Chapter 5,6,7,8 Flashcards
(127 cards)
1
Q
What is the degree of unsaturation?
A
The total number of pi bonds and rings in a compound.
2
Q
How do you calculate the degree of unsaturation from a molecular formula?
A
Start with the formula for a saturated hydrocarbon (CnH2n+2) and subtract 2 hydrogens for each pi bond or ring in the molecule.
3
Q
What suffix is used for alkenes in IUPAC nomenclature?
A
-ene
4
Q
What are the key rules for naming alkenes?
A
- The longest continuous chain containing the carbon-carbon double bond is numbered to give the functional group the lowest possible number.
- For multiple double bonds, use “diene,” “triene,” “tetraene,” etc.
- State substituents before the name of the chain, with numbers to designate position.
5
Q
How are stereoisomers of alkenes named?
A
E/Z prefixes to indicate the arrangement of groups around the double bond.
6
Q
What is a vinylic carbon?
A
A carbon that is part of the double bond.
7
Q
What is an allylic carbon?
A
A carbon directly adjacent to a vinylic carbon.
8
Q
What are the vinyl and allyl groups?
A
Vinyl group: -CH=CH2 Allyl group: -CH2CH=CH2
9
Q
What is the hybridization of carbon atoms in a double bond?
A
sp² hybridization
10
Q
How is a double bond formed?
A
A sigma bond forms by overlap of sp² orbitals, and a pi bond forms by side-to-side overlap of unhybridized p orbitals.
11
Q
Why must all six atoms of a double-bond system be in the same plane?
A
For effective overlap of the p orbitals to form the pi bond, all atoms must lie in the same plane.
12
Q
What determines the kinds of reactions a compound undergoes?
A
The functional group of the compound.
13
Q
What is an electrophile?
A
An electron-deficient species attracted to electron-rich species.
Electrophiles have a positive charge, partial positive charge, or incomplete octet.
14
Q
What is a nucleophile?
A
An electron-rich species that has electrons to share and reacts with electrophiles.
15
Q
What is the difference between a Lewis acid and a Lewis base?
A
Lewis acids accept a share in a pair of electrons, while Lewis bases donate a share in a pair of electrons.
16
Q
What’s the difference between a base and a nucleophile?
A
A base reacts with a proton, while a nucleophile reacts with something other than a proton.
17
Q
What is the mechanism of a reaction?
A
The step-by-step description of the process by which reactants are changed into products.
18
Q
What do curved arrows represent in reaction mechanisms?
A
Curved arrows show the movement of two electrons from a nucleophile toward an electrophile.
19
Q
What are the rules for using curved arrows?
A
- Each arrow represents movement of two electrons
- The tail starts at the electron source (bond or lone pair)
- The head points to where electrons end up (atom or bond)
- Arrows flow from nucleophile to electrophile
20
Q
What is a transition state?
A
The point in a reaction where bonds are partially broken and partially formed.
21
Q
How is a transition state represented in diagrams?
A
Using dashed lines to indicate partially formed or broken bonds, and with square brackets with a double dagger [‡].
22
Q
What is the difference between a transition state and an intermediate?
A
Transition states have partially formed bonds and cannot be isolated, while intermediates have fully formed bonds and can potentially be isolated.
23
Q
What does thermodynamics tell us about a reaction?
A
The relative amounts of reactants and products present at equilibrium.
24
Q
What does kinetics tell us about a reaction?
A
How fast the reaction occurs.
25
How are the equilibrium constant (Keq) and Gibbs free-energy change (ΔG°) related?
ΔG° = -RT ln Keq
26
What does a negative ΔG° indicate about a reaction?
The products are more stable than reactants, and the reaction is exergonic (releases energy).
27
What does a positive ΔG° indicate about a reaction?
: The products are less stable than reactants, and the reaction is endergonic (requires energy).
28
How is ΔG° related to enthalpy and entropy?
ΔG° = ΔH° - TΔS°
29
What is enthalpy (ΔH°)?
The heat given off (negative) or consumed (positive) during a reaction.
30
How can ΔH° be calculated?
ΔH° = Heat required to break bonds - Heat released from forming bonds
31
What happens to entropy when a single molecule forms from two separate molecules?
Entropy decreases (ΔS° < 0) because there is less freedom of motion.
32
What is Le Châtelier's principle?
