aromatic compounds

AROMATIC COMPOUNDS : AROMATIC COMPOUNDS

Slide 2 : What are arenes? Arenes are aromatic hydrocarbons. The term "aromatic" originally referred to their pleasant smells, but now implies a particular sort of delocalised bonding . The arenes are based on benzene rings. The simplest of them is benzene itself, C6H6. The next simplest is methylbenzene (old name: toluene) which has one of the hydrogen atoms attached to the ring replaced by a methyl group - C6H5CH3.

The structure of benzene : The structure of benzene Benzene, C6H6, is a planar molecule containing a ring of six carbon atoms each with a hydrogen atom attached. The six carbon atoms form a perfectly regular hexagon. All the carbon-carbon bonds have exactly the same lengths - somewhere between single and double bonds. There are delocalised electrons above and below the plane of the ring.

Slide 4 : The presence of the delocalised electrons makes benzene particularly stable. Benzene resists addition reactions because that would involve breaking the delocalisation and losing that stability.

Slide 5 : Benzene is represented by this symbol, where the circle represents the delocalised electrons, and each corner of the hexagon has a carbon atom with a hydrogen attached. The structure of methylbenzene (toluene) Methylbenzene just has a methyl group attached to the benzene ring - replacing one of the hydrogen atoms. Attached groups are often drawn at the top of the ring, but you may occasionally find them drawn in other places with the ring rotated.

Slide 6 : Physical properties Boiling points In benzene, the only attractions between neighbouring molecules are van der Waals dispersion forces. There is no permanent dipole on the molecule. Benzene boils at 80°C - rather higher than other hydrocarbons of similar molecular size (pentane and hexane, for example). This is presumably due to the ease with which temporary dipoles can be set up involving the delocalised electrons.

Slide 7 : Methylbenzene boils at 111°C. It is a bigger molecule and so the van der Waals dispersion forces will be bigger. Methylbenzene also has a small permanent dipole, so there will be dipole-dipole attractions as well as dispersion forces. The dipole is due to the CH3 group's tendency to "push" electrons away from itself. This also affects the reactivity of methylbenzene . Melting points Benzene melts at 5.5°C; methylbenzene at -95°C. . Benzene is a tidy, symmetrical molecule and packs very efficiently. The methyl group sticking out in methylbenzene tends to disrupt the closeness of the packing. If the molecules aren't as closely packed, the intermolecular forces don't work as well and so the melting point falls.

Slide 8 : Reactivity Benzene benzene is resistant to addition reactions. . Adding something new to the ring would need to use some of the delocalised electrons to form bonds with whatever it is adding. That results in a major loss of stability as the delocalisation is broken. Instead, benzene mainly undergoes substitution reactions - replacing one or more of the hydrogen atoms by something new. That leaves the delocalised electrons as they were.

Slide 9 : Methylbenzene The tendency of the CH3 group to "push" electrons away from itself also has an effect on the ring, making methylbenzene react more quickly than benzene. MANUFACTURING ARENES Catalytic reforming Reforming takes straight chain hydrocarbons in the C6 to C8 range from the gasoline or naphtha fractions and rearranges them into compounds containing benzene rings. Hydrogen is produced as a by-product of the reactions. For example, hexane, C6H14, loses hydrogen and turns into benzene.

Slide 10 : Similarly, methylbenzene (toluene) is made from heptane.

Slide 11 : The catalyst A typical catalyst is a mixture of platinum and aluminium oxide. With a platinum catalyst, the process is sometimes described as "platforming". Temperature and pressure The temperature is about 500°C, and the pressure varies either side of 20 atmospheres. Converting some of the methylbenzene into benzene The methyl group can be removed from the ring by a process known as "dealkylation".

Slide 12 : The methylbenzene is mixed with hydrogen at a temperature of between 550 and 650°C, and a pressure of between 30 and 50 atmospheres, with a mixture of silicon dioxide and aluminium oxide as catalyst.

Slide 13 : NITRATION OF BENZENE ANDMETHYLBENZENE 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. The mixture is held at this temperature for about half an hour. Yellow oily nitrobenzene is formed.

Slide 14 : The concentrated sulphuric acid is acting as a catalyst . 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.

The nitration of methylbenzene (toluene) : The nitration of methylbenzene (toluene) two isomers formed: 2-nitromethylbenzene and 4-nitromethylbenzene. Only about 5% of the product is 3-nitromethylbenzene. Methyl groups are said to be 2,4-directing.

The halogenation of benzene : The halogenation of benzene Substitution reactions Benzene reacts with chlorine or bromine in the presence of a catalyst, replacing one of the hydrogen atoms on the ring by a chlorine or bromine atom. The reactions happen at room temperature. The catalyst is either aluminium chloride (or aluminium bromide if you are reacting benzene with bromine) or iron.

Slide 17 : The reaction with chlorine. The reaction between benzene and chlorine in the presence of either aluminium chloride or iron gives chlorobenzene. The reaction with bromine The reaction between benzene and bromine in the presence of either aluminium bromide or iron gives bromobenzene.

Slide 18 : Addition reactions In the presence of ultraviolet light (but without a catalyst present), hot benzene will also undergo an addition reaction with chlorine or bromine. The ring delocalisation is permanently broken and a chlorine or bromine atom adds on to each carbon atom. For example, if chlorine gas is bubbled through hot benzene exposed to UV light for an hour, 1,2,3,4,5,6-hexachlorocyclohexane will be formed.

halogenationofmethylbenzene : halogenationofmethylbenzene 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. With chlorine, substitution into the ring gives a mixture of 2-chloromethylbenzene and 4-chloromethylbenzene.

Slide 20 : FRIEDEL-CRAFTS REACTIONS OF BENZENE AND METHYLBENZENE   Friedel-Crafts acylation of benzene Acylation means substituting an acyl group into something - in this case, into a benzene ring. The most commonly used acyl group is CH3CO-. This is called the ethanoyl group, and in this case the reaction is sometimes called "ethanoylation".

Slide 21 : The most reactive substance containing an acyl group is an acyl chloride (also known as an acid chloride). These have the general formula RCOCl. Benzene is treated with a mixture of ethanoyl chloride, CH3COCl, and aluminium chloride as the catalyst. The mixture is heated to about 60°C for about 30 minutes. A ketone called phenylethanone (old name: acetophenone) is formed.

Slide 22 : Friedel-Crafts acylation of methylbenzene (toluene) In acylation, though, virtually all the substitution happens in the 4- position.

Friedel-Crafts alkylation : Friedel-Crafts alkylation Friedel-Crafts alkylation of benzene Benzene reacts at room temperature with a chloroalkane (for example, chloromethane or chloroethane) in the presence of aluminium chloride as a catalyst.