ORGANIC CHEMISTRY CHM 207 CHAPTER 4: AROMATIC COMPOUNDS (BENZENE AND TOLUENE) NOR AKMALAZURA JANI
Aromatic compounds Organic compound that contains a benzene ring in its molecule is known as an aromatic compounds. Sometimes called arenes. Molecular formula: C 6 H 6 Represented as a regular hexagon containing an inscribed circle.
The corner of each hexagon represents a carbon and a hydrogen atom. Can be represented in two abbreviated ways. Structure of Benzene
Kekul Structure of Benzene Each carbon atom must have four covalent bonds. Molecular formula is C 6 H 6 All the hydrogen atoms are equivalent
Resonance Structure Resonance theory: the structure of benzene is a resonance hybrid structure of two Kekul cononical forms. The hybrid structure is often represented by a hexagon containing an inscribed circle.
Hexagonal ring 6 carbon-carbon bonds are equal. Circle delocalised electrons of the benzene ring
CRITERIA OF AROMATIC COMPOUNDS Structure must be cyclic, containing some number of conjugated pi bonds. Each atom in the ring must have an unhybridized p orbital. (The ring atoms are usually sp 2 hybridized or occasionally sp hybridized). The unhybridized p orbitals must overlap to form a continuous ring of parallel orbitals. The structure must be planar (or nearly planar) for effective overlap to occur. Delocalization of the pi electrons over the ring must lower the electronic energy. *Antiaromatic compound: fulfills the first three criteria, but delocalization of the pi electrons over the ring increase the electronic energy.
Huckels rule Used to determine aromaticity for planar, cyclic organic compounds with a continous ring of overlapping p- orbitals. If the number of pi ( ) electrons in the monocyclic system is (4N+2), the system is aromatic. N is 0, 1, 2, 3.. Systems that have 2, 6 and 10 pi electrons for N = 0, 1, 2 is a aromatic. Systems that have 4, 8, and 12 pi electrons for N = 1, 2, 3 are antiaromatic.
Naming Aromatic Compounds
A substituted benzene is derived by replacing one or more of benzenes hydrogen atoms with an atom or group of atoms. A monosubstituted benzene has the formula C 6 H 5 G where G is the group that replaces a hydrogen atom. All hydrogens in benzene are equivalent. It does not matter which hydrogen is replaced by G.
Some monosubstituted benzenes are named by adding the name of the substituent group as a prefix to the word benzene. The name is written as one word. nitrobenzene nitro group ethylbenzene ethyl group
Certain monosubstituted benzenes have special names. These are parent names for further substituted compounds. methyl group toluene hydroxy group phenol
carboxyl group benzoic acid aniline amino group
Three isomers are possible when two substituents replace hydrogen in a benzene molecule. The prefixes ortho-, meta- and para- (o-, m- and p-) are used to name these disubstituted benzenes.
ortho-dichlorobenzene (1,2-dichlorobenzene) mp 17.2 o C, bp 180.4 o C ortho disubstituted benzene substituents on adjacent carbons
meta-dichlorobenzene (1,3-dichlorobenzene) mp 24.82 o C, bp 172 o C meta disubstituted benzene substituents on adjacent carbons
para-dichlorobenzene (1,4-dichlorobenzene) mp 53.1, bp 174.4 o C para disubstituted benzene substituents are on opposite sides of the benzene ring
phenol 3-nitrophenol When one substituent corresponds to a monosubstituted benzene with a special name, the monosubstituted compound becomes the parent name for the disubstituted compound.
Tri- and Polysubstituted Benzenes
When a benzene ring has three or more substituents, the carbon atoms in the ring are numbered. Numbering starts at one of the substituent groups. The numbering direction can be clockwise or counterclockwise. Numbering must be in the direction that gives the substituent groups the lowest numbers.
When the hydrocarbon chain attached to the benzene ring is small, the compound is named as benzene derivative. Example:
Naming compounds that cannot be easily named as benzene derivatives diphenylmethane 4-phenyl-2-pentene Benzene named as a substituent on a molecule with another functional group as its root by the prefix phenyl.
The phenyl group, C 6 H 5 -
If the hydrocarbon chain contains more than three carbon atoms, phenyl is used as part of the name. Examples:
PHYSICAL PROPERTIES OF BENZENE AND ITS DERIVATIVES Benzene derivatives tend to be more symmetrical than similar aliphatic compounds, and pack better into crystals and have higher melting points. Density: - Slightly dense than non-aromatic analogues, but still less dense than water. - halogenated benzenes are denser than water. Insoluble in water Boiling points depends on the dipole moments of compounds.
REACTION OF BENZENE ELECTROPHILIC SUBSTITUTION REACTIONS OF BENZENE stability of -electron system is lost when benzene undergoes addition reactions. benzene and its derivatives undergo substitution reaction rather than addition reactions. product of substitution reactions: aromatic compounds and not saturated compounds.
Mechanism of electrophilic substitution of benzene
ELECTROPHILIC SUBSTITUTION REACTIONS
Reagents, electrophiles and catalysts in electrophilic substitution reactions ReactionsReagentsCatalystsElectrophiles HalogenationCl 2 or Br 2 AlCl 3, AlBr 3, FeCl 3 or FeBr 3 Cl, Br NitrationHNO 3 H 2 SO 4 NO 2 AlkylationRCl RCH=CH 2 AlCl 3 H 2 SO 4 R RCH-CH 3 AcylationRCOClAlCl 3 RCO SulphonationSO 3 H 2 SO 4 SO 3 H
HALOGENATION OF BENZENE
MECHANISM: BROMINATION OF BENZENE
MECHANISM: NITRATION OF BENZENE
MECHANISM: FRIEDEL-CRAFTS ALKYLATION
MECHANISM: FRIEDEL-CRAFTS ACYLATION
Ortho-Para and Meta Directing Substituents When substituted benzenes undergo further substituents, the substituent group present in the benzene derivative will influence electrophilic substitution in 2 ways which are: i) Reactivity ii)Orientation
EFFECTS OF SUBSTITUENTS ON THE REACTIVITY OF ELECTROPHILIC AROMATIC SUBSTITUTION Substituent group present in the benzene ring can influence the rate of reaction of further substitutions. Electron-donating groups make the ring more reactive (called activating groups) thus influence the reaction become faster. Electron-withdrawing groups make the ring less reactive (called deactivating groups) thus influence the reaction become slower.
A substituents group already in the ring influences the position of further electrophilic substitution whether at ortho, meta or para position. Ortho-para directors: the groups that tend to direct electrophilic substitution to the C2 and C4 positions. Meta directors: the groups that tend to direct electrophilic substitution to the C3 position. EFFECTS OF SUBSTITUENTS ON THE ORIENTATION OF ELECTROPHILIC AROMATIC SUBSTITUTION
Effetcs of substituent groups on the benzene ring Activating groups (electron donating) Deactivating groups (electron-withdrawing) -NH 2 -R -OH -OR -NHCOCH 3 -F -Cl -Br -I ortho-para directors meta directors
REACTIONS OF BENZENE DERIVATIVES Alkylbenzene such as toluene (methylbenzene) resembles benzene in many of its chemical properties. It is preferable to use toluene because it is less toxic. The methyl group activates the benzene nucleus. Toluene reacts faster than benzene in all electrophilic substitutions.
Reactions of toluene Reactions of the methyl group Reactions of the benzene ring Substitution -halogenation Oxidation Electrophilic substitutions - H