Reaction Mechanisms - Addition of Alkenes + Equilibrium Flashcards
What is the most common reaction of alkanes?
Substitution
What is the general formula for the substitution for alkanes?
R-H + A-B = R-A + H-B
R-H + A-B = R-A + H-B
What is H substituted with?
H is substituted with A
R-H + A-B = R-A + H-B
What is B in this?
The LG -> limiting?
What is the most common reaction of alkenes?
Addition
What is the general formula for the addition of alkenes?
C=C + A-B = C-C(A-B -> drawn bellow in molecular formula))
What three things can be added to alkenes by a two step polar process?
Addition of halogens
Addition of water (hydration)
Addition of acids
What happens in the addition of halogens?
3
Cl2 and Br2 can be added readily
Happens nearly instantaneously even at room temperature or below
No light or heat is required
Name two halogens that can be readily added to alkenes?
Cl2
Br2
What is not needed for the addition of halogens?
2
Light
Heat
Heat is not needed for the addition of halogens, what is this similar to?
Similar to the substitution of alkanes
What happens in the addition of water?
2
An acid catalyst is required because a neutral H2O molecule is not acidic enough to provide protons to start the reaction
Its a multistep mechanism
What is needed in the addition of water?
An acid catalyst
Why is an acid catalyst needed in the addition of water?
Because a neutral H2O molecule is not acidic enough to provide protons to start the reaction
What is needed to start the reaction of hydration?
Protons
What happens in the addition of acids?
3
This applies a variety of acids
The first step involves the addition of a H+ to the C=C bond
X- is then added (acid with negative charge)
How many products can be formed using a symmetric reagent and/or alkene?
One one addition product is possible
How many products can be formed using an unsymmetrical reagent/and or alkene?
Two products are possible
What are regioisomers?
Isomers which have the same functional groups but attached at different positions
What is it called if an addition reaction involving both an unsymmetric alkene and reagent MAINLY only gives one regioisomer?
The reaction is said to be regioselective
Most reagents used in addition reactions are polar, what does this mean?
There is a + and a - end
e.g. H+Cl-
What is the Markovnikov rule?
2
When an unsymmetrical reagent adds to an unsymmetrical alkene
The electropositive part of the reagent bonds to the C of the C=C that has the greater number of H atoms attached to it
What does the Markovnikov rule apply to?
When an unsymmetrical reagent adds to an unsymmetrical alkene
What are electrophiles?
4
Electron lovers
They are electron poor reagents
They seek electrons
They are often + ions (cations) or otherwise electron deficient species
What are electrophiles often?
Cations
+ ions
What are nucleophiles?
3
Nucleus lovers
They are electron rich
They form bonds by donating electrons to an electrophile
In what direction do the electrons travel when a bond is formed between a nucleophile and an electrophile?
The nucleophile donates electrons to an electrophile
arrow goes from nucleophile to electrophile
What is a carbocation?
3
An ion with a positively charged carbon atom
A carbonium ion
An organic cation in which the positive charge is located on a carbon atom
Why are carbocations highly reactive?
2
The Carbon lacks electrons (only has 6)
The carbon will therefore react with any species that will provide it with 2 electrons (e.g. Nu-)
What is usually the first stage in an addition reaction?
The formation of the carbonium ion
Why is the formation of the carbonium ion the slowest step of addition?
This is because once the carbonium ion is formed it will react quickly to gain 2 electrons
What is the first step in all addition reactions?
Attack by the electrophile
Since attack by the electrophile is the first step what is the overall process often called?
Electrophile addition reaction
What is chemical thermodynamics?
2
The changes in energy that take place when chemical reactions occur
Thermodynamics control the extent to which a reaction goes to completion
What is chemical kinetics?
The velocity or rate at which the concentrations of reactants and products change
What is a state of equilibrium?
When the concentrations of reactants and products no longer change
When is a reaction said to go to completion?
When equilibrium lies extensively (>99.9%) on the side of the products
What does the equilibrium constant tell us?
The direction that is favoured by the system
What does it mean if Keq > 1?
The formation of the products C and D will be favoured
What does it mean if Keq < 1 ?
The preferred direction for the reaction is from right to left (A+B) is favoured
What is the equation for Keq?
Keq = ([C]^c[D]^d)/([A]^a[B]^b)
What happens when products are lower in energy (more stable) than the reactant?
(2)
Heat will be given off in the reaction
An exothermic reaction
When will an exothermic reaction occur?
When the products are lower in energy (more stable) than the reactants
What is enthalpy?
The term for heat energy associated with a chemical reaction
What is the difference in enthalpy between products and reactant called?
Delta H
