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Week three of summer session 2, 2021

LOGIstics: 

Midterm two will cover Ch 14-16. Although students typically perform better on this midterm, many say that the material is more difficult. 

Chapter 14: Delocalized Pi Systems

Picture

​​Concept #1: Stability of carbocations: vinyl & allylic

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Rank the above structures in order of increasing stability (1st = least stable)
​Answer: e < c < b < a < d < f
Note that: vinyl carbocations are the most unstable & substituted allylic carbocations are more stable than tertiary carbocations. 

Likewise, allylic anions and radicals are more stable than alkane anions and radicals.
Note: this allows both SN1 and SN2 Reactions to speed up on allylic positions. With a primary or secondary allylic leaving group, the reaction will undergo SN1 with a poor nucleophile and SN2 with a strong nucleophile. Tertiary allylic leaving groups will only undergo SN1 reactions.

Reaction #1: Electrophilic 1,2 vs 1,4 addition of H-X to conjugated dienes
(Kinetic vs. Thermodynamic)

Picture
Conditions favoring kinetic control: 
  • short reaction time
  • low temperatures

Conditions favoring thermodynamic control:
  • longer reaction time
  • heat
Note: Instead of using HBr or HCl, we can use Br2 or Cl2 and we still get 1,2 & 1,4 addition of both halogens. (Replace the Hydrogen with a halogen in the product)

REACTION #2: ALLYLIC HALOGENATION

Recommended to watch the following video at 2x speed
​Note: While "NBS" (N-Bromosuccinimide) adds an allylic Br, "NCS" is used to add an allylic Cl
Note: In Professor Nasiri's course, she often asks for the MAJOR product. For NBS, this would be the MORE SUBSTITUTED alkene product. 

​REACTION #3: formation of allylic Grignard and lithium reagents

Once we form the allylic halogen, we may turn it into allylic Grignard and Lithium reagents. These may be used as nucleophiles for synthesis by reacting with alkyl halides, epoxides, and ketones/aldehydes.
​
Picture
Note: TMEDA, along with THF, are solvents that polarize the C-Li bond, increasing basicity & nucleophilicity

​REACTION #4: Diels-Alder

Diels Alder is a famous reaction that reacts a diene and dienophile under heat to form a cyclopentene. It provided the first reliable method of synthesizing complex ring structures due to its consistent regioselectivity and stereochemistry (via Wikipedia).
Regiochemistry of Diels-Alder:  (#1 & 2 are most important-- #3 & 4 are less important)
*rather than memorizing the products, try drawing out the resonance for each, and then match the + side of the diene with the - side of the dienophile and vice versa
Picture
EDG = Electron donating group; EWG = Electron withdrawing group (source- from wikipedia)
Stereochemistry of Diels-Alder:
Picture
Image is from AATC workshop #12 (navigate: "Useful resources" -> "Che 118B" -> "AATC Resources" -> "Workshops" -- #11 & 12 have diels-alder practice)
  • Outside groups (left of the inside group) add SYN to the endo groups (left of the exo group)
  • Inside groups (right of the outside group) add SYN to the exo groups (right of the endo group)
Overall: Groups on the left of both molecules add syn, and groups on the right of both molecules add syn to each other.
Molecular orbitals of Diels-Alder:
A question that pops up every few quarters is the following: 
Picture
There are two possible answers depending on whether you use the HOMO (highest occupied molecular orbital) or LUMO (lowest unoccupied molecular orbital) of the diene. Drawing one of the two above structures will get you full points for this question.

Below, is a diagram that demonstrates how to get the above answer.
First, there are four energy levels for diene molecular orbitals, each with four molecular orbitals. The dashed lines represent nodes, where the electron density switches orientation (there is one node beginning at level 2, two nodes beginning level 3, and three nodes beginning at level 4). The four pi electrons found in our diene are found on levels 1 and 2, and there are no electrons found in level 3. Therefore level 2 becomes our HOMO and level 3 is our LUMO. 
There are two energy levels for our dienophile molecular orbitals, each with two molecular orbitals. Once again, the same pattern emerges where we have zero nodes in level 1 and one node on level 2. Level 1 contains both pi electrons, HOMO, while level 2 becomes our LUMO, because it has no pi electrons. 

The diels-alder reaction occurs when we have electrons from the HOMO of either diene or dienophile traveling to create a bond by entering the LUMO of the other molecule. 
Picture
Drawn with iPad

​​REACTION #5: Electrocyclic reactions


CHAPTER 15: Benzene & Aromaticity

nomenclature of benzene derivatives

Naming priority:​ Benzoic Acid > Benzaldehyde > Acetophone (a 2 carbon ketone coming off benzene) > Phenol > Thiols (SH) > Aniline > Toluene + Other hydrocarbon benzenes

determining aromaticity:

Anti-aromatic: follows all rules for aromaticity except Huckel's rule (4n + 2)

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  • Home
  • Courses
    • Organic Chemistry Che118A (Fall 2021) >
      • Che 118A Table of Contents
      • Che118A Introduction
      • Week One
      • Week Two
      • Week Three
      • Week Four
      • Week Five
      • Week Six
      • Spectroscopy Resources >
        • Reactions after MT2
    • Organic Chemistry Che118B (Summer Session) >
      • Che 118B Table of Contents
      • Che118B Introduction
      • Reaction Summaries
      • Week One
      • Week Two
      • Week Three >
        • Lectures (Ch 14-15)
        • Practice Questions
      • Week Four >
        • Lectures (Ch 15-16)
        • Practice (Midterm 2)
      • Week Five >
        • Lectures (Ch 17)
        • Practice Questions
      • Week six >
        • Lectures (Ch 18)
        • Practice (Final)
    • Organic Chemistry Che118C (Quarter-based) >
      • Che 118C Table of Contents
      • Che 118C Introduction
      • Reaction Summaries >
        • Chapter 19 (MT 1 part 1)
        • Chapter 20 (MT 1 Part 2)
        • Chapter 21 (MT 2 Part 1)
        • Chapter 22 (Mt 2 Part 2)
        • Chapter 23 (Final Part 1)
        • Chapter 24 (Final Part 2)
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    • Che 118C >
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