Post on 12-Apr-2017
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