Aromatic Compounds Flashcards
general fromula of benzene
C6H6
properties of benzene
colourless liquid
boiling point of 80.1 degrees celsius
immiscible in water
burns with a smokey flame due to high percentage of C in formula
present in Coal Tar (byproduct of coal production)
bonding in benzene
each C atom is sp2 hybridized ie. during the bonding each C atom has 3 sp2 hybrid orbitals and 1 unhybridized p orbital
characteristics of the bonding in benzene
each C-H bond is a sigma bond formed by the overlapping of sp2 hybrid orbital in C and one s hybrid in hydrogen
each C-C bond is a sigma bond formed by the overlapping of sp2 hybrid orbitals of C
How do the 6 electrons move in an area above and below the nuclei of the C atom to make a compound aromatic?
the unhybridized p orbitals of eachC atom overlaps above and below the plane of the nuclei
each p orbital overlaps with the 2 adjacent p orbitals
what is resonance?
the movement of electrons from one bond to another
features of the structure of benzene
- each CCC bond angle is 120
- the benzene molecule is planar
- all C-C bonds are identical in length and in strength (does not have double bonds and single bonds)
- the C-C bonds are intermediate in lengthand strength between the C-C single bond and C=C double bond
- the enthalpy change of hydrogenation is -209kJ
steps in the mechanism of electrophilic substitution
- since all 6 C atoms are identical the electrophile can approach any of the C atoms
- as the electrophile approaches, the C-H bond begins to get weaker and the pi electrons gravitate toards that C atom
- an unstable intermediate is formed in which the electrophile and he H are partially bonded to the same C atom
- the unstable intermediate quickly breaks down the H and leaves as H+ and the C electrophile bond is formed
the symmetical movement of pi electrons is restored
what is the nitrating mixture?
concentrated sulphuric acid and concentrated nitric acid
nitration of benzene
when benzeneis reacted with nitrating mixture, the benzene is converted to a pale yellow liquid called nitrobenzene
the mixture is placed in a water bath at 5 degrees Celsius
what is a halogen carrier?
carriers which induce polarity of the non-polar halogen molecules helping with the generation of electrophiles
chlorination of benzene
when Cl gas is passed through benzene at room temperature in the presence of a halogen carrier, the Cl atom will substitute H and chlorobenzene is formed
properties of methylbenzene
colourless liquid
boiling point 111 degrees Celsius
insoluble in water
obtained from Coal Tar
good solvent
parts of methylbenzene
the side chain
the aromatic nucleus (ring)/ phenyl group
what are the reactions of the side chain/ methyl group?
substitution
oxidation
substitution reaction of the side chain
when Cl gas is passed through methylbenzene boiling under reflux in the presence of sunlight, Cl atoms will substitute the H of the CH2 group
follows free radical substitution mechanism
oxidation of the methylbenzene side chain
when methylbenzene is heated under reflux with oxidizing agents (H2SO4 or acidified KMnO7), the side chain is converted to a carboxylic group and benzoic acid is formed
can occur with other alkyl groups but the final product is always benzoic acid
what are the positions of the C atoms in methylbenzene?
the C atom with the CH3 group is 1
two number 2 C atoms on either side of the 1
two number 2 C atoms below the position 2 atoms
one number 4 C atom
characteristics of methylbenzene involving reactions with the aromatic ring
the C atoms of the ring are not equivalent
the C atoms differ in their distances from the C atom with the CH3 group
how do positions work with electrophiles?
when an electrophile is approaching the ring, it will be directed to a particular position
the position to which the electrophile is directed is determined by the nature of the group
types of groups
activating
deactivating
what is an activating group?
electron donating groups which push electrons into the ring and increasing the density at all the C atoms
what happens when an activating group is present?
- the electrophile substitution reactions take place more easily than with benzene
- the incomin electrophile will go to the second and fourth C atoms
what are some common activating groups?
CH3/ C2H5
OH
NH2
what are deactivating groups?
electron withdrawing group which pull electrons away from the ring, reducing the electron density at all C atoms
what happens when a deactivating group is present?
the third carbon atom is the most negative due to the electron density being reduced more at the 2nd and 4th C atom
- the electrophilic substitution reaction takes place less readily than with benzene
- incoming electrophiles are directed to the third position
common deactivating groups
COOH
NO2
CHO
chlorination of methylbenzene
when methylbenzene is reacted with Cl at rom temperature in the presence of a halogen carrier a mixture of 2-chloromethylbenzene and 4-chloromethylbenzene is formed
a mixture of isomers is formed by the isomer in which the electrophile goes to the fourth position and is always formed in greater quantity
nitration of benzene
when methylbenzene is reacted with the nitration mixture at 30 degrees Celsius, a mixture of 2-nitromethylbenzene and 4-nitromethylbenzene is formed
if the conditions are adjused, further nitration will occur and 2,4,6-trinitromethylbenzene is formed
what is 2,4,6-trinitromethylbenzene?
also called dynamite or TNT
it is a high explosive and when it is detonated, it decomposes into large volumes of gases
