Unit 4.2 - Aromaticity Flashcards
When do radicals form?
Hen a covalent bond breaks
Give an example of when a radical forms
When you crack a large alkane
Describe a radical
Has an unpaired electron and is very reactive
What type of reactions do radicals form?
Very rapid ones
How are radicals represented?
With a dot
Nucleophiles
Ions or molecules which can donate a lone pair of electrons, forming a covalent bond
What are Nucleophiles attracted to?
Positive charges or positive centres of charge
Examples of positive centres of charge
Nuclei, carbon-halogen bonds
How are Nucleophiles charged?
Negatively or neutrally
What do we need to remember to show on Nucleophiles?
Lone pairs of electrons
Three types of reagent in organic chemistry
Radicals
Nucleophiles
Electrophiles
Electrophiles
Ions or molecules which can accept a lone pairs of electrons, forming a covalent bond
What are electrophiles attracted to?
Negative charges or negative centres of charge
Which part of an alkene will be attacked by an electrophile and why?
The pi bond since it’s a negative centre of charge
Negative centre of charge of an alkene
Pi bond
How are electrophiles charged?
Positively or neutrally
With what do we need to show lone pairs - Nucleophiles or electrophiles?
Nucleophiles
What are the 5 types of reactions we need to learn?
Additional
Substitution
Elimination
Oxidation/reduction (redox)
Hydrolysis
Additional reaction description
2 molecules are brought together to produce 1 molecule
A + B ——> C
Must normally be a double bond available
Example of an addition reaction
Electrophilic addition
What type of reactions do alkenes always undergo?
Electrophilic addition
Substitution reaction description
An atom or group of atoms replaced another atom or group of atoms in a molecule
Normally the of the group which is introduced must be similar to the group which is replaced
Examples of substitution reactions
Nucleophilic substituion
Free radical substitution
Example of a free radical substation reaction
Photo chlorination of methane
Elimination reaction description
A simple molecule such as water is eliminated from the molecule
AB ——> A + B
A double bond often forms as a result of the reaction (opposite of addition)
During which type of reaction does a double bond often form?
Elimination
What would an elimination reaction be known as if water was the eliminated molecule?
Dehydration
Redox reaction description
Consists of both oxidation and reduction - can’t have one without the other
All of the possible ways of defining oxidation
Loss of electrons
Gain of oxygen
Loss of hydrogen
All of the possible ways of defining reduction
Gain of electrons
Loss of oxygen
Gain of hydrogen
3 examples of redox reactions
Combustion
Total oxidation of primary alcohols
Dichromate (VI) reduced to Cr3+
Colour change when dichromate (VI) is reduced to Cr3+
Orange to dark green
Hydrolysis reaction description
Chemical reaction between the organic compound and water
Very slow = needs a catalyst
Examples of catalysts used in hydrolysis reactions + why they’re used
Sodium hydroxide
HCl
Very slow reactions
Example of a hydrolysis reaction
Nucleophilic substitution
What type of compound is benzene?
Aromatic
Simplest aromatic compound
Benzene
Describe benzene at room temperature
A colourless liquid
Give 2 features of benzene
Distinctive smell
Highly toxic
What is benzene soluble in and what isn’t it soluble in and why?
Insoluble in water (immiscible)
Soluble in non-polar solvents
It’s a non-polar molecule
Describe benzene in water
Insoluble (immiscible)
Immiscible meaning
Doesn’t mix
What type of intermolecular forces are formed when benzene in dissolved in non-polar solvents?
Van der Waals
What type of molecule is benzene?
Non-polar
What type of molecules do polar solvents dissolve?
Polar molecules (and vice versa for non-polar solvents and molecules)
Empirical formula of benzene
CH
Molecular formula of benzene
C6H6
What structure for Benzene did August Kekule suggest?
A six-membered carbon ring of alternating double and single bonds
What were the issues with the Kekule model for Benzene?
Could not explain some of its reactions
Which of benzene’s reactions could the Kekule model not explain?
It should react as an alkene and easily undergo addition reactions with Br2 and decolorize bromine water (no decolorisation = not classified as a true alkene)
What did Kekule propose to explain why his benzene model did not react as an alkene as it should?
That benzene had 2 forms and suggested that one form changed to the other so quickly that an approaching molecule would have no time to react with it by addition
Resonance arrow
<——>
Give an example of when a resonance arrow is needed
To show the two resonance Kekule forms of benzene
How does the actual structure of benzene compare to the structure first predicted?
It’s a lot more stable
Cyclohexene formula
C6H10
What type of molecule is cyclohexene?
An alkene
What happens to cyclohexene on hydrogenation?
It forms the cyclic alkane cyclohexane
Cyclohexane formula
C6H12
Why would it be expected that the enthalpy of hydrogenation of benzene would be 3x that of cyclohexene?
