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
Concept #1: Stability of carbocations: vinyl & allylic
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.
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.
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)
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Conditions favoring kinetic control:
Conditions favoring thermodynamic control:
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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: 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.
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
*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
EDG = Electron donating group; EWG = Electron withdrawing group (source- from wikipedia)
Stereochemistry of Diels-Alder:
Image is from AATC workshop #12 (navigate: "Useful resources" -> "Che 118B" -> "AATC Resources" -> "Workshops" -- #11 & 12 have diels-alder practice)
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Molecular orbitals of Diels-Alder:
A question that pops up every few quarters is the following:
A question that pops up every few quarters is the following:
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.
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.
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)
