6 Reactions of Alkanes

Summary

This educational video delves into the chemical properties of alkanes, a type of hydrocarbon, focusing on their reluctance to react due to high bond enthalpies and low bond polarity. It explains the concept of complete and incomplete combustion, providing equations for both. Additionally, the video explores alkanes' reactions with halogens under UV light, detailing the free radical mechanism involved in these substitution reactions, from initiation to termination, and emphasizing the importance of understanding homolytic fission in the context of these chemical processes.

Takeaways

• 🔍 Alkanes are part of the homologous series and are characterized by their saturation, meaning they have no double or triple bonds.
• 🔄 Alkanes are generally unreactive due to their high bond enthalpies and low bond polarity between carbon and hydrogen atoms.
• 🔥 All hydrocarbons, including alkanes, can undergo combustion, which is a reaction with oxygen, resulting in carbon dioxide and water as products.
• 🌐 Complete combustion of alkanes involves the formation of carbon dioxide and water, with the number of carbon dioxide molecules equal to the number of carbon atoms and the number of water molecules equal to half the number of hydrogen atoms.
• 🚫 Incomplete combustion occurs when there is a lack of oxygen, leading to the formation of carbon or carbon monoxide, which is a poisonous gas.
• 🌞 Alkanes can react with halogens under the influence of bright light, such as UV light, in a process known as a substitution reaction.
• 💡 The reaction of alkanes with halogens involves a free radical mechanism, starting with the formation of free radicals from the halogen molecules.
• 🔄 The propagation step in the free radical mechanism involves the reaction of a free radical with an alkane, leading to the formation of new radicals and products.
• 🔚 The termination step in the free radical mechanism occurs when two radicals collide and react, forming a stable molecule without unpaired electrons.
• 📚 It is important to understand the concept of homolytic fission, where a bond breaks and each atom retains one electron, leading to the formation of free radicals.

Q & A

What are homologous series in organic chemistry?

Homologous series refer to a group of organic compounds that have similar chemical properties and a gradation in physical properties, differing by a constant unit, typically a -CH2- group.

Why are alkanes considered saturated hydrocarbons?

Alkanes are considered saturated hydrocarbons because their molecules contain only single bonds between carbon and hydrogen atoms, meaning they have the maximum number of hydrogen atoms possible for the given number of carbon atoms.

What is the significance of bond enthalpy in the reactivity of alkanes?

The bond enthalpy, or bond dissociation energy, indicates the strength of a bond. Alkanes have high bond enthalpies, meaning their carbon-hydrogen and carbon-carbon bonds are strong and difficult to break, which contributes to their unreactive nature.

How does bond polarity affect the reactivity of alkanes?

Bond polarity arises from an electronegativity difference between atoms. Since carbon and hydrogen have similar electronegativities, the bonds in alkanes are nonpolar, and there is an even distribution of electron density, making alkanes less likely to participate in reactions.

What are the products of complete combustion of alkanes?

In complete combustion, alkanes react with oxygen to produce carbon dioxide (CO2) and water (H2O) as the primary products.

How can you balance the chemical equation for the complete combustion of methane?

The balanced equation for the complete combustion of methane (CH4) is CH4 + 2O2 → CO2 + 2H2O. The number of carbon dioxide molecules equals the number of carbon atoms, and the number of water molecules equals twice the number of hydrogen atoms in the alkane.

What is incomplete combustion and why does it occur?

Incomplete combustion occurs when there is insufficient oxygen to fully react with the fuel. This can lead to the formation of carbon monoxide (CO) and carbon (C), which are dangerous due to their toxicity and potential to cause fires.

What are the conditions required for alkanes to react with halogens?

Alkanes react with halogens under conditions of bright light, specifically ultraviolet (UV) light, which is necessary to initiate the reaction through the formation of free radicals.

What is a free radical mechanism and why is it important in the reaction of alkanes with halogens?

A free radical mechanism is a chemical reaction that involves the participation of free radicals, which are molecules with unpaired electrons. This mechanism is important in the reaction of alkanes with halogens because it explains the initiation, propagation, and termination steps of the reaction, which would not easily occur due to the unreactive nature of alkanes.

What is the term used to describe the breaking of a bond in a free radical mechanism?