Because cyclohexene only has 1 C=C bond whereas benzene is a cyclic alkene that’s a triene, therefore it has 3 C=C bonds
How does the actual value for the enthalpy change of hydrogenation of benzene compare to the theoretical value? Why?
It’s considerably less than calculated
Benzene is more stable than predicted
Why is benzene more stable than predicted?
Because of the stabilisation provided by pi electron delocalisation
How do we work out the stabilisation energy of benzene?
It’s the difference between the theoretical value and the calculated value of its enthalpy of hydrogenation
How would we work out the enthalpy of formation of benzene using bond energies?
Remember that benzene contains 3 C-C and 3 C=C bonds and remember BERP
Chemical formula of benzene
C6H6
Structure of benzene
Planar cyclic structure
How are the carbon atoms arranged in benzene?
In a regular hexagon
What do the electrons of the 6 different carbon atoms in benzene do?
3 of the 4 outer electrons in each carbon atom are involved in forming single covalent bonds - two between the carbon atoms and one between the carbon and hydrogen atoms
What type of framework is given rise to from the arrangement of the outer electrons of each carbon atom in benzene?
A Trigonal planar framework
What does the Trigonal planar framework of benzene give rise to and what is this similar to?
The hexagonal structure
Is a bit similar to graphite
Why does each carbon have an electron left that are found in the p orbitals of benzene?
Because 3 of the 4 electrons in each carbon atom are involved in forming single covalent bonds, and so each carbon atom has one left
What do the total of 6 electrons left for each carbon atom (those that are not involved in forming single covalent bonds) in benzene do?
These electrons are found in p orbitals at 90 degrees to the plane of the molecule and they interact to give a pi electron delocalised bond around the molecule
Which electrons are found in p orbitals at 90 degrees to the plane of the molecule and they interact to give a pi electron delocalised bond around the molecule of benzene?
The 1 electron from each carbon atom (6 in total) that are not involved in forming single covalent bonds
What is responsible for stabilising benzene?
The pi electron delocalisation
What does the pi electron delocalisation do to benzene?
Stabilises it
How can the pi electron delocalisation be imagined in benzene?
As a charge cloud above and below the plane of the ring
Why is benzene not a conductor of electricity even though there are delocalised electrons?
The electrons are delocalised around the ring
How is the delocalised bond in benzene represented in images?
By a circle
How could the benzene ring be described in terms of electrons?
An area of high electron density
Describe the ring in the middle of benzene
Electron rich
What does a benzene ring attract due to it being an area of high electron density?
Electrophiles
What does delocalisation do to benzene compared to a structure containing alternate single and double bonds?
Stabilises it
What stabilises benzene?
Delocalisation
Explain the three forms of evidence for the delocalisation in benzene
- The C-C bonds in benzene are of the same length and intermediate in length between C-C and C=C bonds (hence “regular hexagon”)
- Benzene unlike the alkenes does not undergo addition reactions but undergoes substitution reactions. If benzene underwent addition reactions, the stability afforded by the pi electron delocalisation would be lost (the ring would break). The stability is retained when substitution reactions occur. Thus benzene does not undergo addition reactions with bromine or hydrogen bromide
- Benzene is not oxidised by potassium manganate (VII) solution, unlike the alkenes.
Why can’t benzene undergo addition reactions and what type of reactions does it undergo instead?
If it underwent addition reactions, the stability afforded by the pi electron delocalisation would be lost (the ring would break)
Substitution reactions happen instead, where the stability is retained
What type of reactions does benzene undergo rather than what? Why?
Substitution rather than addition
To retain the stabilisation afforded to the molecule by pi electron delocalisation
Pi electron delocalisation
Pi electrons at 90 degrees to the plane of the molecule
What type of reactions are the two main reactions of benzene?
Electrophilic substitution
What happens during the nitration of benzene?
A nitro group (-NO2) replaces a hydrogen atom in the benzene ring
Nitro group
-NO2
-NO2
Nitro group
Reagents for the nitration of benzene
Concentrated sulphuric acid and concentrated nitric acid
Conditions for the nitration of benzene
Reflux (heat), <50 degrees Celsius
Product of the nitration of benzene
Nitrobenzene, C6H5NO2
Equation for the nitration of benzene
C6H6 + HNO3 ——> C6H5NO2 + H2O
How is nitrobenzene formed?
As a yellow oily liquid
What type of compound is nitrobenzene?
Toxic
Can benzene be nitrated further after its nitration?
Yes under more extreme conditions
Mononitration
Nitration with only 1 nitro group
What type of reaction is the mononitration of benzene an example of?
Electrophilic substitution
Electrophile during the mono nitration of benzene
The nitryl cation, NO2+
How is the nitryl cation NO2+ formed in the reaction mixture during the nitration of benzene?
