C25 Aromatic Chemistry Flashcards
describe, in terms if orbital overlap, the similarities and differences between the bonding in the Kekules model and the delocalised model of benzene.
similarities
- orbital overlap (sideways overlap of p orbitals)
- pi bond (pi bond/system/ring above and below atoms/ring/plane)
differences
- kekule = alternating pi bonds, localised pi electrons, 2 electrons in pi bond, 3 pi bonds
- delocalised = all p orbital overlap/ pi electrons spread around the ring/ 6 electrons in pi bond
explain the relative resistance of chlorination in benzene compared with an alkene.
reactivity of alkene
- pi electrons are localised / pi bond is localised
reactivity of benzene
- electrons are delocalised/ pi ring is delocalised
in alkene
- higher electron density
- more susceptible to electrophilic attack
what is an example of a redox reaction of phenol with a metal?
e.g. phenol + Na => sodium phenoxide (salt) + Hydrogen
What are the conditions and reagents for the electrophilic substitution of Benzene with Alkyl groups?
- Haloalkane e.g. C2H5Cl or C2H5Br
- presence of AlCl3 or FeBr3 (acts as a halogen carrier catalyst)
- Benzene
what is the functional groups on a phenol?
- phenol
-OH group
what is the pH of an alcohol?
7 (it is neutral)
what happens to phenol in water?
- when dissolved in water phenol partially dissociates forming phenxide ion and proton.
- phenol is less soluble in water than alcohols, because of the non-polar benzene ring
what does delocalised mean?
electrons are shared between more than 2 atoms
explain what the curly arrow in an electrophilic substitution represents.
the movement of a pair of electrons from a covalent bond breaking.
Explain the relative resistance to bromination of benzene?
Benzene:
- contains delocalised pi electron ring
- low electron density
- so its unable to induce a dipole on Br-Br and attract electrophiles
needs halogen carrier to induce dipole on Br2
Explain the relative resistance bromination of Phenol.
Phenol:
- 1 of lone pairs of electrons from oxygen atom is partially donated into the pi electron ring
- high electron density
- is able to induce a dipole in Br-Br and attract it as an electrophile
why is it easier to brominate phenol than Benzene?
phenol:
- delocalised pi electron ring
- 1 lone pair of electron from oxygen atom is partially donated into pi electron ring
- higher electron density than Benzene
- so it can induce a dipole in Br2 and attract it as an electrophile
What are the conditions and reagents for the electrophilic substitution of Benzene with NO2?
Concentrated sulphuric and nitric acid
50*C for Monosubstitution
70*C for disubstitution
why is phenol a weak acid?
it partially dissociates to produce a p+
Explain the relative resistance to bromination of cyclohexene.
cyclohexene:
- has localised pi electrons
- high electron density
- so its able to induce a dipole in Br-Br and attract it as an electrophile
what colour does phenol turn litmus paper?
Red
how does the nitration of benzene show that sulphuric acid is acting as a catalyst?
the sulphuric acid is regenerated at the end.
What are the conditions and reagents for the electrophilic substitution of Benzene with Halogens?
- Halogen carrier e.g. FeBr3 and AlCl3
- room temperature and pressure
- Benzene
why is the mechanism for the nitration of benzene described as substitution?
H+ substituted for NO2+
what does localised mean?
electrons are shared between 2 atoms.
Are phenols weaker or stronger acids than carboxylic acids?
they are weaker than carboxylic acids and they only react with strong bases e.g. NaOH.
phenols don’t react with weak bases like metal carbonates
what is an example of the neutralisation of phenol with a base?
e.g. acid + base => salt + water
phenol + KOH => potassium phenoxide + water
what is the Qualitative test for the identification of phenol, Benzene, and Cyclohexene?
add Bromine
- Bromine is decolourised by cyclohexene
- Bromine decolourises and a white precipitate is formed with Phenol
- there is no observed changed with Benzene
write an equation for the generation of the electrophile NO2+.
HNO3 + H2SO4 —> NO2+ + HSO4- + H2O
describe what happens to the p - orbital electrons in the C atoms in the benzene.
they’re at right angles to plane of ring and overlap sideways, above and below the ring.
what is benzene?
colourless sweet smelling, highly flammable
- found naturally in crude oil, is a component of petrol
- a carcinogen, can cause cancer
why is benzene not used in schools?
it is a carcinogen
what is the IUPAC name of Benzene?