If a system at equilibrium is disturbed, the system will adjust to offset the disturbance.
33
How does decreasing the concentration of a product affect the equilibrium?
The reaction will shift to form more of that product to maintain the equilibrium constant.
34
What is catalytic hydrogenation?
The addition of H2 to a double bond of an alkene to form an alkane, requiring a metal catalyst (often Pd/C) to weaken H2
35
What is the heat of hydrogenation?
The absolute value of ΔH° for the hydrogenation reaction, indicating the relative stability of an alkene(smaller --> stable)
36
How does alkyl substitution affect alkene stability
Stability increases with the number of alkyl groups attached to the sp² carbons: R2C=CR2 > R2C=CHR > RHC=CHR > RHC=CH2 > H2C=CH2
37
Why are trans isomers generally more stable than cis isomers?
Cis isomers have steric strain due to electron cloud overlap when large groups are on the same side of the double bond.
38
What is the free energy of activation (ΔG‡)?
The energy barrier that must be overcome for reactants to be converted into products.
39
How is ΔG‡ calculated?
ΔG‡ = Free energy of the transition state - Free energy of the reactants
40
What is the difference between thermodynamic and kinetic stability?
Thermodynamic stability relates to ΔG° (negative = stable product) Kinetic stability relates to ΔG‡ (large = stable reactant because it reacts slowly)
41
What are the three main factors affecting reaction rate?
1. Number of collisions between reacting molecules 2. Energy of collisions (sufficient to overcome energy barrier) 3. Orientation of collisions (proper alignment for reaction)
42
How does temperature affect reaction rate?
Increasing temperature increases the kinetic energy of molecules, leading to more frequent and energetic collisions.
43
What is the rate constant (k)?
A proportionality constant that reflects how easily the transition state barrier is reached.
44
What is the Arrhenius equation?
k = Ae^(-Ea/RT) Where k = rate constant, A = frequency factor, Ea = activation energy, R = gas constant, T = temperature in Kelvins
45
What is the rule of thumb for temperature effects on reaction rate?
A 10°C increase in temperature roughly doubles the reaction rate.
46
What is the rate-determining step?
The slowest step in a reaction, which dictates the overall reaction rate.
47
What is a catalyst?
A substance that increases reaction rate by providing an alternative pathway with a lower activation energy, without being consumed in the reaction.
48
How do catalysts affect the equilibrium constant?
Catalysts do not change the equilibrium constant or the amount of product formed, only the rate at which it is formed.
49
What are common types of catalysts?
Acids, bases, and nucleophiles.
50
What are enzymes?
Biological catalysts (proteins) that speed up reactions in living systems.
51
What is molecular recognition in enzyme function?
The ability of an enzyme to selectively bind to specific substrates through intermolecular interactions like hydrogen bonds, London dispersion forces, and dipole-dipole interactions.
52
What catalytic mechanisms does lysozyme use to cleave bacterial cell walls?
Lysozyme uses both acid and nucleophilic catalysts at its active site.
53
What is the general mechanism for electrophilic addition reactions of alkenes?
Slow step: Electrophile (E+) attacks the π bond, forming a carbocation
Fast step: Nucleophile (Nu-) attacks the carbocation
Overall transformation: C=C + E-Nu → E-C-C-Nu
54
Arrange the following carbocations in order of increasing stability.
Tertiary carbocation
Primary carbocation
Secondary carbocation
Methyl carbocation
Increasing stability: CH₃⁺ < RCH₂⁺ < R₂CH⁺ < R₃C⁺ (Methyl < primary < secondary < tertiary)
The stability increases with more alkyl substituents due to hyperconjugation.
55
What is hyperconjugation and how does it stabilize carbocations?
Hyperconjugation is the** delocalization of electrons** from an **adjacent** σ bond orbital** into the **empty p orbital** of the carbocation. This distributes the positive charge over more atoms, stabilizing the carbocation.
56
State Markovnikov's Rule for the addition of HX to alkenes.
In the addition of HX to an alkene, the **hydrogen adds to the carbon with more hydrogens**, and the** halide adds to the carbon with fewer hydrogens.**
This is because the** electrophile (H+) adds to form the more stable carbocation.**
57
What is the mechanism for acid-catalyzed hydration of alkenes?