The term used to describe the breaking of a bond in a free radical mechanism is 'homolytic fission', which results in two free radicals, each with an unpaired electron.

What are the possible products of the reaction between alkanes and halogens, and why are they significant?

The reaction between alkanes and halogens can produce haloalkanes (e.g., CH3Cl) and hydrogen halides (e.g., HCl). These products are significant because they demonstrate the substitution of hydrogen atoms in alkanes with halogen atoms, and the reaction can lead to various other products depending on the reaction conditions and the presence of radicals.

Outlines

00:00

🔍 Understanding Alkanes' Unreactivity

This paragraph introduces the topic of alkanes and their chemical reactions. It explains why alkanes are unreactive due to their high bond enthalpies and low bond polarity. The high bond enthalpy means strong bonds that are hard to break, and the low bond polarity between carbon and hydrogen atoms results in an even distribution of electrons, leading to a lack of regions with high or low electron density for reactions to occur. The paragraph sets the stage for further exploration of alkanes' reactions, particularly with halogens and their combustion processes.

05:02

🔥 Alkanes in Combustion: Complete and Incomplete

This section delves into the combustion of alkanes, a reaction with oxygen that is crucial for their use as fuels. The paragraph explains the concept of complete combustion, where alkanes react with sufficient oxygen to form carbon dioxide and water. It provides a step-by-step guide to writing balanced chemical equations for complete combustion, emphasizing the relationship between the number of carbon and hydrogen atoms in the alkane and the resulting carbon dioxide and water molecules. The paragraph also touches on incomplete combustion, which occurs when there is a lack of oxygen, potentially leading to the formation of carbon or carbon monoxide, the latter being a poisonous gas. The summary underscores the importance of balanced equations and the products formed in both types of combustion reactions.

10:03

⚛️ Free Radical Mechanism in Halogen Reactions

The final paragraph discusses the reaction of alkanes with halogens, which requires the presence of bright UV light. These reactions are substitution reactions, where a hydrogen atom in the alkane is replaced by a halogen atom. The paragraph introduces the concept of a free radical mechanism, detailing the initiation, propagation, and termination steps involved in these reactions. It explains how UV light causes the formation of free radicals from halogen molecules, which then react with alkanes to form new compounds and additional radicals. The summary highlights the importance of understanding the mechanism behind these reactions, including the role of homolytic fission in forming free radicals and the potential formation of various products beyond those represented in the simplified reaction equations.

Keywords

💡Homologous Series

A homologous series is a group of compounds that have similar chemical properties and structures, differing by a constant unit, typically a -CH2- group. In the video, alkanes and alkenes are mentioned as examples of homologous series. This concept is foundational for understanding the structural similarities and differences that lead to their chemical behaviors.

💡Alkanes

Alkanes are a type of hydrocarbon with the general formula CnH2n+2, where n is an integer. They are characterized by single bonds between carbon atoms and are considered saturated hydrocarbons. The video focuses on alkanes, explaining their unreactive nature due to high bond enthalpies and low bond polarity.

💡Bond Enthalpy

Bond enthalpy is the energy required to break a chemical bond. In the context of the video, alkanes are noted to have high bond enthalpies, which means their carbon-hydrogen and carbon-carbon bonds are strong and difficult to break, contributing to their stability and unreactive nature.

💡Bond Polarity

Bond polarity refers to the separation of charge within a bond due to differences in electronegativity between atoms. The video explains that carbon-hydrogen bonds in alkanes are nonpolar because carbon and hydrogen have similar electronegativities, leading to an even distribution of electron density and a lack of reactive sites.

💡Combustion

Combustion is a chemical reaction between a fuel and an oxidant, typically oxygen, that releases energy in the form of heat and light. The video discusses complete and incomplete combustion reactions of alkanes, emphasizing the production of carbon dioxide and water in complete combustion and the potential formation of carbon monoxide in incomplete combustion.

💡Complete Combustion

Complete combustion is a reaction where a hydrocarbon reacts with sufficient oxygen to form carbon dioxide and water. The video provides examples of writing balanced equations for complete combustion of alkanes like methane and ethane, highlighting the stoichiometric relationships between carbon dioxide, water, and the hydrocarbon.

