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in this video we're going to talk about
organic chemistry
now this video is for those of you who
are taking
the first semester of organic chemistry
in college
organic chemistry focuses on organic
compounds compounds that contain
carbon atoms now carbon likes to form
four bonds
but you also need to know the number of
bonds that the other elements like to
form
so elements in the first group of the
periodic table
like hydrogen they like to form one bond
beryllium for example likes to form two
bonds
boron which has three valence electrons
it likes to form three bonds
carbon has four valence electrons
carbon likes to form four bonds
nitrogen nitrogen likes to form three
bonds
in organic chemistry oxygen likes to
form two
and elements like fluorine and the other
halogens chlorine
bromine iodine they like to form one
bond
now there are cases where chlorine
bromine and iodine conform seven bonds
but we're really not going to cover that
in this video i do have another video on
title lewis structures which
goes into that detail for those of you
who might be curious
now understanding that this is important
because it helps us to draw
lewis structures so for instance let's
say if we want to draw the lewis
structure of
water h2o knowing that
oxygen likes to form two bonds and
hydrogen
likes to form one we can start with a
structure that looks like this
now oxygen likes to have eight electrons
each bond represents two electrons so
that's a total of four
to get to eight we need to add two lone
pairs
so that is the lewis structure for water
now the bond between hydrogen and oxygen
is known as an h-bond or a hydrogen bond
which is a special type of covalent bond
hydrogen bonds occur whenever
h is directly attached to nitrogen
oxygen or fluorine
hydrogen bonding explains why water has
such a high boiling point
now let's say if we have methyl fluoride
how can we draw the lewis structure
for that molecule
so we know that carbon likes to form
four bonds so we're going to put that in
the middle
hydrogen likes to form one bond
fluorine also likes to form one bond so
this is the best lewis structure that we
can make
and if you want you can add three lone
pairs to fluorine so that it has eight
electrons around it
elements in the second row like carbon
nitrogen oxygen and fluorine they like
to have eight electrons
they don't always have to have eight but
ideally speaking
they like to have eight so that they can
be stable
but that's the structure of methyl
fluoride
now the carbon fluorine bond is a polar
bond
carbon has an electronegativity value of
2.5
and the electronegativity value of
fluorine is 4.0
whenever the electronegativity
difference between two elements
is 0.5 or more the bond is said to be
polar
so this is a polar bond it's not a
hydrogen bond
because fluorine is not directly
attached to hydrogen
but it's a polar covalent bond now you
might be wondering
what does it mean for a bond to be polar
a polarized object is an object that is
neutral overall
but has charge separation that is one
side is positive and the other side is
negative
so this is a polarized object
the bond is polar because fluorine
carries
a partial negative charge due to the
fact that
it's more electronegative than carbon so
it pulls electrons toward itself
and carbon being less electronegative
has a partial
positive charge so due to that charge
separation
we can say that the carbon fluorine bond
is polar
now the carbon hydrogen bond that's a
non-polar bond
the reason being is the
electronegativity difference between
carbon and hydrogen is less than 0.5
carbon we said its electronegativity
value is 2.5
hydrogen is 2.1 so the difference
between the two
is 0.4
so understand this anytime you see a
hydrocarbon
or a bond that contains carbon and
hydrogen
those bonds are non-polar bonds
because sometimes you need to determine
if a molecule is polar
or nonpolar so for instance methane ch4
that's a non-polar molecule because it
only contains carbon and hydrogen
so far we've considered two types of
covalent bonds
polar covalent bonds and another special
type which are hydrogen bonds
and we also talked about nonpolar
covalent bonds like the carbon hydrogen
bond that's a non-polar covalent bond
we need to understand the difference
between a covalent bond
and an ionic bond
in a covalent bond the electrons are
being shared
they can be shared equally or unequally
so in the case of the hydrogen molecule
h2 the electrons in this bond
are being shared equally between these
two atoms
because the atoms are identical so
that's known as
a nonpolar covalent bond which i'll put
cb for covalent bond in the case of
hydrogen fluoride that is a
polar covalent bond
hydrogen has an electronegativity of 2.1
fluorine has a value of 4.0
so hydrogen is going to be partially
positive
fluorine will be partially negative and
this is a special type of
a polar covalent bond that's a hydrogen
bond
so those are some two examples of
covalent bond you have
