Module 6 Flashcards
What evidence is there to reject the Kekulean model of benzene?
Relatively unreactive
- Benzene does not undergo electrophilic addition or decolourise bromine water as it would if there were double bonds
Carbon-carbon bond length
- X-ray diffraction has shown all of the bonds to be the same length (a length in between single and double)
Hydration Enthalpies
- The enthalpy of hydrogenation is less than 3x that of cyclohexene (which would be the expected value if Kekule was right)
Describe the structure of the delocalised model of benzene?
Adjacent p-orbitals overlap sideways to form a ring of electron density above the below the plane of bonding (known as a pi-system)
What reagents are required to nitrate benzene and what kind of reaction is it?
Electrophilic Substitution reaction, requiring nitric acid and sulphuric acid (to form NO2+ electrophile)
What reagents are required to halogenate benzene, and what kind of reaction is it?
Electrophilic substitution, requiring FeBr3 (to form Br+ electrophile), or the equivalent Fe-halogen compound
What reagents are required to alkylate benzene, and what kind of reaction is it?
Electrophilic substitution, requiring a haloalkane and a halogen carrier (e.g. FeCl3)
What reagents are required to acylate benzene, and what kind of reaction is it?
Electrophilic substitution, requiring an acyl-chloride and a halogen carrier (e.g. FeCl3)
What does phenol ‘behave as if it were’?
A weak acid, it can partially dissociate protons from its hydroxide group
How do you bromate phenol and what type of reaction is it?
Electrophilic substitution, add 3Br2, no catalyst is required
When phenol is nitrated (adding nitric acid), which 2 products are formed?
2-Nitrophenol and 4-Nitrophenol
Why is phenol more reactive than benzene?
The lone pair of electrons from the p-orbital of the OH is donated into the pi-system and makes the ring more electron dense, increasing susceptibility to electrophile attack
What effect(s) does the NH2 (anime) group have on the benzene ring?
It ‘activates’ it, allowing it to react more readily with electrophiles and also directing to positions 2 and 4 substitutions
What effect(s) does the NO2 (nitro) group have on the benzene ring?
It ‘deactivates’ it, making it less able to react with electrophiles and also directing to position 3 substitutions
What is the (general) relationship between?
All activating groups are 2 and 4 directing (except for halogens)
All deactivating groups are 3 directing
What are the alternate names of the 2,3 and 4 positon-substitutions?
2 is also known as ortho
3 is also known as meta
4 is also known as para
What are the conditions and reagents required for the oxidation of an aldehyde, and what does it form?
Potassium dichromate and sulphuric acid, under reflux, to produce butanoic acid
How can you reduce an aldehyde?
2[H] (from the NaBH4) and water, to produce a primary alcohol
How can you reduce a Ketone?
2[H] (from the NaBH4) and water, to produce a secondary alcohol
How can you form hydroxynitriles from a carbonyl compound?
Add sulphuric acid and sodium-cyanide (to form hydrogen cyanide in-situ) - good for increasing chain length
How can carbonyls react by nucleophilic addition with NaBH4, and how with NaCN?
Where an intermediate is formed and then protonated, making an alcohol or a hydroxynitrile respectively
How can you test for a carbonyl group?
Add Brady’s reagent (2,4-DNP in methanol and sulphuric acid), if positive, a yellow/orange ppt forms
