BTEC Applied Science - Unit 5 Chemistry - Free radical substitution in Alkanes

Summary

This chemistry lesson focuses on free radical substitution in alkanes, crucial for BTEC Applied Science Unit 5. It covers key terms like halogenation and homolytic fission, then delves into the reaction mechanism in three stages: initiation (requiring UV light to form free radicals), propagation (involving reactions between free radicals and alkanes), and termination (where free radicals combine to end the reaction). The instructor also guides viewers through past exam questions, enhancing understanding of the concept.

Takeaways

• 🔬 Halogenation is the process of adding a halogen to an alkane.
• 🌐 Free radicals are species with an unpaired electron, represented by a dot.
• 💥 Homolytic fission is the breaking of a covalent bond to form two free radicals.
• 🔍 The reaction mechanism outlines the step-by-step process of a chemical reaction.
• 🌀 Free radical substitution in alkanes involves three steps: initiation, propagation, and termination.
• 🌞 UV light is required to initiate the reaction by converting halogens into free radicals.
• 🔄 Propagation involves two steps: a free radical reacts with an alkane to form a new free radical and a halogenated product.
• 🔚 Termination occurs when two free radicals combine to form a non-free radical species, ending the reaction.
• 🧪 The overall reaction of free radical substitution results in a halo alkane and hydrogen halide (HX).
• 📚 Past exam questions highlight the importance of understanding the stages of the reaction and the role of free radicals.

Q & A

What is the main topic of the video script?

The main topic of the video script is the free radical substitution in alkanes, specifically for BTEC Applied Science Unit 5 Chemistry.

What is the significance of halogenation in alkanes?

Halogenation is significant in alkanes because it involves the addition of a halogen to an alkane, which is a key reaction in organic chemistry leading to the formation of haloalkanes.

What does the term 'free radical' represent?

A 'free radical' represents a species with an unpaired electron, often represented by a dot, such as Cl• for a chlorine free radical.

What is homolytic fission and how does it relate to the reaction mechanism?

Homolytic fission is the process where a covalent bond breaks to form two free radicals, each with an unpaired electron. It is a key step in the reaction mechanism of free radical substitution in alkanes.

How does UV light play a role in the initiation step of the reaction?

UV light provides the energy required for homolytic fission of the halogen molecule (like Br2), resulting in the formation of two halogen free radicals, which is the first step in the reaction mechanism.

What is the role of the alkane in the propagation step of the reaction?

In the propagation step, the alkane reacts with a halogen free radical, leading to the substitution of a hydrogen atom with a halogen and the formation of a new carbon-centered free radical.

How does the second propagation step differ from the first?

The second propagation step involves the newly formed carbon-centered free radical reacting with another halogen molecule, leading to the formation of the haloalkane product and another halogen free radical.

What is the purpose of the termination step in the reaction?

The termination step is crucial as it ends the chain reaction by combining two free radicals to form a stable, non-radical molecule, preventing further reaction.

Why is it important to understand the reaction mechanism for free radical substitution?

Understanding the reaction mechanism helps in predicting the outcome of chemical reactions, controlling reaction conditions, and understanding the role of each step in the overall process.

How does the video script assist students in preparing for exams?

The video script provides a detailed explanation of the reaction mechanism, key terms, and practice questions, which can help students understand the concepts better and prepare for exam questions.

What is the significance of the term 'propagation' in the context of the reaction?

The term 'propagation' refers to the middle steps of the reaction mechanism where the chain reaction is sustained by the continuous formation and reaction of free radicals.

Outlines

00:00

🔬 Introduction to Free Radical Substitution in Alkanes

This paragraph introduces the concept of free radical substitution in alkanes, particularly aimed at BTEC Applied Science Unit 5 Chemistry. It covers key terms such as halogenation, free radicals, and homolytic fission. The paragraph explains that a free radical is a species with an unpaired electron, typically represented by a dot. It also discusses the reaction mechanism, which is a step-by-step sequence of processes leading to a chemical change. The overall reaction involves a molecule of alkane reacting with a halogen (like Br2), resulting in the substitution of a hydrogen with a halogen atom and the formation of HBr. The process requires UV light and is the first step in the free radical substitution mechanism.

05:02

🌐 Understanding the Propagation Stage of Free Radical Substitution

The second paragraph delves into the propagation stage of the free radical substitution reaction. It requires two steps: the first involves a free radical (from the initiation step) reacting with an alkane, removing a hydrogen atom to form HBr and creating a new free radical on the alkane. The second step sees this new free radical react with another halogen molecule, resulting in the substitution of a hydrogen with a halogen and the formation of a haloalkane. The paragraph also covers the termination step, where two free radicals combine to form a non-free radical species, effectively ending the reaction. Examples include the combination of two bromine free radicals to form Br2 or the joining of two alkyl free radicals to form a larger molecule.