non-polar covalent bonds and polar
covalent bonds
so make sure you understand the
difference here the electrons are shared
equally as the key word
and in this case the electrons are
shared unequally
fluorine is going to have a tighter hold
on those electrons
now let's talk about ionic bonds
so in an ionic bond the electrons are
not shared they're transferred
a good example is the reaction between
sodium metal
and chlorine sodium has one valence
electron
chlorine has seven
sodium is a metal and chlorine
is a non-metal metals like to give away
the electrons
because they're electropositive
nonmetals like to
accept or acquire electrons because
they're electronegative
so sodium is going to give one electron
to chlorine a half arrow represents the
flow of one electron
a full arrow represents the flow of two
electrons
so in this reaction sodium is going to
turn into
the sodium plus ion which is known as a
cation
positively charged ions are called
cations
now chlorine when it acquires an
electron
it becomes chloride so now it has eight
electrons
it's gonna now have a negative charge
negatively charged ions are known as
anions
now going back to physics you know that
opposite charges attract one another
so a positively charged ion will feel a
force of attraction
to a negatively charged ion so these two
charges they're attracted to each other
and that force of attraction that
electrostatic force
is what keeps the ions in an ionic
crystal together
so that creates that ionic bond the
electrostatic force of attraction that
pulls the sodium and the chloride ions
together
so now you know the difference between
an ionic bond
and a covalent bond
so now we're going to spend some time
drawing lewis structures
of certain organic compounds but before
we do let's talk about
some common names of alkanes
alkanes are saturated organic compounds
meaning that the carbon atoms are filled
with hydrogen atoms
methane is a one carbon alkane
it's m e t h a n e
a two carbon alkane c two h six this is
known as
ethane
c3h8 this is called propane
c4 h10 this is known as
butane
so alkanes generally follow this formula
c n
h two n plus two
so a five carbon alkane will have twelve
hydrogen atoms
if n is five then it's going to be h two
times
five plus two so that's ten plus two you
get twelve
a five carbon alkane is known as pentane
a six carbon alkane is known as
hexane you may want to take some notes
by the way
because you'll need to know this at
least up to 10.
the seven carbon alkane that's heptane
next we have octane
and then after that this is that's a 20.
this is not a or if you want to call it
no name
and then c10h22 that's known as
decade
now let's talk about how to draw the
lewis structure of c2h6
c2h6 we know that's ethane
and you can write the condensed
structure like this
it's ch3ch3
so this tells you how many hydrogen
atoms are on each carbon
first carbon has three hydrogen atoms
and that's attached to the second carbon
which also
has three hydrogen atoms
so that's how you can draw the lewis
structure of ethane
now what about c2h4
how would you draw the lewis structure
for that
so this time each carbon atom is going
to have
two hydrogen atoms instead of
three because there's a total of four
and you want to make sure that
the four hydrogen atoms are distributed
equally
so what bond do we need between the two
carbon atoms
well we know that carbon likes to form
four bonds
so the only way this is going to happen
is if we put a double bond
between the two carbon atoms
and so this is known as an alkene
alkenes contain at least one double bond
alkanes do not contain double bonds
so a two carbon alkene is known as
ethene
now what about this one c2h2 how can we
draw the lewis structure for that
well we have a total of two hydrogen
atoms
so we're going to put one hydrogen atom
on each carbon
and in order for the two carbon atoms to
have
four bonds we need to put a triple bond
in the middle
because carbon likes to form four bonds
and so whenever you have a hydrocarbon
with a triple bond you now have what is
known as
an alkyne a two carbon alkyne is known
as
ethyne
the common name for this is acetylene
and the common name for this compound is
known as
ethylene
alkenes and alkynes are known as
unsaturated compounds because they don't
contain the maximum number of hydrogen
atoms
per carbon atom alkanes are known as
saturated compounds so make sure you
keep this in mind
now let's focus on the carbon-carbon
bonds
what would you say which of these bonds
is the longest bond
the carbon-carbon single bond the double
bond or
the carbon-carbon triple bond
which one is the longest and which one
is the shortest
you need to know that the carbon carbon
single bond
is longer than the carbon-carbon double
bond
and that's longer than the carbon-carbon
triple bond
the length of the carbon-carbon single
bond is 154 picometers
which is 1.54 angstroms
one angstrom is 100 picometers the cc
double bond
is 133 picometers in ethene
and in ethyne i mean ethyne
it's uh 120.
