Brainscape's Knowledge GenomeTM


Top Flashcards (Certified)
All Flashcards

Orgo II > Ch 14 Conjugated Compounds and Ultraviolet Spectroscopy > Flashcards

Ch 14 Conjugated Compounds and Ultraviolet Spectroscopy Flashcards

1
Q

What are conjugated compounds?

A

Molecules where multiple bonds alternate with single bonds, creating a system of conjugated π-electrons

Examples include alkenes, alkynes, and aromatic compounds.

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

Define conjugation in the context of conjugated compounds.

A

Interaction of electron density from π-bonds over the entire conjugated system

Common examples are dienes and aromatic compounds.

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

How does conjugation affect the stability of compounds?

A

Increases stability due to resonance structures

Significant in aromatic systems where delocalization leads to aromatic stability.

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

What are the spectroscopic properties of conjugated compounds?

A

Absorption of light in the UV-visible range due to extended π-systems

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

What are electronic transitions in conjugated systems?

A

π-electrons transition to higher energy levels when exposed to light

Involves excitation from bonding to antibonding molecular orbitals.

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

What are the two main types of conjugated compounds?

A
  • Dienes
  • Aromatic Compounds

Examples include 1,3-butadiene for dienes and benzene for aromatic compounds.

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

What is the principle of UV spectroscopy?

A

Molecules absorb specific wavelengths of UV light based on their electronic structure

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

What does the Beer-Lambert law relate to in UV spectroscopy?

A

Absorbance (A) to concentration (c) and path length (l) of the sample

Formula: A = ε · c · l.

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

What are π → π* transitions?

A

Electrons in bonding π-orbitals are excited to antibonding π*-orbitals

Most common in conjugated systems.

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

What are n → π* transitions?

A

Electrons in non-bonding orbitals are excited to antibonding π* orbitals

Often seen in molecules with heteroatoms.

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

What does λ_max represent in UV spectroscopy?

A

The wavelength at which a molecule absorbs UV light

Dependent on the extent of conjugation.

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

What is a Bathochromic shift?

A

A shift to longer wavelengths of UV absorption as conjugation increases

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

List some applications of UV spectroscopy.

A
  • Studying structure of conjugated compounds
  • Determining degree of conjugation
  • Identifying functional groups
  • Monitoring reaction progress
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What factors affect UV absorption?

A
  • Conjugation
  • Substituents

Electron-donating groups shift absorption to shorter wavelengths, while electron-withdrawing groups shift to longer wavelengths.

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

True or False: Conjugation decreases the stability of organic molecules.

A

False

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

Fill in the blank: The extended π-system in conjugated compounds allows for the absorption of light in the _______ range.

A

UV-visible

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

What is the significance of understanding the relationship between conjugation and UV absorption properties?

A

Essential for interpreting UV spectra and gaining insights into molecular structure and behavior

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

What is conjugation in organic chemistry?

A

Conjugation refers to the overlap of p-orbitals across adjacent bonds, allowing for electron delocalization and stabilization of the molecule.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

How do conjugated alkenes differ from isolated double bonds?

A

In conjugated alkenes, double bonds are alternated with single bonds, allowing for delocalization of π-electrons across the conjugated system.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

What is an example of a conjugated compound?

A

1,3-butadiene, where the electrons of two double bonds are in conjugation.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

What characterizes 1,3-dienes?

A

1,3-dienes have two double bonds separated by a single bond, creating conjugation.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

What is the significance of electron delocalization in conjugated compounds?

A

Electron delocalization stabilizes the molecule, allowing π-electrons to spread over multiple atoms rather than being confined to one bond.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

What is the role of conjugation in aromatic compounds?

A

In aromatic compounds like benzene, π-electrons are delocalized over the entire ring, contributing to stability and unique reactivity.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

What is the effect of longer conjugation in polyenes?