💡Incomplete Combustion

Incomplete combustion occurs when there is insufficient oxygen, leading to the formation of carbon monoxide and carbon instead of carbon dioxide. The video warns of the dangers of carbon monoxide, a poisonous gas, and illustrates how incomplete combustion can occur in scenarios like gas heaters lacking sufficient oxygen.

💡Halogens

Halogens are a group of non-metal elements in Group 17 of the periodic table, including fluorine, chlorine, bromine, iodine, and astatine. The video discusses the reaction of alkanes with halogens under the influence of UV light, resulting in substitution reactions where hydrogen atoms are replaced by halogen atoms.

💡Free Radical Mechanism

A free radical mechanism involves reactions that proceed through free radicals, which are highly reactive species with unpaired electrons. The video explains the initiation, propagation, and termination steps in the reaction of alkanes with halogens, emphasizing the role of UV light in generating free radicals and the subsequent chain reactions.

💡Homolytic Fission

Homolytic fission is the process of bond breaking where a bond's electrons are equally shared between the two atoms, resulting in two free radicals. The video uses the example of a chlorine molecule splitting into two chlorine radicals under UV light, which is a key step in the free radical mechanism of alkane-halogen reactions.

💡Propagation Step

In a free radical mechanism, the propagation step involves the reaction of a free radical with another molecule, leading to the formation of new products and new free radicals. The video illustrates this with the reaction of a chlorine radical with methane, resulting in the formation of HCl and a methyl radical, which can then react further.

Highlights

Introduction to the sixth film in a series about the standard level organic topic.

Focus on alkanes and their chemical reactions.

Explanation of why alkanes are unreactive due to high bond enthalpies and low bond polarity.

Alkanes are saturated, meaning they have no double bonds, making them less reactive.

Bond enthalpies in alkanes are high, making them difficult to break.

Carbon-hydrogen bonds in alkanes are non-polar due to similar electronegativities of carbon and hydrogen.

All hydrocarbons, including alkanes, can burn well, making them suitable as fuels.

Complete combustion of alkanes involves oxygen and produces carbon dioxide and water.

Equations for complete combustion reactions can be balanced by matching carbon dioxide to carbon atoms and water to hydrogen atoms.

Incomplete combustion occurs when there is a lack of oxygen, leading to the formation of carbon or carbon monoxide.

Incomplete combustion reactions can be balanced with any combination of carbon and carbon monoxide, as long as water is formed.

Alkanes can react with halogens under bright light, a process called a substitution reaction.

The reaction of alkanes with halogens requires UV light to initiate.

Free radical mechanisms are involved in the reactions of alkanes with halogens.

Initiation in free radical mechanisms involves the formation of free radicals, such as chlorine radicals from chlorine molecules.

Propagation steps in free radical mechanisms involve radicals reacting with other molecules, such as alkanes, to form new radicals.

Termination steps in free radical mechanisms occur when two radicals collide and form a stable molecule.

Homolytic fission is the breaking of a bond into two identical parts, which is crucial in free radical mechanisms.

Understanding the free radical mechanism is important for contrasting with other types of reactions.

Encouragement to practice writing equations for combustion and reactions with halogens to solidify understanding.

