Aromantic compounds Flashcards
what is the molecular formula of benzene?
C6H6
when is phenyl used?
you get a phenyl group C6H5 by removing hydrogen from a benzene ring and the group will always be attached to something else e.g phenol,phenylethanone etc.
Explain Kekule’s ideas but the issues behind it
benzene was cyclic and had alternate single and double bonds
Issues:
1.benzene doesn’t undergo electrophilic addition reactions with bromine-alkenes do
2.Benzene has relatively low reactivity unlike alkenes.
3.Benzene has a regular planar structure with equal C-C bond lengths (C=C bonds shorter than C-C)
Evidence against Kekule: reaction with bromine
Benzene doesn’t undergo electrophilic addition with bromine like alkenes do. Bromine water remains orange unless it undergoes electrophilic substitution with the usage of a halogen carrier such as Fe,FeBr3,AlBr3 etc. to make Br+ more reactive
Evidence against Kekule: Bond length
Benzene has a regular structure with equal C-C bond lengths i.e. its planar. However Kekule didn’t have a regular structure with C=C bonds being shorter than C-C bonds.
Evidence against Kekule: Enthalpy of hydrogenation
its less exothermic than expected giving benzene greater stability. Therefore Kekule is wrong because he predicted a more exothermic enthalpy of hydrogenation
Explain structure of delocalised Benzene model
- planar cyclic molecule
- triganol planar 120 degrees around each C atom
- 3 sigma bonds per C atom the 4th electron being in a p-orbital
- p-orbitals overlap sideways above and below plane of ring forming pi bond.
- 6 pi electrons delocalised over 6 C atoms leading to further stability
- benzene doesn’t undergo addition reactions as stability would be lost.
Why are higher temperatures not used in nitration?
multi substitution would occur
Why are Friedel-Crafts reactions important?
leads to C-C bond formation
Why do phenols not react with metal carbonates?
Phenol isn’t acidic enough.
Comparison of alkene, benzene and phenol: electrons
alkene- pi electrons localised between 2 C atoms + high electron density
benzene-6 pi electrons delocalised over 6 C atoms + lower electron density than alkenes
phenol-6 pi electrons delocalised over 6 C atoms + lone pair in p-orbital on O delocalised into ring activating it + higher electron density than benzene
Comparison of alkene, benzene and phenol: attracting electrophiles
alkene-polarises Br2 molecule and attracts it
benzene-only weakly polarises Br2 molecule needs halogen carrier to make powerful electrophile Br+
phenol-polarises Br2 molecules and attracts it
Comparison of alkene, benzene and phenol: type of reaction
alkene-electrophilic addition colour change orange to colourless
benzene-electrophilic substitution
phenol-electrophilic substitution colour change orange to colourless and formation of white precipitate 2,4,6-tribromophenol
define activating group
lone pair delocalised into benzene ring activating it higher pi electron density of C 2 and 4
define deactivating group
withdraw electron density from benzene ring deactivating it. higher pi electron density at C 3
