Aromatic compounds

XVII. Aromatic hydrocarbons Delocalized electrons over all the atoms in a ring is called aromatic system The simplest aromatic compound: benzene

XVII. Aromatic hydrocarbons Benzene or Benzol or 1,3,5 cyclohexatriene C6H6: - carbon-carbon bonds in benzene are of the same length of 140 picometres (pm). - The C-C bond lengths are greater than a double bond (134pm) but shorter than a single bond (147pm). - This intermediate distance is explained by electron delocalization: the electrons for C-C bonding are distributed equally between each of the six carbon atoms.

XVII. Aromatic hydrocarbons Delocalized electrons The electrons in the molecule are not associated with a single atom or covalent bond Delocalized electrons are part of a pi electron system that extends over several adjacent atoms In some molecules we could observed, that the Lewis structure can’t represent the correct electron structure of the molecule e. g. benzene

XVII. Aromatic hydrocarbons Delocalized electrons In a Lewis diagram, two carbons can be connected by one or two covalent bonds, but in the observed benzene molecule the bond lengths are longer than double bonds yet shorter than single bonds. Therefore one calls the two lewis structures resonating structures and the real molecule is considered to be their average, called a resonance hybrid. Resonance structures of the same molecule are connected with a double-headed arrow

XVII. Aromatic hydrocarbons Delocalized electrons Resonance often translates to a delocalization of electrons in pi orbitals . For example, in benzene 6 pi electrons completely delocalised over all 6 carbon atoms, thus contributing something like half-bonds.

XVII. Aromatic hydrocarbons Benzene -The sigma electrons are more reactive than the pi elecrons - Because pi electrons are delocalized. Delocalized electrons are always less reactive This delocalisation of electrons is known as aromaticity, and gives benzene great stability. This enhanced stability is the fundamental property of aromatic molecules that differentiates them from molecules that are non-aromatic

XVII. Aromatic hydrocarbons Benzene In benzene the C are in sp2 hybrid state The benzene ring is in flat plane The delocalized pi electrons are above and under the plane of the ring forming electron cloud around the whole carbon ring

XVII. Aromatic hydrocarbons 1. Homocyclic aromatic compounds: The aromatic ring contains only C atoms e.g. benzene 2. heterocyclic aromatics one or more of the atoms in the aromatic ring is of an element other than carbon: oxygen, nitrogen or sulfur e.g. pyridine; but imidazole not only pi electrons are delocalized!!!!! 3. five-membered rings e.g. pyrrol 4. Polycyclic aromatic hydrocarbons: two or more simple aromatic rings fused together by sharing two neighbouring carbon atoms e.g. naphthalene.

XVII. Aromatic hydrocarbons reaction of aromatics: SUBSTITUTION!!!!! only Derivatives of benzene: -The benzene ring is perfectly symmetrical -The reactivity of the carbn atoms in the ring is the same -Any of the H atoms can be substitued e.g. Substitution of 1 H of the ring with halogen chlorobenzene

XVII. Aromatic hydrocarbons What happens when we want to substitute more than 1 H on the ring?? Direction rule!!!! The firs substituent directs the next one The direction of the second substituent on benzene is the result of a distortion of the aromatic system by the first substituent The second can be in - ortho - meta - para position

XVII. Aromatic hydrocarbons Orto meta para Pozition of the substituents

XVII. Aromatic hydrocarbons Direction rule – Why??? After the first substituent the 6 C atoms are not equivalent any more! Reason: bond polarization the whole pi electron system is distorted it will be inhomogen First substituents: e.g. Cl, NO2 – electron taking groups CH3 - electron sending group

XVII. Aromatic hydrocarbons Derivatives of benzene: Nomenclature : e.g. Nitro benzene Para-chloro – nitrobenzene o,p-dichloro nitrobenzene

XVII. Aromatic hydrocarbons Representatives: Alkyl substitution 1. Toluene (methyl benzene) C6H5CH3 - water-insoluble liquid - Toluene is a common solvent, able to dissolve: paints, paint thinners, many chemical reactants, rubber, printing ink and disinfectants.

XVII. Aromatic hydrocarbons 2. Phenols ( hydroxy benzene) C6H5OH -OH group(s) are attached to the aromatic ring 2.1. Phenol Very toxic It causes skin burning It is also used in the production of drugs (it is the starting material in the industrial production of aspirin), weedkiller, and synthetic resins

XVII. Aromatic hydrocarbons 2.2 Resorcin, or resorcinol Used externally it is an antiseptic and disinfectant, is used 5 to 10% in ointments in the treatment of chronic skin diseases such as psoriasis and eczema In large doses it is a poison causing giddiness, deafness, salivation, sweating and convulsions

XVII. Aromatic hydrocarbons 2.3 Hydroquinone two hydroxyl groups bonded to a benzene ring in a para position reducing agent which is soluble in water In human medicine, hydroquinone has been used as a topical application in skin whitening to reduce the color of skin. This use is banned in some countries (e.g. France) because of fears of a cancer risk.

XVII. Aromatic hydrocarbons 3. Aniline: (C6H5NH2 ) Aniline is toxic by inhalation of the vapor, absorption through the skin or swallowing. It causes headache, mental confusion and in severe cases can cause convulsions. It can affect the nervous system causing tiredness, loss of appetite, headache and dizziness. Uses: disinfectant, industrial dyes – to stain clothes

XVII. Aromatic hydrocarbons 4. Benzoik acid (C6H5COOH) Uses: Benzoic acid and its salts are used as a food preservative, represented by the E-numbers E210, E211, E212, and E213 Benzoic acid inhibits the growth of mold, yeast and some bacteria In medicine: is used for the treatment of fungal skin diseases

XVII. Aromatic hydrocarbons 5. Salicylic acid 6. aspirin Ca2+

XVII. Aromatic hydrocarbons 7. Trinitrotoluene (TNT) CH3

XVII. Aromatic hydrocarbons 8. Naphtalene, naphthalin used as an antiseptic and insecticide, especially in mothballs as a household fumigant large amounts of naphthalene may damage or destroy red blood cells.