so we need to understand are is that
triple bonds are
short bonds whereas single bonds
are long bonds because sometimes you may
get a test question that
asks you something about bond length
which of these bonds is the shortest
or which of these bonds is the longest
so single bonds are longer than triple
bonds keep that in mind
so now that we've talked about bottom
left
let's talk about bond strength which
bond
is the strongest the single bond the
double bond
or the triple bond what would you say
the single bond is the weakest
the triple bond is the strongest
why is that well it's easier
to break one bond instead of three bonds
imagine trying to break a pencil it's
easier breaking one pencil
than trying to break three pencils at a
time three bonds are stronger than one
now let's talk about sigma and pi bonds
a single bond contains one
sigma bond all single bonds
are sigma bonds a double bond
contains one sigma
and one pi bond
the triple bond contains one sigma
and two pi bonds now which
bond is stronger a sigma bond or a pi
bond
you need to know that sigma bonds are
stronger
than pi bonds so it's harder
to break this bond versus
just one of the pi bonds in the triple
bond not all three bonds but
one of the pi bonds so in summary
a triple bond is stronger than
a single bond because you're comparing
three bonds to one
however a sigma bond is stronger than a
pi bond
when you're comparing one bond with one
bond so i'm just going to say that one
more time
sigma bonds are stronger than pi bonds
but triple bonds are stronger
than single bonds
now the next thing we're going to talk
about is bond order
is the bond order for a single bond
a double bond and a triple bond this one
is pretty straightforward
for a single bond the bond order is one
for a double bond the bond order is 2
and for triple bond the bond order is 3.
so that's just something to know
now let's talk about hybridization what
is the hybridization
of the carbon atoms highlighted in green
would you say it's s sp2 sp3
sp dsp3 d2 sp3 what would you say
a quick and simple way to determine the
hybridization
around a certain carbon atom is to count
the number of
atoms attached to that particular
carbon atom and the number of lone pairs
that it has
so this particular carbon atom is
attached to four
other atoms so you could think of it as
having
four groups around it so the
hybridization is going to be s1
p3 because the exponents add up to four
now let's say if we want to determine
the hybridization of this carbon atom
that carbon is attached to three other
atoms
so it has three groups around it the
hybridization is going to be s1
p2 or sp2 hybridized
and then for this particular alkyne the
carbon atom has
two atoms attached to it so for two
groups it's going to be
sp hybridized so that's a quick and
simple way to determine the
hybridization
of a carbon atom
here's a question for you so let's say
we have a lewis structure that looks
like this
what is the hybridization of
not the atoms but the bond what is the
hybridization of the ch bond
and also of
this particular ch bond what would you
say
if you were to get a test question that
asks you that question how would you
determine the hybridizations
of this bond highlighted in red
and this bond highlighted in blue
what you need to do is determine the
hybridization of
the atoms that are connected to those
bonds
so what is the hybridization of this
carbon atom
that carbon is attached to four other
atoms
so it has four groups and so it's going
to be
sp3 hybridized
now you need to look at the other atom
the hydrogen
hydrogen is only attached to one atom so
what do you think the hybridization of
hydrogen
is going to be this is going to be s
so that's the hybridization of the ch
bond
that's going to be sp3 dash
s you simply just write both of these
together
now what about the ch bond highlighted
in blue
what is the hybridization of that ch
bond feel free to pause the video if you
want to try it
so let's start with the carbon atom that
carbon is attached to two
other atoms so it's going to be sp
hybridized and hydrogen is always
s hybridized so that particular ch bond
is going to be sp dash s hybridized
now another question that you might be
asking given
this compound is how many sigma and pi
bonds
are in this compound feel free to pause
the video
and try
so first let's count the number of sigma
