Module 6 - chapter 25 P2 Flashcards
What evidence is there suggesting that the kekulé model is incorrect?
- no rapid decolourisation of bromine water observed under standard conditions (benzene should undergo electrophilic addition due to the c=c bonds, suggesting that benzene doesn’t have c=c bonds in its structure). benzene is also resistant to reaction
- ALL c-c bonds in benzene are the same length of 0.139nm - intermediate between the length of a single bond and a double bond (don’t say sigma/pi). Concluding that benzene doesn’t have single and double bonds but bonds intermediate in length. This was found by X-ray diffraction which is used to measure the length of bonds in a molecule.
- the enthalpy of hydrogenation is less exothermic than expected from studies of cyclohexene - if following the kekulé model the enthalpy would be expected to be x3 the enthalpy of cyclohexene bc of the 3 double bonds (-120KJmol-1 x 3 = -360) but the actual enthalpy is -208. The actual benzene structure is more stable than expected.
What is the kekulé model?
Benzene is a ring of 6 carbon atoms joined by alternate single and double bonds
What is the delocalised model of benzene?
- benzene is planar, cyclic, hexagonal hydrocarbon, C6H6
- only three electrons are used by each carbon atom for bonding (bonding to 2Cs and 1H) so each C has one electron in a p-orbital at right angles to the plane of the sigma bonds.
- adjacent p-orbital electrons overlap sideways in both directions, 3 above and 3 below the plane of the carbon atoms in the ring structure to give electron density above and below the ring. 6e- delocalised in total bc 3e- used by each c for bonding so only 1e- delocalised per c.
- the six electrons occupying this DELOCALISED PI SYSTEM (not pi bonds bc pi bonds only contain 2e-) are said to be delocalised (spread over more than 2 atoms = over 6 atoms) and in the p orbital overlap ring
- bond angles = 120 (trigonal planar)
Properties of benzene
- benzene is the simplest aromatic hydrocarbon (an arene)
- colourless, sweet smelling, highly flammable liquid
- found naturally in crude oil, is a component of petrol and also found in cigarette smoke
- classified as a carcinogen (can cause cancer)
- liquid at room temperature, immiscible in water (only London forces between molecules)
Nitro group
NO2
OH group
Phenol
Amino
NH2
Rule when drawing groups on benzene
Phenols and methyl groups always position 1
Uses of benzene
Polystyrene, dyes and fibres (polyesters), explosives (TNT = 2,4,6 trinitrotoluene), pharmaceuticals
How does electrophilic addition occur with benzene?
Electrophiles (electron pair acceptors) attack a benzene ring, a catalyst is required (H2SO4)
When showing mechanism show curly arrow from the centre circle to the electrophile as the electrons are attracted to the electrophile
Reagents for nitration of benzene (nitrating mixture) and why
HNO3 and H2SO4 both CONCENTRATED, H2SO4 to catalyse bc reaction is slow
Equation for the nitration of benzene
Benzene + HNO3 —(H2SO4 50C)—> nitrobenzene + H2O
Equation for generation of electrophile for nitration of benzene
HNO3 + H2SO4 -> NO2+ + HSO4- + H2O
Electrophile = NO2+ (nitronium ion) NOT HNO3
Equation for regeneration of catalyst for nitration of benzene
H+ + HSO4- -> H2SO4
What type of bond is formed in the nitration of benzene?
Daitive covalent bond
OR
Coordinate bond
Not “daitive coordinate”
Why is the temperature key for electrophilic substitution of benzene?
To maintain a good rate of reaction and if the temperature rises above 50C, further substitution reactions may occur to form dinitrobenzene
Reaction with excess nitric acid (2HNO3) above 50C (70C) -> 1,3 dinitrobenzene
How does halogenation of benzene occur?
A halogen carrier is used (something with 3 halogens) as a catalyst.
E.g. AlCl3, FeCl3, AlBr3, FeBr3
The halogen carrier is generated in situ (in the reaction vessel) from the metal and halogen
Equation for the generation of electrophile for halogenation of benzene
Br2 + FeBr3 -> FeBr4- + Br+
Br-Br bond broken by heterolytic fission
(Bromination)
Equation for the halogenation of benzene
Benzene + halogen -> halogenated benzene + H+
Room temp and pressure
Equation for regeneration of catalyst with halogenation of benzene
FeBr4- + H+ -> FeBr3 + HBr
What is alkylation?
