Mechanism of Some Typical Complex Reactions

Mechanism of Some Typical Complex Reactions

1.1 basic concepts of chain reaction: chain-initiating step

chain-propagating steps

chain-terminating step

The chain-initiating step is usually a reaction in which a molecule is decomposed by thermal collision or radiation. In some cases, a molecule named as initiator, which can readily decompose and produces radicals, is introduced into the system. Some activated species, such as free atoms, radicals, carbonium ions, act as chain initiating species, which is usually called chain carriers.

1 Chain Reaction

1

2

3

4

5

2

2

2

2

2

Br 2Br

Br H HBr H

H Br HBr+Br

H +HBr H +Br

2Br Br

k

k

k

k

k

§11.9

Owing to the participation of active intermediate, the

activation energy of chain reactions, usually ranging between

0 ~ 40 kJ mol-1, is much lower than that of other kinds of

reactions of 40 ~ 400 kJ mol-1. Therefore, chain reactions can

proceed rapidly.

chain-propagating steps

A chain reaction is one in which some intermediates are

consumed and regenerated in a cycle of reactions the net

result of which is to carry forward the overall reaction.

2

3

2

2

Br +H HBr+H

H +Br HBr+Br

k

k

Basic characteristics of chain reaction:

1) with active radicals participating;

2) low activation energy of chain propagation

3) wide existence

Examples for Chain reactions:

1) H2 + Cl2 ; H2 + Br2 ;

2) Pyrolysis (thermal decomposition) of many organic

molecules.

3) polymerization:

1.2 rate equation of chain reaction

Polymerization initiated by initiator

X-X X

X + RCH=CH2 XRCH-CH2

XRCH-CH2 + RCH=CH2 XRCH-CH2-RCH-CH2

……XR(CH-CH2)n-1CH-CH2 + RCH=CH2 XR(CH-CH2)nCH-CH2

XR(CH-CH2)nCH-CH2 +X XR(CH-CH2)nCH-CH2X

XR(CH-CH2)n + XR(CH-CH2)m XR(CH-CH2)n (CH-CH2) mRX

k2

k3

112

1 22 2

3

[R-CH=CH ][X-X]k

r kk

Just likes that of H2+Cl2

1.3 Branched chain reactions and explosion

According to Arrhenius equation:

aE

RTk Ae

the rate constant of the reaction increases exponentially with temperature.

If an exothermic reaction is constrained to take place in a

limited volume and the heat cannot be dissipated,

temperature will increase. The higher temperature will, in

turn, accelerate the reaction and produce more heat, which

causes a still higher temperature. This will finally result in a

catastrophe-an explosion, which is named as thermal

explosion.

The most extensively studied branched chain reaction is the formation of water.

2 H2(g) + O2(g) 2 H2O (g)

It was found that, the rate of the reaction is highly dependent on the surface of container, temperature, and pressure.

Explosion of H2-O2 mixture

explosion limit;

region of explosion;

region of normal reaction.p1 p2 p3

Stable peninsula

Explosion peninsula

The complete mechanism for the reaction is still not

unambiguously established. One simple sequence consists

of the following elementary steps was proposed:

pre

ssu

re

temperature

600 oC

Initiation: 2H 2H

Propagation:

2 2 2H H O H O HO

2k2 2HO H H O H

Branching:

gaseous termination:

Wall termination:

k32H O HO O

4k2O H HO H

5k22H M H M

2HO H M H O M

H wall destroyed

HO wall destroyed

For certain reaction under appropriate conditions,

propagating steps occur in which the loss of a single

propagator is overcompensated for by the production of two

or more other propagating species, such chain reaction is

branched chain reaction.

Initiation: 1kA R

Propagation: 2kR A P R

gaseous termination: gkR

wkR Wall termination:

21 2

2

[A]

[A](1 ) w g

k krk k k

Under low pressure, the free range of molecules is large, the wall termination is dominate, therefore, the explosion depends on the volume and the nature of the vessel.

At high pressure, the free range of molecules is small, the gaseous termination is dominate, therefore, the explosion does not depend on the volume and the nature of the vessel.

21 2

2

[A]

[A](1 ) w g

k krk k k

21 2

2

[A]

[A](1 ) w g

k krk k k

When = 1, the denominator (kw + kg) has certain value, a

nd the reaction will take place at a moderate rate.

When > 1, i.e., branched chain reaction, k2[A](1-) is of

minus value, if k2[A](1-) + kw + kg 0, the reaction rate

, and explosion occurs.