bonds
every single bond is a sigma bond so we
have one
two three four five
and there's one sigma bond in the triple
bond so we have a total
of six sigma bonds
now how many pi bonds do we have we know
that a double bond contains one pi bond
a triple bond contains two pi bonds
so this molecule have six sigma bonds
and two pi bonds so that's how you can
determine the number of sigma and pi
bonds
in an organic compound
now the next topic of discussion is how
to calculate the formal charge
of an element so let's use carbon as an
example
calculate the formal charge of
each carbon atom for these three
situations
if you know how to do it feel free to go
ahead and try
now there's a formula that will help you
to calculate the formal charge of an
atom
and here it is the formal charge is
going to be
equal to the number of valence electrons
of the element minus
the number of bonds and dots attached to
that element
so for the first example
carbon has four bonds i mean
let me say it again carbon has four
valence electrons
it's in group 4a of the periodic table
and in this example it has 3 bonds 1 2
3 and it doesn't have any dots around it
so 4 minus 3 is 1.
so this particular carbon atom has a
formal charge
of plus one so we could say that it has
a positive formal charge
now moving on to the next one it's going
to be
four valence electrons minus three bonds
and one dot
so we got three bonds one dot that's
four minus four
so that's zero so this one is neutral
it doesn't have a charge
now for the last example on the right
the valence electrons of carbon will
still be the same
we still have three bonds but this time
we have two dots
one lone pair is equivalent to two dots
so three plus two is
five four minus five is negative one
so this particular carbon atom has a
negative charge
we said that positive let me say that
again we said that positively charged
ions are called
cations and negatively charged
ions are known as anion when you add a
carbon to it
this is called a carbocation
and on the right this is called a carb
anion carb anions have negative charges
now when you have an odd number of
electrons you have what is known as a
radical
so a one carbon radical is known as a
methyl radical
radicals tend to be neutral
now let's work on some more examples go
ahead and calculate the formal charge
of the sulfur atom
and also calculate the formal charge
of the nitrogen atom
go ahead and take a minute to work on
that example for those two examples
rather
by the way for those of you who are
looking for a specific topic but
for some reason you're not finding it in
this video when you get a chance
do a youtube search type in organic
chemistry
playlist i have two playlists an older
one
and a newer one the newer one is going
to be more helpful
so in that playlist chances are you you
could find
a topic that you might be looking for so
just
feel free to take a look at that as well
i'll be posting some links
in the description section below of this
video
with some other topics that might help
you with
the first exam that you might be taking
in organic chemistry
so feel free to check that out as well
so let's begin let's start with sulfur
let's write the formula so the formal
charge is going to be the number of
valence electrons
minus the sum of the bonds and dots
around that element
now how many valence electrons does
sulfur have
sulfur is found in group 6a
of the periodic table if you don't have
one you could do a google search
you could find it in google images and
then you could pull up the periodic
table
if you look for sulfur it's right under
oxygen and it's in group
6a which means that sulfur has six
valence electrons in this structure it
only has
one bond attached to it now how many
dots does it have
well we have three lone pairs which is
equivalent
to six dots so this
is six minus seven so the sulfur atom
has a negative one formal charge
now for nitrogen in the ammonium ion
nitrogen has five valence electrons it's
in group
5a of the periodic table in this
structure it has
four bonds no dots so 5 minus four is
one
so this particular nitrogen atom has a
plus one
or some may say a one plus formal charge
so that's how you could determine the
formal charge
of an element now let's go back to this
structure
because there's another question that
i'm going to ask that teaches another
concept
