The Halogenation of Alkanes
Summary
TLDRThis lecture delves into the radical reactions of alkanes, emphasizing their general unreactivity due to the lack of polarity. It outlines the three phases of halogenation: initiation, where light or heat breaks the halogen bond; propagation, involving radical reactions with alkanes to form new radicals; and termination, where radicals combine to form stable products. The 'quick product method' simplifies predicting reaction outcomes, and the process is characterized as a radical chain reaction, highlighting the importance of understanding these mechanisms for organic chemistry.
Takeaways
- đ Alkanes are generally unreactive compounds due to the similar electronegativities of carbon and hydrogen, resulting in no polarity.
- đ To induce reactions in alkanes, they must be paired with highly reactive radicals.
- đ Halogenation of alkanes is a three-phase radical substitution reaction involving initiation, propagation, and termination steps.
- đĄ The initiation step involves the homolytic cleavage of a halogen molecule (like Cl2) under light or heat, generating radicals.
- đ Propagation steps start and end with radicals but involve different radicals, maintaining the chain reaction by producing new radicals.
- đ In the propagation phase, a radical reacts with an alkane, leading to the formation of a new radical and a product (e.g., CH3Cl from CH4 and Cl radical).
- đ« Termination steps occur when radicals react with each other, forming non-radical products and ending the chain reaction.
- đ The overall reaction mechanism can be quickly summarized using the 'quick product method', which involves replacing a CH bond in the alkane with a halogen bond.
- đŹ The reaction is driven by light or heat, which is crucial for the initial formation of radicals.
- đ Students should be able to draw the descriptive mechanisms for these reactions, identify initiation and propagation steps, and understand the characteristics of each.
Q & A
Why are alkanes generally unreactive?
-Alkanes are generally unreactive because they do not have any polar bonds. The electronegativities of carbon and hydrogen are roughly the same, making the molecule non-polar. This lack of polarity means alkanes are neither nucleophilic nor electrophilic.
What is necessary to get alkanes to react?
-To get alkanes to react, they must be paired with reactive radicals. These radicals are extremely reactive and will force the alkanes to undergo a reaction.
What is halogenation of alkanes?
-Halogenation of alkanes is a three-phase radical substitution reaction involving initiation, propagation, and termination steps.
What occurs during the initiation step of alkane halogenation?
-During initiation, a halogen molecule (e.g., Cl2) reacts with light or heat, causing homolytic bond cleavage and forming two reactive radicals.
How does the propagation step work in the halogenation of alkanes?
-In propagation, the radical formed in the initiation step reacts with an alkane (e.g., methane). This reaction produces a new radical (e.g., a methyl radical) and continues with the methyl radical reacting with another halogen molecule to form products and regenerate the initial radical.
What is the termination step in alkane halogenation?
-Termination occurs when two radicals meet and react to form a non-radical product, effectively ending the chain reaction. Possible terminations include two Cl radicals forming Cl2, two methyl radicals forming ethane, or a Cl radical and a methyl radical forming chloromethane.
What is the significance of homolytic bond cleavage in radical reactions?
-Homolytic bond cleavage is significant because it splits a bond evenly, giving one electron to each atom involved. This process creates radicals, which are essential for initiating and propagating radical chain reactions.
Why is light or heat necessary in the initiation step?
-Light or heat provides the energy needed to break the bond between halogen atoms in a halogen molecule, forming reactive radicals that can then react with alkanes.
What is meant by 'propagation' in the context of radical chain reactions?
-Propagation refers to the steps in which radicals react with stable molecules to form new radicals, thereby sustaining the chain reaction. Each propagation step produces a different radical, which continues the reaction.
What are the products of the halogenation of methane with chlorine under light or heat?
-The products of the halogenation of methane with chlorine under light or heat are chloromethane (CH3Cl) and hydrogen chloride (HCl).
What is a radical chain reaction?
-A radical chain reaction is a type of chemical reaction in which radicals are formed and react with stable molecules to produce new radicals, creating a chain of reactions. It involves initiation, propagation, and termination steps.