Protonation of alkene to form carbocation
Nucleophilic attack by water
Deprotonation to form alcohol
Overall: CH₂=CH₂ + H₂O → CH₃CH₂OH (with H₂SO₄ catalyst)
58
What product is formed from acid-catalyzed addition of alcohols to alkenes?
Ethers are formed through acid-catalyzed addition of alcohols to alkenes.
Mechanism:
Protonation of alkene
Nucleophilic attack by alcohol
Deprotonation to yield ether
59
When do carbocation rearrangements occur during alkene reactions?
Carbocation rearrangements occur when they lead to formation of a more stable carbocation. They happen through:
1,2-hydride shifts: H atom moves with its electrons
1,2-methyl shifts: CH₃ group moves with its electrons
60
What is unique about the regioselectivity of hydroboration-oxidation compared to other alkene additions?
Hydroboration-oxidation shows anti-Markovnikov regioselectivity:
Boron adds to the less substituted carbon
OH group ends up at the less substituted carbon after oxidation
Overall: syn addition of water to alkene with anti-Markovnikov orientation
61
What is the mechanism and stereochemistry of halogen (X₂) addition to alkenes?
Formation of cyclic halonium ion intermediate
Nucleophilic attack by halide ion
Stereochemistry: Anti addition (halogens add to opposite sides of the double bond)
Product: Vicinal dihalide
62
How do peroxyacids react with alkenes?
Peroxyacids react with alkenes to form epoxides in a concerted mechanism:
Oxygen transfer from peroxyacid to alkene
Stereochemistry: Syn addition
Product: Three-membered cyclic ether (epoxide)
63
What happens during ozonolysis of alkenes?
Ozonolysis cleaves the C=C bond:
Reductive workup (Zn/HOAc or Me₂S) gives carbonyl compounds
Products: Aldehydes and/or ketones where the C=C bond was located
64
What is regioselectivity in alkene reactions?
Regioselectivity refers to the preferential formation of one constitutional isomer over another in a reaction where multiple isomers are possible.
65
What's the difference between a stereoselective reaction and a stereospecific reaction?
Stereoselective reaction: Forms one stereoisomer preferentially over another
Stereospecific reaction: Different stereoisomers of the reactant form different stereoisomers of the product
A stereospecific reaction is also stereoselective, but a stereoselective reaction is not necessarily stereospecific.
66
Differentiate between syn and anti addition to alkenes.
**Syn addition:** Both new groups add to the **same face/side** of the double bond
Examples: **Hydrogenation (H₂/Pd), hydroboration, epoxidation**
**Anti addition: **New groups add to **opposite faces/sides of the double bond
**
Examples: **Halogenation (X₂)**, **halohydrin formation**
67
How does the stereochemistry of the starting alkene (cis vs. trans) affect the stereochemistry of the products in addition reactions?
For** syn addition:**
**Cis** alkene → **erythro** product (acyclic) or **cis **product (cyclic)
**Trans** alkene → threo product (acyclic) or **trans product (cyclic)**
For** anti addition**:
**Cis** alkene → threo product (acyclic) or trans product (cyclic)
**Trans** alkene → erythro product (acyclic) or** cis product (cyclic)**
68
What is unique about the stereoselectivity of enzyme-catalyzed alkene reactions?
Enzyme-catalyzed reactions are completely stereoselective, forming only one stereoisomer of the product.
Example: Fumarase catalyzes the addition of water to fumarate (trans-butenedioate) to form only (S)-malate, not the (R)-enantiomer.
Enzymes can distinguish between stereoisomers because they are chiral proteins with specific three-dimensional binding sites.
69
What is the stereochemical outcome of catalytic hydrogenation of alkenes?
Catalytic hydrogenation (H₂/Pd) is a syn addition:
H₂ adds to the same face of the double bond
The stereochemistry depends on the starting alkene:
Hydrogenation of cis-alkene → erythro product (acyclic) or cis product (cyclic)
Hydrogenation of trans-alkene → threo product (acyclic) or trans product (cyclic)
70
How does the formation of a carbocation intermediate affect the stereochemistry of alkene addition reactions?
With carbocation intermediate (e.g., HX addition):
Both syn and anti addition occur
Four stereoisomers are formed
No stereospecificity
Without carbocation intermediate (e.g., X₂, H₂, or peroxyacid addition):
Either exclusively syn or anti addition
Stereospecific reaction
The cis and trans alkenes give different stereoisomeric products
71
How do you name alkynes?
