Aromatic Compounds • Aromatic compound: a hydrocarbon that contains one or more benzene-like rings. • arene: a term used to describe aromatic compounds. • Ar-: a symbol for an aromatic group derived by removing an -H from an arene. • Kekulé structure for benzene (1872).
Benzene • Resonance structure for benzene (1930s) • The theory of resonance, developed by Linus Pauling, provided the first adequate description of the structure of benzene. • According to the theory of resonance, certain molecules and ions are best described by writing two or more Lewis structures; the real molecule or ion is a resonance hybrid of these structures. • Each individual Lewis structure is called a contributing structure. • We show that the real molecule is a resonance hybrid of the two or more Lewis structures by using a double-headed arrow between them.
Benzene • Here are two contributing structures for benzene; • The resonance hybrid has some of the characteristics of each Lewis contributing structure. • The length of a carbon-carbon bond in benzene (140 pm), for example, is midway between that of a carbon-carbon single bond (146 pm) and a double bond (134 pm).
Nomenclature • Monosubstituted alkylbenzenes are named as derivatives of benzene; for example, ethylbenzene. • The IUPAC system retains certain common names for several of the simpler monosubstituted alkylbenzenes;
Nomenclature • The common names for these monosubstituted benzenes are also retained. • phenyl group (C6H5- or Ph-): the substituent group derived by removal of an H from benzene.
Nomenclature • When two substituents occur on a benzene ring, three isomers are possible; they may be located by: • numbering the atoms of the ring or • using the locators ortho (o), meta (m), and para (p).
Nomenclature • For three or more substituents: • If one of the substituents imparts a special name, name the molecule as a derivative of that parent. • If none of the substituents imparts a special name, number the substituents to give the smallest set of numbers, and list them in alphabetical order before the ending "benzene".
PAHs • Polynuclear aromatic hydrocarbon (PAH) • A hydrocarbon that contain two or more benzene rings, with each pair of rings sharing two adjacent carbon atoms.
Reactions of Benzene • By far the most characteristic reaction of aromatic compounds is substitution at a ring carbon. • This reaction is called aromatic substitution. • Some groups that can be introduced directly on the ring are the halogens, the nitro (-NO2) group, and the sulfonic acid (-SO3H) group. • Halogenation
General Mechanism • The first stage • Suppose the electrophile is a positive ion X+. • Two of the electrons in the delocalized system are attracted towards the X+ and form a bond with it. This has the effect of breaking the delocalization, although not completely.
General Mechanism • The ion formed in this step isn't the final product. It immediately goes on to react with something else. It is just an intermediate. • There is still delocalization in the intermediate formed, but it only covers part of the ion. When you write one of these mechanisms, draw the partial delocalization to take in all the carbon atoms apart from the one that the X has become attached to. • The intermediate ion carries a positive charge because you are joining together a neutral molecule and a positive ion. This positive charge is spread over the delocalized part of the ring. Simply draw the "+" in the middle of the ring. • The hydrogen at the top isn't new - it's the hydrogen that was already attached to that carbon. We need to show that it is there for the next stage.
General Mechanism • Stage Two • Here we've introduced a new ion, Y-. Where did this come from? You have to remember that it is impossible to get a positive ion on its own in a chemical system - so Y- is simply the negative ion
General Mechanism • A lone pair of electrons on Y- forms a bond with the hydrogen atom at the top of the ring. That means that the pair of electrons joining the hydrogen onto the ring aren't needed any more. These then move down to plug the gap in the delocalized electrons, so restoring the delocalized ring of electrons which originally gave the benzene its special stability.
Reactions of Benzene • Nitration • A value of nitroarenes is that the nitro group can be reduced to a primary amino group:
Reactions of Benzene • Sulfonation • An application of sulfonation is in the preparation of synthetic detergents.
FRIEDEL-CRAFTS ALKYLATION • Alkylation means substitution of an alkyl group. A hydrogen on the ring is replaced by a group like methyl or ethyl, etc. in the presence of a catalyst.
FRIEDEL-CRAFTS ALKYLATION • The formation of the electrophile • The electrophile is CH3+. It is formed by reaction between the chloromethane and the aluminum chloride catalyst. • Step 1
FRIEDEL-CRAFTS ALKYLATION • Step 1 • The hydrogen is removed by the AlCl4- ion which was formed at the same time as the CH3+ electrophile. The aluminum chloride catalyst is re-generated in this second stage.
FRIEDEL-CRAFTS ACYLATION • Acylation – substitution of an acyl group. • An acyl group is an alkyl group attached to a carbon-oxygen double bond. If "R" represents any alkyl group, then an acyl group has the formula RCO-. • The most commonly used acyl group is CH3CO-. This is called the ethanoyl group. • Benzene is treated with a mixture of ethanoyl chloride, CH3COCl, and aluminum chloride as the catalyst. A ketone called phenylethanone is formed.
FRIEDEL-CRAFTS ACYLATION • The formation of the electrophile • Step 1
FRIEDEL-CRAFTS ACYLATION • Step two • The hydrogen is removed by the AlCl4- ion which was formed at the same time as the CH3CO+ electrophile. The aluminum chloride catalyst is re-generated in this second stage.
Phenols • The functional group of a phenol is a hydroxyl group bonded to a benzene ring. • Name substituted phenols either as derivatives of phenol or by common names.
Phenols • Some phenols found in nature
Phenols as Antioxidants • Autoxidation: a reaction that converts an R-H group to an R-O-O-H (hydroperoxide) group.
Phenols as Antioxidants • Autoxidation is a radical chain reaction: • chain initiation: Formation of a radical from a nonradical compound. • radical: An atom of molecule with an unpaired electron.
Phenols as Antioxidants • chain propagation: Reaction of a radical to form a new radical. • propagation step 1: • propagation step 2:
Phenols as Antioxidants • Hydroperoxides: • are unstable. • Under biological conditions, they degrade to short-chain aldehydes and carboxylic acids with unpleasant "rancid" smells. • Similar formation of hydroperoxides in the low-density lipoproteins deposited on the walls of arteries leads to cardiovascular disease in humans. • In addition, many effects of aging are thought to be the result of hydroperoxide formation and their subsequent degradation.
Phenols as Antioxidants • Vitamin E is a natural antioxidant. • BHT and BHA are synthetic antioxidants. • These compounds are radical scavengers. • They form stable radicals and thus break the cycle of chain propagation steps; they prevent further formation of destructive hydroperoxides.