How can you quickly determine the products of an alkane halogenation reaction?
-To quickly determine the products of an alkane halogenation reaction, replace one hydrogen atom in the alkane with a halogen atom, and the other halogen atom will form a hydrogen halide as a side product.
Outlines
đŹ Alkanes and Radical Reactions
This paragraph introduces the concept of alkanes reacting via radical reactions. Alkanes are generally unreactive due to their non-polar bonds and lack of nucleophilic or electrophilic properties. To induce a reaction, alkanes must be paired with reactive radicals. The lecture focuses on the halogenation of alkanes, which is a three-phase radical substitution reaction involving initiation, propagation, and termination steps. The example of methane reacting with a halogen (Cl2) under light or heat conditions is used to illustrate the process, resulting in the formation of CH3Cl and HCl. The role of light (hΜ) in the reaction mechanism is also explained, highlighting the importance of understanding the descriptive mechanism in organic chemistry.
đĄïž Mechanism of Alkane Halogenation
This paragraph delves deeper into the mechanism of alkane halogenation, explaining the steps involved in the reaction. The initiation step involves the homolytic cleavage of Cl2 by light or heat, generating two chlorine radicals. These radicals then react with alkanes, such as methane, in the propagation step. The reaction mechanism is characterized by the movement of unpaired electrons, leading to the formation of a methyl radical and a halogen radical. The propagation step continues as the methyl radical reacts with another Cl2 molecule, resulting in the formation of the desired product and a side product, HCl. The paragraph also discusses the termination step, where radicals combine to form non-radical species, effectively ending the reaction. The quick product method is introduced as a shortcut for identifying products in organic chemistry tests, emphasizing the substitution of CH bonds with halogen bonds in alkanes.
đ Understanding Radical Chain Reactions
The final paragraph summarizes the key points discussed in the lecture. It reiterates that alkanes are unreactive compounds due to their non-polar nature and lack of nucleophilic or electrophilic properties. To facilitate their reaction, alkanes must be paired with reactive radicals, as demonstrated in the halogenation process. The paragraph also emphasizes the importance of understanding the three phases of the halogenation reaction: initiation, propagation, and termination. The lecture concludes by encouraging students to practice drawing the descriptive mechanisms for these reactions and to understand the characteristics of each step, such as the homolytic bond cleavage in initiation and the continuous generation of radicals in propagation. The overall goal is to help students prepare for exams by reinforcing their understanding of radical chain reactions and radical substitution reactions.
Mindmap
Keywords
đĄAlkanes
đĄRadical Reactions
đĄHalogenation
đĄInitiation
đĄPropagation
đĄTermination
đĄHomoytic Bond Cleavage
đĄNon-Polar Bonds
đĄQuick Product Method
đĄRadical Chain Reaction
đĄElectrophilic and Nucleophilic
Highlights
Alkanes are generally unreactive compounds due to the similar electronegativity of carbon and hydrogen, resulting in no polarity.
To induce alkanes to react, they must be paired with highly reactive radicals.
Halogenation of alkanes is a three-phase radical substitution reaction involving initiation, propagation, and termination steps.
Initiation in halogenation involves homolytic bond cleavage of halogens by light or heat, creating radicals.
Propagation steps are characterized by reactions starting and ending with radicals, but not the same radical, sustaining the chain reaction.
The quick product method simplifies identifying products in halogenation reactions without detailing the mechanism.
Radical chain reactions are initiated by light or heat, which generate radicals that react with alkanes, creating a chain of reactions.
Radical substitution reactions involve the substitution of a CH bond in an alkane with a halogen bond.
The role of light (hΜ) in the mechanism is to provide the energy for the initiation step.
Methane reacts with chlorine under light or heat to produce chloromethane and HCl as products.
The propagation step involves the alkane's hydrogen being replaced by a halogen, forming a new radical.
Termination steps occur when radicals combine to form stable, non-radical molecules, ending the chain reaction.
Different combinations of radicals can lead to various termination products, such as Cl2 or CH3Cl.
