alkenes_Mechanisim1
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Reaction Mechanisms
Mechanism: a detailed, step-by-step description of how a reaction occurs.
A reaction may consist of many sequential steps. Each step involves a
Reaction coordinate.
Example
CH3 - H + Br
CH3 + H - Br DH = 109 kJ
Almost formed radical.
•Endothermic
•Transition state resembles the (higher
energy) proபைடு நூலகம்ucts.
The carbocation intermediate is very reactive. It does not obey the octet rule
(electron deficient) and is usually present only in low concentration.
Rate Determining Step. The rate at which the carbocation is formed controls the rate of the overall reaction. The energy of activation for this process is critical.
:Br-
Br
Regioselectivity (Orientation)
H - Br
Or
H
Secondary + Br carbocation
H
+ Br
Primary carbocation
Br
H
2-Bromo-propane
Secondary carbocation more more stable and more easily formed.
Flat, planar. Can react on either side of the plane.
Very reactive and present only in very low concentration.
Step 2 of the Mechanism
:Br-
Br
Mirror objects
Only a small
[H3C H Br]
amount of radical character remains.
Almost broken.
Almost formed.
Electrophilic Additions
– Hydrohalogenation using HCl, HBr, HI – Hydration using H2O in the presence of H2SO4 – Halogenation using Cl2, Br2 – Halohydrination using HOCl, HOBr – Oxymercuration using Hg(OAc)2, H2O
Mechanism
Step 1
Step 2
Now examine Step 1 Closely
Electron rich, pi system.
Showed this reaction earlier as an acid/base reaction. Alkene was the base.
New term: the alkene is a nucleophile, wanting to react with a positive species.
A + B Intermediate Intermediate C + D
Step 1 is the “slow step”, the rate determining step.
Step 1: endergonic, high energy of activation. Slow process
transformation of the structure.
For the step
C + A-B C-A + B
Transition State Three areas to be aware of.
Reactants
Products
Energy of Activation. Energy barrier.
2. The concentration of an intermediate is usually quite low. The Energies of Activation for reaction of the Intermediate are low.
3. There is a transition state for each step. A transition state is not a stable structure.
• Electron releasing, alkyl groups, -CH3, stabilize the carbocation making it easier to form.
• Electron withdrawing groups, such as -CF3, destabilize
the carbocation making it harder to form.
4. The reaction coordinate can be traversed in either direction: A+B C+D or C+D A+B.
Hammond Postulate
The transition state for a step is close to the high energy end of the curve. For an endothermic step the transition state resembles the product of the step more than the reactants. For an exothermic step the transition state resembles the reactants more than the products.
Alkene Reactions
Reactivity above and below the molecular plane!
Pi bonds
Plane of molecule
Addition Reactions
A-B
A B
Important characteristics of addition reactions
Increasing Ease of Formation
Factors Affecting Carbocation Stability Inductive
1. Inductive Effect. Electron redistribution due to differences in electronegativities of substituents.
Syn addition: Both A and B come in from the same side of the alkene. Both from the top or both from the bottom.
Anti Addition: A and B come in from opposite sides (anti addition).
An increase in H during a reaction favors reactants. A decrease favors products.
An increase in entropy (eg., more molecules being formed) during a reaction favors products. A decrease favors reactants.
Step 2: exergonic, small energy of activation. Fast Process.
Characteristics of two step
Reaction
1. The Intermediate has some stability. It resides in a valley.
Carbocations
Electron deficient. Does not obey octet rule. Lewis acid, can receive electrons. Electrophile.
sp2 hybridized.
p orbital is empty and can receive electrons.
Energy Changes in a Reaction
• Enthalpy changes, DH0, for a reaction arises from changes in bonding in the molecule.
– If weaker bonds are broken and stronger ones formed then DH0 is negative and exothermic.
followed by reduction
Electrophilic Addition
We now address regioselectivity….
Regioselectivity (Orientation)
The incoming hydrogen attaches to the carbon with the greater number of hydrogens. This is regioselectivity. It is called Markovnikov orientation.
DG0: if positive equilibrium favors reactants (endergonic), if negative favors products (exergonic). DG0 = DH0 – TDS0
Step 1: Step 2:
Multi-Step Reactions
Orientation (Regioselectivity)
If the doubly bonded carbons are not equivalent which one get the A and which gets the B.
Stereochemistry: geometry of the addition.
Br H
1-Bromo-propane
Carbocation Stabilities
Order of increasing stability: Methyl < Primary < Secondary < Tertiary Order of increasing ease of formation: Methyl < Primary < Secondary < Tertiary
– If stronger bonds are broken and weaker ones formed then DH0 is positive and endothermic.
Gibbs Free Energy
Gibbs Free Energy controls the position of equilibrium for a reaction. It takes into account enthalpy, H, and entropy, S, changes.
Acidic molecule, easily ionized.
We had portrayed the HBr earlier as a BronstedLowry acid.
New term: the HBr is an electrophile, wanting to react with an electron rich molecule (nucleophile).
