Organic Chemistry Reaction Mechanism - Chemistry Video Lecture - Class 11

every day you go through the same
process wake up from a good night's rest
and go to school but as you know this is
not as simple as it sounds there are
many steps involved right from waking up
to taking a shower
to eating breakfast
to commuting to school
this is something you know but did you
know that this mechanism of going to
school has a deep connection to
chemistry what is that meet me in the
studio to find out
hi I am d'hubert Oh Goswami
welcome to the logical world of
chemistry today we will unravel the
mechanism behind chemical reactions in
today's episode we will study reaction
mechanisms homolytic and heterolytic
cleavage by which the reactions take
place and then we will look at reaction
intermediates like free radicals carbo
cations and carbonyls and finally we
will discuss some important questions
and their answers in exam decoded nearly
all reactions occur not in a single step
but in a series of steps reaction
mechanism helps in understanding what
changes are taking place at each step
while reactants are converted into
products so what is the reaction
mechanism well consider the process of
you going to school so you sleeping in
your bed is the reactant and the desired
end result at least by your parents and
teachers is to have you in school hmmm
but jokes aside it's not as simple as it
sounds there are many steps in this
reaction a you wake up B you take a
shower C have breakfast
and D go to school I am sure you got
that
so let's translate this into chemistry
take a look at how chemists believe
hydrogen and iodine react to form
hydrogen iodide
so a chemical reaction is a series of
changes by which substances react with
each other to form new substances the
formula equation we write for a reaction
is the sum of all the steps in a
reaction but the overall reaction does
not tell us how the reaction is actually
proceeding that is what is the mechanism
of the reaction now why do we need to
know this mechanism well study of the
reaction mechanism helps in
understanding the changes taking place
at each step of a particular reaction it
also helps in understanding the
reactivity of compounds further knowing
reaction mechanisms can help us in the
synthesis of new compounds
let me explain with the help of an
example what happens when methane and
chlorine react in the presence of light
take a look at the reaction on your
screen you may think does the
transformation of methane to methyl
chloride involve one step or several if
so what are these steps what is the
function of light in the reaction the
answers to all these questions can be
found by understanding the reaction
mechanism the reaction mechanism can be
defined as the sequential account of
each step describing details of electron
movement energetics during bond cleavage
and bond formation and the rates of
transformation of reactants into
products now let's go back to the
process of you going to school if you
remember you are sleeping so we can say
you were the substrate but you may have
continued to sleep unless someone woke
you up in short as a substrate you need
an attacking reagent to begin the
process now let's translate this into
organic chemistry an organic reaction is
believed to take place by the attack of
the reagent on an organic molecule this
molecule is also called substrate this
leads to the formation of a number of
intermediates these intermediates
finally lead to the formation of
products and byproducts
just as our journey from one station to
another includes various intermediate
stations a reactance journey to reach
its product also encounters various
intermediates one thumb rule you must
always remember is when a reaction takes
place that is when reactants form
products the bonds in the reactants will
be cleaved first this process is known
as bond fission a covalent bond can get
cleaved by either of these two ways
homolytic cleavage or heterolytic
cleavage but knowing the names is not
enough so let us first study homolytic
cleavage in detail in this type of
cleavage when the covalent bond breaks
each atom gets one electron each from
the shared pair of electrons
whenever homolytic cleavage takes place
it leads to the formation of neutral
species with an unpaired electron these
species are called free radicals free
radicals are denoted by putting a dot
over the symbol of their atom or group
of atoms let us understand homolytic
cleavage more clearly with the help of
an example
in the presence of UV light chlorine
molecule undergoes homolytic cleavage to
form free radicals of chlorine an
important point to remember here is that
homolytic fission is denoted by the
fishhook shaped
half arrows these arrows are used to
show the movement of single unpaired
electron
since homolytic cleavage results in the
