Reaction Mechanisms - Addition of Alkenes + Equilibrium

Question 1

What is the most common reaction of alkanes?

Answer 1

Substitution

Question 2

What is the general formula for the substitution for alkanes?

Answer 2

R-H + A-B = R-A + H-B

Question 3

R-H + A-B = R-A + H-B

What is H substituted with?

Answer 3

H is substituted with A

Question 4

R-H + A-B = R-A + H-B

What is B in this?

Answer 4

The LG -> limiting?

Question 5

What is the most common reaction of alkenes?

Answer 5

Addition

Question 6

What is the general formula for the addition of alkenes?

Answer 6

C=C + A-B = C-C(A-B -> drawn bellow in molecular formula))

Question 7

What three things can be added to alkenes by a two step polar process?

Answer 7

Addition of halogens

Addition of water (hydration)

Addition of acids

Question 8

What happens in the addition of halogens?

3

Answer 8

Cl2 and Br2 can be added readily

Happens nearly instantaneously even at room temperature or below

No light or heat is required

Question 9

Name two halogens that can be readily added to alkenes?

Answer 9

Cl2

Br2

Question 10

What is not needed for the addition of halogens?

2

Answer 10

Light

Heat

Question 11

Heat is not needed for the addition of halogens, what is this similar to?

Answer 11

Similar to the substitution of alkanes

Question 12

What happens in the addition of water?

2

Answer 12

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

Question 13

What is needed in the addition of water?

Answer 13

An acid catalyst

Question 14

Why is an acid catalyst needed in the addition of water?

Answer 14

Because a neutral H2O molecule is not acidic enough to provide protons to start the reaction

Question 15

What is needed to start the reaction of hydration?

Answer 15

Protons

Question 16

What happens in the addition of acids?

3

Answer 16

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)

Question 17

How many products can be formed using a symmetric reagent and/or alkene?

Answer 17

One one addition product is possible

Question 18

How many products can be formed using an unsymmetrical reagent/and or alkene?

Answer 18

Two products are possible

Question 19

What are regioisomers?

Answer 19

Isomers which have the same functional groups but attached at different positions

Question 20

What is it called if an addition reaction involving both an unsymmetric alkene and reagent MAINLY only gives one regioisomer?

Answer 20

The reaction is said to be regioselective

Question 21

Most reagents used in addition reactions are polar, what does this mean?

Answer 21

There is a + and a - end

e.g. H+Cl-

Question 22

What is the Markovnikov rule?

2

Answer 22

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

Question 23

What does the Markovnikov rule apply to?

Answer 23

When an unsymmetrical reagent adds to an unsymmetrical alkene

Question 24

What are electrophiles?

4

Answer 24

Electron lovers

They are electron poor reagents

They seek electrons

They are often + ions (cations) or otherwise electron deficient species

Question 25

What are electrophiles often?

Answer 25

Cations + ions

Question 26

What are nucleophiles? | 3

Answer 26

Nucleus lovers They are electron rich They form bonds by donating electrons to an electrophile

Question 27

In what direction do the electrons travel when a bond is formed between a nucleophile and an electrophile?

Answer 27

The nucleophile donates electrons to an electrophile | arrow goes from nucleophile to electrophile

Question 28

What is a carbocation? | 3

Answer 28

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

Question 29

Why are carbocations highly reactive? | 2

Answer 29

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-)

Question 30

What is usually the first stage in an addition reaction?

Answer 30

The formation of the carbonium ion

Question 31

Why is the formation of the carbonium ion the slowest step of addition?

Answer 31

This is because once the carbonium ion is formed it will react quickly to gain 2 electrons

Question 32

What is the first step in all addition reactions?

Answer 32

Attack by the electrophile

Question 33

Since attack by the electrophile is the first step what is the overall process often called?

Answer 33

Electrophile addition reaction

Question 34

What is chemical thermodynamics? | 2

Answer 34

The changes in energy that take place when chemical reactions occur Thermodynamics control the extent to which a reaction goes to completion

Question 35

What is chemical kinetics?

Answer 35

The velocity or rate at which the concentrations of reactants and products change

Question 36

What is a state of equilibrium?

Answer 36

When the concentrations of reactants and products no longer change

Question 37

When is a reaction said to go to completion?

Answer 37

When equilibrium lies extensively (>99.9%) on the side of the products

Question 38

What does the equilibrium constant tell us?

Answer 38

The direction that is favoured by the system

Question 39

What does it mean if Keq > 1?

Answer 39

The formation of the products C and D will be favoured

Question 40

What does it mean if Keq < 1 ?

Answer 40

The preferred direction for the reaction is from right to left (A+B) is favoured

Question 41

What is the equation for Keq?

Answer 41

Keq = ([C]^c[D]^d)/([A]^a[B]^b)

Question 42

What happens when products are lower in energy (more stable) than the reactant? (2)

Answer 42

Heat will be given off in the reaction An exothermic reaction

Question 43

When will an exothermic reaction occur?

Answer 43

When the products are lower in energy (more stable) than the reactants

Question 44

What is enthalpy?

Answer 44

The term for heat energy associated with a chemical reaction

Question 45

What is the difference in enthalpy between products and reactant called?

Answer 45

Delta H