Reaction Mechanisms
Mechanism: a detailed, step-by-step description of how a reaction occurs.
A reaction may consist of many sequential steps. Each step involves a
Reaction coordinate.
Example
CH3 - H + Br
CH3 + H - Br DH = 109 kJ
Almost formed radical.
•Endothermic
•Transition state resembles the (higher
energy) proபைடு நூலகம்ucts.
The carbocation intermediate is very reactive. It does not obey the octet rule
(electron deficient) and is usually present only in low concentration.
Rate Determining Step. The rate at which the carbocation is formed controls the rate of the overall reaction. The energy of activation for this process is critical.
:Br-
Br
Regioselectivity (Orientation)
H - Br
Or
H
Secondary + Br carbocation
H
+ Br
Primary carbocation
Br
H
2-Bromo-propane
Secondary carbocation more more stable and more easily formed.
Flat, planar. Can react on either side of the plane.
Very reactive and present only in very low concentration.
Step 2 of the Mechanism
:Br-
Br
Mirror objects
Only a small
[H3C H Br]
amount of radical character remains.
Almost broken.
Almost formed.
Electrophilic Additions
– Hydrohalogenation using HCl, HBr, HI – Hydration using H2O in the presence of H2SO4 – Halogenation using Cl2, Br2 – Halohydrination using HOCl, HOBr – Oxymercuration using Hg(OAc)2, H2O
Mechanism
Step 1
Step 2
Now examine Step 1 Closely
Electron rich, pi system.
Showed this reaction earlier as an acid/base reaction. Alkene was the base.
New term: the alkene is a nucleophile, wanting to react with a positive species.
A + B Intermediate Intermediate C + D
Step 1 is the “slow step”, the rate determining step.
Step 1: endergonic, high energy of activation. Slow process
transformation of the structure.
For the step
C + A-B C-A + B
Transition State Three areas to be aware of.
Reactants
Products
Energy of Activation. Energy barrier.
2. The concentration of an intermediate is usually quite low. The Energies of Activation for reaction of the Intermediate are low.
3. There is a transition state for each step. A transition state is not a stable structure.
• Electron releasing, alkyl groups, -CH3, stabilize the carbocation making it easier to form.
• Electron withdrawing groups, such as -CF3, destabilize
the carbocation making it harder to form.
4. The reaction coordinate can be traversed in either direction: A+B C+D or C+D A+B.
Hammond Postulate
The transition state for a step is close to the high energy end of the curve. For an endothermic step the transition state resembles the product of the step more than the reactants. For an exothermic step the transition state resembles the reactants more than the products.
Alkene Reactions
Reactivity above and below the molecular plane!
Pi bonds
Plane of molecule
Addition Reactions
A-B
A B
Important characteristics of addition reactions
Increasing Ease of Formation
Factors Affecting Carbocation Stability Inductive
1. Inductive Effect. Electron redistribution due to differences in electronegativities of substituents.
Syn addition: Both A and B come in from the same side of the alkene. Both from the top or both from the bottom.
Anti Addition: A and B come in from opposite sides (anti addition).
An increase in H during a reaction favors reactants. A decrease favors products.
An increase in entropy (eg., more molecules being formed) during a reaction favors products. A decrease favors reactants.
Step 2: exergonic, small energy of activation. Fast Process.
Characteristics of two step
Reaction
1. The Intermediate has some stability. It resides in a valley.
Carbocations
Electron deficient. Does not obey octet rule. Lewis acid, can receive electrons. Electrophile.
sp2 hybridized.
p orbital is empty and can receive electrons.
Energy Changes in a Reaction
• Enthalpy changes, DH0, for a reaction arises from changes in bonding in the molecule.
– If weaker bonds are broken and stronger ones formed then DH0 is negative and exothermic.
followed by reduction
Electrophilic Addition
We now address regioselectivity….
Regioselectivity (Orientation)
The incoming hydrogen attaches to the carbon with the greater number of hydrogens. This is regioselectivity. It is called Markovnikov orientation.
DG0: if positive equilibrium favors reactants (endergonic), if negative favors products (exergonic). DG0 = DH0 – TDS0
Step 1: Step 2:
Multi-Step Reactions
Orientation (Regioselectivity)
If the doubly bonded carbons are not equivalent which one get the A and which gets the B.
Stereochemistry: geometry of the addition.
Br H
1-Bromo-propane
Carbocation Stabilities
Order of increasing stability: Methyl < Primary < Secondary < Tertiary Order of increasing ease of formation: Methyl < Primary < Secondary < Tertiary
– If stronger bonds are broken and weaker ones formed then DH0 is positive and endothermic.
Gibbs Free Energy
Gibbs Free Energy controls the position of equilibrium for a reaction. It takes into account enthalpy, H, and entropy, S, changes.
Acidic molecule, easily ionized.
We had portrayed the HBr earlier as a BronstedLowry acid.
New term: the HBr is an electrophile, wanting to react with an electron rich molecule (nucleophile).