Aromatic Compounds
CH 16
- the name of this functional group class is historically derived from the pleasant aromas associated with many of its constituent compounds discovered in past centuries - although not all aromatic compounds known today share the same distinction (many are in fact, odorless or even unpleasant), they are the principle components in such pleasant smelling substances as cinnamon, licorice, wintergreen, clove & vanilla
CLASSIFICATION –
- also known as arenes, many aromatic compounds possess the benzene ring functional group:
G's FG = aromatic compound or arene benzene ring or aryl grp
- other compounds which do not contain a benzene ring are also referred to as “aromatic” based on their similarity in physical & chemical behavior to benzene
STRUCTURE OF BENZENE –
- there are several characteristics of benzene which, although initially seemed contradictory, helped finally to determine its enigmatic structure a) Unsaturated Molecular Formula – - benzene ‘s molecular formula (MF = C6H6 ), indicates a high level of unsaturation - recall the calculation of centers of unsaturation (CU):
CU = (2n + 2) - # H's 2 [2(6) + 2] - 6 2 where n = #C's
For C6H6: CU =
CU = 4
benzene is a highly unsat'd HC
- from the many possible combinations of rings and multiple bonds consistent with four centers of unsaturation, F.A. Kekule’ (1866) reckoned correctly that benzene is comprised of one sixmembered ring and three alternating (conjugated) double bonds
b) Resistance to Addition Reactions – - although benzene has double bonds, it does not undergo typical alkene reactions such as electrophilic addition - benzene is, in fact, very stable towards hydrogenation, halogenation, hydration & addition of hydrogen halides:
NR
EX.
H2 X2 C6H6 H2 O HX NR NR NR
c) Preference for Substitution Reactions – - instead of addition reactions like alkenes, benzene readily undergoes substitution reactions typical of alkanes: EX.
C6H6 + Cl2 Fe C6H5 Cl + HCl
-
- the name of this functional group class is historically derived from the pleasant aromas associated with many of its constituent compounds discovered in past centuries - although not all aromatic compounds known today share the same distinction (many are in fact, odorless or even unpleasant), they are the principle components in such pleasant smelling substances as cinnamon, licorice, wintergreen, clove & vanilla
CLASSIFICATION –
- also known as arenes, many aromatic compounds possess the benzene ring functional group:
G's FG = aromatic compound or arene benzene ring or aryl grp
- other compounds which do not contain a benzene ring are also referred to as “aromatic” based on their similarity in physical & chemical behavior to benzene
STRUCTURE OF BENZENE –
- there are several characteristics of benzene which, although initially seemed contradictory, helped finally to determine its enigmatic structure a) Unsaturated Molecular Formula – - benzene ‘s molecular formula (MF = C6H6 ), indicates a high level of unsaturation - recall the calculation of centers of unsaturation (CU):
CU = (2n + 2) - # H's 2 [2(6) + 2] - 6 2 where n = #C's
For C6H6: CU =
CU = 4
benzene is a highly unsat'd HC
- from the many possible combinations of rings and multiple bonds consistent with four centers of unsaturation, F.A. Kekule’ (1866) reckoned correctly that benzene is comprised of one sixmembered ring and three alternating (conjugated) double bonds
b) Resistance to Addition Reactions – - although benzene has double bonds, it does not undergo typical alkene reactions such as electrophilic addition - benzene is, in fact, very stable towards hydrogenation, halogenation, hydration & addition of hydrogen halides:
NR
EX.
H2 X2 C6H6 H2 O HX NR NR NR
c) Preference for Substitution Reactions – - instead of addition reactions like alkenes, benzene readily undergoes substitution reactions typical of alkanes: EX.
C6H6 + Cl2 Fe C6H5 Cl + HCl
-