3 with RCH2Br to form primary amines. What might then happen?

multiple substitutions, then secondary, tertiary ammonium ions might form, hence produces a low yield of primary amine, use excess ammonia to maximise the yield

how to make primary amine from halogenoalkanes?

halogenoalkanes and conc. NH3 in ethanol, heat in a sealed tube, lead to further substitution

how to get from nitrile to primary amine? why is this better than using halogenoalkanes?

reduction using nickel catalyst, only primary amine forms

reduction using LiAlH4, in dry ether

does not lead to further substitutions

how to form an phentlamine salt from nitrobenzene?

reduction using Tin and conc. HCl

Room temperature

Equation for the reaction of nitrobenzene to phenylamine

C6H5NO2 + 6[H] → C6H5NH2 + 2H2O

what mechanism is used for forming amides from acyl chlorides and amines?

nucleophilic addition elimination

draw mechanism showing reaction between ethanoyl chloride and amine

draw mechanism showing reaction between halogenoalkanes and amine

recrystallisation practical to purify a solid

• choose a solvent

• the solid should be soluble when in hot solvent and insoluble in cold solvent

• record the mass of solid, pour in a beaker with minimum amount of hot solvent

• filter out the insoluble impurities

• add minimum amount of hot solvent to dissolve the residue on the side of conical flask

• put the rest of solution in ice cold water bath, leave to crystallise

• filter using a vacuum to filter out the pure solids, the liquid left is the soluble impurities

• rinse the crystals with minimum volume of ice-cold solvent to wash away any solution containing impurities

• allow solvent to evaporate

what is a zwitterion

• a molecule/ion in neutral condiiton having separate positive and negative ions

• when ph low, tend to form positive ions and ph high, negative

• when amino acid add to water, zwitterion is formed

are amino acids, apart from glycine, chiral?

• yes, amino acids have a chiral C atom and exist as two enantiomers

• though in nature, only one of these enantiomers is present

• these enantiomers are able to rotate the plane of plan-polarised light in the opposite directions

• these amino acids are usually called the 2-aminoacids by their IUPAC names

Explain why recrystallisation would separate pure solids from impurities

• the pure solid is less soluble in cold solvent than hot

• when cooled, pure solids comes out of solution, leaves the impurities behind

What happens when phenol reacts with sodium carbonate

Nothing because phenol is not acidic enough, phenol is a weak acid

Recrystallisation

• dissolve in minimum quantity of hot solvent

• filter to remove insoluble impurities

• allow to cool and crystallise

• filter crystals to remove soluble impurities under vacuum

• allow crystals to dry and determine melting temperature

• compare to known values in a database

Steam distillation

• separate an immiscible organic liquid from an aqueous mixture

• the organic compound needs to have a higher boiling point than water

• the organic compound is distilled with water at a lower temperature and prevents decomposition