B. Mechanism Consider the mechanism shown below for chlorination of a monosubsti
ID: 303927 • Letter: B
Question
B. Mechanism Consider the mechanism shown below for chlorination of a monosubstituted aromatic ring Cl CI-CI: Cl FeCl CI Fe +HC+FeCl a) In this reaction is the aromatic ring behaving as an electrophile or a nucleophile? b) Why is FeCla considered a catalyst in the reaction shown? c) Which would you expect to be faster, an electrophilc aromatic substitution in which the Z group is an electron donating group or an electron withdrawing group? Explain your answer d) Below is a chart showing electron donating and withdrawing groups. Electron-Withdrawing Groups (EWG) weak EWG very strong EWG Electron-Donating Groups (EDG) 0 OR eak EDG very strong EDG e) Look closely at the EDG groups, what do you notice about the atom which is attached directly to the ring? Are there similarities between these atoms? (R stands for any carbon attachment, not all lone pairs have been drawn in) f) Now, look at the EWG groups except for the weak EWG (X stands for a halogen) What do you notice about the atom which is attached directly to the ring? Look at the bonds to these atoms. Look at partial charges or formal charges on these atoms Again, are there similarities? NOTE: All EWGs except the halogens are meta directing and are deactivating. The halogens are ortho/para directing. All EDGs are ortho/para directing and activatingExplanation / Answer
a)In this reaction is the aromatic ring behaving as an electrophile or a nucleophile?
Ans: The benzene/aromatic ringbehaves as a nueclophile as it is the source of the electrins being donated to the electrophile Cl+.
b) Why is FeCl3 considered a catalyst in the reaction shown?
Ans: A catalyst is a substance that speeds up a chemical reaction, but is not consumed by the reaction; and hence can be recovered chemically unchanged at the end of the reaction. FeCl3 isconsidered a catalyst as it facilitates the addition of electrophile and is regenerated at the end of the reaction.
c) Which would you expect to be faster, an electrophillic aromatic substitution which Z is an electron donating group or an electron withdrawing group? Explain?
Ans: The reaction would proceed faster when Z is an electron donating group. This is because the intermediate cabocation formed in the reaction is stabilized due to the electron density received from the electron donating groups. Hence the reaction proceeds faster.
e) Look closely at the EDG groups, what do you notice about the atom which is attached directly to the ring? Are there similarities between these atoms?
Ans: All the strong EDG groups are the ones that are directly linked to electronegative atoms like O and N. The similarity is that these atoms have atleast one lone pair of electrons. In the weak EDGs The loan pair is either absent (R) or is involved in resonce with the extended double bond (NH(C=O)R).
f) Look at the EWG groups except for the weak EWGs. What do you notice about the atom that is directly attached to the ring? What are the similarities?
Ans: The smilarity amongst the EWG groups is that they are directly linked to a carbon atom connected to one or 2 electronegative atoms like O, X= I,F,Cl,Br. In very strong EWGs the ring is directly linked to a carbon that is attached to more than 2 electronegative atoms or directly to the eectronegative atom itself. Even in case of nitrile CN, it is treated as 3 Ns connected to a carbon .
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