Why does Benzene undergo only electrophilic substitution reactions?
This property can be attributed to the remarkable stability of Benzene, due to the 6 delocalised electrons forming a ᴨ cloud of electrons. Instead of the electrons forming three stationary C==C bonds, they form a delocalized ring which gives benzene greater stability, and this is seen in the enthalpy change when breaking the delocalized ring of electrons in benzene.
Comparing the structures of benzene and 1, 3, 5-cyclohexatriene:
One would expect to have similar enthalpy changes for breaking the delocalized ring of benzene and the 1, 3, 5 pi bonds of the 1, 3, 5-cyclohexatriene, but in real life it is around 150 kJ/mol in excess of the amount of energy needed to break the three pi bonds in 1, 3, 5-cyclohexatriene.
This is why addition reactions are unlikely to occur with benzene molecules. In order for a molecule to get added to benzene, the delocalized ring of electrons needs to be broken, and since the energy required for this to occur is so high, it is not an easy thing to do.
In the case of electrophilic substitution reactions, the delocalized ring of electrons remain as they are, therefore it does not need a large amount of energy hence the reactions occur more readily.
What are electrophiles? Why does benzene only undergo electrophilic but not nucleophilic substitution?
By definition, a molecule which forms a covalent bond by accepting a pair of electrons is called an electrophile. Any molecule, ion or atom that is electron deficient in any way can behave as an electrophile.
In contrast, any molecule which forms a covalent bond by donating a pair of electrons is called a nucleophile. Nucleophiles are usually rich in electrons and seek out positive atoms or molecules, which is usually located in the nucleus of an atom – hence the name Nucleophile.
If we look at the structure of benzene, we can see that although it possesses a neutral overall charge, the delocalized electron cloud forms an area of negative charge which attracts positively charged electrophiles or the positive end of polar molecules. Nucleophiles, possessing a negative charge, are not attracted to this delocalized electron cloud.
So how does an electrophilic reaction occur (reaction mechanism)?
This is best explained with the help of diagrams.
1. Electrophile is attracted to the benzene molecule.
2. Two of the delocalized electrons are used to form a bond with the electrophile. The remaining two pi bonds in the benzene molecule are unaffected, so delocalization is present but not across the whole benzene molecule.
The percent yield of products that was calculated for this reaction was about 81.2%, fairly less pure than the previous product but still decently pure. A carbon NMR and H NMR were produced and used to identify the inequivalent carbons and hydrogens of the product. There were 9 constitutionally inequivalent carbons and potentially 4,5, or 6 constitutionally inequivalent hydrogens. On the H NMR there are 5 peaks, but at a closer inspection of the product, it seems there is only 4 constitutionally inequivalent hydrogens because of the symmetry held by the product and of this H’s. However, expansion of the peaks around the aromatic region on the NMR show 3 peaks, which was suppose to be only 2 peaks. In between the peaks is a peak from the solvent, xylene, that was used, which may account to for this discrepancy in the NMR. Furthermore, the product may have not been fully dissolved or was contaminated, leading to distortion (a splitting) of the peaks. The 2 peaks further down the spectrum were distinguished from two H’s, HF and HE, based off of shielding affects. The HF was closer to the O, so it experienced more of an up field shift than HE. On the C NMR, there are 9 constitutionally inequivalent carbons. A CNMR Peak Position for Typical Functional Group table was consulted to assign the carbons to their corresponding peaks. The carbonyl carbon, C1, is the farthest up field, while the carbons on the benzene ring are in the 120-140 ppm region. The sp3 hybridized carbon, C2 and C3, are the lowest on the spectrum. This reaction verifies the statement, ”Measurements have shown that while naphthalene and benzene both are considered especially stable due to their aromaticity, benzene is significantly more stable than naphthalene.” As seen in the reaction, the benzene ring is left untouched and only the naphthalene is involved in the reaction with maleic
This week’s lab was the third and final step in a multi-step synthesis reaction. The starting material of this week was benzil and 1,3- diphenylacetone was added along with a strong base, KOH, to form the product tetraphenylcyclopentadienone. The product was confirmed to be tetraphenylcyclopentadienone based of the color of the product, the IR spectrum, and the mechanism of the reaction. The product of the reaction was a dark purple/black color, which corresponds to literature colors of tetraphenylcyclopentadienone. The tetraphenylcyclopentadienone product was a deep purple/black because of its absorption of all light wavelengths. The conjugated aromatic rings in the product create a delocalized pi electron system and the electrons are excited
The positive charge on the phosphorus atom is a strong EWG (electron-withdrawing group), which will trigger the adjacent carbon as a weak acid. 5 Very strong bases are required for deprotonation such as an alkyl lithium, however in this experiment 50% sodium hydroxide was used as reiterated. Lastly, the reaction between ylide and aldehyde/ketone produces alkene. (Eq. 1) As shown in equation 2, the reaction between the phosphonium salt and the sodium hydroxide produces the ylide/carbanion that is stabilized due to the positive charge on phosphorus and the conjugation that occurs in the benzene ring as shown by the structure B in equation 2.