formation of free radicals
therefore organic reactions which
proceeded by homolytic cleavage are also
called free radical reactions before we
move further let's take a break but I
will leave you with something to keep
your gray cells going which scientist is
said to be the founder of radical
chemistry if you want more information
and questions on this topic you can log
on to www.nasa.gov
welcome back I am the brother Goswami
and today we are discussing reaction
mechanism before the break I had asked
you to identify the scientist who is
said to be the founder of radical
chemistry and the answer is Moses
Gomberg in 1900 Moses Gomberg reported
the generation of the first organic free
radical the triphenylmethyl radical the
radical was formed by reacting triphenyl
chloromethane with silver metal in
benzene he is therefore known as the
founder of radical chemistry now coming
back to free radicals as we discussed
earlier free radicals are species
containing an unpaired electron
if the unpaired electron is on carbon
atom then they are called
alcohol free radicals alcohol free
radicals can be classified as primary
secondary and tertiary depending on the
class of the carbon atom carrying the
unpaired electron let's talk some more
about them a primary free radical is the
one in which an unpaired electron is
present on the primary carbon recall
that primary carbon is that carbon which
is bonded to only one carbon a secondary
free radical is one in which the
unpaired electron is present on the
secondary carbon a secondary carbon is
that carbon which is bonded to two
carbons
a tertiary free-radical is the one in
which the unpaired electron is present
on the tertiary carbon as you know a
tertiary carbon is that carbon which is
bonded to three carbons as far as the
stability of alkyl radicals is concerned
tertiary free radicals are more stable
than secondary free radicals which are
in turn more stable than primary free
radicals after studying homolytic
cleavage and free radicals let us now
move on to heterolytic cleavage in this
type of cleavage the covalent bond
breaks in such a way that one of the
fragments takes both the electrons of
the shared pair leaving none on the
other heterolytic cleavage is
unsymmetrical and results in the
formation of two charged particles do
you know what the species produced
during heterolytic cleavage called carbo
cations and carbon ions are formed
during heterolytic cleavage well the
species are called carbo cations if they
have a carbon atom possessing a positive
charge carbo cations are electron
deficient because they have only six
electrons in their valence shell let us
now study the classification of carbo
cations carbo cations can be classified
as primary secondary and tertiary
depending upon whether one two or three
carbons are directly attached to the
positively charged carbon
ethyl carbo-cation is a primary carbo
cation
isopropyl carbo-cation is a secondary
carbo cation and tertiary butyl carbo
cation is a tertiary carbo cation it's
time to take up a question from one of
our viewers
hello sir i'm arvind and i have some
doubts regarding carbo cations can you
please explain the stability of
different carbo cations I am glad you
asked me that question as we know carbo
cations are electron deficient species
when groups like the alkyl group which
have electron donating inductive effect
are present they release electron
density towards the positively charged
carbon atom this stabilizes the carbo
cation therefore more the number of
alkyl groups the greater will be the
stability of the carbo cation therefore
the tertiary carbo cation is more stable
than a secondary carbo cation which in
turn is more stable than a primary carbo
cation to understand it better let's see
this example which of these carbo
cations is more stable now we know that
a tertiary carbo cation is more stable
than a primary carbocation
we can see that the first structure
represents a primary carbo cation and
the second structure represents a
tertiary carbo cation therefore the
second carbo cation that is tertiary is
more stable than the first one that is a
primary carbo cation but what happens if
during heterolytic cleavage the species
formed contains a carbon atom possessing
a negative charge well such species are
called carbon ions carbon ions may be
defined as a group of atoms that contain
a carbon atom bearing a negative charge
like carbo cations Carbon ions are also
classified as primary secondary and
tertiary depending on whether one two or
three carbon atoms are attached to the
carbon bearing negative charge in
general primary secondary and tertiary
carbon ions are represented as shown the
alkyl groups may be same or different
now just as different carbo cations vary
in their stability carbon ions also vary
in their stability order of stability of
carbon ions can again be explained on
the basis of electron releasing
inductive effect of alkyl groups greater
the number of alkyl groups on the carbon
bearing negative charge greater would be
the electron density released by the
alkyl groups so the negative charge on
carbon Ann gets intensified making the
carbon ion more unstable therefore the
primary carbon ion is more stable than
secondary carbon ion which is more
stable than tertiary carbon and that's
all the theory for today I hope this
lesson has given you clarity on reaction
mechanisms it's time now for a short
break stay tuned because coming up next
is exam decoded
welcome to chemistry 11 I am the brother
and we are exploring the world of
chemical reactions before stepping ahead
on to our next segment let's do a quick
recap of today's topic in today's
episode we studied that the sequential
account of each step taking place in a
reaction is referred to as the reaction
mechanism the reaction mechanism
describes details of electron movement
and energetics during bond cleavage and
bond formation a covalent bond can get
cleaved by either homolytic cleavage or
heterolytic cleavage free radicals are
formed during homolytic cleavage and
carbo cations and carbon ions are formed
during heterolytic cleavage I hope now
you are ready to get tested so let's
move on to exam decoded
welcome to exam decoded that
much-awaited segment of our episode
where I will give you lots of tips and
advice to help you become a topper so
let's get on to our rapid-fire round I
will ask you five questions and you have
to answer them quickly in not more than
five seconds here is the first question
for you the detailed step-by-step
description of a chemical reaction is
called a reaction mechanism G isomerism
C catenation the hybridization the
correct answer is a it's called reaction
mechanism second question which one of
the following represents an example of
carbo-cation j CH 3 + g CH 4 c CH 3 CH 2
- G C 2 H 6 the correct answer is a next
which is the most stable carbo cation a
tertiary G primary C secondary D ethyl
carbo-cation if you chose a you are
right just two more to go heterolytic
cleavage results in the formation of
free radicals true or false
it is definitely false as heterolytic
cleavage produces carbo cations and
carbon ions the last question
free radicals contain an unpaired
electron true or false
this is true now let's see how you
scored if you got all five questions
right we will call you a high flyer you
did well this shows that your concepts
are quite clear if you got four or three
right then you are an achiever and a
little more study can get you on the top
but if you got three or less you need to
go through the topic once more for more
help on this topic you can always log on
to www.wymt.com a Kansan comes under the
unit organic chemistry and carries a
weight age of seven marks this makes it
critical from exam point of view the
questions asked from this topic can be
very short answer type questions for one
mark and short answer type questions for
two or three marks these would typically
test your knowledge ability and
conceptual ability let us now see how to
answer one such question in order to get
full marks the question is explain what
are free radicals give one example each
of a primary secondary and tertiary free
radical also mention the order of
stability of free radicals
this question carries three months let
me show how you can score three on three
if this question comes in your exams
begin by explaining that free radicals
are an atom or group of atoms which
contains an unpaired electron they are
formed during homolytic cleavage this
definition will fetch you half mark
ethyl radical isopropyl radicals and
tertiary butyl radical are examples of
primary secondary and tertiary free
radicals respectively
mentioning examples of each category
along with proper classification will
fetch you half mark each as for the
stability of free radicals tertiary free
radical is more stable than secondary
free radical which is more stable than
primary radical mentioning the correct
order of stability of free radicals will
fetch you another one mark
well I hope you got that if you didn't
you can always log on to www.wymt.com
it's now time to say goodbye but don't
worry I will leave you with some food
for thought did you know that free
radicals effect the aging process how
does a very common antioxidant like
vitamin C help in reducing the aging
process oxidation reactions can produce
free radicals which start chain
reactions that damage cells antioxidants
like vitamin C terminate these chain
reactions by removing free radical
intermediates and inhibit other
oxidation reactions by getting oxidized
themselves just one benefit of knowing
the reaction mechanism keep thinking
till next time this is Debra T wishing
you all the best goodbye