AROMATIC CHEMISTRY BENZENE Definition of AROMATIC/ALIPHATIC?? The structure of benzene Percentage composition by mass gave the empirical formula as C 6 H 6, which led Kekulé to suggest the Structure shown.
However, two sets of information contradicted this simplistic approach to the bonding in benzene: 1. The chemistry of benzene: the reactions that benzene undergoes do not reflect the presence of 3 C=C double bonds (addition reactions, etc) 2. The ring is a regular hexagon, planar, with C-C bonds lengths that lie between those of C-C and C=C The problem was solved by representing the ring as a hybrid:
This is called a resonance hybrid. Be careful! Benzene does not oscillate between these two forms. This model merely intends to respect the fact that the 6 bonds are equivalent and are an intermediate between the C-C and the C=C bonds.
Delocalisation in the Benzene Ring The carbon atoms are sp 2 hybrids 120º between them A regular hexagon is formed as a result, with sigma bonds between the C atoms and between the C and H atoms. The single electrons in the p-orbital of the C atoms together form bonding that gives Benzene its unique character. Key ideas are: lateral overlap occurs. a pi molecular orbital is formed. this is delocalised.
Evidence for the structure of Benzene There are 2 important ways of confirming the structure of benzene and showing that it differs from the hypothetical cyclohexatriene. 1. Enthalpy changes of hydrogenation (these can be calculated using enthalpy of combustions) Consider the hydrogenation of cyclohexene:
The difference of 152 kjmol -1 is due to the stabilising effect of the delocalisation. We can represent this on an energy diagram.
2. Total bond enthalpy of benzene vs. Cyclohexatriene 6C (g) + 6H (g) Sum of the bond enthalpies of benzene C 6 H 6 (g) H = +5598 kjmol-1 H 1 = +31 kjmol-1 C 6 H 6 (l) H2 = -49 kjmol-1 6C (s) + 3H 2 (g)
Using average bond enthalpy data, we can calculate the hypothetical total bond enthalpy for cyclohexatriene: C=C +611 kjmol-1 C-C +346 kjmol-1 C-H +413 kjmol-1 Hypothetical total bond enthalpy = NB: The enthalpy change for the formation of Benzene is, theoretically 252kJ/mol but experimentally +49kJ/mol
We know that the calculated figure for benzene (the experimental figure) is a lot more than this theoretical figure for cyclohexa-triene (by how much?) Why is this the case? By two different methods, we have calculated the stabilising effect of the delocalisation in benzene to be 152 and 169 kjmol-1. The difference between these 2 figures can be explained in terms of the error due to the use of average bond enthalpies (not great here). Also, the enthalpy of atomisation of C is difficult to measure accurately and would affect the calculation in the second method.
Other evidence for the proposed structure of Benzene See X-ray diffraction map, p196
Identify the peaks (and those missing!)
Try to account for the fragments
Elegant simplicity.
THE NAMES OF AROMATIC COMPOUNDS An aromatic compound is one which contains a benzene ring. Often, more than one name is acceptable and it's common to find the old names still in use. The phenyl group You get a methyl group, CH 3, by removing a hydrogen from methane, CH 4. You get a phenyl group, C 6 H 5, by removing a hydrogen from a benzene ring, C 6 H 6. Like a methyl or an ethyl group, a phenyl group is always attached to something else.
Aromatic compounds with only one group, where the name is based on benzene (the benzene ring is sometimes called the nucleus ) Chlorobenzene Nitrobenzene C 6 H 5 Cl Methylbenzene (ethylbenzene, etc). The old name for methylbenzene is toluene. C 6 H 5 CH 3 C 6 H 5 NO 2 Name these molecules
Isomerism (chloromethyl)benzene Notice the brackets around the (chloromethyl) in the name, so that you indicate that the chlorine is part of the methyl group and not attached to the ring (a ring substituent ). The chlorine here is known as a side chain substituent. If more than one of the hydrogens had been replaced by chlorine, the names would be (dichloromethyl)benzene or (trichloromethyl)benzene. All of these are based on methylbenzene and so the methyl group is given the number 1 position on the ring.
Other molecules involving benzene Benzenecarboxylic acid Benzoic acid is the older name. It has a carboxylic acid group, -COOH, attached to the benzene ring. Phenylamine (old name aniline). Phenylamine is a primary amine and contains the -NH2 group attached to a benzene ring Note: Phenyl (C6H5-) can be abbreviated to Ph- the radical from benzene is not benzyl This used to be used for the phenylmethyl radical (C6H6CH2-)
phenylethene (old name styrene) phenol phenylethanone phenyl ethanoate
2-hydroxybenzoic acid (2-hydroxybenzenecarboxylic acid) benzene-1,4-dicarboxylic acid 2,4,6-trichlorophenol 2,4,6-trichlorophenol is the familiar antiseptic TCP.
Note: The alkyl group or radical (R-) is derived from the alkanes, eg. Methyl, ethyl, etc. An aryl group (Ar-) or radical derives from an aromatic radical eg. Phenyl, chlorophenyl, etc An acyl group or radical (RCO- or ArCO-) is derived from removing the OH from a carboxylic acid or the Cl from an acid chloride).