The substitution of a hydrogen into the benzene ring by an alkyl group.
A haloalkane reacts in the presence of halogen carrier
It increases the number of c-c bonds in a compound.
What is used as the catalyst for alkylation?
Halogen carrier, AlCl3
What is the equation for the generation of the electrophile for alkylation?
C2H5Cl + AlCl3 -> AlCl4- + C2H5+
Could be any haloalkane
Equation for alkylation
Benzene + haloalkane -> alkylbenzene + H+(hydrogen halide)
Equation for regeneration of catalyst for alkylation
H+ + AlCl4- -> AlCl3 + HCl
What is acylation?
Addition reaction of benzene with an acyl group (could be halide or oxide) in the presence of a halogen carrier (AlCl3) as a catalyst. An aromatic ketone is formed
This is benzene with a ketone attached to the ring with the carbon chain from the acyl chloride attached where the Cl was
Benzene ring
Carbon with double bonded oxygen bonds to ring
Carbon chain attached to ketone in place of Cl
What is an acyl chloride?
COCl, eg ethanoyl chloride = CH3COCl
Naming benzene with a carboxylic acid attached
… carboxylic acid
Benzoic acid
Naming benzene with an aldehyde attached
Benzaldehyde
Benzenecarbaldehyde
Carboxylic acid group
C=O
-OH
Aldehyde group
C=O
-H
What type of reactions are nitration, halogenation, alkylation and acylation?
Electrophilic substitution
Cyclohexene v benzene
Cyclohexene only one double bond, benzene alternating
Benzene has lower electron density than cyclohexene as the electrons are delocalised
Benzene is more stable so requires more energy to bond with halogens
What does a curly arrow represent?
Movement of a pair of electrons
In an intermediate benzene how many electrons are involved in the pi bonding?
4 involved in the pi bonding as 2 are used in the daitive coordinate bond with the substituent
Differences between cyclohexene and benzene reacting with bromine/a halogen
Cyclohexene reacts by regular electrophilic addition
Benzene requires a halogen carrier, bromine is not decolourised without one
What is phenol?
Benzene with OH group
Cyclohexene v benzene
Cyclohexene only one double bond, benzene alternating/ delocalised ring
Benzene has lower electron density than cyclohexene as the electrons are delocalised
Benzene is more stable so requires more energy to bond with halogens
Why does benzene only react with bromine if a halogen carrier is present?
Benzene is too stable to react with non-polar molecules.
Because benzene has delocalised electrons above and below the plane of the carbon atoms in the benzene ring, resulting in insufficient electron density around any two carbon atoms to polarise the bromine molecule, therefore the reaction can’t take place so the electrophile must be generated
In alkenes the reaction can occur because the electrons are localised
Is NaOH a strong or weak base?
Strong, metal hydroxides are strong bases
Why are phenols stronger acids than alcohols but less acidic than carboxylic acids?
alcohols can’t react with strong or weak bases, phenol can react with strong bases, carboxylic acids are stronger acids because they can react with weak bases but the others can’t
there is a positive inductive effect with alcohols because with ethanol for example, the ethoxide ion accepts a proton due to the oxygen being more electronegative than the carbons and hydrogens so the lone pair is attracted. the electron density of the oxygen increases so the ethoxide ion accepts protons more easily
Is sodium carbonate a strong or weak base?
Weak, metal carbonates are weak bases, only carboxylic acids are strong enough to react with sodium carbonate
sodium carbonate dissociates into carbonic acid which is a stronger acid than phenol
Does phenol react with strong or weak bases?
Only strong bases because it is a weak acid
examples of halogen carriers?
AlCl3, FeBr3, AlBr3
what does benzene reacting with ethanoyl chloride form? what type of reaction is this?
Phenylethanone, electrophilic substitution acylation
how to distinguish between phenol and carboxylic acids?
react with sodium carbonate, only carboxylic acids will produce CO2 gas in reaction as phenol is not acidic enough to react
reaction of phenol with sodium hydroxide
forms sodium phenoxide and water, the proton is removed from the phenol and replaced with the sodium in ionic bonding
uses of phenyl esters?