Cyclohexa-1,3,5-triene
compared to alkenes how does benzene react with water when in the kekule structure?
it is less reactive
what is the geometry and bond angle around each carbon in Benzene?
120*, trigonal planar, has 3 bonding regions and no lone pairs.
what is the 1st piece of evidence to disapprove Kekules model?
1 - lack of reactivity of benzene
benzene doesn’t undergo electrophilic addition reaction to decolourise bromine water from orange to colourless under normal conditions.
what is the benzene ring called when its attached to an alkyl chain with a functional group?
the prefix phenyl is used in the name.
e.g. phenylethanone and 2-phenyloctane
is Benzene soluble in water? explain why.
no because its non-polar
what are the main features of the delocalised model of Benzene?
- Benzne is planar, cyclic, hexagonal Hydrocarbon containing 6 carbon and 6 hydrogen atoms.
- each carbon atom uses 3 of its available 4 electrons in bonding to 2 other Carbon atoms and to 1 hydrogen atom.
- each carbon atom has 1 electron in a p-orbital at right angles to the plane of the bonded carbon and hydrogen atoms.
- adjacent p-orbital electrons overlap sideways, in both directions, above and below the plane of the carbon atoms to form a ring of electron density.
- this overlapping of the p-orbitals creates a system of pi-bonds, which spread over all 6 of the carbon atoms in the ring structure.
- the 6 electrons occupying this system of pi-bonds are said to be delocalised.
what is the empirical formula of Benzene?
CH
in benzene, 3 of electrons in outer shell of each carbon form bonds with 2 C and H. the 4th electron is in what type of orbital?
P-orbital
what is the name of the electrophile NO2+?
nitronium ion
what is the 3rd piece of evidence to disapprove Kekules model
3 - hydrogenation enthapies
if benzene did have Kekules structure then it’d be expected to have an enthalpy of hydrogenation that is x3 of cyclohexene.
enthalpy of hydrogenation of cyclohexene = 120KJmol^-1
the expected enthalpy of hydrogenation of Kekules structure of benzne = 3 x -120 = -360KJ mol^-1.
however, the actual enthalpy of hydrogenation of benzene is -208 KJ mol^-1, meaning 152 KJ mol^-1 less energy is produced.
therefore, the actual Benzne structure is more stable than the theoretical Kekule model of benzene.
this shows that benzene is less exothermic than expected.
what is the 2nd piece of evidence to disapprove Kekules model of Benzne?
2 - the lengths of the carbon-carbon bonds in benzene
using X-ray diffraction to measure the length fo the bonds in a molecule.
when benzene was examined in 1929, it was found that all the bonds in benzene were 0.139nm in length. this is between the bond length of a single bond (0.153nm) and a double bond (0.134nm)
what are the carbon - carbon bonds like if the benzene adopt the kekule structure?
alternating long C-C and short C=C bonds
what is the 3 main pieces of evidence that disapprove Kekules model of benzene?
1 - lack of reactivity of benzene
2 - the length of carbon-carbopn bonds in benzene
3 - hydrogenation enthalpies
what is the basis for benzene compounds being described as aromatic?
many benzene compounds have pleasant smells.
how many electrons in the outer shell of a carbon atom?
4
what is an electrophile?
an electron pair acceptor (species attracted to areas of high electron density)
what is the molecular formula of Benzene?
C6H6
what is another term for an aromatic hydrocarbon?
Arene
summarise the unusual bonding in benzene.
p - orbitals overlap forming a delocalised pi system
What’s the state of benzene at room temp
Liquid
What’s the Bond angle of benzene
120*
What technique was used to find bond lengths of benzene
X ray diffraction
What happens to the 4th electron in the p-orbital of each carbon atom in benzene?
It delocalises to from rings of electron density above and below the hexagon, forming rings of delocalised electron density above/below the hexagon.