A weak peak was at a position between 1600-1620 cm-1 can also be seem in the IR, which was likely to be aromatic C=C functional group that was from two benzene rings attached to alkynes. On the other hand, the IR spectrum of the experimental diphenylacetylene resulted in 4 peaks. The first peak was strong and broad at the position of 3359.26 cm-1, which was most likely to be OH bond. The OH bond appeared in the spectrum because of the residue left from ethanol that was used to clean the product at the end of recrystallization process. It might also be from the water that was trapped in the crystal since the solution was put in ice bath during the recrystallization process. The second peak was weak, but sharp. It was at the position of 3062.93 cm-1, which indicated that C-H (sp2) was presence in the compound. The group was likely from the C-H bonds in the benzene ring attached to the alkyne. The remaining peaks were weak and at positions of 1637.48 and 1599.15 cm-1, respectively. This showed that the compound had aromatic C=C function groups, which was from the benzene rings. Overall, by looking at the functional groups presented in the compound, one can assume that the compound consisted of diphenylacetelene and ethanol or
The weight of the final product was 0.979 grams. A nucleophile is an atom or molecule that wants to donate a pair of electrons. An electrophile is an atom or molecule that wants to accept a pair of electrons. In this reaction, the carboxylic acid (m-Toluic acid), is converted into an acyl chlorosulfite intermediate. The chlorosulfite intermediate reacts with a HCL. This yields an acid chloride (m-Toluyl chloride). Then diethylamine reacts with the acid chloride and this yields N,N-Diethyl-m-Toluamide.
As a result, the laboratory experiment was determined to be successful and the two product samples obtained and completed calculations displayed that overall bromide was a stronger nucleophile as the chloride ion was more electronegative than bromide, which allowed it to hold electrons in closerE. In conclusion, since bromide is less electronegative and has more electrons, it was able to share the unpaired electrons more easily than chlorideA. These results were expected, as the alkyl bromide would be the major product of procedure A as it followed the SN2 mechanism which was based on nucleophile strength and the product from procedure B would be a near-equal mixture as it followed the SN1 reaction mechanismC. The methods used during this experiment allowed for a successful completion and determination of the better nucleophile, but other additions and observations would have been interesting and beneficial as well. A possible examination of the two sample products collected using pH tested values or observation of sample spotted chromatography paper under a
There is another type of mechanism called an elimination mechanism that is competing with the substitution mechanisms to attack the molecules. There are two types of elimination reactions, E1 and E2. E1 and SN1 mechanisms compete with each other as E2 and SN2 mechanisms compete with each other. To ensure that our experiment favors the substitution reaction, an environment must be created in which the leaving group, H2O, is a weak base and the nucleophile, bromine, has strong polarity. This is obtained in this experiment by adding sulfuric acid to the reacting solution. Sulfuric acid not only donates protons but also acts as a dehydrating catalyst to push the reaction more toward the products. Without sulfuric acid
Metals contain a sea of electrons (which are negatively charged) and which flow throughout the metal. This is what allows electric current to flow so well in all metals. An electrode is a component of an electric circuit that connects the wiring of the circuit to a gas or electrolyte. A compound that conducts in a solution is called an electrolyte. The electrically positive electrode is called the anode and the negative electrode the cathode.
...teraction of the HOMO of the diene and the LUMO of the dienophile. This reaction was done at relatively low temperatures as the dry ether has a boiling point of 34.6 °C. At low temperature the endo preference predominates unless there is extreme steric hindrance, which in this case there is not. The endo product forms almost exclusively because of the activation barrier for endo being much lower than for exo. This means that the endo form is formed faster. When reactions proceed via the endo for the reaction is under kinetic control. Under kinetic control the adduct is more sterically congested, thus thermodynamically less stable. The endo form has a lower activation energy, however, the EXO form has a more stable product. As this is a symmetrical Diels-Alder reaction there is not two possible isomers of the product.
The electrophile is positively charged, so it will not go to the ortho and para positions, but to the meta positions in greater abundance. Therefore, the majority of EWGs (with the exception of halogens) are meta directors. In this experiment, a meta director is used. If the product added to the ortho or para positions produces a carbocation intermediate that has a positive charge on a carbon that is directly touching the EWG. This carbocation intermediate has more energy, and is therefore less stable.
We successfully achieved our goal of synthesizing benzhydrol, but we did not successfully reach the goals of the completion of the synthesis or purification of benzhydrol.
electrophile (electron pair acceptor) with an attached leaving group. This experiment was a Williamson ether synthesis usually SN2, with an alkoxide and an alkyl halide. Conditions are favored with a strong nucleophile, good leaving group, and a polar aprotic solvent.
These reactions take place unexpectedly. However, the rate of an unexpected reaction may not be very great. This is because an energy hurdle must first be defeated.
Time - The longer time can let more copper ions from the anode to the cathode if the current are the same. There are still more factors which can affect the mass deposited during electroplating. 3). Distance between two electrodes - If the distance between the two electrodes is greater, the copper ions require to travel more from the anode to the cathode.
From these properties of bonds we will see that there are two fundamental types of bonds--covalent and ionic. Covalent bonding represents a situation of about equal sharing of the electrons between nuclei in the bond. Covalent bonds are formed between atoms of approximately equal electronegativity. Because each atom has near equal pull for the electrons in the bond, the electrons are not completely transferred from one atom to another. When the difference in electronegativity between the two atoms in a bond is large, the more electronegative atom can strip an electron off of the less electronegative one to form a negatively charged anion and a positively charged cation. The two ions are held together in an ionic bond because the oppositely charged ions attract each other as described by Coulomb's Law.