- polyesters
- some types of liquid crystal displays (LCDs)
bromination of phenol
doesn’t require halogen carrier and can occur at room temp because high electron density ring induces dipole on bromine.
RAPIDLY decolourises bromine water because bromine substituted into ring.
white precipitate of 2,4,6 tribromophenol formed and HBr
nitration of phenol
dilute nitric acid at room temp and no sulfuric acid catalyst required
forms isomers of 2-nitrophenol and 4-nitrophenol and water
why is phenol more reactive than benzene?
lone pair of electrons from oxygen in pi system so increased electron density in the delocalised ring and electrophiles are more attracted to the ring.
higher electron density can polarise molecules e.g. bromine so no halogen carrier is required
what is activating the delocalised ring of electrons and how does this affect reaction?
when a lone pair of electrons is added to the delocalised ring of electrons, makes the reaction occur more readily due to increased electron density
e.g. NH2 activates the ring because it isn’t electronegative enough to attract electrons
what is second substitution called?
disubstitution
what is deactivating the delocalised ring of electrons and how does this affect reaction?
electrons are removed from the delocalised ring by being attracted by an electronegative group. decreased electron density in the ring so reactions occur less readily and more slowly - catalyst, high temp, high conc and/or halogen carrier required
e.g. NO2 very electronegative so deactivates ring
directing effect of NO2
positions 3 and 5
directing effect of NH2
positions 2 and/or 4
how does the directing effect link to activating/deactivating groups?
all activating groups (and halogens) direct to positions 2 and 4
all deactivating groups direct to positions 3 and 5
property of phenol
bactericidal
Acid definition
Proton donor
Base definition
Proton receiver
Properties of phenol
Colourless solid at room temp whereas benzene is liquid at room temp. This is because phenol can hydrogen bond so the mpt is higher
Dissolves in water due to hydrogen bonds but less soluble than alcohols due to non-polar benzene ring
Dissociates in water slightly as behaves as a weak acid
Equation :
C6H5OH(aq) + H2O C6H5O-(aq) + H3O+(aq)
Produces proton and phenoxide ion
What are alkylation and acylation also known as?
Friedel-Crafts alkylation/acylation
Which homologous series is benzene part of?
Arene
what does aromatic mean?
contains a benzene ring : historically was used to classify derivatives of benzene because many pleasant-smelling compounds have a benzene ring, despite many odourless compounds containing a benzene ring the term is still used
what are compounds with one substituted group called?
monosubstituted
naming as benzene/phenyl
in monosubstituted aromatic compounds the benzene ring is considered the parent chain so the compound is named “alkyl/halo/nitro benzene”. When the benzene ring is connected to an alkyl chain with a functional group or with seven or more carbon atoms benzene is the substituent so phenyl is used as the prefix instead, e.g. phenylethanone or phenyloctane
what are the exceptions to the phenyl/benzene naming rule?
BENZOIC ACID (benzenecarboxylicacid)
PHENYLAMINE
BENZALDEHYDE (benzenecarbaldehyde)
how do you list substituent groups in a benzene ring?
alphabetical order and using the lowest numbers possible
how does the delocalised model of benzene account for its observed stability?
the pi-bond electron density is spread out above and below the ring rather than concentrated areas of electron density in the separate bonds as in the kekule model. the presence of delocalised electrons makes it more stable (by resonance and loss of internal energy)
use of nitrobenzene
starting material for dyes, pharmaceuticals (paracetamol) and pesticides
how to show the role of a catalyst/halogen carrier in reactions
show stages of formation of electrophile and regeneration of catalyst
how can halogen carriers be generated in situ?
by the reaction of the metal and the halogen in the reaction vessel, e.g. Al or Fe as reacting metal
what exception is there for the reaction conditions when halogenating benzene
bromination of benzene can occur at rtp in the presence of a halogen carrier
official name for alkylation
Friedel-Crafts alkylation
which direction does the arrow go in mechanisms in relation to the electrophile?
towards electrophile from inner ring