How do rings of electron density affect the stability of benzene
Makes benzene very unstable, even though its unsaturated (aromatic stability)
Why does benzene have a relatively high MP
Close packing of flat hexagonal molecules when solid
Why’s benzene attacked by electrophiles
High electron density above/below ring due to delocalised electrons
Nitration of benzene is what type of reaction
Electrophilic substitution reaction
What ion’s used to nitrate benzene
NO2+
What’s the cataclysm in nitration of benzene
Sulfuric acid
What type of catalyst is used for a friedel-crafts reactions
Halogen carrier
E.g. AlCl3
Why does benzene not react directly with halogens
Aromatic ring is too stable
What happens to the 4th electron in the p-orbital of each carbon atom in benzene?
It delocalises to form rings of electron density above and below the hexagon, forming rings of delocalised electron density above/below the hexagon.
What is happening when AlCl is formed in terms of electrons?
The lone pair of electrons on the chlorine atom is forming a coordinate bond to Al
How could you use a Friedel-Crafts mechanism to add a methyl group to a benzene ring?
Use a halogenoalkane and AlCl3 to create an electrophile that can attack benzene
What reactions can you carry out to show the weak acidity of phenol?
A neutralisation reaction with NaOH occurs but absent when you react phenol with carbonates
What is the relative ease of electrophilic substitution of phenol compared to benzene and why?
It is easier for electrophilic substitution to occur with phenol because oxygen lone pair of electrons from the -OH group are partially delocalised into ring therefore this increases the electron density of the ring thus electrophiles are more attracted phenol
What is the directing effect of electron donating groups OH and NH2?
They direct group to the 2 and 4 position of the ring in electrophilic substitution of aromatic compounds
What is the directing effect of electron withdrawing group NO2?
NO2 directs atoms to the 3 position of the ring in electrophilic substitution of aromatic compounds
What’s the formula of benzene
C6H6
What’s the empirical formula of benzene
CH
What’s the state of benzene at room temp
Liquid
What’s the bond angle of benzene
120*
What are the 3 features of benzene that don’t support kekule’s model
Benzene is resistant to addition reactions
Enthalpy change of hydrogenation of benzene is more stable than predicted
All the C bonds are the same length
What technique was used to find the bond lengths of benzene
X ray diffraction
What happens to the 4th electron in the p-orbital of each C atom in benzene
It delocalises to form rings of electron density above and below the hexagon, forming rings of delocalised electron density above/below the hexagon
How do the rings of electron density affect the stability of benzene
Makes benzene very stable, even though it is unsaturated (aromatic stability)
Why does benzene have a relatively high MP
Close packing of flat hexagonal molecules when solid
Is benzene soluble in water
No because its non-polar
Why’s is benzene not used in schools
Is a carcinogen
How do you name compounds containing a benzene ring
-benzene, or phenyl- ; can designate position on ring using numbers if there is more than one substituent
Why’s benzene attacked by electrophiles
High electron density above/below ring due to delocalised electrons
Nitration of benzene is what type of reaction
Electrophilic substitution reaction
Which ion is used to nitrate benzene
NO2^+
What’s the catalyst in nitration of benzene
Sulfuric acid
What type of catalyst is used for Friedel-Crafts reaction
A halogen carrier (e.g. AlCl3)
Why does benzene not react directly with halogens
The aromatic ring is too stable
What is happening when AlCl4^- is formed in terms of electrons
The lone pair of electrons on the chlorine atom s forming a coordinate bond to Al
How do you use a Friedel-crafts mechanism to add a methyl group to a benzene ring
Use a halogenoalkane and AlCl3 to create an electrophile that can attack benzene
What reaction can you carry out to show the acidity of phenol
Neutralisation reaction with NaOH occurs but absent when you react phenol with carbonates
What’s is the relative ease of electrophilic substitution of phenol compared to benzene and why
It’s easier for electrophilic substitution to occur with phenol because oxygen lone pair of electrons from the -OH group are partially delocalised into ring therefore this increases the electron density of the ring thus electrophiles are more attracted to phenol
What is the directing effect of electron donating groups OH and NH2
They direct group to the 2 and 4 position of the ring in electrophilic substitution of aromatic compounds
What’s the directing effect of electron withdrawing group NO2
NO2 directs atoms to the 3 position of the ring in electrophilic substitution of aromatic compounds