how many bonding electrons and
nonbonding electrons are present in this
ion
what would you say
a lone pair
represents a pair of non-bonding
electrons because they're only attached
to one atom
so they're non-bonding electrons in a
bond
you have two bonding electrons
so to count the number of bonding
electrons this is going to be
2 4 6 eight
so this particular ion has a total of
eight
bonding electrons because it has four
bonds
each bond contains two bonding electrons
now how many nonbonding electrons do we
have
two four six there's a total of six
nonbonding electrons which equates to
three lone pairs
so make sure you understand that
constant one lone pair
is equal to two nonbonding electrons
and one bond equates to two bonding
electrons now let's continue
our discussion of drawing the lewis
structures of organic compounds
as well as considering the functional
groups of those compounds
so let's consider ch3ch2
oh and let's compare that with
ch3 cho
so feel free to pause the video and go
ahead and draw the two
lewis structures so
for the one on the left we have a carbon
that is connected
to three hydrogen atoms
and then it's attached to a ch2 that is
a carbon with
two hydrogen atoms and that's connected
to an o h group
keep in mind oxygen likes to have two
bonds
and it's going to have two lone pairs so
that's the lewis structure for this
molecule
now whenever you see an oh group this is
the functional group
of an alcohol
and the fact that we have two carbons
this is
associated with the alkane ethane
but because we have an alcohol we can
name
it ethanol so alcohols have the suffix
o-l
so this is ethanol
now what about the structure on the
right so we have a methyl group
that is a ch3 and that is
attached to a carbon atom now it's cho
instead of o-h
when you see c-h-o this is the
functional group of an aldehyde
if we were to write o-h this is not
going to work because
carbon doesn't have four bonds
so that tells you that we need a
different structure
an aldehyde contains a carbonyl group
which is basically a carbon double
bonded to an oxygen
and then here's the hydrogen so in this
case every carbon atom
has four bonds every hydrogen atom has
one bond
and the oxygen atom has two bonds which
is what they usually have in neutral
molecules
and of course the oxygen is going to
have two lone pairs as it did here
so this is a carbonyl
and this particular carbonyl which is
attached to a hydrogen
that is known as an aldehyde
so that's the name of the functional
group a two carbon aldehyde
is called ethanol instead of ethanol
so it ends in a l as opposed to ol
the common name for this aldehyde is
acetaldehyde
now let's consider these two examples
ch3och3
and ch3 co
ch3 notice the difference so go ahead
and draw the lewis structure for these
two organic compounds
for the first one we have a methyl group
that's a carbon with three hydrogen
atoms
and then we have an oxygen in the middle
as we said before oxygen likes to form
two bonds
and then another method group to the
right of that
so this particular compound has a
functional group known as an ether
an ether is basically an oxygen atom
between two carbon atoms that's an ether
now because this particular ether has
two methyl groups
one to the left one to the right this is
known as
a dimethyl ether the prefix die
means two tri means three
tetra means four penta means five
now how would you draw the lewis
structure for the compound on the right
so we know that we have a methyl group
on the left
and then there's a carbon atom now where
do we put the oxygen atom
if we put it in the middle
this carbon atom is going to have two
bonds
so to fix that problem we need to put it
up here we need to make
another carbonyl functional group and
then
write the methyl group so in this case
every carbon atom has
four bonds the oxygen atom has two bonds
and every hydrogen atom has one bond
that's why it's important to know
the number of bonds each element like to
make because
it makes drawing the lewis structures a
lot easier
now even though we have a carbonyl
functional group
the name of the functional group for
this particular molecule is different
than an aldehyde
this is known as a ketone
when the carbonyl group is in the middle
of the chain is the ketone
when it's at the end of a chain it's an
aldehyde
so to name this particular ketone it's
called
