aromatic compounds

Question 1

what are aromatic compounds?

Answer 1

hydrocarbons, 4n + 2 shared electrons (Huckel’s rule)

Question 2

how are electrons organised in aromatic compounds?

Answer 2

electrons occupy molcular orbital that encompasses all C atoms arising from superposition of pz orbitals

Question 3

properties of benzene

Answer 3

6 carbon ring C6H6:

• bond length: all identical (140pm / C-C is 154, C-H is 134)
• inert to reactions that break aromatic system -> sharing of electrons lowers energy by a lot therefore high energy has to be achieved to break it
• apolar molecule (doesn’t react with water)
• H can be substituted
• benzene in substituent compound -> phenyl group
• scaffold for many drugs

Question 4

give examples of benzene based compounds

Answer 4

• aspirin
• ibuprofene
• acetaminophen
• phenylalanine

Question 5

properties of benzene compounds

Answer 5

• less dense than water (denser than hydrocarbons)
• halogenated benzene is more dense than water
• aromatic hydrocarbons are apolar (insoluble in water) and used for solvents for other organic compounds
• when containing polar functioning groups (OH, COOH) becomes more soluble in water
• resistant to reactions that break aromatic system
• flammable

Question 6

what is the main benzene reaction and what are its properties?

Answer 6

electrophilic aromatic substitution:

• addition of electrophile breaks the aromatic system (sigma complex)
• exit of leaving group allows to obtain a substituted aromatic compound
• groups attached to ring may activate or inactivate ring (electron donors or withdrawing) for further substitution or direct it at specific positions