Nomenclature of Alkynes Video Lecture | Chemistry Class 11 - NEET

leah here from leh Versailles calm and
in this video I will show you how to
name alkynes using the IU PAC system
before we can name alkynes you have to
know how to recognize them in a molecule
an alkyne occurs when you have two pi
bonds between the same two set of
carbons also known as a carbon two
carbon triple bond the hybridization of
the carbon that holds the alkyne is SP
which means that it has a bond angle of
180 degrees and the shape of the
molecule will be linear let's try a
simple alkyne for example C Triple C
single bound to H this is a sign
commonly called acetyl lean if you
compare this to a two carbon alkane will
have the H C single bound CH but we also
have two additional sets of hydrogen's
as we place each PI bond we have to
remove two hydrogen's which gives the
alkyne a hydrogen deficiency index of
two and the formula of cnh2n
-2 this is compared to the formula of an
alkane for cnh2n plus 2 due to the
difference of four hydrogen's
when drawing an alkyne in lime structure
remember that the bond angle is 180
degrees people will often draw an alkyne
almost like an alkane by adding two
bonds but this is incorrect and this is
because the alkyne is linear and has to
be drawn as a straight line when reading
this structure though don't forget that
the triple bond starts and ends on two
carbons but you also have two additional
carbons on the end of the chain to name
an alkyne you follow the same rules that
you would for standard I you pack naming
and that means we first identify the
parent chain given that the alkyne
occurs in the middle of the molecule I
can start numbering from the right or
the left and I have a total of 4 carbons
giving me a first name of Bute because i
have a triple bond in the molecule i get
a last name of YN e where ein i place
number 2 in front of the IM to specify
where the triple bond occurs on the
molecule and this gives me a final name
of 2-butene let's try another example
for this example recognize that we have
a terminal alkyne because the triple
is at the end of the molecule I start by
highlighting the parent chain when the
alkyne is your highest priority you
start numbering from the side that will
give it the lowest number they start
numbering from the left and get a total
of five for our first name of pent since
the triple bond occurs on number one I
have one line for a final name of one
pen time for this example I have a
substituted alkyne and once again I
start by highlighting my parent chain I
have the option of numbering from the
left where I hit a methyl group at
carbon two or numbering from the right
where I hit the alkyne at carbon two
when you have a substituent and a triple
bond that both have the same number the
substituent gets highest priority and
that means I have to start numbering
from the left seven carbons in my parent
chain gives me a first name of Hecht
hitting my triple bonded carbon five
gives me a last name of five ein and my
methyl group substituent on carbon 2
gives me two methyl putting the name
together I have two methyl five hep time
a dying is not as scary as it sounds it
simply means that you have to carbon the
carbon triple bonds on the same molecule
will use this molecules an example and
once again we start by highlighting the
parent chain in this case I have a
terminal alkyne on the left and so we
start numbering from the left to get a
total of seven carbons for a first name
of HEPT
since I have two triple bonds occurring
on carbon 1 and carbon 5 I start with
the last name of dine to show that I
have two triple bonds but I also have to
include both numbers giving me 1 comma 5
putting this name together with the
numbers preceding the dine I get kept 1
5 dine if I choose to put the numbers in
the front of the molecule I have
something interesting occur I have 1 5
Hecht dine but because T and D are both
consonants and don't flow together I
have to add the letter A so I get 1 5
hepta dine to make the molecule read
better in the next video we'll look at
how to name a molecule that has both an
alkene and an alkyne in the same
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