Chapter 13 Part 2 Benzene and its Derivatives Benzene and its Derivatives

Aromatic Compounds Aromatic compound: Aromatic compound: a hydrocarbon that contains one or more benzene-like rings – arene: – arene: a term used to describe aromatic compounds – Ar-: – 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) – developed by Linus Pauling, resonance hybrid – according to the theory, 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 contributing structure – 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 structures that can be drawn for benzene – the resonance hybrid has some of the characteristics of each Lewis structure – the length of a carbon-carbon bond is midway between that of a carbon-carbon single bond and a double bond

Reactions of Benzene By far the most characteristic reaction of aromatic compounds is substitution at a ring carbon aromatic substitution – this reaction is called aromatic substitution – some groups that can be introduced directly on the ring are the halogens, the nitro (-NO 2 ) group, and the sulfonic acid (-SO 3 H) group Halogenation

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

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 (C 6 H 5 - or Ph-): – phenyl group (C 6 H 5 - or Ph-): the substituent group derived by loss 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"

Phenols as Antioxidants Read this portion for interest only.

End Chapter 13 Benzene and its Derivatives

PAHs Polynuclear aromatic hydrocarbon (PAH) Polynuclear aromatic hydrocarbon (PAH) – a hydrocarbon that contain two or more benzene rings, with each pair of rings sharing two adjacent carbon atoms

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 – Don’t memorize these.

Phenols as Antioxidants Autoxidation is a radical chain reaction – chain initiation: – chain initiation: formation of a radical from a nonradical compound

Phenols as Antioxidants – chain propagation: – 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