This document discusses first order complex reactions. It defines a complex reaction as a multi-step reaction involving reaction intermediates. A first order complex reaction is one where the overall reaction rate depends on the concentration of a single reactant. Determining the order of a complex reaction requires experimental analysis, as the reaction orders may not equal the stoichiometric coefficients. An example of a first order complex reaction provided is the hydrolysis of tert-butyl bromide, which proceeds through a tert-butyl radical intermediate and has an overall first order dependence on the organic reactant concentration.

1. TOPIC:
FIRST ORDER COMPLEX REACTION
Mehran University of engineering
and technology, jamshoro
Prepared by:
Abu Umeer (14CH42)
u.ahmed8@yahoo.com
Madni Alam (14CH38)
Chaman Lal (14CH39)
Assigned by:
Prof: Dr: Abdul Rehman Memon 1

2. Complex Reaction:
• It is a Multi-step reaction.
• These are reactions involving simultaneous
breakdown by a sequence of reaction steps.
• Slowest step of complex reaction is called rate
determining step.
• E.g. A B C
B is a reaction intermediate.
• Three types of reaction;
a) OPPOSING REACTION (REVERSIBLE)
b) SIDE REACTIONS (PARALLEL)
c) CONSECUTIVE REACTIONS
K K’
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3. ….cont:
a) OPPOSING REACTION (REVERSIBLE):
• The simplest case is, in which both reactions are of first order
A B
b) SIDE REACTIONS (PARALLEL):
• Here the reacting substance can be removed by two or more
reactions occurring simultaneously,
EXAMPLE:
– Purified insulin degrades by two mechanisms-
deamidation and polymerization.
K
K’
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4. ….cont:
c) CONSECUTIVE REACTIONS:
• Simplest is one where both the reaction is of first order.
• If K2 > K1 then B can be considered as unstable intermediate and rate
determining step for overall reaction would be conversion of A to B.
A B C
Examples: -
Degradation of chlorbenzodiazepine by hydrolysis to lactam
form and further to benzophenone.
k1 k2
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5. Order of Reaction:
• This is the number of concentration terms that
determine the rate.
• Consider the reaction:
A + B C + D
• The overall equation is,
Rate = k [A]x [B]y
• The overall order of reaction is x+y
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6. …..cont:
• Reactions may be classified according to the order
of reaction, which is the number of reacting species
whose concentration determines the rate at which
the reaction occurs.
• Types of order of reaction:
1. Zero order reaction
2. First order reaction
3. Second order reaction
4. Third order reaction & Higher
5. Pseudo order reaction
6. Complex reactions28-Sep-16 6

7. 1st order reaction:
 When the reaction rate depends on the first power
of concentration of a single reactant, it is
considered to be first order.
K = 2.303 log (a/a-x)
t
 Unit of K for first order is time-1 i.e. SI unit is (sec)-1.
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8. First Order Complex Reactions:
• The complex reactions in which reaction rate
depends on a single reactant, are known as first
order complex reactions.
• Radioactive decay processes are all first order.
• Complex reactions may or may not have reaction
orders equal to their stoichiometric coefficients.
• Reaction orders can be determined only
by experiment.
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9. ……cont:
• There is no detectable difference between first
order and zero order at less than 15%
decomposition.
• Many companies take the attitude of plotting by
first order in situation where the order is
unknown.
• Establishing the true order of reaction can often
be difficult, due to the fact that strength changes
at 25ºc are small and that unit–to –unit and assay
variation make such a distinction difficult
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10. EXAMPLE :
• The reaction between Trimethylbromomethane
(tert-butyl bromide) and the hydroxide ions from
sodium hydroxide solution is first order complex
reaction :
• The overall reaction replaces the bromine atom in
the organic compound by an OH group.
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11. …..cont:
• The reaction takes place in two elementary steps :
1. In the first elementary step
Trimethylbromomethane breaks to give ions :
Carbon-bromine bonds are reasonably strong, so this
is a slow change .
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12. …..cont:
• In the second elementary step ,tert-butyl radical
reacts with hydroxide ion to give “Butyl Alcohol” :
Because carbon-oxygen bonds are strong, once the
OH group has attached to the carbon atom, it tends
to stay attached. The reaction is fast, & Butyl Alcohol
is the final product.
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13. …..cont:
• The tert-butyl radical is the reaction
intermediate.
• The slow step of a reaction is known as
the rate determining step. SO,
rate = k [C4H9Br]
• The reaction is first order with respect to the
organic compound.
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14. Summary:
• Complex reaction is a Multi-step reaction.
• Order of reaction is the number of concentration
terms that determine the rate.
• first order complex reactions are those in which
reaction rate depends on a single reactant.
• Reaction orders can be determined only
by experiment.
• There is no detectable difference between first
order and zero order at less than 15%
decomposition.
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