Replace "-ane" ending with "-yne"
Terminal alkynes: Triple bond at end of chain (HC≡CH: ethyne/acetylene)
Internal alkynes: Triple bond in middle (CH₃C≡CCH₃: 2-butyne)
Number chain to give triple bond lowest possible number
Multiple functional groups? List alphabetically
72
: What is the structure of an alkyne?
Triple bond = 1 sigma bond + 2 pi bonds
Carbon atoms are sp-hybridized
Linear geometry (180° bond angles)
Pi bonds form cylinder of electrons around sigma bond
Internal alkynes are more stable than terminal ones
73
Why are terminal alkynes acidic?
Terminal alkynes (RC≡CH) have pKa ≈ 25
More acidic than: alkenes (pKa ≈ 44) and alkanes (pKa > 60)
Why? sp-hybridized carbon is more electronegative
Strong bases like NaNH₂ can remove the terminal H
Creates acetylide ions (RC≡C⁻) which are good nucleophiles
74
: What happens when HX adds to alkynes?
First addition: Makes vinyl halides
Follows Markovnikov's rule (H⁺ to less substituted carbon)
Second addition (excess HX): Makes **geminal dihalides**
**Terminal alkynes:** H⁺ adds to terminal carbon
**Internal alkynes:** **H⁺ adds to give more stable carbocation**
75
How do alkynes react with water?
**Terminal alkynes:** Form methyl ketones (RC≡CH → RCOCH₃)
**Internal alkynes:** Form ketones
**Symmetrical:** Single ketone product
**Unsymmetrical:** Mixture of two ketones
**Mechanism:**
Add H⁺ to triple bond
Water attacks
Forms **enol intermediate**
Enol **converts to ketone** (**more stable**)
76
What products form from hydroboration-oxidation of alkynes?
**Terminal alkynes:** Form** aldehydes (anti-Markovnikov)**
BH3/R2BH for internal alkyne due to less steric hindrance
R2BH for terminal alkyne
77
How can alkynes be reduced to alkenes or alkanes?
**Complete reduction: **H₂, Pd/C → alkane
To **cis-alkene**: H₂, **Lindlar catalyst**
**Lindlar **= Pd/BaSO₄ + quinoline (poisoned)
Adds **H₂ from same side (syn addition)**
To **trans-alkene**: Na/NH₃ (liquid)
Adds **H₂ from opposite sides (anti addition)**
Only works for** internal alkynes**
**Dissolving metal reduction**
78
How can we make internal alkynes from terminal ones?
Step 1: Remove terminal H with strong base (NaNH₂)
Step 2: Add alkyl halide (R-X)
Result: RC≡CH → RC≡CR'
Works best with: Primary or methyl halides
Can repeat to add different R groups
Allows carbon chain extension
79
How do terminal and internal alkynes differ in their reactions?
Terminal alkynes (RC≡CH):
Hydration → methyl ketones
Hydroboration → aldehydes
Can form acetylide ions
Acidic hydrogen
Internal alkynes (RC≡CR'):
Hydration → ketones
Can be reduced to trans alkenes
No acidic hydrogen
More stable than terminal alkynes
80
81
What are delocalized electrons?
Electrons shared among multiple atoms, not just two.
82
How do delocalized electrons influence molecular properties?
They significantly influence stability, pKa values, and reaction products.
83
What is a prime example of delocalization in bonding?
Benzene (C6H6) is an example of delocalization.
84
What unusual property does Benzene exhibit?
Benzene forms only one monosubstituted product and three disubstituted products.
85
What was Kekulé's proposal regarding Benzene's structure?
Alternating single/double bonds in a six-membered ring.
86
What did electron diffraction confirm about Benzene?
It confirmed a planar structure with all C-C bonds at 1.39 Å, showing delocalization.
87
What hybridization do the carbons in Benzene undergo?
Each carbon is sp2 hybridized.
88
What forms the doughnut-shaped clouds of electrons in Benzene?
The remaining p orbitals on each carbon overlap side-to-side.
89
Define resonance hybrid.
The actual structure of a molecule with delocalized electrons.
90
What are resonance contributors?
Approximate structures with localized electrons used to represent delocalized electrons.
