Benzene Flashcards
Hydrogenation
Hydrogenation is an addition reaction in which hydrogen atoms are added all the way around the benzene ring. A cycloalkane is formed.
These reactions destroy the electron delocalisation in the original benzene ring, because those electrons are being used to form bonds with the new hydrogen atoms.
Raney nickel catalyst at 150c (finely divided nickel catalyst)
Producing a smoky flame
Combustion is hardly ever complete, especially if they are burnt in air. The high proportion of carbon in the molecules means that you need a very high proportion of oxygen to hydrocarbon to get complete combustion.
The arenes tend to burn in air with extremely smoky flames - full of carbon particles.
Sulfonation of Benzene
Reflux at room temperature with fuming sulphuric acid.
Fuming sulphuric acid, H2S2O7, can usefully be thought of as a solution of sulphur trioxide in concentrated sulphuric acid.
The product is benzenesulphonic acid.
Sulfonation of Methylbenzene
Methylbenzene is more reactive than benzene because of the tendency of the methyl group to “push” electrons towards the ring.
The effect of this greater reactivity is that methylbenzene will react with fuming sulphuric acid at 0°C, and with concentrated sulphuric acid if they are heated under reflux for a shorter period.
As well as the effect on the rate of reaction, with methylbenzene you also have to think about where the sulphonic acid group ends up on the ring relative to the methyl group.
Methyl groups have a tendency to “direct” new groups into the 2- and 4- positions on the ring (assuming the methyl group is in the 1- position). Methyl groups are said to be 2,4-directing.
In the case of sulphonation, the exact proportion of the isomers formed depends on the temperature of the reaction. As the temperature increases, you get increasing proportions of the 4- isomer and less of the 2- isomer.
This is because sulphonation is reversible. The sulphonic acid group can fall off the ring again, and reattach somewhere else. This tends to favour the formation of the most thermodynamically stable isomer. This interchange happens more at higher temperatures.
The 4- isomer is more stable because there is no cluttering in the molecule as there would be if the methyl group and sulphonic acid group were next door to each other.
Nitration of Benzene
Nitration happens when one (or more) of the hydrogen atoms on the benzene ring is replaced by a nitro group, NO2.
Benzene is treated with a mixture of concentrated nitric acid and concentrated sulphuric acid at a temperature not exceeding 55°C.
The concentrated sulphuric acid is acting as a catalyst and so isn’t written into the equations.
At higher temperatures there is a greater chance of getting more than one nitro group substituted onto the ring. You will get a certain amount of 1,3-dinitrobenzene formed even at 50°C. Some of the nitrobenzene formed reacts with the nitrating mixture of concentrated acids.
Nitration of Methylbenzene
Methylbenzene reacts rather faster than benzene - in nitration, the reaction is about 25 times faster. That means that you would use a lower temperature to prevent more than one nitro group being substituted - in this case, 30°C rather than 50°C. Apart from that, the reaction is just the same - using the same nitrating mixture of concentrated sulphuric and nitric acids.
Just as with benzene, you will get a certain amount of dinitro compound formed under the conditions of the reaction, but virtually no trinitro product because the reactivity of the ring decreases for every nitro group added. From an experimental point of view this is just as well. Trinitromethylbenzene used to be called trinitrotoluene or TNT!
Substitution Halogenation of Benzene
Benzene reacts with bromine in the presence of a catalyst, replacing one of the hydrogen atoms on the ring by a bromine atom.
The reactions happens in the dark. at room temperature.
Strictly speaking iron isn’t a catalyst, because it gets permanently changed during the reaction. It reacts with some of the bromine to form iron(III) bromide, FeBr3.
The reaction between benzene and bromine in the presence of either aluminium bromide or iron gives bromobenzene. Iron is usually used because it is cheaper and more readily available.
Addition Halogenation of Benzene
In the presence of ultraviolet light (but without a catalyst present), hot benzene will also undergo an addition reaction with bromine. The ring delocalisation is permanently broken and a bromine atom adds on to each carbon atom.
Free radical reaction
Substitution Halogenation of methylbenzene
It is possible to get two quite different substitution reactions between methylbenzene and bromine depending on the conditions used. The bromine can substitute into the ring or into the methyl group.
Substitution into the ring
Substitution in the ring happens in the presence of aluminium chloride (or aluminium bromide if you are using bromine) or iron, and in the absence of UV light. The reactions happen at room temperature.
This is exactly the same as the reaction with benzene, except that you have to worry about where the halogen atom attaches to the ring relative to the position of the methyl group.
Methyl groups are 2,4-directing, which means that incoming groups will tend to go into the 2 or 4 positions on the ring - assuming the methyl group is in the 1 position. In other words, the new group will attach to the ring next door to the methyl group or opposite it.
With chlorine, substitution into the ring gives a mixture of 2-chloromethylbenzene and 4-chloromethylbenzene.
Alkylation
Substituting an alkyl group involves refluxing benzene with a halogenoalkane in the presence of a halogen carrier catalyst (AlCl3)
Alcl4- is formed
Acylation
Similar to alkylation but with an acyl chloride producing a ketone
Phenol + bromine water
Multi-electrophilic substitution takes place without heating or a halogen carrier.
Decolourised bromine water changes to a white precipitate which smells anticeptic.
Chlorine produces TCP
Phenol + dilute nitric acid
Multi-electric substitution producing 2,4,6-trinitrophenol
Doesn’t require sulfuric acid and isn’t concentrated .
OH group activates the benzene ring
