Jesse KerrMay 21st 2014

Molecule to Organism

Electrophilic Aromatic Substsituion Lab

Results:

Determining the Products and their Ratios in Nitration of

Bromobenzene

Bromobenzene (1.0 mL, 9.55 mmol) was nitrated using nitric acid and

sulfuric acid. 4-nitrobromobenzene (0.433g, 2.14 mmol) and 2-nitrobromobenzene

(0.1694g, 0.84 mmol) were obtained in low yield (combined 2.98 mmol (31.20%

yield)). The isomeric ratio of these two products was found to be 71.8% para and

28.2% ortho. The melting point of the isolated 4-nitrobromobenzene was found to

be 128.2 – 130.4 ° C.

FTIR analysis of the 4-nitrobromobenzene isomer yielded 3098(m),

1921(m), 1800(m), 1675(m), 1599(m), 1568(m), 1511(m-s), 1471(m), 1357(m-s),

1276(w-m), 1172(w)1104(w-m), 1066(m), 1010(w), 839(vs), 738(s-vs). (Figure 1)

NMR spectra of 4-nitrobromobenzene yielded 7.80- 7.98 ppm (d, 2H, δ H1, H6),

8.02 – 8.17 ppm (d, 2Hm, H2, H4). (Figure 2)

Figure 1: FTIR (Nicolet) spectra of 4-nitrobromobenzene

Figure 2: NMR spectra of 4-nitrobromobenzene

Product Formed in Competitive Nitration of Acetanilide and Methyl

Benzoate

Methyl benzoate (5.50 mmol, 0.609 mL) and acetanilide (5.50mmol, 0.61 mL)

were subjected to a competitive nitration by nitric acid combined with sulfuric acid.

A crude product (0.5864 g) was purified by recrystallization (19.69 % yield) and

found to have a melting point of 221.2- 223.9 ° C. Thus, the crude product consisted

of 4-nitroacetanilide (3.26 mmol), giving a percent yield of 59.27 %.

Discussion

Nitration of Bromobenzene

The nitration of bromobenzene yielded the ortho and para isomers in

approximately a 30: 70 ratio. This is consistent with reported observations.1 The

bromine substituent, like other halogens, is an ortho/para director as well as a weak

deactivator of the ring. These two experimental observations can be explained by

resonance and inductive effects, respectively. In the first step of electrophilic

aromatic substition reactions, a benzene π bond acts as a nucleophile and adds to an

electrophile. The intermediate that is formed after this step is called an arenium ion,

and consists of a benzene ring with a positive charge that is to a greater or lesser

degree delocalized throughout the ring. When this arenium ion has a substituent

present in which an atom bonded directly to the benzene has one or more unshared

pairs of electrons, this substituent has the ability to contribute to the resonance

forms stablilizing the intermediate. However, whether or not this substituent can

contribute to the resonance forms depends on where the electrophile adds to the

ring. Meta addition of an electrophile allows for three resonance forms, where the

positive charge is delocalized between the two ortho positions and the para

positions from the substituent. Ortho/ para addition of an electrophile, however,

allow for four resonance forms, causing the intermediate to have the positive charge

delocalized between the two meta positions, the carbon the substituent is bonded

to, and on the substituent itself. This greater number of resonance forms and thus

greater stability of the arenium ion explains why halogens and other substituents

with lone pairs on the atoms bonded directly to the aromatic ring are ortho/ para

directors. Nitration of bromobenxene creates a higher ratio of para to ortho

products primarily because of steric hindrance; it is more sterically favorable for an

electrophile to add on to the carbon para to the bromine substituent than to add

ortho, directly next to the carbon attached to the bromine.