91
What is the analogy used to explain resonance contributors?
Rhinoceros is the hybrid, unicorn/dragon are contributors.
92
What moves when drawing resonance contributors?
Only electrons move; atoms never move.
93
What type of electrons can move in resonance structures?
Only pi (π) electrons and lone-pair electrons can move.
94
What does stability of resonance contributors depend on?
Factors include incomplete octet, charge placement, and charge separation.
95
What is delocalization energy?
The extra stability a compound gains from having delocalized electrons.
96
Is a resonance hybrid more stable than any single resonance contributor?
Yes, a resonance hybrid is always more stable.
97
What influences the magnitude of delocalization energy?
The number of stable resonance contributors and their equivalence.
98
What defines aromatic compounds?
Cyclic p cloud, planar structure, and 4n + 2 p electrons.
99
What is Hückel's Rule?
The p cloud must contain 4n + 2 p electrons.
100
Provide an example of an aromatic compound.
Benzene (6 pi e-).
101
What characterizes antiaromatic compounds?
They contain 4n p electrons and meet cyclic p cloud criteria.
102
Provide an example of an antiaromatic compound.
Cyclobutadiene (4 pi e-).
103
What defines non-aromatic compounds?
They fail cyclic p cloud criteria.
104
How does aromaticity relate to acidity?
If the conjugate base is aromatic, it is more stabilized, making the parent compound more acidic.
105
What stabilizes carboxylic acids compared to alcohols?
The carboxylate ion's negative charge is delocalized over two oxygens.
106
What is the stability order for allylic and benzylic cations?
Tertiary > Secondary > Primary.
107
What does Molecular Orbital (MO) Theory explain?
It explains electron delocalization and stability through atomic orbital combinations.
108
What is the result of in-phase p orbital overlap?
It leads to bonding molecular orbitals.
109
What happens when out-of-phase p orbitals overlap?
It leads to antibonding molecular orbitals.
110
How many MOs are formed by 1,3-butadiene's four p electrons?
Four MOs (two bonding, two antibonding).
111
What contributes to the stability of 1,3-butadiene?
Its electrons occupy the lower-energy bonding MOs.
112
How do isolated dienes react?
React like typical alkenes; electrophile adds to the sp2 carbon bonded to the most hydrogens.
113
What are the two types of products formed from conjugated dienes during electrophilic addition?
* 1,2-addition product: Direct addition across one double bond.
* 1,4-addition product: Conjugate addition, electrophile adds to C1 and nucleophile to C4.
114
What is the mechanism for the addition of electrophiles to conjugated dienes?
Involves formation of a resonance-stabilized allylic cation intermediate.
115
What are the conditions that favor the kinetic product?
Favored under mild conditions (low temperature).
116
What characterizes the thermodynamic product?
The more stable product (lower overall energy), favored under vigorous conditions (high temperature).
117
In the reaction of HBr with 1,3-butadiene, which product is often the kinetic product?
1,2-addition product.
118
In the reaction of HBr with 1,3-butadiene, which product is often the thermodynamic product?
1,4-addition product.
119
How are kinetic and thermodynamic products formed for the diene reactions
Lower temperature traps the reaction at the first stable product formed (kinetic), while higher temperature allows products to interconvert and reach the most stable product (thermodynamic).
120
What is a Diels-Alder reaction?
A concerted reaction between a conjugated diene and a dienophile to form a cyclic compound.
121
What type of reaction is the Diels-Alder reaction classified as?
A pericyclic reaction.
122
What is the role of electron-withdrawing groups on the dienophile in a Diels-Alder reaction?
They increase the reactivity of the dienophile, making it a better electrophile.
123
How does molecular orbital theory explain the Diels-Alder reaction?
Through the overlap of the diene's HOMO with the dienophile's LUMO.
124
What is syn addition in the context of Diels-Alder reactions?
Substituents on the dienophile maintain their relative stereochemistry in the product.
125
What distinguishes endo products from exo products in Diels-Alder reactions?
Endo: substituent points under the diene; Exo: substituent points away.
126
What conformation must the conjugated diene be in to react in a Diels-Alder reaction?
The s-cis conformation.
127
What is retrosynthetic analysis in the context of Diels-Alder reactions?
Identifying reactants from a Diels-Alder product by breaking sigma bonds and recreating the diene and dienophile.
