Armomatic Chemistry Flashcards
What is benzenes skeletal structure and formula ?
•C6H6
•A hexagon with a circle in the middle
What is another name for arenes ?
Aromatic compounds, as they were first found in sweet smelling dyes
What is the most common type of reaction for benzene ?
Substitution reactions in which one H is substituted for another functional group
What is the shape of benzene ?
Flat, regular hexagon. Bond angle = 120 degrees.
What is the bond length between C atoms in benzene ?
It’s between the length of C-C and C=C.
What happens to the 4th electron in the p subshell of each C atom in benzene ?
It delocalises to form rings of electron density above and below the hexagonal structure
What effect on the stability of benzene does its areas of electronegativity have?
It makes benzene more unstable, even though it is unsaturated
Describe the structure of cyclohexa-1,3,5-ene
A 6 carbon hexagonal molecule with double bonds on every other carbon
What is the thermochemical evidence that benzene is more stable than cyclohexa-1,3,5-ene?
•Hydrogenation of cyclohexene = -120kJmol-1
•Hydrogenation of cyclohexa-1,3-ene =-240kJmol-1
•Hydrogenation of cyclohexa-1,3,5-ene = -360kJmol-1
•Benzene hydrogenation is -208kJmol-1 so is more stable
Apart from thermochemical data, why is cyclohexa-1,3,5-triene not a suitable model for benzene ?
•Benzene is symmetrical whilst C135 is not
•C135 readily undergoes addition reactions whilst benzene does not
•Both form different isomers when halogenated.
What is the appearance of benzene at 298k 100kPa ?
Colourless liquid
Why does benzene have a relatively high melting point ?
There is close packing of the flat hexagonal molecules in the solid phase.
Is benzene soluble in water ?
No, it is non polar
Why is benzene dangerous ?
It is carcinogenic
How do you name molecules containing a benzene ring ?
Why is benzene attacked by electrophiles?
There is a high electron density above and below the ring
What is delocalisation energy and what is the effect of this on benzenes reactions?
•The large amount of energy required to break the aromatic ring apart
•Results in the ring typically staying intact during reactions.
What is seen when benzene is combusted ? Why ?
•Smokey flames due to soot from incomplete combustion
•This is due to a high carbon:hydrogen ratio.
Draw / describe a general electrophilic substitution mechanism of benzene using El+ to represent an electrophile
*Arrow from cirlce of benzene ring to El+
*Benzene now has a half ring and El as well as H on the same carbon
*Arrow from C-H to delocalised ring + charge.
What ion (name and formula) is used to nitrate benzene ?
*NO2+ (+charge is on the nitrogen)
*It is called a nitronium ion or nitryl cation.
How is the NO2+ ion generated ? (conditions and equation)
*Concentrated H2SO4 and concentrated HNO3
*H2SO4 + HNO3 -> H2NO3+ + HSO4-
*H2NO3+ -> H2O + NO2
*Overall: H2SO4 + HNO3 -> HSO4- + NO2+ + H2O.
How is the H2SO4 catalyst regenerated in the nitration of benzene ?
HSO4- + H+ -> H2SO4 (H+ is lost from the substitution of benzene)
Draw / describe a mechanism and write an overall equation for the nitration of benzene.
*C6H6 + HNO3 ->C6H5NO2 + H2O
*Arrow from inner circle of benzene ring to N of NO2+
*Benzene now has both NO2 and H bonded to the same carbon, and a semi circle with +
*Arrow from C-H to + of inner circle.
What are the uses of nitrated arenes?
*Production of explosives e.g. TNT as it releases lots of heat and gas on explosion
*To make aromatic amines that are used for industrial dyes.