The low percent yield (30.2 %) achieved in the nitration of bromobenzene

may be explained by two factors, one due to experimental error, the other due to the

weakly deactivating charcter of the bromine substituent. The first factor is that the

experimenters did not add the correct amount of sulfuric acid (approximately one-

third the amount) that the lab called for. This would have led to a lower

concentration of the nitrosoniom cation and thus less electrophile to react with the

benzene ring. The second factor refers to the fact that bromine is a weakly

deactivating substituent of benzene. Although this factor likely cannot account for

the low percent yield found in the lab, in general, due to bromine’s inductive effects,

bromobenzene reacts slower in electrophilic aromatic substition than does benzene

itself. This is because the arenium ion is made more positive and less stable by the

withdrawing of electrons from the ring by electron withdrawing groups such as

halogens.

The cystallization process selectively crystallized the 4-nitrobromobenzene

isomer from our crude product. This is supported by the experimental melting

point value of 128.2 – 130.4 ° C, nearly identical to reported values in the literature.2

It is also supported by our FTIR spectra: The triad of peaks at 1921(m), 1800(m),

and 1675(m) are all characteristic of a para di-substituted benzene ring, as are the

two peaks at 839(vs) and 738(s). 1599(w-m), 1511(w-m), and 1357(w-m) indicated

an aromatic ring and single and double bonded carbons.

Competitive Nitration of Acetiline and Methyl Benzoate

In the competitive nitration of acetiline and methyl benzoate, acetaniline was

the only product nitrated by the nitrosoniom ion. The conclusion that 4-

nitroacetanilide was the only product formed is supported by the melting point

obtained from the purified recrystallization product. This melting point, 221.2-

223.9 ° C is very similar to literature values for this molecule. 3 This selective

nitration of acetaniline instead of methyl benzoate is explained by the fact that the

amide substituent with the phenyl bonded to the nitrogen is an electron-donating

group and is moderately activating towards electrophilic aromatic substituion,

whereas the ester group with the phenyl on the carbonyl carbon is an electron

withdrawing group and is moderately deactivating towards electrophilic aromatic

substitution.

In the case of acetanalide nitration was predicted at both the ortho and para

position. The reason for these directing effects is the same as for bromobenzene. In

the case of methyl benzoate, however, nitration was predicted at the meta positions

of the molecule. This is due to resonance effects in the arenium ion intermediate.

Whether the electrophile adds at the ortho, meta, or para position, there will be

three resonance forms that delocalize the positive charge through the arenium ion.

When the electrophile adds meta, those three resonance forms consist of the two

ortho positions and the para position. These resonance forms are all equally stable.

When the electrophile adds ortho or para, however, the positive charge is

delocalized between the two meta positions and the carbon directly bonded to the

ester. This latter resonance form in meta addition is very unstable because the

positive charge is on a carbon with an electron withdrawing group present, thus

further destabilizing the ion. Therefore, in ortho/ para addition this resonance form

contributes little to cation delocalization, and it is as if there were only two

resonance forms, making meta addition more favorable. However, although meta

addition is the most favorable option for ekectrophilic substition of methyl

benzoate, it is still much less favorable than substitution on acetaniline or even

unsubstituted benzene.

The melting point of the purified product illustrated that not only had

acetaniline been the only substrate nitrated, but nitration had only occurred at the

para position on acetaniline as well. The reason for this is similar to bromobenzene;

it is due to steric hindrance of the rather bulky amide group. This large substituent

makes attack at the para position by the electrophile much more likely, and thus is

the only product formed.

After obtaining melting point of our purified product, we were able to

conclude that all of our product had been 4-nitroacetanilide, thus giving us the

number of moles of this product formed and a percent yield. The larger percent

yield in this nitration than that of bromobenzene was likely due to activation of the

ring by the electron donating amide, as well as experimental factors. The lack of any

methyl 3-nitrobenzoate shows that this group was sufficiently deactivating to allow

for no substitution at all (in the presence of acetaniline).

Bibliography

1 Banerjee, Dhruv K. "Ortho and Para % of Key Reactions." Utkarshchemistry.13 Nov. 2013.

2 CSID:21171513, http://www.chemspider.com/Chemical-Structure.21171513.html

3 CSID:7407, http://www.chemspider.com/Chemical-Structure.7407.html