Students should practice writing out the descriptive mechanisms for halogenation reactions to solidify their understanding.
The alkane's lack of polarity makes it neither nucleophilic nor electrophilic, requiring the presence of reactive radicals for reactions.
The lecture emphasizes the importance of understanding the three phases of radical reactions for alkanes: initiation, propagation, and termination.
The quick product method is a practical tool for organic chemistry exams, allowing for rapid product identification.
The lecture concludes with a summary of key points, reinforcing the understanding of alkane reactivity and halogenation mechanisms.
Transcripts
in this online lecture we're going to
discuss how alkanes react via radical
reactions and let's look at our key
points first number one alkanes are
unreactive compounds in general and
number two here we're going to see that
in order to get alkanes to react
you must pair them with reactive
radicals and number three here
halogenation of alkanes is a three phase
radical substitution reaction which
involves initiation a propagation step
and a termination step now let's look at
an alkane right here and let's remind
ourselves about some of the properties
of alkanes
for instance remember alkenes don't have
any polar bonds the electronegativity of
carbon and hydrogen are roughly the same
so there's no polarity for this molecule
another thing that's true that means
therefore the molecule is not very
nucleophilic and it's also not
electrophilic neither the carbons nor
the hydrants are partially positive or
partially negative so this is why
alkenes are not very reactive molecules
so in order to get them to react what
we're going to have to do is place them
near reagents that are extremely
reactive this in turn will give the
alkane no other option but to react with
the molecule so let's see how that works
here here is our overall reaction notice
she got ch4 methane a typical alkane and
we're reacting a halogen cl2 with hv or
heat and we're getting ch3cl and HCL as
products the hv is actually h nu which
comes from an equation in physics which
member means energy equals h times the
frequency of light with H being Planck's
constant so we're interpreting this h v
and I'm calling it V just to make it
simple here we know that that means the
addition of light will see what role
this plays in the mechanism in a few
seconds now in organic chemistry it's
very important that we know the
descriptive mechanism of how this
reaction goes down and we should know
it's
broken up into three parts with the
first part being called initiation and
we're gonna see these steps are
appropriately named which will help us
remember these four hour or go tests so
here's how it all begins the cl2 in this
reaction reacts with either light or
heat what the light in heat does is
simply break the bond between the two
CLS and notice our arrow movement here
we have single headed arrows which means
the movement of one electron the result
of this error movement would look like
this notice what we're getting here are
two radicals remember radicals have that
unpaired electron sitting there which
makes these species very reactive and
very unstable so notice in this
initiation step we went from a non
radical to a radical and the type of
bond cleavage performed here if you
remember is homolytic bond cleavage so
let's pause for a second here and point
out the key feature of a typical
initiation reaction it simply involves
like we said a non radical turning into
a radical this is initiation because the
non radical that we're turning into the
radical is now becoming a very reactive
species so this kind of gets the
reaction going this then brings us to
the second part of this descriptive
mechanism which is called propagation
what we end up doing here is taking the
radical and now reacting it with our
alkane and remember going back to our
overall reaction we're choosing to react
methane as our alkane so going back here
we're writing out our methane right here
and we're just emphasizing one of his CH
bonds and notice here we have the CL
radical that's very reactive now is
forced to react with the alkane with
this type of mechanism the unpaired
electron moves this way and one of the
electrons in the CH bond moves this way
to meet up with him and another electron
in the CH bond jumps up on top of the
carbon notice again that CH bond is
breaking homolytically
for products we end up with something
that looks like this
notice the CL radical is now bonded to
the H that was originally bonded to the
methane and the methane has simply
become a methyl radical so now the
methyl being radicalized let's say
further reacts but this time he reacts
with a CL 2 molecule now remember here
where did this CL 2 molecule come from
if we go back to the overall reaction
remember we have the CL 2 right here and
the CL 2 were reacting at this point
simply a CL 2 that hasn't react with
either the heat or light which means it
hasn't been homolytically cleaved it's
still intact so going back here we are
let's look at the arrow movement here in
this step the radical electron here and
the methyl meets up with one of the
electrons in the CL bond and another
electron in the CL 2 bond jumps up on
