Y13 Aromatics Flashcards
Aromatic compound definition
Contains at least one benzene ring
Arene definition
Aromatic hydrocarbon which contains at least one benzene ring
What are the 3 problems with Kekule’s benzene model?
- Reactivity
- Bond length
- Enthalpy change of hydrogenation
Explain why reactivity is a problem with Kekule’s benzene model?
Benzene doesn’t undergo electrophilic addition reactions, doesn’t react with Br2, and usually undergoes substitution reactions
Explain why bond length is a problem with Kekule’s benzene model?
Carbon to carbon bond lengths in benzene are all equal and a C-C single bond is longer than a C=C double bond
Explain why enthalpy change of hydrogenation is a problem with Kekule’s benzene model?
Enthalpy change of hydrogenation is less exothermic than expected which tell us benzene is a more stable molecule than Kekule’s model suggests
When is benzene the suffix?
When small alkyl groups, NO2, and halogens are attached
What is the prefix when NO2 is a substituent to benzene?
Nitro-
When is benzene the prefix?
When the benzene ring is attached to an alkyl chain with a functional group or with 7+ carbons
What is the prefix of benzene?
Phenyl-
Describe the delocalised model of benzene (4 bullet points)
Each carbon has a spare electron in a p-orbital
P-orbitals overlap sideways
Form an electron pi cloud above and below the ring
Pi electrons are delocalised and electron density is more spread out
What 3 things must be remembered when drawing out the electrophilic substitution of benzene mechanism?
- How many e- are spread over how many carbons e.g. 4e- spread over 5C in step 2
- Benzene ring is broken in step 2
- on the carbon where the benzene ring is broken
What is the conversion, reagents, and conditions of the halogenation of benzene?
Conversion: Benzene –> chloro/bromobenzene
Reagents: Cl2/Br2
Conditions: Halogen carrier catalyst (Fex3/Alx3)
What 3 equations must be included with the halogenation of benzene?
- Forming electrophile
- Curly arrows (mechanism)
- Reforming catalyst
What is the standard equation for forming an electrophile in the halogenation of benzene?
Alx3 + x2 –> Alx4- + x+
(Same with Fex3)
What is the curly arrow equation for the halogenation of benzene the same as?
Electrophilic substitution of benzene mechanism (broken ring, +)
What is the standard equation for reforming the catalyst in the halogenation of benzene?
Alx4- + H+ –> Alx3 + Hx
What is the conversion, reagents, conditions, and type of reaction for the alkylation of benzene?
Conversion: Benzene –> alkylbenzene
Reagents: Haloalkane
Conditions: Alx3/Fex3
Type: Electrophilic substitution
What is the conversion, reagents, conditions, and type of reaction for the acylation of benzene?
Conversion: Benzene –> aromatic ketone
Reagents: Acyl chloride
Conditions: Alx3/Fex3
Type: Electrophilic substitution
What is the method for purifying an organic solid (4 steps)?
- Dissolve impure solid in MINIMUM volume of HOT solvent
- COOL
- FILTER under REDUCED PRESSURE
- Wash with cold solvent and leave to DRY
Why does the minimum volume of solvent have to be used when purifying an organic solid?
If too much solvent is used, more of the organic compound will remain in solution on cooling
Why does hot water have to be used when purifying an organic solid?
Ensures maximum recrystallisation
Why must the sample be cooled in ice when purifying an organic solid?
Organic compound insoluble in cold solvent so will recrystallise and impurities stay dissolved in solution
Why must the sample be washed with cold water when purifying an organic solid?
Any remaining impurities are washed off and removed
What is the difference between purifying an organic liquid and an organic solid?
Organic liquid: Separating funnel
Organic solid: Recrystallisation
What must be included when drawing the apparatus of recrystallisation?
Buchner flask, Buchner funnel, moistened filter paper, rubber tubing
What is the conversion, reagents, conditions, and type of reaction for the nitration of benzene?
Conversion: Benzene –> nitrobenzene
Reagents: Conc. HNO3
Conditions: Conc. H2SO4, reflux, 50°C
Type: Electrophilic substitution
Why is a temp. of 50°C used for the nitration of benzene?
Prevents further substitution
What is the equation for forming the electrophile for the nitration of benzene?
HNO3 + H2SO4 (catalyst) –> HSO4- + H2O + NO2+ (electrophile)
What is the equation for the catalyst (H2SO4) being reformed for the nitration of benzene?
HSO4- + H+ (from mechanism) –> H2SO4
What is the comparison between the electrons in benzene and cyclohexene?
Benzene: Delocalised (6e- spread over 6C)
Cyclohexene: Localised (2e- spread over 2C)
What is the comparison between the electron densities of benzene and cyclohexene?
Benzene: Low
Cyclohexene: High
Is Br2 polarised or not with benzene and cyclohexene?
Benzene: No
Cyclohexene: Yes
Is a catalyst required for the reaction of Br2 with benzene or cyclohexene?
Benzene: Yes (generates more powerful electrophile Br+)
Cyclohexene: No
What is the comparison between the attraction for electrophiles of benzene and cyclohexene?
Benzene: Low
Cyclohexene: High
What are the 2 ACTIVATING groups and how do they affect further substitution and electron density?
Activating groups: OH, NH2
Further substitution happens more readily
Increase electron density
What is the 1 DEACTIVATING group and how does it affect further substitution and electron density?
Deactivating group: NO2
Further substitution happens less readily
Decreases electron density
Which carbons do activating groups direct substituents to?
2,4,6-directing
Which carbon does the deactivating group direct substituents to?
Only 3-directing
What conditions are required for activation?
No catalyst required, room temp.
What conditions are required for deactivation?
Halogen carrier catalyst, high temp.
Phenol definition
OH group DIRECTLY bonded to benzene ring
What can phenols act as and what do they produce when they dissociate in water?
Weak acids, phenoxide ion
What is the order of acidity and what does each react with?
Least acidic: Alcohols - don’t react with NaOH or Na2CO3
Phenols - react with NaOH but not Na2CO3
Most acidic: Carboxylic acids - react with both NaOH and Na2CO3
What is the test for carboxylic acids and what is the observation?
Add carbonate, effervescence produced
What are the conditions, observations, and number of substitutions when phenol reacts with bromine water?
Conditions: No catalyst, room temp.
Observations: White ppt. of 2,4,6-tribromophenol, Br2 water decolourised
No. of substitutions: 3
What is the condition and observation when phenol reacts with dilute HNO3?
Condition: Room temp.
Observation: Mixture of 2-nitrophenol and 4-nitrophenol formed
What is the comparison of the reactivity of phenol and benzene?
Lone pair of electrons in a p-orbital on the oxygen atom of the OH group in phenol which is donated to pi cloud and is delocalised
Electron density of pi cloud higher in phenol than benzene and has a stronger attraction to electrophiles
Electron density in pi cloud of phenol is sufficient to polarise Br2 without the need for a halogen carrier catalyst