10:02

📝 Examining Past Exam Questions on Free Radical Substitution

The final paragraph focuses on applying the knowledge of free radical substitution to past exam questions from BTEC Applied Science Unit 5. The script guides viewers to practice answering questions about the reaction mechanism, including identifying the type of bond fission in the initiation step and the products of the propagation steps. It also addresses a complex question about the chlorination of methane, explaining each stage of the reaction: initiation (formation of chlorine free radicals), propagation (reaction of chlorine free radical with methane and subsequent formation of chloromethane), and termination (combination of two chlorine free radicals to form Cl2). The paragraph concludes with an encouragement for viewers to engage with the content and provide feedback.

Keywords

💡Free Radical Substitution

Free Radical Substitution is a type of chemical reaction where a hydrogen atom in an alkane is replaced by a halogen atom. In the context of the video, this process is central to understanding how alkanes react with halogens under certain conditions. The video script describes this as a step-by-step mechanism involving free radicals, which are species with unpaired electrons.

💡Halogenation

Halogenation refers to the process of adding a halogen to an alkane, which is a key reaction type in organic chemistry. The video explains that halogenation involves the substitution of a hydrogen atom in an alkane with a halogen atom, and it is represented in the script with the example of bromine (Br2) reacting with an alkane to form a halo alkane and HBr.

💡Free Radical

A free radical, as mentioned in the video, is a species with an unpaired electron, often represented by a dot. Free radicals are crucial in the halogenation of alkanes because they initiate the reaction and propagate the substitution process. The video uses the term 'Cl dot' to represent a chlorine free radical, illustrating how it participates in the reaction mechanism.

💡Homolytic Fission

Homolytic fission is the breaking of a covalent bond to form two free radicals, each with one of the original electrons. The video script explains this concept by showing how a bond between two chlorine atoms can break, resulting in two chlorine free radicals. This process is essential for the initiation step of the free radical substitution reaction.

💡Reaction Mechanism

The reaction mechanism is the step-by-step sequence of elementary reactions that occur in a chemical reaction. In the video, the mechanism for free radical substitution in alkanes is detailed across three steps: initiation, propagation, and termination. Understanding the mechanism is vital for grasping how the overall chemical change happens.

💡Initiation Step

The initiation step is the first phase of the free radical substitution reaction where a halogen molecule is split into two free radicals by UV light. The video script specifies that this requires UV light and uses the example of Br2 being split into 2Br dots to illustrate this step.

💡Propagation Step

Propagation steps are the middle stages of the free radical substitution reaction where the free radical reacts with the alkane to form a new free radical and a product. The video describes two propagation steps: one where a bromine free radical reacts with an alkane to form HBr and a carbon-centered free radical, and another where this new radical reacts with another halogen molecule to form the halo alkane.

💡Termination Step

Termination is the final stage of the free radical substitution reaction where two free radicals combine to form a stable molecule, ending the reaction. The video script gives examples such as two bromine free radicals combining to form Br2, illustrating how the reaction can be terminated.

💡UV Light

UV light is necessary to provide the energy required to break the bond in the halogen molecule, initiating the reaction. The video script mentions that UV light is needed for the homolytic fission of Br2 to form two Br free radicals, which is a critical step in the halogenation of alkanes.

💡Halo Alkane

A halo alkane is the product of the halogenation reaction where a hydrogen atom in an alkane is replaced by a halogen atom. The video script describes the formation of a halo alkane as the outcome of the free radical substitution reaction, highlighting it as a key product of the process.

💡HX

HX refers to the hydrogen halide that is formed as a byproduct during the halogenation of alkanes. In the video, HBr is given as an example of HX, which is formed when a hydrogen atom in the alkane is replaced by a bromine atom, illustrating the substitution reaction's outcome.

Highlights

Introduction to free radical substitution in alkanes, relevant for BTec Applied Science and other A-Level specifications.

Key terms: Halogenation is the addition of a halogen to an alkane.

Free radicals are species with unpaired electrons, often represented with a dot.

Homolytic fission is the breaking of a covalent bond to form two free radicals.

Reaction mechanism shows the step-by-step sequence that leads to a chemical change.

Free radical substitution is a reaction where a hydrogen in an alkane is substituted by a halogen.

The reaction requires UV light to initiate.