Transcripts

hello and welcome to the sixth in a

series of films about the standard level

organic topic in the first five films

we've looked at what we mean by

homologous series and we've looked at

two different homologous series the

alkanes and alkenes here we're going to

be focusing on the alkanes once again

and in particular on their chemical

reactions so hopefully by the end of

this film you will understand first of

all what it is about alkanes that means

they don't particularly like reacting

with other things and then for the

things that they do like reacting with

you're going to be able to write some

equations so you'll be able to write

equations for complete and incomplete

combustion reactions and also for the

reactions of alkanes with halogens and

what is more for the reactions with

halogens you know what we mean by a free

radical mechanism okay so if we start

off by looking at why it is that alkanes

don't really like reacting with other

things you might remember that we termed

alkanes and saturated when we were

introducing this analogous series and

that means that they're already full if

you like that we have to break some

bonds and remove some atoms before we

can put new atoms into the molecule so

one way of explaining why they're so

unreactive would be to look at the bond

enthalpies in the molecules right

because if you've got a high bond

enthalpy that means you're a bond that

is difficult to break and the bonds

between carbon single bonds between

carbons and hydrogen's and carbons and

carbons tend to have quite a high bond

enthalpy so they're hard to break so

before we can add anything here we've

got to break quite strong bonds and

that's a difficult thing to do we could

also look at this in terms of bond

polarity and this is a little bit more

complicated but we don't have to

understand it fully okay bond polarity

arises as a result of an

electronegativity difference between

atoms and this can lead to a kind of

imbalance of electron density around the

molecule and quite often reactions will

happen because things will attack

regions of high or low electron density

however if you've got atoms that have

quite similar electronegativities then

the bonds won't be very polar

so because carbon and hydrogen are quite

similar in their electronegativity

carbon hydrogen bonds aren't very polar

the electrons are quite evenly spread

spread throughout the molecule and so

what these molecules don't react very

much okay so you don't have to explain

that in a great deal of detail but you

do need to be able to say that because

their bond enthalpies are high and

because the bond polarity is low alkanes

tend to be unreactive molecules now then

let's have a look at some of the

reactions that they do take part in now

all hydrocarbons not just alkanes but

all hydrocarbons can burn quite well

which is why we use them as fuels so

combustion is a reaction with oxygen

combustion just being another name for

burning so if we think about how we're

going to write equations for this well

we can think of any particular alkane

like methane for example and if we're

going to write an equation for it then

we're going to need to include oxygen as

one of our reactants because combustion

is a reaction with oxygen now

we're talking here about complete

combustion complete combustion means

that the carbons get as much oxygen as

they can and so the hydrogen so do the

hydrogen's when we come to deciding the

products so potentially if carbon were

to combine with oxygen you could imagine

it forming carbon monoxide or carbon

dioxide but if we're to give it its

maximum amount of oxygen it forms carbon

dioxide there is only one oxide of

hydrogen and that's water so in any

complete combustion reaction these are

going to be our products carbon dioxide

and water writing the equation now is

just simple that's left really is to

balance it now the number of carbon

dioxide's will always be the same as the

number of carbons in your hydrocarbon

and the number of water's will always be

the same as half the number of

hydrogen's in your hydrocarbon so

there'll be two water molecules here

right one carbon dioxide now all that's

left is to do the oxygens and we've got

two oxygen atoms here another two there

so we need four on the left

so we need two O twos now you could say

this is a little bit easier when the

number of carbon atoms in the

hydrocarbon or in the alkane is an odd

number and we'll see why in just a

moment because I'll just do one quickly

for ethane where we've got an even

number of carbon atoms and once again

combustion involves oxygen and if it's

complete combustion then we're going to

be making carbon dioxide and water and

this time we're going to make two carbon

dioxides and we're going to make three

waters now what you'll see with any even

at any alkane that contains an even

number of carbons you'll end up with an

odd number of oxygens on this side okay

so we've got four oxygen atoms here in a

seven sorry three there so seven in

total so we need three and a half o2 s

now that equation can be left just like

that if you like but some people get a

little bit upset by the presence of

these half numbers so if we wanted to we

could double everything in this equation

we could make that two key things seven

oxygens makes four carbon dioxide's and

six water's either one of those

equations being just as good as the

other the key point about complete

combustion equations is not only that

they balance but they contain carbon

dioxide and water as their products now

if we can trust that with incomplete

combustion this is when there's a lack

of oxygen so often gas heaters for

example get tested to see if they're

working properly which means is there a

sufficient supply of oxygen to the fuel

because if there isn't then the carbon

in the alkane could just be left as

carbon or more importantly or more

dangerously it could turn into carbon

monoxide instead of carbon dioxide and

the danger with that is it's a very

poisonous gas but it doesn't smell of

anything so if you're producing it in

your home it's not really a very good

thing okay so if we think about writing

balanced equations now what we need to

make sure we do is that we include at

least one of those things or any