the other CL again homolytically 4
products we end up with something that
looks like this notice we get the CL
radical but we also get this thing right
here as a product and let's remind
ourselves here remember this is the
product of the overall reaction and
remember we also get this as a side
product it's in these propagation steps
is where we get these things notice here
is one of the products right here and
there's that side product HCl over here
however let's focus now on what it means
to be a propagation step notice the
characteristics here propagation steps
start with radicals and end with
radicals but however not the same
radical it's a different radical in this
case CL radical going to methyl radical
and in the other propagation step we
have the methyl radical going to the CL
radical make sense of this
think about it propagation means to keep
going if we're just simply going from
Run radical to another we're able to
keep this reaction going by producing
radicals which will then go on to react
with other non radicals which mean
these propagation steps will keep
happening as the reaction proceeds
however we should also look at how these
reactions end think about the big
picture here for a second as this
reaction is proceeding what we have in
our reaction mixture is a buildup of
radicals notice right here shaded in
green again we got CL radicals floating
around and we got those methyl radicals
floating around what's possible here is
in the reaction mixture that these
radicals can actually meet up if they do
that's what defines what's called the
termination step of this mechanism for
instance let's say 2 CL radicals happen
to meet up in the reaction mixture if
they do this is the electron movement
these two electrons pair and we end up
with this as a result notice we're
getting a non radical CL 2 molecule here
but this is not the only possibility
remember we also have methyl radicals
floating around in solution if these two
meet up it would be the same type of
mechanism here 2 electrons pairing and
giving us this right here as a product
another non radical and the last
possible combination is a CL radical
meeting up with a methyl radical in this
case this is the electron movement and
we end up with this as a result again
make sense of this these are termination
steps because we're going from radicals
to non radicals and these non radicals
are not going to be very reactive so
this is like an end point of the
reaction so to help us remember this
lets know that all termination steps
involve a radical turning into a non
radical so there it is our overall
reaction right here but however let's
talk about quick product method here
remember on an organic chemistry test we
sometimes have to quickly get to the
product without mulling through the
mechanism and for this quick product
method is very simple what we're doing
is we're taking the alkane right here
and we're noticing that if it happens to
be reacted with some kind of Hal
in this case cl2 with either heat or
light then what we're doing here is
simply replacing one of the CH bonds of
the alkane with AC halogen bond in this
case CCL and then just in case as a side
product the other CL in this case gets
bonded to a hydrogen
another example here just to make sure
you got this reaction down looks
something like this notice in this case
we have on alkane which is ethane but
we're reacting it with BR 2 with either
light or heat and notice again all
that's happening here is we're replacing
one of the CH bonds with here AC BR bond
and notice we're getting this time hbr
as a side product again very simple
quick product method some vocab here we
should know is that the type of reaction
that we're looking at here is called a
radical chain reaction the big picture
here is that the light and heat in the
initiation step that splits the CL 2 or
the BR 2 creates the original radicals
and those radicals then go on to react
with alkenes which then propagate into
other radicals which creates a chain of
radical reactions hence the term radical
chain reaction another term that can
describe this reaction is a radical
substitution reaction we notice this
when we think of quick product again a
CH bond in an alkane is being
substituted and turning into AC
connected to a halogen bond a smart or
go students should be able to draw out
the descriptive mechanisms for these
reactions they can write out the
initiation Union even tell you its
initiation and of course show you the
characteristics of initiation they could
then take you to propagation and talk
about all the possible termination steps
so to prepare for your exam you should
take a blank sheet of paper and try to
by yourself write this whole process out
it'll really help you wrap your brain
around what's happening here so what did
we learn here
points number one we saw that again
alkanes are unreactive compounds because
they have no polarity they're not
nucleophilic they're not electrophilic
so therefore we also saw that in order
to get alkanes to react
we must pair them with reactive radicals
and by doing that key point number three
halogenation of alkanes is a three phase
radical substitution reaction we
initiate then we propagate and then we
terminate
you
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