The overall reaction equation involves an alkane and a halogen forming a halo alkane and HX.

The reaction mechanism consists of three steps: initiation, propagation, and termination.

Initiation step requires UV light to break the halogen molecule into two free radicals.

Propagation step involves two stages: the free radical reacts with alkane to form a new free radical and HX.

The second propagation step involves the newly formed free radical reacting with halogen to complete the substitution.

Termination step occurs when two free radicals combine to form a non-free radical species, ending the reaction.

Past exam questions are used to illustrate the application of the reaction mechanism.

Homolytic fission is identified as the type of bond fission shown in the initiation step.

The alkane with a free radical is used up in the second propagation step.

In a termination step, free radicals can combine to form a halogen molecule or join together to form a larger molecule.

The lesson concludes with a suggestion to like, subscribe, and comment if the content was useful.

Transcripts

[Music]

welcome to my channel on chemistry

lessons make sure you like and subscribe

so this is aimed at btec applied science

unit 5 chemistry but it is relevant for

other a level specifications for the

free radical substitution in alkanes

so in terms of the btec applied science

specification it's learning aim a2 in

unit 5 and it's down the bottom here the

free radical substitution in alkenes

now if you haven't looked at the

introduction to alkanes video i suggest

you do that before you go any further

with this video

let's start with some key terms then

so the term halogenation

means the adding of a halogen to an

alkane so this is the halogenation of

alkanes and it's adding a group seven

atom to the alkane

the term free radical is given to

something or a species that has an

unpaired electron and we usually see

that

or represent it as a dot so cl dot that

would represent a chlorine free radical

homolytic fission

this is when the covalent bond breaks to

form two free radicals for example the

covalent bond between two chlorines

if that bond breaks so that one electron

goes to one chlorine and the other

electron goes to the other chlorine you

end up with

two chlorine free radicals

that's homolytic fission

we're going to use the term reaction

mechanism and the mechanism of a

reaction just shows the step-by-step

sequence that lead to the overall

chemical change so in other words

chemical reactions don't just happen

instantly as one step it's usually

over a series of sequences or steps and

that's known as the reaction mechanism

so let's look at the overall reaction to

start with them so this is known as the

free radical substitution that's because

free radicals are involved and it's a

substitution reaction what we're going

to be doing is one of the hydrogens and

it can be any of the hydrogens in the

alkane i've just circled one of them

there

one of the halogens will swap with one

of the hydrogens so our product

in this case

that one hydrogen that i've circled is

going to be substituted which means

swapped

with one of the halogens

leaving behind h br

so what's happened in this chemical

reaction

is

a molecule of an alkane

has reacted with a halogen in this case

br2 halogen is anything from group 7

this could have been cl2 it could have

been i2 so it's a halogen and one of the

halogens in the br2 has swapped places

with the h so now we have hbr as a

product and our halo alkane as a product

in order to do that we did need uv light

so this would be our overall equation

here it's one molecule of our alkane

reacting with one diatomic molecule of

the halogen

forming the halo alkane and hx or the

hbr in this case

so now we're going to look at the actual

reaction mechanism instead of the

overall equation

and as we've seen from earlier slides

the reaction mechanism is going to show

us the step-by-step sequences that

happen in that chemical reaction and the

free radical substitution in alkanes

always takes place over three steps or

there's three sequences to it

so the first sequence or the first step

is known as the initiation step

and this step requires uv light

and there's only one step in this

section

so the halogen i'm going to stick with

bromine

so the initiation takes the halogen

and turns it into

two

halogen free radicals so we end up with

br2 makes two br dot and it's important

to show this dot because i'm showing

that it's a free radical

and uv light is needed so we often write

uv light above this arrow uv light is

needed

to break that bond in the bromine so

what's actually happening here is that

uv light is providing the energy for

this bond to undergo homolytic vision so

that's what's actually happening

but we need to be aware of that equation

br2

goes to 2br dot

now if this was halogenation using

chlorine then all we'd simply do is have

cl2 going to 2cl dot

so the second stage of the free radical

substitution is known as the propagation

stage and this requires two steps

so what we need is we take our free

radical

from the previous step so the free

radical from the initiation is needed

now and this time it reacts with

our

alkane now i'm going to show a

structural formula of ethane

so the free radical is going to react

with the alkane and what it does is it

removes one of the hydrogens to become

hbr

so that br free radical removes one of

the hydrogens

i'm going to remove the hydrogen on the

first carbon so that now becomes ch2

ch3

but what i must remember to do here is

where i've just removed the hydrogen

that carbon becomes a free radical so

it's important here to recognize that in

a propagation step

we have a free radical at the start with

a non-free radical

and it forms a free radical

and a non-free radical

and that's the first step of propagation

the second step of propagation

requires

this new free radical that we've just

formed so i'm just

repeating this new free radical from

here

this time it will react with the halogen

and not

the free radical so it goes back to br2

so it's reacting with br2 here

and now what we do is we form our

product so the bromine now will be

bonded to that carbon that was a free

radical so the bromine is now bonded to

that carbon

and you can see the substitution is now

complete we've now

substituted the hydrogen on that carbon

for the halogen

but it also leaves us with another

free radical

so again we can recognize it as a

propagation because we've got one free

radical in the reactants with a non-free

radical

producing a free radical in the products

and the non-free radical so there's

always one free radical on either side

in a propagation step that's our two

propagation steps there

and the final stage of the mechanism is

a termination step this is where

the reaction can be terminated which

means finished and it's whenever two

free radicals come together to form a

non-free radical so there are actually

several different possibilities here

the one that i like best would be to

take the free radical that we've seen in

the previous slide

with the halogen free radical

they will come together

to form

our product

so all that's happening is the free

radical

is joining so this electron

and this electron are pairing up to make

a covalent bond between that c and br so

that would be classed as a termination

step

another possibility would be to take

a br free radical with a br free radical

and they can come together to form

br2 that's also classed as a termination

step

and again there's nothing stopping me

from taking this free radical

and reacting it with another identical

free radical

and this is going to be slightly more

complicated to draw but it's going to be

ch3 and a ch2

bonded to the ch2 and a ch3 that's

slightly more complicated to get your

head around but what's happening is

this electron and this electron are

forming a covalent bond so we're

literally joining those two ch2s

together in the middle here

so we're going to look at a couple of

past exam questions now and these were

from unit 5 b tech applied science so i

suggest you pause the video

you have a go at these questions

yourself and when you're ready to hear

the answers unpause the video

so the first question is asking us to

complete this sentence a free radical is

a species with an unpaired electron

part two what type of bond fission is

shown in the initiation that's homolytic

fission

part three is slightly complex we've got

to recognize

that

y

is used up in the second step so y has

to be

our

alkane with a free radical so it's a dot

c 4

h 9

because what happens in that first

initiation in that first propagation

step sorry

this broad mean free radical will remove

the hydrogen or one of the hydrogens so

it becomes hbr

and then what i've done is one of those

hydrogens has now gone so that's why it

becomes c4h9 and we mustn't forget to

draw that dot to show that that c4h9 is

now a free radical

so z so in the second propagation step

this newly formed free radical y

reacts with another halogen

br2

and part four finally the reaction ends

with a termination step suggest what

happens to the free radicals in a

termination step well two free radicals

can come together

in this case sorry it's bromine so i'm

going to

go with bromine so two bromine free

radicals can come together

to form bromine

so the two free radicals come together

to form the halogen

okay so the second exam question then so

again i'm going to suggest you pause the

video and have a go at this one and when

you're ready to hear the answer you

won't pause the video

now this is probably your worst

nightmare or the worst possible free

radical question a six mark question

but let's check the the question so

we've been told here that chloromethane

is produced when chlorine reacts with

methane so i'm just gonna have a look at

the overall equation so methane is going

to react with chlorine

to make

chloromethane

and i know that's a substitution

so one of the chlorine substitutes with

one of the h's that's the overall

equation

i know that we would need uv light to do

that

and it's asking me to explain the three

stages so i'm actually just going to get

straight in there and say the initiation

is the first stage

initiation is the first stage or the

first step and that requires

the chlorine to become two chlorine free

radicals with uv light

so that's the first stage

the second stage

is the propagation steps and again i'm

naming each stage

this requires two steps

it's going to be the chlorine free

radical

reacting with methane so i'm being

specific to the question don't just copy

down the ones that you've done for

revision make sure it matches the

question this question is the

chlorination of methane

so the first step the chlorine free

radical will take one of the h's to

become hcl

and i've now formed a chlorine free

radical not chlorine a ch3 free radical

because i've taken off one of the

hitches

the second step for propagation is this

newly formed ch3 free radical

with the halogen

to make

our

product clch3

you could put ch3cl it's correct

plus a chlorine free radical so notice

how

the propagation steps one free radical

in the reactants and one free radical in

the products

and the third and final

is known as the termination

and i just need to give one example i'll

put two chlorine free radicals coming

together

to make a chlorine cl2 molecule

that's going to get me all six marks

hopefully you found this lesson useful

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