Alkenes preparation from alkyl halides, alcohol (Part - 14) - Hydrocarbons, Class 11 Video Lecture - Class 11 | Best Video for Class 11

hello friends this video on hydrocarbons
part 14 is brought to you by exile fear
car no more fear from exam now let's
understand the preparation of alkenes
how do you prepare our keys you can play
our games from Alekhine's from alkyl
halide British machine Undine lights and
tell you what is best in line light from
alcohol so we typically use all these
four mechanism to prepare alkenes let's
start from ultimate so all kinds if you
see if I add hydrogen to it it becomes
alkenes right those all kinds are all
the more than Sasha so if I add hydrogen
to it it will become a cheese but there
is an issue by our hydrogen thus triple
bond will break double bond and double
crown will also break into single bond
right so what should we do we should not
be able to will not be able to use the
tottenham catalyst as we have used in
the last example where we are just
putting the platinum metals and it got
hydrogenated can we use it is we can but
the quality will not trade right
sometimes we'll get double bond from
nice triple bond or you do the control
class nation control dehydrate relation
so one option is you can do using
lindlar catalyst and alkene thus
obtained in this has cysts geometry
similar to my dehydrogenation using the
metal not in them so what is the linda's
catalyst it is nothing but it is 5
person palladium by weight you dig 5%
pallidum by weight and this has a great
property that only triple Borden tricks
there is a triple bond carbon double
bond but double bond will not be will
not become into single bond using
linda's catalyze this converts only
Tripper to double bond and this catalyst
is repaired by reduction of palladium
chloride take Valium chloride you reduce
it you get this catalyst this has lost
surface area
hold it as a sim additions we get since
Joe vertical alkies correct this is one
way to prepare alkie the other way is
you have this alkyne we would use with
sodium in liquid ammonia and this forms
trans alkene so in the previous case we
saw that both the hydrogen get all on
the same side because generally very how
this metal surface kind of stuff right
example is the PD catalyst also you have
this triple bond then hydrogen will be
able to Percy inside same thing right
but in this case it is at an sedition
one hydrogen on this side another in
from this side
correct so this is my free radical type
of reaction so here I use sue diem so
the first step is to form free radical
on I the reaction mechanism goes like
this the sodium metal reacts with this
alkynes to form a my ons free radical
anion carbon ion again same right for
example let's say I take this guy so I
have R then I have C triple bond C
Ornish correct the moment I give a
sodium guy here sodium free radical this
bond will break homolytically correct
the business sodium so what will happen
is this guy and she this the next step
is on C the free radical here the double
bond one bond is broken now this guy
will get and in the meantime sodium was
the free radical suit even transmits
electron to this guy sodium will becomes
any class and these electrons will come
here the negative
right now I have a liquid ammonia from
liquid ammonia this guy will attract
hydrogen correct so this will become on
this add liquid ammonia so what happen
is this guy will get something like this
this double wouldn't be here is my are
here times are - right so on this I will
get hydrogen bomb plus y -3 and na na x3
will become na and this tube or NH u any
correct because now this from Emory I
got hydrogen in a dashed here right so
it is something like this it is three
for two in his three dot in s so this is
our H plus this H does attacked here in
its got form here and I got NH 2 any
correct
now on this now again I have a second
sodium second Sunil I add seconds will
be second shooting radical P article
this is again give electron to this time
so this will become something like this
see - and again I had an STD so this
will give the hydrogen to this guy and
this NH 2 - will react with error gets
an a-plus actually with sodium so I will
get something like this and here I will
get a link so then it's - this is what I
do you see this is Mike transactions so
this very action Michael so I have my
sodium phenolate again because it is
sodium and this will result in the
mobile I take V Z so this will break
actually and now for this carbon it will
give a negative charge will become
costing itself right and then then this
guy will attract one hydrogen and
whatever nh-2 miles you have reacted so
doing to form this now again one sodium
radical will give this guy a negative
charge right give electro to this guy
and now this carbon will again in fact
when hydrogen comes ammonia and you form
something else so as you have discussed
the preparation of alkane with the
Mallik I'm using hydrogenation similarly
we do here also will you add the
hydrogen in the presence of the catalyst
same reaction we have as platinum
platinum catalyst so I have my alkene
here alkyne here you add hydrogen to it
using these catalysts and you get only
one more reaction I have this is my
alkyne in time I got et so fine I got
floki now we'll talk about generation of
alkyne using alkyl halides
so alkyl halides are my anything in this
RX form maybe CS 3 X or CA 2 h 5x of
this form when you heat this with
alcoholic potassium hydroxide in alcohol
this takes out one halogen acid HX maybe
a CLS via digs out that acid from the
alkali halide to form alkyne with my
final problem and this reaction is
called the hydrohalogenation because you
are removing halogen acid dehydrates
one hydrogen and one halogen is removed
right it's called d hydrohalogenation
there's something called dehydration is
different where you remove the halogen
here it's called d high to an
elimination because you remove hydrogen
and how is it correct this is example of
beat elimination because you are
removing no high position from the beta
carbon I'd lose my carbon alpha carbon
atom from which I am I recon halogen
removed and from the beta carbon atom
hydrogen is called beta elimination
correct because these two are removed
and they are from two different
he has gone beta elimination and they
are next to each other and this is the
rate of reaction iodine is very fast
moving has less chlorine is less on the
move let's talk about the alkyl group
tertiary is the Max is the very fast
