Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Organic Chemistry 2 > Delocalized π Systems (Chapter 14) > Flashcards

Delocalized π Systems (Chapter 14) Flashcards

You may prefer our related Brainscape-certified flashcards:
Chemistry 101 flashcards AP® Chemistry flashcards Periodic Table flashcards Organic Chemistry 101 flashcards
1
Q

Conjugation

A

The π–overlap of three or more p orbitals on adjacent atoms in a molecule.

E.g. Allylic Compounds, Conjugated Dienes

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Stability of Dienes

A

Conjugated > Isolated > Cumulated

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

If an intermediate allylic cation/radical is achiral, then the product following addition is…?

A

Racemic

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

If an intermediate form is more unstable, then the activation energy required to reach that form is…?

A

Greater

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Why is an allyl carbocation more stable than a typical carbocation?

A

The allyl cation is stablized via resonance.

This same resonance-stabilizing effect is seen in allyl radicals and allyl anions.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

Allyl Compound

A

A planar and conjugated system consisting of a primary carbon bonded to a double-bonded carbon.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Why are cumulated dienes non-planar?

A

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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

Cumulated Diene

Allene

A

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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

Allyl C–C Bond Lengths

A
  • 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.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

Where do the reactions of allyl compounds occur?

A

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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

Why does radical allylic halogenation occur faster than radical alkane halogenation?

A

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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

Conjugated Diene

A

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.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Isolated Diene

Nonconjugated Diene

A

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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Why is a conjugated diene more stable than an allyl compound?

A

The conjugated diene exhibits more resonance forms as a result of greater electron delocalization.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

Why are conjugated dienes more stable when in the s–trans conformer than in the s–cis conformer?

A

The s–cis conjugated diene conformer experiences steric repulsions between the inward-pointing hydrogens on the terminal carbons.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Kinetic Control

A

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.

17
Q

Reversibility in Conjugated Diene Electrophilic Addition

A
  • Reactions under kinetic control are irreversible.
  • Reactions under thermodynamic control are reversible.
18
Q

Likely Conditions of Kinetic Control

A
  • Low Temperate
  • Short Reaction Time
19
Q

Likely Conditions of Thermodynamic Control

A
  • High Temperature
  • Long Reaction Time
20
Q

Thermodynamic Control

A

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.

21
Q

Intermediate Stability and Reaction Kinetics

A

The more stable the transition/intermediate state is, the faster the product will form.

22
Q

Why is the cyclic halonium ion not formed during the dihalogenation of conjugated dienes?

A

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.

23
Q

Diels-Alder Reaction

A

A concerted and stereospecific [4+2] cycloaddition reaction in which all six bonds of the six-membered cycle are undergoing changes.

24
Q

Electron-Donating Groups (EDGs)

Diels-Alder

A
  • —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.

25
Q

Electron Withdrawing Groups

Diels-Alder

A
  • —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.

26
Q

What is the sterochemical result of monoaddition to an achiral intermediate?

A

Racemic Mixture

27
Q

Why do allylic halide SN2 reactions occur faster than haloakane SN2 reactions?

A

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.

28
Q

Octet Structure and Resonance Forms

A

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.

29
Q

Why is a doubly-bonded (sp2) carbocation less stable than a singly-bonded (sp3) carbocation?

A

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.

30
Q

Why does the kinetic product (of a conjugated diene) form with a lower activation barrier than the thermodynamic product?

A

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.)

31
Q

What causes the increased reactivity of extended π systems?

A

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.

32
Q

Why does the cyclohexene produce of the Diels-Alder reaction not undergo further reactions with dienes?

A

The C=C double bond of the cyclohexene is sterically hindered, so additional diene compounds are unlikely to attack.

33
Q

How does stereochemistry impact the kinetics of the Diels-Alder reaction?

A
  • 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.
34
Q

Addition of H—X to Diene

A

Monosubstituted Cycloalkene

The thermodynamic product and the kinetic product of the addition mechanism are identical.

35
Q

Addition of R—OH to Diene w/ Acid

A

Monosubstituted Cycloalkene

The thermodynamic product and the kinetic product of the addition mechanism are identical.

36
Q

Radical Stability Trend: C—H Bonds

Six Types

A
  1. CAllylic—H
  2. 3° Csp3—H
  3. 2° Csp3—H
  4. 1° Csp3—H
  5. Csp2—H
  6. 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.)

37
Q

BDE Trend: C—H Bonds

Six Types

A
  1. Csp—H (BDE = 126)
  2. Csp2—H (BDE = 111)
  3. 1° Csp3—H (BDE = 101)
  4. 2° Csp3—H (BDE = 99)
  5. 3° Csp3—H (BDE = 97)
  6. 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.)

38
Q

Acidity Trend: C—H Bonds

Six Types

A
  1. Csp—H
  2. CAllylic—H
  3. Csp2—H
  4. 3° Csp3—H
  5. 2° Csp3—H
  6. 1° Csp3—H
39
Q

Why is a 3° radical/ionic Carbon more stable than a 1° radical/ionic Carbon?

A

Hyperconjugation

Organic Chemistry 2 (12 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions