The Halogenation of Alkanes

Summary

This 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

• 😀 Alkanes are generally unreactive compounds due to the similar electronegativities of carbon and hydrogen, resulting in no polarity.
• 🔍 To induce reactions in alkanes, they must be paired with highly reactive radicals.
• 🌟 Halogenation of alkanes is a three-phase radical substitution reaction involving initiation, propagation, and termination steps.
• 💡 The initiation step involves the homolytic cleavage of a halogen molecule (like Cl2) under light or heat, generating radicals.
• 🔄 Propagation steps start and end with radicals but involve different radicals, maintaining the chain reaction by producing new radicals.
• 🔗 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).
• 🚫 Termination steps occur when radicals react with each other, forming non-radical products and ending the chain reaction.
• 📚 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.
• 🔬 The reaction is driven by light or heat, which is crucial for the initial formation of radicals.
• 📝 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

00:00

🔬 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 (hν) in the reaction mechanism is also explained, highlighting the importance of understanding the descriptive mechanism in organic chemistry.

05:02

🌡️ 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.

10:05

🔄 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.

Keywords

💡Alkanes

Alkanes are a class of hydrocarbons with the general formula CnH2n+2, characterized by single covalent bonds between carbon atoms. They are known for their unreactivity due to the lack of polarity in their bonds, as the electronegativities of carbon and hydrogen are similar. In the video, alkanes are discussed in the context of their reactions with reactive radicals, highlighting their need for external reactivity to undergo chemical changes.

💡Radical Reactions

Radical reactions involve species with unpaired electrons, making them highly reactive. The video script explains that alkanes, being generally unreactive, require the presence of reactive radicals to undergo reactions. The process of halogenation of alkanes is described as a radical reaction, initiated by light or heat, which breaks the bond in a halogen molecule to form radicals.

💡Halogenation

Halogenation is a chemical reaction where a halogen atom is added to an organic molecule. In the context of the video, halogenation of alkanes is a three-phase radical substitution reaction involving initiation, propagation, and termination steps. The script uses the example of methane reacting with chlorine under light or heat to form methyl chloride and hydrochloric acid.

💡Initiation

Initiation is the first step in a radical chain reaction, where a non-radical species is converted into a radical. In the video, the initiation step is illustrated by the homolytic cleavage of a chlorine molecule (Cl2) under the influence of light or heat, resulting in two chlorine radicals, which are essential for starting the reaction.

💡Propagation

Propagation steps are central to radical chain reactions, where radicals react with other molecules to form new radicals, thus continuing the chain. The video describes propagation as involving the reaction of a chlorine radical with methane, forming a methyl radical and a chlorine molecule, and then the methyl radical reacting with another chlorine molecule to form the final product and regenerate a chlorine radical.

💡Termination

Termination steps in radical reactions involve the combination of radicals to form non-radical species, effectively ending the chain reaction. The video script mentions that termination can occur when two radicals, such as two chlorine radicals or a methyl radical and a chlorine radical, meet and react, forming stable, non-reactive molecules like dichlorine (Cl2) or methane (CH4).

💡Homoytic Bond Cleavage

Homolytic bond cleavage is a process where a bond breaks such that each atom retains one of the bonding electrons, resulting in two radicals. This type of bond cleavage is crucial in the initiation and propagation steps of the halogenation reaction described in the video, where bonds in chlorine molecules are broken to form radicals.

💡Non-Polar Bonds

Non-polar bonds are covalent bonds where the electronegativities of the bonded atoms are similar, resulting in an even distribution of electron density. The video emphasizes that alkanes have non-polar bonds between carbon and hydrogen atoms, contributing to their unreactivity. This lack of polarity is a key factor in why alkanes require reactive radicals to undergo reactions.

💡Quick Product Method

The quick product method is a simplified approach to predict the products of a reaction without detailing the full mechanism. In the video, this method is used to quickly determine the products of halogenation reactions, where a CH bond in an alkane is replaced by a bond to a halogen atom, with a hydrogen halide as a side product. This method is useful for quickly identifying the outcome of a reaction in an exam or study context.

💡Radical Chain Reaction

A radical chain reaction is a type of chemical reaction that involves a series of steps where radicals are formed, react with other molecules, and are regenerated, leading to a chain of reactions. The video script describes the halogenation of alkanes as a radical chain reaction, initiated by light or heat, propagated by the reaction of radicals with alkanes, and terminated when radicals combine to form non-radical species.

💡Electrophilic and Nucleophilic

Electrophilic and nucleophilic are terms used to describe the reactivity of molecules in chemical reactions. Electrophiles are electron-deficient species that tend to accept electrons, while nucleophiles are electron-rich species that tend to donate electrons. The video script notes that alkanes are neither electrophilic nor nucleophilic due to the similar electronegativities of carbon and hydrogen, which contributes to their unreactivity.

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 (hν) 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