then secondly and then primary let's see
the reaction mechanism is in this case
my alcoholic
potage he'll give me water which - Oh H
- can react what can attack this ocean -
here attack this hydrogen this hydrogen
what will do it will break this bond
phone is born here of this carbon will
have 5 born now this carbon will break
this one because again there's a wish -
will attack this hydrogen correct in the
movie did I miss - this bond breaks and
you form this bond double bond and since
this carbon will have 5 born on this one
bond will break because it's a good
living group is how much is a good
leaving group actually and they'll come
on all right
and thus if you see if you this you see
you get this the double bond alright so
if you see the whole reaction to all
this reaction so in this whole reaction
there too is a pair of two things the
first is the living look it is very good
leaving group leave early moment I
attack you as man and this this is used
right and then what you father's and
thus this fold break here this will
break here and get a check out you get
something okay
let's talk about arcing preparation from
vicinal diol lights so what are national
dialects so in which two halogens are a
task in to Edison cover they are called
missing and I highlight example I have
CC notice I have some X right so this
they are gone listener die lights so how
to prepare our key from this you react
this with zinc loose metal zinc metal
loose it has to be loose I'll tell you
why and inform general x2 and will get
ugly
this is called D halogenation please
note the Ramos was dehydrogenation
because you removing hypership solution
in this case you're removing halogens
only Scott D have education correct
example I have CST brcs to be our Indian
cuisine you get it clean and chain yeah
same thing this reaction you get o
alkene let's probe in here now let's see
the reaction mechanism so I have it
suppose this guy correct I have some X
here and some except when it suppose it
will be take bromine only here to
bromine here and I have hydrogen
hydrogen okay now when you have this
jean guy in zinc metal that's why I'm
looking for the loose metals I have zinc
surface here Jade surface so what happen
is this jingle form born with these two
options because J and X 2 is a stable
compound right so if I am born with this
and this Librium correct so with this
what happen it will form something like
this this bond is broken will be a
double bond here right and I have zinc
we are in this surface
or NEX chlorine or hiding and you get
this this is one easy correct so just
opposite of hydrogenation using the
metal catalyst here you use the metal
and this metal absorbs this halogens and
from 7vr do and you get this female so
normal prepare alkenes from alcohols by
acidic dehydration so the concept is
pretty simple actually you see any
alcohol where is our Oh H so in this
case Oh H is a poor living group so
living room but if you add H+ to it if
you a - first way it gets divided those
are h2 plus in this case this guy is a
bubbly that's what we do actually
so you see alcohols are eating with
sulfuric acid forms early killings with
elimination of one water molecule so
this is one water bottle you see this is
regulated this is the reaction we have
this is my alcohol and you heat them
sulfuric acids concentrate if you get it
in here and this is called dehydration
acid it the - of alcohol because in this
case you removing water one can write
hydrogen which correct this is gone SCD
dehydration of alcohol and this reaction
is also gone vida ignition because if
you see hydrogen oxygen which is uniform
two different comas and they are nearby
it is called be dilation
please note why we use sulfuric acid and
why we don't use mineral acid for
example HCL hbr
HHI why because if we use these what
happens in a moment which - is remove
first thing is that we remove that then
this CL - or BR -
can attack so in that case it will be a
substitution reaction
wait you move wash - and you see a -
will come and attack this position and
this will be a sufficiently action but
if you use h2so4 so if you see it breaks
and it forms h + + h SS o - alright or
if use h3po4 it gives s + NH 2 3 or 4 -
so these ions are very big and they are
not good nucleophile you can use s3 p or
full also this becomes these are very
big and they are not good nucleophiles
they're not going to dignify so they
won't attack right and thus you'll be
able to dehydrate this correct so if you
see the reaction here what happens is
for example i have this guy right so i
have see oh oh a CH and other carbon
again put here right the moment you add
X class so this guy comes here and you
get are this needed Oh H - girls wait
the same thing I put ROH to class all
right so you'll get something like this
and this guy is a good leaving group so
this guy will leave the whole thing will
leave and you'll get something like this
then CH h equal plus correct now if I
had used SC l BF h we are hie then the
chlorine CL - in this solution would
have attacked it but since I am using
h2so4 and which has given h+ + HS / - so
H is a 4 - is a poor liquify it took
more time
so what in happen is in this case this
guy will break and form a water the H+
will come here and in form of this
okay it correct this is a even mechanism
so in this case the third degree
alcohol will react more fast because
this plus you get right the carbo
cations third degrees more stable so
this will be more fast this is the slow
process actually but it is a three
degree cation VB all the one fast
because TDD cation is more stable to
really is less stable correct
so three degree cation is more fast it
happens fast and needs less templates
but if you have two degree or one degree
of carbon right in that case attorney
and three cation is carbo cation is not
a stable so you need higher temperature
so the reactivity series will be three
degree has more reactive degrees less
and one degrees all the so if you know
the reaction together we can tell
actually y HC l hy h iy is to here is
not used Y of h2so4 use or Y is 3 P or
for reduced right because this is let it
be if you find and what is the order so
if you have three degree or interior
bonding they are called which will react
fast so if you know the reaction because
this carbo cation is formed this
reaction it they can have in the
intermediate so the more stable the
carbo cation the fast will be the
reaction good correct thank you
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