WEEK Four OF SUMMER SESSION 2, 2021

CLASS LOGISTICS

Midterm 2 will cover Chapter 15 and 16. 
Notes from review session: 
[che_118b]_review_session_midterm_2.pdf
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Chapter 15 continued: Aromaticity and benzenes


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electrophilic aromatic substitution & Reaction #1: halogenation

Substitution of an unsubstituted benzene's hydrogen with an electrophile (E+)

Reaction #2: Sulfonation of benzene

​Reaction #3: Nitration of benzene

reaction #4 & 5: ​friedel-crafts alkylation + Acylation

Note: both reactions do not occur with a deactivating group/ meta-directing group attached

Chapter 16: Electrophilic addition and synthetic reactions of substituted benzenes

Concept #1: Electron donating vs electron withdrawing groups

Activating (aka Electron Donating) groups: 
-R, -Ph, -OH, -OR, -NH2, -NHR, -NR2, -NHCOR
(these increase the rate of Electrophilic Aromatic Substitution)

Deactivating (aka Electron Withdrawing) groups:
-X, -CN, -SO3H, -CO2H, -CO2R, --CHO, --COR,
-NO2, -N+R3 (Nitrogen with 3 additional groups attached), -CF3, -CCl3
(these decrease the rate of E.A.S.)

-ALL activating groups are ortho and para directors
-Deactivating groups are meta directing EXCEPT for halogens, which are ortho and para directors

Reaction #1: reduction of nitrobenzene to aniline

Picture
Nitrobenzene is a meta-director, while aniline is a ortho-/para-director
  • We reduce our nitrobenzene to aniline using any of the three reagents on top of the reaction arrow.
  • Zn(Hg), HCl = "Clemensen reduction" (mechanism not taught)
  • The oxidation of aniline (the reverse reaction) is catalyzed by a peroxycarboxylic acid (MCPBA is often used)

REACTION #2: REDUCTION OF a carbonyl

  • Same reagent as one of the reagents that reduces nitrobenzene (Clemensen reduction)

REACTION #3: REDUCTION OF A CARBONYL

  • If we have a "benzylic hydrogen"-- a hydrogen that's attached to a carbon attached to a benzene, we cleave the rest of the chain off and turn the carbon off of the benzene into a carboxylic acid.

  • Note: we don't need the acid work-up step because in class, we use KMnO4 in acidic conditions (see above picture)
  • I recommend speeding up Leah4Sci's videos 

REACTION #4: Protecting anilines by reacting with acyl halides (Aniline -> acetanilide)

  • The NH2 of anilines are more nucleophilic than the double bonds of the benzene, so NH2 will compete as a nucleophile with reagents such as friedel crafts. Therefore, we must first "protect" the NH2 by reacting it with an acyl chloride (CH3COCl). Then we can do reactions such as friedel crafts.
  • Additionally, amines (NH2 in this case) is much stronger of an activating group than the amide (NHCOCH3 in this example), This can have important synthesis ramifications.
  • To reverse the reaction, we can react the newly formed amide (NHCOCH3) with H2O in acid or base and heat.

REACTION #5: Phenols to anisoles

  • OH (on benzene = phenol) is a stronger activating group than OCH3 (on benzene = anisole), similar to how NH2 is a stronger activating group than NHCOCH3. This has synthesis ramifications.

REACTION #6: desulfonation

  • SO3H is a deactivating, meta-directing group
  • Formation of the SO3 electrophile occurs by two H2SO4's reacting with one-another under heat 
  • This reaction is primarily used in synthesis to "position block" a benzene carbon.