Chem 240: Ch#10- Radical Reactions

Front 1

Ionic Reactions:

Back 1

those in which covalent bonds break heterolytically & in which ions are involved as reactants, intermediates, or products

Front 2

A general homolysis:

Back 2

Front 3

What must be supplied to cause homolysis of covalent bonds? In what 2 ways is it done?

Back 3

Energy must be supplied in the form of
1. heating
2. irradiation w/ light.

Front 4

Hydrogen Abstraction:

Back 4

Front 5

When atoms combine to form molecules, what is released as covalent bonds form?

Back 5

energy (the molecules of the products have lower enthalpy than the separate atoms)

Front 6

Bond formation is what type of process?

Back 6

exothermic

Front 7

energy must be supplied to do what with the bonds?

Back 7

break covalent bonds

Front 8

reactions in which only bond breaking occurs are always what type of process?

Back 8

endothermic

Front 9

Homolytic bond dissociation energies:

Back 9

energies required to break covalent bonds homolytically

Front 10

Table 10.1: Single-Bond Homolytic Dissociation Energites DH @ 25 degrees C.

Back 10

Front 11

For bond breaking delta H is ____(1)____, and for bonds forming delta H is ___(2)_____

Back 11

1: positive
2. negative

Front 12

Energy diagram showing stability of primary, secondary, tertiary radicals:

Back 12

Front 13

The general stability of alkyl radicals:

Back 13

Front 14

Halogenation:

Back 14

- substitution reaction of an alkane w/ a halogen

- a halogen atom replaces 1/ more of the H atoms of the alkane.

(methane, ethane, and other alkanes react w/ the first 3 members of the halogen family: F, Cl, Br... alkanes do not react appreciably w/ I)

Front 15

Why does multiple substitution reactions almost always occur?

Back 15

b/c all H atoms attached to C are capable of reacting w/ F, Cl, or Br.

Front 16

Back 16

Front 17

Elaborate on the statement "chlorination is unselective"

Back 17

Chlorination of most higher alkanes gives a mixture of isomeric monochloro products as well as mroe highly halogenated compounds.

- Cl is relatively unselective, meaning it does not discriminate greatly among the different types of H atoms (primary, secondary, tertiary) in an alkane.

Front 18

Explain an exception in the halogenation of an alkane (or cycloalkane) whose H atoms are all equivalent?

Back 18

Neopentane

- it can form only one monohalogenation product, and the use of a large excess of neopentane minimizes polychlorination.

Front 19

The Chlorination of Methane:

Back 19

1. the reaction is promoted by heat or light.

2. the light-promoted reaction is highly efficient.

Front 20

A Mechanism for the reaction: Radical Chlorination of Methane:

Back 20

Front 21

Describe each step in a chain reaction:

Back 21

1. Initiation step (radicals are created)

2. propagation step (one radical generates another)

3. Termination step (basically all possible radials combine)

Front 22

Describe each step in a chain reaction:

Back 22

1. Initiation step (radicals are created)

2. propagation step (one radical generates another)

3. Termination step (basically all possible radials combine)

Front 23

Entropy:

Back 23

measures the relative disorder/ randomness of a system

Front 24

For a chemical system, entropy of the molecules can be related to what? and what it associated with?

Back 24

related to the # of degrees of freedom available to the molecules & their constituent atoms.

- degrees of freedom is associated w/ ways in which movement/ changes in relative position can occur.

Front 25

What are the 3 degrees of freedom in a molecule?

Back 25

1. Translational: associated w/ movements of the whole molecule through space

2. Rotational: associated w/ the tumbling motions of the molecule

3. vibrational: associated w/ the stretching & bending motion of atoms about the bonds that connect them

Front 26

If the atoms of the products of a reaction have more degrees of freedom available than they did as reactants, the entropy change (delta S) for the reaction will be?

Back 26

positive

Front 27

atoms of the products that are more constrained (have fewer degrees of freedom) than the reactants, has a _________ entropy change (delta S).

Back 27

negative results

Front 28

a low energy of activation means a reaction will take place ________?

Back 28

rapidly

Front 29

a high energy of activation means that a reaction will take place ________?