Formed when the concentrated sulphuric acid reacts with the concentrated nitric acid
Which area attracts the electrophile during the nitration of benzene? Why?
The delocalised pi electrons
Provide an electron rich area
What does the electrophile form a bond with during the nitration of benzene?
With the delocalised pi electrons (an electron rich area)
When is delocalisation partially lost during the nitration of benzene?
When an unstable intermediate is formed
How is stability restored after the unstable intermediate during the nitration of benzene?
A hydrogen atom is lost to give the product in order to restore the delocalisation and hence the systems stability
Where exactly do we draw the arrow during the intermediate stage of the nitration of benzene?
Start it from the centre of the C-H bond
What does the dotted line represent during the reaction mechanism of the nitration of benzene?
The broken delocalised ring
Curly arrows
Represent the movement of a pair of electrons
In which direction are curly arrows always from and to?
Always from negative to positive
What happens during the halogenation of Benzene?
A halogen atom (Cl, Br) replaces a hydrogen atom on the benzene ring
What type of reaction is the halogenation of benzene?
Substitution
Will benzene decolorize bromine on its own?
No
What is the catalyst required for in the halogenation of benzene known as?
A halogen carrier
Reagents for the halogenation of benzene
Chlorine or bromine
Catalyst for the halogenation of benzene
Aluminium chloride or iron (III) chloride (halogen carrier)
Conditions for the halogenation of benzene
Room temperature, dry
Possibly products of the halogenation of Benzene
If Cl2 was used —> chlorobenzene C6H5Cl
If Br2 was used —> bromobenzene C6H5Br
Equation for the halogenation of benzene (using Cl2)
C6H6 + Cl2 ——> (C6H5Cl) + HCl
Describe the mechanism for the chlorination of benzene
The chlorine molecule is polarised as it gets close to the benzene molecule. The partial positive charge of the chlorine acts as the electrophile.
Aluminium chloride accepts a chloride ion to form AlCl4- (aluminium is electron deficient). Also an electron is given back from C-H in the middle to regain stability
The aluminium chloride is re-formed
Where acts as the electrophile during the halogenation of benzene?
The partial positive charge of the chlorine atom
What is the halogenation of benzene a good example to show in terms of catalysts? Explain
Shows how catalysts remain unchanged during a reaction
AlCl3 shows this
What happens during the Friedel-Crafts Alkylation reaction?
An alkyl group replaces a hydrogen atom in the benzene ring
Typical alkyl groups
CH3 methyl
CH3CH2 ethyl
CH3CH2CH2 propyl
All of the benzene reactions to learn
Nitration
Halogenation
Friedel-Crafts Alkylation
Alkaline hydrolysis of 1-chlorobutane and chlorobenzene
Equation for the Friedel-Crafts Alkylation reaction
C6H6 + CH3Cl ——> C6H5CH3 + HCl
What is used as a catalyst/halogen carrier during the Friedel-Crafts Alkylation reaction? Why?
Aluminium chloride
Since we need to remove the chlorine from the initial halogen carrier
Catalyst for the Friedel-Crafts Alkylation reaction
Aluminium chloride
Conditions for the Friedel-Crafts Alkylation reaction
Room temperature, dry
Product of the Friedel-Crafts Alkylation reaction
Methylbenzene C6H5CH3 with chloromethane
What would the product of the Friedel-Crafts Alkylation reaction be if the reagent is chloroethane?
Ethylbenzene
What is the alkaline hydrolysis of 1-chlorobutane and chlorobenzene reaction for?
A comparison reaction to compare their reactivity with alkaline NaOH
1-chlorobutane equation
CH3CH2CH2CH2Cl
Chlorobenzene equation
C6H5Cl
Method for the alkaline hydrolysis of 1-chlorobutane and chlorobenzene reaction
1.) 1cm3 of each compound is places in a test tube with 2cm3 of ethanol and 2cm3 of aqueous sodium hydroxide and placed in a water bath at 60 degrees Celsius for 10 minutes
2.) to this mixture is added a mixture of 2cm3 of dilute nitric acid and 2cm3 of aqueous silver nitrate solution similarly warmed for 10 minutes
3.) the mixture is allowed to stand to observe any changes
What are we comparing in the alkaline hydrolysis of 1-chlorobutane and chlorobenzene reaction?
How easily chlorine substitutes for an OH in both of the compounds
What does OH- act as in the alkaline hydrolysis of 1-chlorobutane and chlorobenzene reaction?
A nucleophile
What should react the strongest in the alkaline hydrolysis of 1-chlorobutane and chlorobenzene reaction? Give the expected observations
1-chlorobutane should react more readily (white ppt) than the chlorobenzene (no reaction)
Explain in detail why 1-chlorobutane reacts more readily than chlorobenzene in the experiment
Chlorobenzene does not react readily with Nucleophiles such as the hydroxide ion because the lone pair of electrons on the chlorine atom interacts with the pi delocalised electron system of the benzene ring. The lone pair of electrons on the chlorine atom are partially delocalised into the ring system. The C-Cl bond is strengthened.
