What makes benzene so stable? Each carbon is bonded to the other carbon by sigma bonds. The unused p-orbital which is perpendicular to the plane forms a pi bond that extends above and below the plane. each electron is free to move, i.e. the electrons are said to be delocalised No repulsion among electrons, therefore the system is stable. Benzene resists addition reactions because that would involve breaking the delocalisation and losing that stability.
Naming benzene Chloro- chlorobenzene Methyl – methylbenzene Nitro – nitrobenzene Hydroxy – phenol Amino – phenylamine Carboxylic acid – benzenecarboxylic acid
Reaction mechanism • Benzene undergoes electrophilic substitution • Cannot undergo addition reactions as this requires too much energy.
The 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 50°C • At higher temperatures more than one nitro group substituted onto the ring. You will get a certain amount of 1,3-dinitrobenzene formed even at 50°C. • Notice that the new nitro group goes into the 3 position on the ring. Nitro groups "direct" new groups into the 3 and 5 positions.
Halogenation (Friedel Crafts) • Benzene reacts with chlorine or bromine, but only in the presence of a catalyst. The catalyst is either aluminium chloride (or aluminium bromide if you are reacting benzene with bromine) or iron. • The catalyst polarises the halogen molecule, accepting a pair of electrons from the partial negative halogen
Effect of existing functional group on the benzene ring • When one group is already attached to the benzene ring, an incoming group may attach itself at three possible positions. • Methyl group (electron donating) ‘directs’ incoming groups to the 2,4 position on the benzene ring. • Nitro group (electron withdrawing) ‘directs’ incoming groups to the 3 position on the benzene ring.
Inductive effect • Methyl group tends to be electron donating and push the electrons density towards the carbon atom. It is said to have a positive inductive effect. • Nitro, and halogens tends to be electron withdrawing and pull the electrons density away from the carbon atom. It is said to have a negative inductive effect.
Conjugation • Conjugation is the term used to describe an arrangement of chemical bonds in which two double bonds are separated by one single bond. Chemical structures with this configuration are very stable. • This effect results from the ability of the electrons in conjugated bonds to delocalize, or spread their effect throughout the molecule`
Methylbenzene The intermediate formed has the positive charge concentrated on positions 2(ortho), 4(para) and 6(ortho) relative to the incoming electrophile. The electron donating groups lessens the positive charge , if it is present at those sites. Thus stabilizing the ring
Electron donating group • An electron-donating group attached to a benzene ring will increase the rate of reaction of further substitution. It will also direct the incoming group to the 2 or 4 position.
Nitrobenzene The intermediate formed has the positive charge concentrated on positions 2, 4 and 6 relative to the incoming electrophile. The electron withdrawing groups increases the positive charge , if it is present at those sites. Thus destablizingthe ring
Electron withdrawing • An electron-withdrawing group attached to a benzene ring will decrease the rate of reaction of further substitution. It will also direct the incoming group to the 3position.
Phenol Derivative of benzene
Donation of lone pair of electrons from oxygen increases the electron density. Activates the ring, making it more reactive than benzene Causes O – H to dissociate more easily, making it a weak acid.
PHENOL • Reacts with a strong base to from a salt and water • Reacts with aqueous bromine
phenol • OH activates the ring, causing substitution on the 2,4 and 6 positions • This is seen in the addition of bromine and nitro groups