propanone
think of propane for a three carbon
alkane
so a three carbon ketone is going to be
propanone
ketones have the suffix o and e
so notice the difference between a
ketone and aldehyde
a ketone will have the carbonyl group in
the middle
and aldehyde will have the carbonyl
group at the end
so an aldehyde has a hydrogen that's
attached
to the carbonyl group a ketone does not
have
the hydrogen attached to the carbonyl
group and that's how you could
distinguish
a ketone from an aldehyde even though
they both have
the carbonyl functional group
now here's the next one ch3 co2
ch3 versus ch3
and then ch2 times 3
co2h go ahead and draw the lewis
structures for
those two molecules
so i'm not going to completely draw it
out i'm going to expand
the condensed structures so instead of
writing c with three hydrogens i'm going
to leave it as ch3
i'm going to expand this part
it's not going to be like this because
carbon doesn't have four bonds
instead the ch3 is going to be attached
to a carbon
we're going to have a carbonyl group and
then an oxygen
attached to a ch3 so this functional
group is known as an
ester an ester has
two oxygen atoms and it has a carbonyl
group
on the right we have a ch3
that's a methyl group and then we have
three methylene groups
so ch2 ch2 ch2
and then we have co2h which you can
write as cooh
but we're going to expand that further
so we're going to have a carbonyl group
and the hydroxyl group
so when you combine a carbonyl group and
a hydroxyl group like that
you get a functional group called a
carboxylic acid
now that particular carboxylic acid has
a total of
five carbon atoms a five carbon alkane
is known as
hexane so a five carbon
carboxylic acid is going to be called
wait let me take that back a five carbon
alkane is not hexane but pentane
hexane has six carbons so a five carbon
carboxylic acid
is going to be called pentanoic acid
so that's how you can name that
particular carboxylic acid
now as for the ester we have a methyl
group
and then here we have a two carbon group
with two oxygens which is called
ethanoate so combined this is called
methyl ethyno8
if you want more examples on how to name
esters
just type in ester nomenclature
in the youtube search bar and you'll see
some videos
come up that has more examples on how to
name esters
but that's how you can name this
particular one it's called methyl
ethanoate now here's a challenge problem
go ahead and expand
this particular structure
you don't have to worry about naming it
but go ahead and expand it
what i would do is i would start with
the ch
group there so we have a carbon
attached to a hydrogen
and then that carbon has two methyl
groups attached to it
so we have one over here and then
another one on top
so this ch is the ch that we see here
and then to the right there is another
ch next to it so i'm going to put carbon
with a hydrogen so that's the ch
of this group and then attached to that
is an oh
which we can put here and then a ch3
the ch trees or the methyl groups
they're always at the end
of a structure never in the middle
the ch groups tend to branch out and ch2
groups
tend to be in the middle of a structure
so let me give you another example
so let's say we have ch3
times 3 and then the carbon
and then a ch with
a br and then
a ch2 and then another ch2
times two followed by a ch string go
ahead and expand that structure
so for this one i'm going to start with
a carbon atom
that carbon has three methyl groups
connected to it
so here's the first one here is the
second one
and here is the third one next we have a
ch group
so i'm going to put a carbon and then
the hydrogen
and then there's a bromine atom which is
going to be attached to this carbon
because carbon likes to form four bonds
next we have a ch2 group which tends to
be in the middle
so that's a carbon with two hydrogen
atoms and then we have two more ch2
groups
here's the first one and here is the
second one
and then we have a ch3 group at the end
so that's how you can quickly expand a
structure that looks like that
so keep in mind the methyl groups are at
the end
of a structure like this
the ch2 groups they're in the middle
and the ch groups tend to branch off
they're also in the middle
but they tend to have other special
elements attached to it like a bromine
or
an oh group
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