Relative Rates of Electrophilic Aromatic Substitution
RELATIVE RATES OF ELECTROPHILIC AROMATIC SUBSTITUTION MICA FELICIANO UNIVERSITY OF THE PHILIPPINES, DILIMAN, QUEZON CITY 1101, PHILIPPINES DATE SUBMITTED: 13 FEBRUARY 2014 DATE PERFORMED: 6 FEBRUARY 2014
ANSWERS TO QUESTIONS 1. Arrange the compounds used in the experiment in order of increasing reactivity towards Br2 in CH3COOH. Explain. Nitrophenol < Chlorobenzene < Benzene < Acetanilide < Phenol < Aniline The presence of a substituent in the substituted aromatic ring affects the ring’s reactivity towards the electrophile, and the orientation/position of the electrophile in the aromatic ring when it undergoes electrophilic aromatic substitution. A substituent that is electron-donating is considered an activating group – that is, it makes the aromatic ring more reactive towards the electrophile. Moreover, most activators attach the electrophile either in the ortho or para position of the ring (with respect to the original substituent). On the other hand, a substituent that is electron-withdrawing makes the ring less reactive, and is called a deactivating group. Deactivators are generally meta directors, with the exception of halogens, which are weakly deactivating but ortho-para directing. Of the six compounds separately reacted with Br2, nitrophenol should be least reactive because of its strong deactivating substituent, NO2. Next to nitrophenol is chlorobenzene with a weakly deactivating halogen substituent, which is then followed by the unsubstituted benzene. The last three compounds, acetanilide, phenol, and aniline, contain the activating groups, NHOR, OH, and NH2, respectively. The order of the three is determined by the relative strength of the activating substituent. 2. Do your experimental results agree with the theoretical data? If not, what are the possible sources of error? Actual: Nitrophenol < Benzene < Chlorobenzene < Acetanilide < Phenol/Aniline The general trend in the experimental result is close to the theoretical data shown in 1, except for benzene and chlorobenzene which switched positions in the trend. This may be attributed to an excess in the volume of chlorobenzene (or shortage in that of benzene) which were added to Br2 that made the reaction, observed as color disappearance/change, more evident. Also, possible contamination of the reagent bottles or droppers used could have also affected the reactions. 3. Predict order of reactivity (from least to most) of the ff. compounds towards chlorination. A. Toluene, nitrobenzene, anisole, methylbenzoate
Nitrobenzene < methylbenzoate < toluene < anisole
B. Benzene, acetophenone, bromobenzene, benzyl alcohol
Acetaphone < bromobenzene < benzene < benzyl alcohol C. Styrene, benzaldehyde, aniline, iodobenzene
Benzaldehyde < iodobenzene < styrene < aniline 4. Explain the effect of solvent in the reaction of acetanilide and Br2 in Part B. Acetanilide readily reacts with Br2 via electrophilic aromatic substitution because of its electron-donating substituent. Mixing Br2 with acetic acid, however, further speeds up the reaction since acetic acid is a polar solvent that acts as a catalyst by polarizing Br 2. Meaning to say, the electrons shared by the two Br atoms move closer to one of them, making one Br atom electrophilic (and the other nucleophilic) such that it can react with the aromatic ring. On the other hand, mixing Br2 with cyclohexane slows down the reaction because the solvent is nonpolar. REFERENCES Bruice, P. (2004). Organic Chemistry (4th ed.). Pearson Prentice Hall. McMurry, J. (2012). Organic Chemistry (8th ed.). Cengage Learning. Solomons, G., Fryhle, C., & Snyder, S. (2014). Organic Chemistry (11th ed.). John Wiley & Sons.