How is the C-Cl bond strengthened in chlorobenzene?
The lone pair of electrons on chlorine are partially delocalised into the ring system
What does a stronger C-Cl bond lead to for chlorobenzene?
It means that the chlorine is less likely to be substituted with the OH-
Which reaction does chlorobenzene not undergo like 1-chlorobutane and why?
Hydrolysis
As the C-Cl bond is stronger in chlorobenzene and not as easily broken
what was the issue with the kekule structure of benzene?
if it had the kekule structure, it would have different bond lengths and so no regular hexagon
which halogens will react when heating an aromatic halogeno with aqueous sodium hydroxide?
the chlorines attached to the aliphatic carbon will react, but those attached to benzene remain
how do we work out what reaction occurred to produce a product?
we think of the normal reaction that would occur and work in reverse
which halogens react and which don’t when heating an aliphatic halogeno with aqueous sodium hydroxide?
the halogens attached to the aliphatic carbon will react, but those attached to the benzene remain
% yield equation
Actual/theoretical x 100
Why will benzene not react with OH- ions to give phenol as one of the products?
The delocalised ring of electrons in benzene will repel the negative OH- ion
Write an equation for the formation of the species that attacks the benzene ring when chlorobenzene is formed
AlCl3 + Cl2 ——> AlCl4 + Cl+
Why is it important in the nitration of benzene that the temperature does not exceed 50 degrees Celsius?
As further substitution/nitration into the ring can take place
Why would it be expected that the enthalpy of hydration of benzene is 3x that of cyclohexene?
Since benzene has 3 double bonds
Name two neutrally charges nucleophiles
H2O and NH3
melting point of phenol and solid or liquid?
low
solid
melting point of phenol and solid or liquid?
low
solid
solubility of phenol
insoluble in cold, soluble in hot water
why is phenol only soluble in hot water?
it can form hydrogen bonds with water but the overall molecule is polar
toxicity of phenol
highly toxic
is phenol corrosive?
yes
equation for the reaction between phenol and NaOH
C6H5OH + NaOH —><— C6H5ONa + H2O
what colour does litmus not turn with phenol and why?
red
not acidic enough
why is phenol more acidic than alcohols?
it’s easier for phenol to donate its proton
conditions for the reaction of phenol with ethanol chloride
room temperature, dry
observations of the reaction of phenol with ethanoyl chloride + explanation
white musty fumes
HCl
conditions for the reaction between phenol and bromine
room temperature, aqueous solution
what forms when aqueous bromine reacts with phenol
a white precipitate of 2,4,6 -tribromophenol
how do you know that 2,4,6-tribromophenol has been formed during the reaction between phenol and bromine?
there’s a smell of antiseptic
what can the reaction with neutral aqueous iron (III) chloride be used for?
to differentiate between a phenol and an alcohol
ethanoyl oxy structure
C2O2H3
systematic name for aspirin
2-ethanoyloxybenzenecarboxylicacid
uses of aspirin
anaesthetic, pain relief, lower temperate, anti-inflammatory, prevention of strokes and heart attacks
why does aspirin prevent strokes and heart attacks?
it’s a blood thinner so it stops clotting
disadvantages of aspirin
bleeding risks, aspirin poisoning from overdosing, ulcers, allergic reaction, overdose can be fatal
(CH3CO)2O name
ethanoic anhydride
ethanoic anhydride
(CH3CO)2O
what does ethanoic anhydride do during the production of aspirin?
esterifies the phenol group
Delocalisation energy of benzene
The extra stability that benzene has as a result of having delocalised electrons (experimental - predicted)
Give a large scale use of a named organic halogen containing compound if your choice
DDT
Pesticide
Why is chlorine by itself not an organic halogen?
It’s only an organo halogen compound if the carbon and chlorine are bonded together
When reacting things with benzene rings, e.g acidified potassium dichromate (VI) or NaOH, which elements react and which don’t?
Only those that are parts of the aliphatic chains react, not those directly bonded to the benzene ring
Example of a neutrally charged electrophile
Br2
How does the Br-Br (for example) bond become polarised in the halogen action of benzene?
By electron attraction from the pi electron system and from polarisation from the electron deficient atom in the catalyst
Catalyst for the bromination of benzene
FeBr3
Why is there little reaction between benzene and bromine under normal conditions in the absence of a catalyst?
In the absence of a catalyst, there is little induced polarisation of the bromine molecular by the pi electron system
Catalyst for the hydrogenation of alkenes and benzene?
Nickel
Reagents for the nitration of benzene
Concentrated sulphuric and concentrated nitric acid
Radical
Species with an unpaired electron
