CH 21 - Benzene and Aromaticity

aromatic compounds are

highly saturated cyclic compounds

benzene is the

parent name

phenyl is the

name of the substituent

benzene is unexpectedly reactive toward a number of reagents that

react with other unsaturated functional groups such as alkenes and alkynes

• Br2 and H2/Pd

when benzene does react with Br2,

• in the presence of a catalytic FeCl3

• it undergoes substitution rather than addition

when bromobenzene is reacted with Br2/FeCl3,

three isomeric dibromobenzenes are formed

benzene model

• all C-C bonds are the same lengths

• all bond angles are 120

• all carbons are sp2 hybridized

• each carbon has a single unhydribzed 2p orbital with a single electron

molecular orbital theory of benzene hold that the

six parallel 2p orbitals combine to form 6 pi MOs

• 3 bonding and 3 anti bonding

to be aromatic a compound must

• be cyclic

• have one 2p orbital on each atom of the ring

• be planar

• have a closed loop of (4n + 2) pi electrons in the cyclic arrangement of 2p orbitals

benzene substiutnet is called

phenyl group, Ph-

a toluene substituent is called

benzyl group, Bn-

disubtituated benzenes - ortho

1,2 position

disubtituated benzenes - meta

1,3 positions

disubtituated benzenes - para =

1,4 positions

polycyclic arenas - 2 rings is called

naphthalene

phenols are weak acids and react with strong bases such as NaOH to form

water-soluble salts

phenoxide anions will react with alkyl halides to

produce phenyl ethers

strong oxidizing agents will convert phenols into

quinones

quinones can eb be reversible reduced to

hydroquinones

benzylic cations and radicals are stabilized by

resonance

benzylic oxidation

reduces any carbon substituents into a carboxylic acid

• H2CrO4

• must have at least one benzylic hydrogen to occur

benzylic halogenation

adds -Br to benzylic position

• NBS