Worksheet #18

Part 1: Naming benzene rings 1. Monosubstituted benzene compounds are named by combining the name of the substituent with the parent name -benzene. For example, benzene with a bromine instead of one hydrogen is named bromobenzene. Draw the molecules for each name: a. chlorobenzene

b. nitrobenzene

c. ethylbenzene

d. sec-butylbenzene

2. There are many monosubstituted benzenes with nonsystematic names; a few of them are commonly seen in 112A/B and 113A. Draw these: a. toluene (a -CH3 substituent) b. phenol (an -OH substituent)

c. aniline (an -NH2 substituent)

d. benzaldehyde (a -CHO substituent)

e. benzoic acid (a -CO2H substituent)

f. styrene (a vinyl substituent)

3. Disubstituted benzenes are named using the prefixes ortho (o), meta (m) and para (p). These prefixes tell you how one substituent is positioned relative to the other. Ortho means the substituents are on adjacent carbons, meta means they are 1,3 relative to each other, and para means they are 1,4 to each other (which means they are opposite each other on the benzene ring). Draw: a. m-dibromobenzene b. o-chloroaniline

c. m-propyltoluene

d. p-tert-butylbenzaldehyde

Worksheet #18 4. Benzenes with more substituents are named using locants (position numbers) for substituents, chosen so the lowest possible numbers are used, just as with naming cyclohexane derivatives. The non-systematic names you saw in problem 2 can be used as parent names, particularly if it simplifies the overall name; that has been suggested below when appropriate. Draw and name: a. A benzene with two methyl groups para, and a chlorine meta to one of them

b. A phenol with a bromine and a chlorine, both ortho to the -OH (hydroxy) group

c. A toluene with three nitro groups all meta to each other (this is TNT!)

d. An aniline with two isopropyl groups meta to each other and to the -NH2 group

Part 2: Rules of aromaticity (15.3) AROMATIC

ANTI-AROMATIC

NON-AROMATIC

Flat (planar) (hint: lots of sp2)

Flat (planar)

Not flat, OR

Cyclic

Cyclic

Not cyclic, OR

Conjugated (all members have p-orbital)

Conjugated

Not conjugated, OR

Passes Hückel rule: has 4n+2 electrons

Fails Hückel rule: has 4n electrons

Odd number of electrons

(e.g. 2, 6, 10, 14, 18, 22, 26, 30, 34….)

(e.g. 0, 4, 8, 12, 16, 20….)

Very stable

Very reactive

Stable/reactive is not a rule, just a general observation. Note: the # electrons for Hückel test are the pi electrons + lone pair electrons in the pi system. Electrons lateral to the ring don’t count towards Hückel (e.g. the lone pair on N in pyridine below) Note: conjugated means all members of the ring have a p-orbital aligned perpendicular to the plane of the ring and overlapping. All these are conjugated: -

CH

+

CH

N H

N

Part 3: Identifying aromaticity

How many electrons are in the pi system? Are these aromatic, anti-aromatic or non-aromatic? H

N

N benzene

pyrrole

pyridine

Worksheet #18 H -

H N

H +

C

C

N O N

N

pyridine

pyrimidine furan

NH2

+

CH

N NH tropilium

O

uracile

phenanthrene

O N N H

OH N H

NH2

naphthalene

pyrrolidine

histidine

Draw in the lone pairs; draw the lone pair inside the ring if the lone pair is in the aromatic pi system, and outside the ring if lone pairs are lateral to the ring. Indicate how may electrons each N or O contributes to the pi system (if there is one). Is it aromatic, anti-aromatic or non-aromatic?

N N pyridine

N pyrimidine

O

H

N pyrrole

H N

N

N H pyrrolidine

NH2

furan

NH uracile

O