Delocalized π Systems (Chapter 14) Flashcards
Conjugation
The π–overlap of three or more p orbitals on adjacent atoms in a molecule.
E.g. Allylic Compounds, Conjugated Dienes
Stability of Dienes
Conjugated > Isolated > Cumulated
If an intermediate allylic cation/radical is achiral, then the product following addition is…?
Racemic
If an intermediate form is more unstable, then the activation energy required to reach that form is…?
Greater
Why is an allyl carbocation more stable than a typical carbocation?
The allyl cation is stablized via resonance.
This same resonance-stabilizing effect is seen in allyl radicals and allyl anions.
Allyl Compound
A planar and conjugated system consisting of a primary carbon bonded to a double-bonded carbon.
Why are cumulated dienes non-planar?
The central sp-hybridized carbon of the allene is unable to form two parallel C=C double bonds, so one of the π bonds must be perpendicular to the other.
The central carbon has two p orbitals available for π-bond formation; these p orbitals are perpendicular to one another, so the two π bonds that the central carbon can form must be perpendicular.
Cumulated Diene
Allene
A nonconjugated diene in which the double bonds occur successively and share a single sp-hybridized carbon.
The two π bonds of a cumulated diene are perpendicular to one another.
Allyl C–C Bond Lengths
- The two C–C bonds of an allyl are identical.
- Both C–C bonds are between a C—C single bond and a C=C double bond in length.
Where do the reactions of allyl compounds occur?
Terminal Carbons
The positive/negative charge of an allyl cation/anion is located on the two terminal carbons (NOT the internal carbon) due to resonance.
Why does radical allylic halogenation occur faster than radical alkane halogenation?
The bond-dissociation energy of the allylic C—H bond is significantly lower than BDE of an alkane C—H bond.
I.e. The activation energy barrier of homolytic allylic C—H cleavage is smaller than the AE barrier of homolytic alkane C—H cleavage.
Conjugated Diene
A planar compound containing two C=C double bonds joined by one single C—C bond.
- Only the conjugated portion of a conjugated diene compound is planar.
- Bond lengths within the conjugated portion of the conjugated diene are between a double C=C bond and a single C—C bond.
Isolated Diene
Nonconjugated Diene
A non-planar and nonconjugated compound containing two C=C double bonds separated by at least one sp3-hybridized carbon.
No delocalized π system is present in isolated dienes.
Why is a conjugated diene more stable than an allyl compound?
The conjugated diene exhibits more resonance forms as a result of greater electron delocalization.
Why are conjugated dienes more stable when in the s–trans conformer than in the s–cis conformer?
The s–cis conjugated diene conformer experiences steric repulsions between the inward-pointing hydrogens on the terminal carbons.
Kinetic Control
The ratio of products is determined by the relative rates of the competing reaction pathways (i.e. the stability of the competing transition states).
The product with the most stable carbocation intermediate will be the major product of the reaction.
Reversibility in Conjugated Diene Electrophilic Addition
- Reactions under kinetic control are irreversible.
- Reactions under thermodynamic control are reversible.
Likely Conditions of Kinetic Control
- Low Temperate
- Short Reaction Time
Likely Conditions of Thermodynamic Control
- High Temperature
- Long Reaction Time
Thermodynamic Control
The ratio of products is determined by the stability of the multiple products.
The most stable product will be the major product of the reaction.
Intermediate Stability and Reaction Kinetics
The more stable the transition/intermediate state is, the faster the product will form.
Why is the cyclic halonium ion not formed during the dihalogenation of conjugated dienes?
The allylic cation intermediate (formed by initial monohalogenation) is more stable than the cyclic halonium ion.
No stabilization via electron delocalization is observed following cyclic halonium formation.
Diels-Alder Reaction
A concerted and stereospecific [4+2] cycloaddition reaction in which all six bonds of the six-membered cycle are undergoing changes.
Electron-Donating Groups (EDGs)
Diels-Alder
- —R (Hyperconjugation + Induction)
- —OR (Resonance)
- —OCOR (Resonance + Induction)
- —OH (Resonance)
- —NHCOR (Resonance)
- —NR2 (Resonance)
- —NHR (Resonance)
- —NH2 (Resonance)
Electron-donating groups are para–directing and ortho–directing.