A

Longer conjugation generally leads to greater stabilization due to more extensive delocalization of electrons.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
What is the relationship between conjugation and light absorption?
Conjugation lowers the energy gap between the ground state and excited state, allowing compounds to absorb light in the UV or visible spectrum.
26
What are the common electronic transitions measured in UV spectroscopy?
* π → π* transitions * n → π* transitions
27
What does the Beer-Lambert Law describe?
The absorption of light is proportional to the concentration of the compound and the path length through the sample.
28
What is λ_max in UV spectroscopy?
λ_max is the wavelength at which a molecule absorbs the most UV light.
29
What is a bathochromic shift?
A shift to longer wavelengths (red shift) in the absorption spectrum due to increased conjugation.
30
What is a hypsochromic shift?
A shift to shorter wavelengths (blue shift) when electron-withdrawing groups are attached.
31
Define chromophore.
A chromophore is a part of the molecule responsible for absorbing light, typically involving conjugated π-systems.
32
Define auxochrome.
An auxochrome is a substituent that influences the absorption spectrum but does not absorb light itself.
33
How do aromatic compounds behave in UV spectroscopy?
Aromatic compounds like benzene typically absorb UV light around 260 nm due to π → π* transitions.
34
What is the UV absorption feature of conjugated carbonyl compounds?
Conjugated carbonyl compounds exhibit strong absorption in the UV range due to lower energy gaps from conjugation.
35
What is the application of UV spectroscopy in structural elucidation?
UV spectra help determine the conjugation in a molecule by examining λ_max and the type of transitions.
36
How is UV spectroscopy used in monitoring chemical reactions?
It tracks changes in conjugated systems during reactions, such as polymerization or reactions altering conjugation.
37
What is the importance of UV spectroscopy in quantitative analysis?
It correlates absorbance with concentration for compounds containing conjugated systems.
38
Fill in the blank: The principle of UV spectroscopy involves measuring the absorption of UV light, which promotes electrons from a lower energy orbital to a _______.
[higher energy orbital]
39
What results when two orbitals of the same phase overlap?
A bonding molecular orbital (y molec) ## Footnote This type of overlap leads to increased electron density between the nuclei, stabilizing the bond.
40
What results when two orbitals of opposite phase overlap?
An antibonding molecular orbital (y* molec) ## Footnote This overlap decreases electron density between the nuclei, leading to destabilization.
41
Are conjugated alkadienes thermodynamically more stable than isomeric isolated alkadienes?
Yes ## Footnote Conjugation allows for electron delocalization, which contributes to greater stability.
42
What does Beer’s law relate to?
Absorbance, molar absorptivity, concentration, and path length ## Footnote Beer’s law is expressed as A = e x c x ℓ.
43
What is the absorption maximum (lmax) of 2,5-Dimethyl-2,4-hexadiene in methanol?
242.5 nm (e = 13,100) ## Footnote This value indicates the wavelength at which the compound absorbs light.
44
What is the relationship between the number of conjugated multiple bonds and wavelength absorption?
The greater the number of conjugated multiple bonds, the longer the wavelength at which the compound absorbs light ## Footnote This is due to the stabilization of the excited state.
45
What compound has an absorption maximum at 497 nm?
β-Carotene ## Footnote β-Carotene has 11 conjugated double bonds and absorbs blue-green light.
46
What color do we observe when β-Carotene absorbs light at 497 nm?
Red-orange ## Footnote This is the complementary color to the absorbed blue-green light.
47
What region of the spectrum do highly conjugated compounds absorb light in?
Visible region (400-700 nm) ## Footnote This absorption leads to the observed colors of such compounds.
48
What is the relationship between temperature and the ratio of 1,2 vs. 1,4 addition?
Highly dependent on temperature ## Footnote Different temperatures can favor different pathways in reactions.
49
Which conformation of conjugated dienes is lower in energy?
s-trans conformation ## Footnote This conformation has less steric hindrance.
50
What must be true for a reaction to be symmetry allowed?
Conservation of orbital symmetry ## Footnote The phases of the reacting molecular orbitals must be symmetrical.
51
What does the diene contribute in a Diels-Alder reaction?
Electrons from its highest energy occupied orbital (HOMO) ## Footnote This is crucial for the formation of the cycloadduct.
52
What does the dienophile receive in a Diels-Alder reaction?
Electrons in its lowest energy unoccupied orbital (LUMO) ## Footnote This allows for the reaction to proceed effectively.
53
Is the [2 + 2] cycloaddition of two ethylenes thermally allowed?
No, it is thermally forbidden ## Footnote This is due to the antibonding overlap of HOMO and LUMO.
54
What type of cycloaddition is photochemically allowed but thermally forbidden?
[2 + 2] cycloaddition ## Footnote This reaction can occur under light but not under thermal conditions.
55
What are the two classes of dienes?
Conjugated diene, Cumulated diene, Isolated diene
56
How do conjugated dienes react with electrophiles?
They add the same electrophiles as simple alkenes do.
57
What is the unusual behavior of 1,3-butadiene attributed to?
The stability and delocalized nature of the intermediate allylic carbocation.
58
What are the two different reaction products formed from conjugated dienes?
1,2-addition product (simple addition), 1,4-addition product (conjugate addition)
59
What determines the relative proportion of 1,2-addition and 1,4-addition products?
The temperature at which the reaction is carried out.
60
What happens to the product mixture when brought from lower to higher temperature?
The relative amounts of the two products change to reflect the proportions at higher temperature.
61
What is the stability relationship of the 1,4-addition product compared to the 1,2-addition product?
The 1,4-addition product is more stable.
62
What is the mechanism for inter-conversion at equilibrium at elevated temperatures?
The mechanism allows for reversible reactions between the 1,2 and 1,4 products.
63
At lower temperatures, what determines the relative amounts of the products?
The relative rates at which the two additions occur.
64
Which product predominates at lower temperatures and why?
1,2-addition product predominates because it occurs faster.
65
What is the term for reactions where product formation is irreversible due to insufficient energy?
Kinetic control.
66
At higher temperatures, what is the nature of the reactions?
Both reactions are reversible.
67
Which product is favored at higher temperatures and why?
1,4-addition product is favored because it is more stable.
68
What general trend is observed regarding the stability of alkenes?
The greater the number of attached alkyl groups, the greater the stability.
69
What do the numbers in 1,2-addition and 1,4-addition refer to?
Carbons within the C=C–C=C structural unit.
70
What is a regioselective reaction?
A reaction that produces predominantly one constitutional isomer.
71
What is the modern statement of Markovnikov's Rule?
The positive portion of the adding reagent attaches to the carbon atom of the double bond to yield the more stable carbocation.
72
What is the original statement of Markovnikov's Rule?
The hydrogen atom adds to the carbon atom of the double bond that has the greater number of hydrogen atoms.

Orgo II (9 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2025 Bold Learning Solutions. Terms and Conditions