combination of them

okay because how much carbon and carbon

monoxide you produce depends on how much

oxygen you have and if someone tells you

that you're incompletely combusting of

fuel they're not often going to tell you

about the amount of oxygen you've got so

you're kind of free to write a number of

different equations here so for example

I could write carbon as my only carbon

containing product and two h2 O's

because I've still got four hydrogen's

and we have to make water because it's

the only oxide of hydrogen and that

equation is balanced and you could

imagine this reaction happening in a

Bunsen burner where the air hole was

closed because there wouldn't be very

much oxygen and so all the methane would

turn into water and carbon perhaps and

this carbon would make things so

this would make a beaker that you were

heating it would cover it in a black

substance which would be the carbon and

also this carbon would glow very

brightly in the flame so your flame

would be yellow okay so anyway that's a

little bit of a sidetrack the key point

about incomplete combustion reactions is

that they balance but how you make them

balance is kind of up to you because you

can include any amount of carbons and

carbon monoxides in the products that

you want just make sure that you're also

forming water

okay so alkanes are quite good at

burning other reactions they're not so

good at and in particular they're not

very good at reacting with halogens they

might wonder why we're doing this in

that case well because alkenes are quite

good and there's a contrast to be made

but anyway let's stick with our Canes

for the moment alkanes will react with

halogens

but we need bright light to do this so

often you'll see in the equation above

the arrow UV meaning there has to be

bright UV light in order to make these

reactions happen they're called

substitution reactions why they call

that because we're swapping one of the

hydrogen's in the hydrocarbon for a

halogen atom so what you'll see here is

that the CH 4 loses a hydrogen but gains

a bromine atom and then we're left with

a CH

our if Ethan reacts with chlorine we're

going to swap one of the hydrogen's for

chlorine but we'll be left with HCl so

writing the equations is actually quite

simple here what's a little bit more

tricky is the fact that we need to know

something called a mechanism for these

reactions so in that means we need to

know how the reactions happen in other

words we need to be able to describe the

stages that take place now with free

radical mechanisms the first step

involves the formation of a free radical

okay this is called initiation because

it's what gets everything started and

it's why we need the UV light so for

example a chlorine molecule could turn

into two chlorine radicals we're

representing a radical as having a dot

that means it's got an unpaired electron

the UV light causes this split to happen

and if you think about what's holding

two chlorine atoms together in a

chlorine molecule there's a pair of

electrons okay and this type of bond

breaking breaking being fission is

called homo lytic it's called homo lytic

because when we break this bond so lytic

means meaning breaking we end up with

two things that are the same in other

words the electrons in the bond are

shared between the two atoms and we form

two of these so-called free radicals

they're called free radicals because

they have an unpaired electron and this

makes them extremely reactive because

they want to find a partner for this

electron so they'll react with just

about anything including alkanes which

aren't very very reactive so if we

imagine that we had some methane and it

was reacting with chlorine that could

react with a chlorine radical now this

would be a difference that once we'd

form the rear the radicals in the

initiation step we could take part in

what is called a propagation step where

one molecule reacts with another this

takes one of the electrons from a carbon

hydrogen bond and makes the HCL that we

saw on the previous slide however what's

left now

is ch3 because it's lost a hydrogen atom

but it has now got an unpaired electron

so it's called propagation because

although we use up one radical we've

made another and now this radical that

we've just made could potentially react

with something else like perhaps another

chlorine molecule and if it did that

then this electron will pair up with one

of the electrons from this bond and

would make ch3cl and we'd produce yet

another chlorine radical so this

chlorine radical can now take part in

another reaction okay so these are this

is why these are called propagation

steps because although we use up a

radical we make another one the last

step of reaction notice here that we

have made HCl and ch3cl is what is

called a termination step and this is

where two radicals collide because if

two things with an unpaired electron

come together like for example these two

radicals that we've already seen then

they won't produce another radical

they'll produce something that has all

paired electrons and that's the end so

to speak right you could also end up

with two chlorine radicals colliding

together and that would just reform a

chlorine molecule or you could even end

up with two ch3 radicals colliding

together because that would make a

totally different product c2h6 which

would be ether so it's also important to

realize that although we can write quite

nice neat tidy reaction equations for

these like ch4 plus CL 2 makes ch3cl and

HCL it's important to realize that there

will be lots of other products that form

even though we don't necessarily have to

put them into our equation okay and it's

definitely important to know the meaning

of this term homolytic fission because

we'll contrast that with a different

type of fish and later on okay so

hopefully now that you've watched this

film you know why alkenes are unreactive

you can write equations for combustion

reactions and reactions with halogens

and you know what we mean by a free

radical mechanism it's important to

practice practice this stuff obviously

to get it stuck into your memory but if

you've got any questions or comments

please feel free to come and see me

or to post a comment on the YouTube

channel