Electron Withdrawing Groups
Diels-Alder
- —X (Induction)
- —COOH (Resonance + Induction)
- —COOR (Resonance + Induction)
- —COR (Resonance + Induction)
- —CF3 (Induction)
- —CN (Resonance)
- —SO3+H (Resonance + Induction)
- —NO2 (Resonance + Induction)
- —NR3+ (Resonance + Induction)
Electron-withdrawing groups (except for halides) are meta–directing during EAS reactions.
What is the sterochemical result of monoaddition to an achiral intermediate?
Racemic Mixture
Why do allylic halide SN2 reactions occur faster than haloakane SN2 reactions?
The allylic carbon (of the allylic halide) is more electrophilic due to being bonded to a more electron-withdrawing sp2-hybridized carbon.
The strong nucleophile of an SN2 reaction will react more readily/rapidly with the more electrophilic allylic carbon.
Octet Structure and Resonance Forms
If a reaction intermediate possesses a resonance form that satisfies the octet rule, then that intermediate will be the most favored/stable.
The reaction intermediate that possesses the most resonance forms satisfying the octet rule will be the most favored/stable intermediate.
Why is a doubly-bonded (sp2) carbocation less stable than a singly-bonded (sp3) carbocation?
The orbitals of the doubly-bonded carbocation have greater s character, so the empty orbital is held closer to the positively charged nucleus.
The doubly-bonded carbon is more electronegative (than the singly-bonded carbocation) due to sp2-hybridization, so a positive charge on this carbon is less favored.
Why does the kinetic product (of a conjugated diene) form with a lower activation barrier than the thermodynamic product?
The reaction pathway to form the kinetic product possesses a more stable intermediate compound (i.e. the carbocation intermediate is more substituted).
(The more stable intermediate of the kinetic pathway possesses is a more positively charged carbocation.)
What causes the increased reactivity of extended π systems?
Electrophilic addition to extended π systems has a low activation barrier due to the highly delocalized carbocation intermediate.
Minimal input of energy is required to achieve the delocalized carbocation intermediate form.
Why does the cyclohexene produce of the Diels-Alder reaction not undergo further reactions with dienes?
The C=C double bond of the cyclohexene is sterically hindered, so additional diene compounds are unlikely to attack.
How does stereochemistry impact the kinetics of the Diels-Alder reaction?
- Cycloaddition is accelerated when the diene is constrained to the s–cis form.
- Cycloaddition is slow/impossible when the s–cis diene form is inhbited/restricted.
Addition of H—X to Diene
Monosubstituted Cycloalkene
The thermodynamic product and the kinetic product of the addition mechanism are identical.
Addition of R—OH to Diene w/ Acid
Monosubstituted Cycloalkene
The thermodynamic product and the kinetic product of the addition mechanism are identical.
Radical Stability Trend: C—H Bonds
Six Types
- CAllylic—H
- 3° Csp3—H
- 2° Csp3—H
- 1° Csp3—H
- Csp2—H
- Csp—H
The stability of the carboradical compound is determined by the bond-dissociation energy of the C—H bond. (The C—H bond with a greater BDE will create the less stable carboradical intermediate.)
BDE Trend: C—H Bonds
Six Types
- Csp—H (BDE = 126)
- Csp2—H (BDE = 111)
- 1° Csp3—H (BDE = 101)
- 2° Csp3—H (BDE = 99)
- 3° Csp3—H (BDE = 97)
- CAllylic—H (BDE = 87)
The strength of the C—H bond-dissociation energy is determined by the stability of the carboradical intermediate compound. (The more stable carboradical intermediate will have a lower-BDE C—H bond.)
Acidity Trend: C—H Bonds
Six Types
- Csp—H
- CAllylic—H
- Csp2—H
- 3° Csp3—H
- 2° Csp3—H
- 1° Csp3—H
Why is a 3° radical/ionic Carbon more stable than a 1° radical/ionic Carbon?
Hyperconjugation