Back 29

slowly

Front 30

Energy of activation must be determined from experimental data and can not be directly measured (it is calculated) What are certain principles that have been established that enable one to arrive at estimates of energies of activation:

Back 30

Front 31

Thermal cracking:

Back 31

when a compound is heated to a very high temperature, radical reactions take place that produce other products.

Front 32

What does the Energy of activation for a reaction equal to delta H mean?

Back 32

that bonds are broken homolytically but no bonds are formed.

Front 33

What does the Energy of activation for a reaction greater than zero mean?

Back 33

covalent bonds are broken.

Front 34

What does the Energy of activation for a reaction equal to zero mean?

Back 34

these reactions are gas phase reactions in which small radicals combine to form molecules.

Front 35

What is the observed order of reactivities in halogenation?

Back 35

the less reactive, the higher the energy of activation meaning that only a very small fraction of all of the collisions will be energetically effective.

Front 36

A Mechanism for the Reaction: Radical Halogenation of ethane:

Back 36

Front 37

What is the correlation between reactivity of different H atoms and the type of H atoms (primary, secondary, tertiary) being replaced?

Back 37

The tertiary H atoms of an alkane are most reactive, secondary H atom are next most reactive, and primary H atoms are the least reactive

(note: primary H atoms are attached to primary C atoms.. and so forth)

Front 38

What is the geometric structure of most alkyl radicals?

Back 38

trigonal planar at the carbon having the unpaired e-s.

- sp2 hybridized central Carbon.

Front 39

When achrial molecule react to produce a comound w/ a single tetrahedral chirality center, the product will be obtained as a________?

Back 39

racemic form.

Front 40

A Mechanism for the reaction: The Stereochemistry of Chlorination at C2 of Pentane:

Back 40

Front 41

A Mechanism for the Reaction: The Stereochemistry of Chlorination at C3 of (s)-2-Chloropentane:

Back 41

Front 42

What was the cause of getting @ times Markovnikov addition & @ other times getting Anti-markovnikov under the same conditions?

Back 42

Turned out it was due to the presence of organic peroxides in the alkenes... peroxides were formed by the action of atmospheric oxygen on the alkenes

- alkenes that contain peroxides (ROOR) or hydroperoxides (ROOH) reacted w/ HBr...
---- anti-markovnikov addition of HBr occured

- Is a radical chain reaction initiated by peroxides.

Front 43

A Mechanism for the Reaction: Anti-Markovnikov Addition of HBr.

Back 43

Front 44

Describe Markovnikov addition of HBr:

Back 44

the reagent that attacks the double bond first is a proton. Because a proton is small, steric effects are unimportant. It attaches itself to a carbon atom by an ionic mechanism so as to form the more stable carbocation.

Front 45

What type of solvents favor Markovnikov addition of HBr?

Back 45

protic and polar solvents.

Front 46

Does HF, HCl, HI give anit-markovnikov in the presence of peroxides (ROOR)?

Back 46

No

Front 47

Describe the anti-Markovnikov addition of HBr:

Back 47

- occurs in the presence of peroxides
- the reagent attacks the double bond first is the larger bromine atom.
- it attaches itself to the less hindered carbon atom by a radical mechanism, so as to form the more stable radical intermediate.

Front 48

What solvents are preferable in an anti-Markovnikov addition of HBr?

Back 48

nonpolar solvents

Front 49

Polymerizations:

Back 49

reactions by which monomers are joined together

Front 50

A Mechansim for the Reaction: Radical Polymerization of Ethene:

Back 50

Front 51

Acid-catalyzed polymerization:

Back 51

the polymerization of alkenes when they are treated w/ strong acids.

The growing chains are cations rather than radicals.

The catalysts used for cationic polymerizations are usually lewis acids that contain a small amount of water.

Front 52

Concept Map: Mechanism Review of Radical Reactions:

Back 52

Front 53

Acid-catalyzed polymerization:

Back 53

the polymerization of alkenes when they are treated w/ strong acids.

The growing chains are cations rather than radicals.

The catalysts used for cationic polymerizations are usually lewis acids that contain a small amount of water.