Ochem 343, Part 2 Flashcards
SN2 considerations
- sterics
- strength of nucleophile
- solvent influence
Le Chateliers Principle
When a change disrupts an equilibrium, the system adjusts to diminish the change
Zaitser’s Rule
in beta elimination reactions, more substituted alkene usually forms preferentially in E1 and E2.
acidity of H on sp3 vs. sp2 vs. sp C, role in alkyne synthesis via E2
The H on an sp2 hybridized C is MORE acidic (more reactive) than the H on an sp3 c.
H on an sp C is the MOST acidic and very reactive. (more S character = lower in energy).
In Alkyne synthesis via E2 reactions, this is important because you need more than 1 equivalent of strong base to keep reaction moving toward desired products.
Geminal dihalide
A geminal dihalide is 2 halides (X atoms) on the same C
Vicinal dihalide
A vicinal dihalide has 2 halides (X atoms) on adjacent carbons from each other
peroxide
Peroxides are a group of compounds with the structure R−O−O−R.
Initiation def and methods
=Making radicals
Can use peroxides (R-O-O-R)
hv light
What is so favorable about anti-coplanar in E2?
- obitals aligned and pi bonds starting to form, so C on its way to sp2
- favorable orbital arrangement; hyperconjucation happening above and below the plane
- charge; partial charges generally favorable in anticoplanar arrangements
- Low energy conformation trans state is where these groups are stable and spending most of their time.
Why do E2 reactions tend to follow Zaitsev’s Rule?
E2 follows Zaitsev’s because there are typically more anti-coplanar Hydrogens in more branched, or non-terminal, positions.
Why do E1 reactions tend to follow Zaitsev’s Rule?
E1 follows Zaitsev’s because the relative stability of the carbo cation intermediate determines the products, and the more stable carbo cation will be the one on the more substituted position.
Can H-S bonds drive an E2?
No! H-S bonds are much more acidic than O-H
= weaker, more stable base. E2 not possible.
Grignard Reagent
“grin-yard”
Insertion of Mg into a C-halogen bond
Organolithium Reagent
Li in 2 eq. bonds with X (either Cl or Br) and precipitates as a solid salt. The second eq. takes the place of the Cl or Br on the substrate.
Carbene
Carbon nucleophiles.
Can also accept electrons via empty p orbital.
Carbon with 2 attachments and a lone pair.
Approximately sp2 hybridized.
Divalent (no charge).
Highly reactive.
def of carbonion
lone pair on carbon with a negative charge
3 things that carbo cations do:
Carbo cations:
1. act as acids (essentially protonated alkenes).
2. attach nucleophiles.
3. Rearrange.
Like all strong acids, they are INCOMPATIBLE with strong bases.
Grignard reagent
A compound of the form R-Mg-X,
Where X is Br, Cl, or I.
Prepared in ETHER solvent (R-O-R), then ether boiled off.
In reactions, the C is more electronegative than the metal, so C has the partial negative charge.
Reacts as a carbanion, a strong base.
Won’t work with solvent or any H (even H in water is too acidic).
Organolithium reagents
Compounds of the form R-Li.
Prepared in oxygen-free in hydrocarbons.
In reactions, the C is more electronegative than the metal, so C has the partial negative charge.
Reacts as a carbanion, a strong base.
Carbanion
A carbon with 3 bonds and a lone pair, giving it a formal negative charge.
Carbene
A species with a divalent carbon atom—a highly reactive carbon, neutral, with just 2 bonds plus a lone pair.
Can act as an electrophile via empty p orbital, or as a nucleophile via lone electron pair in sp2 orbital (singlet carbene).
When you see metal on C…
Carbon will generally behave as though it has a lone pair in place of that covalent bond.
Tautanimers
Isomers that differ in location of api system and where an H atom is attached.
Alkyl substituent
A carbon chain minus a H.
Alkylating agent adds an alkyl chain.
A cyclic ether
an epoxide
3 member ring with O.
Formed via X2 in water, then W.E.S.
Williamson Ether Synthesis
W.E.S. Strong base (NaH, NaOH) deprotonates an alcohol to make a good O nucleophile. O reacts with a great leaving group (X) to form an ether or epoxide.
Enthalpy
The sum of a system’s internal energy.
Refers to the energy lost and gained by a system.
Exothermic reaction = enthalpy lost, energy released.
Entropy
Order or randomness of a system.
A spontaneous process increases entropy of the universe, decreases entropy of the system.
Ex. Solids have lower entropy.
Heteroatom
Any atom that is not C or H
Homolytic vs heterolytic bond cleavage
Homolytic bond cleavage gives one electron to each atom (free radicals).
Heterolytic bond cleavage gives both electrons to a single atom.
Coulomb’s Law
It takes energy to separate bonds (charged particles)
Common solid precipitates
KCl Protonated pyradine (H attached to N) and Cl
Enol tautamerization under acidic conditions
In acidic conditions, an enol rapidly tautamerizes by
1) Acid protonates the alkene
2) Acid’s conjugate base or water deprotonates the alcohol
Enol tautamerization under basic conditions
Under basic conditions, an enol rapidly tautamerizes by
1) Base deprotonates the alcohol
2) Newly formed water (or conj acid of base) protonates the alkene
halohydrin
X2
solvolysis
The solvent serves as the nucleophile, as seen often in SN1 and E1 reactions.
Why are C-O pi bonds more stable than C-C pi bonds?
C-O pi bonds are more stable than C-C pi bonds, because oxygen’s electronegativity makes it’s p orbitals more stable than carbon’s p orbitals.
Why doesn’t a Grignard solvent work with H present?
Grignard makes a very strong base, so any H would cause us to have an acid/base reaction and get a C-H bond instead of a C-C bond.
When asked to describe the Reggio-chemistry of a reaction, include…
- Where does the nucleophile attach or a pi bond form
- Describe the related energy and stability
When asked to describe the stereochemistry of a reaction, include…
- What step sets the stereochemistry
- Anti, syn, or both
- Describe the related energy and stability
hydride shift
When a hydrogen moves from hyperconjugation (electron donation from filled orbital to empty) to forming a covalent bond.
Similar process to H bonding.
Basic functional group abbreviations
Me
Et
Pr
Bu
