Alkanes, Fractional Distillation, Alkenes, Free Radical

Question 1

Describe the structure of alkanes

draw ethane

Answer 1

Alkanes - saturated hydrocarbons
with the general formula CnH2n+2

The covalent bonds around each carbon atom form a tetrahedral structure, with bond angles of around 109.5 degrees

The bonds in alkanes are a type of covalent bond called a sigma bond

Sigma bonds form when electron orbitals from adjacent atoms directly overlap

A sigma bond contains a pair of electrons - one from each atom on either side of the bond

The pair of electrons in the sigma bond lie directly between the bonding atoms

In alkanes - only sigma bonds are formed

A key feature of sigma bonds is that they are fully rotational (the carbon atoms can rotate relative to each other)

The covalent bonds in alkanes are also relatively strong - and take a lot of energy to break

https://homework.study.com/cimages/multimages/16/ethane206370942586019273.png

Question 2

Describe the properties of alkanes, including how the boiling points vary with chain length

Answer 2

Alkanes are hydrocarbons
Carbon and hydrogen atoms have a very similar electronegativity

This means that alkanes are essentially non-polar molecules
This helps to explain the properties of alkanes

Firstly, alkanes are insoluble in water
This is because water molecules form hydrogen bonds with each other. But because alkanes have no permanent dipoles, they cannot form hydrogen bonds. Therefore alkanes cannot dissolve in water

Unlike many organic molecules, alkanes are generally unreactive
That is because of the strong covalent bonds within alkane molecules
Under certain conditions, alkanes will react

As the length of the carbon chain increases, the boiling point increases
Short chain alkanes have low boiling points and are gases at room temp.
However, longer chain alkanes have higher boiling points
As the carbon chain length increases, alkanes can be liquid and solid at room temp.

This is due to the intermolecular forces in alkanes
Alkanes are non-polar molecules
So the intermolecular forces acting between alkane molecules are induced dipole-dipole interactions (van-der-Waals forces)

When we boil an alkane - we have to break these intermolecular forces
However van der Waals forces are weak forces and do not take a lot of energy to break
That explains why shorter chain alkanes have low boiling points

As the length of the carbon chain increases, the strength of the van der Waals forces increases.

This is because longer chain alkanes have more electrons than shorter chain alkanes (strength of van der waals forces increase with the number of electrons)

Also longer chain alkanes have a greater SA than shorter chain alkanes (FOR THE FORMATION OF VDW forces)
This means that there are many points along the molecules where they can form London forces

So the increased number of electrons and greater SA means that the London forces are greater in long chain alkenes

This means that longer chain alkanes have a higher BP than shorter chain alkanes

Question 3

Describe the properties of alkanes, including how the boiling points vary with branching

Answer 3

Branched chain alkanes have a lower boiling point than straight chain alkanes

The branched isomer has a lower BP than the straight chain isomer

This is because branches prevent alkane molecules from getting closer together

And van der waals forces are strongest over short distances

So in the case of branched chain alkanes, the van der Waals forces are reduced

This explains why branched chain alkanes have lower BP than straight chain alkanes

Question 4

Uses of alkanes

Answer 4

Alkanes are extremely useful compounds

Alkanes are used as fuels

Alkanes are used as the starting materials for the production of a whole range of organic molecules including pharmaceuticals

Question 5

Where do we find alkanes

Answer 5

We find alkanes in crude oil
Crude oil is a fossil fuel - formed underground from the remains of plants and animals

Over millions of years, heat and pressure, convert the chemicals in these remains into crude oil

Question 6

Why is crude oil considered non-renewable

Answer 6

Since we are using crude oil at a faster rate than it can form, crude oil is considered non-renewable

Question 7

Crude oil consists of

Answer 7

Crude oil is a mixture of straight chain and branched chain alkanes
along with other chemicals such as sulfur

Question 8

In order to use these alkanes (found in crude oil), what must be done to them

Answer 8

In order to use these alkanes, we need to separate them. We do this by a process called fractional distillation

fractional distillation separated based on BP

Question 9

What is petroleum

Answer 9

Petroleum is a mixture consisting mainly of alkane
hydrocarbons that can be separated by fractional
distillation

Question 10

Describe how alkanes are separated by fractional distillation

Answer 10

Heat crude oil into a vapour
Vapour passes into a fractionating column (negative temperature gradient)
Separated into fractions due to their differing BP (which depends on their size)
Longer chain fractions with high BP separated at the bottom while those with lower BP (shorter chain fractions) separated towards the top

Fractional distillation is carried out in tall fractionating columns

Fractional distillation depends on the BP of alkanes
(alkanes attracted to each other by intermolecular vdW forces)

if we take an alkane as a gas and cool it to below its BP - it will condense to a liquid

Firstly, the crude oil is heated in a furnace
The temperature of the furnace is hot enough to boil a lot of the alkanes in the crude oil converting them to a gas

Next, the crude oil vapours and liquid pass into the fractionating column
The column is hotter at the bottom and becomes progressively cooler going upwards

Now the crude oil vapours make their way up the column
At different levels in the column we have collecting trays.

These trays have bubble caps which allow vapours to pass upwards

As each alkane moves up the column, at some point it will reach a temperature which is cooler than its BP

Now the alkane condenses back to a liquid and passes out of the column

The alkanes with shorter carbon chains have lower BP, so these are collected near the top of the column

Longer chain alkanes have higher BP so these are collected towards the bottom

Alkanes with very long chains form a thick liquid called bitumen. This is collected from the bottom of the column

Not all of the alkanes will condense
Very short chain alkanes such as methane and ethane are collected from the top of the column as gases

Gases
Petrol/naphtha
Kerosene
Diesel
Lubricating oil
Fuel oil
Bitumen

Question 11

What does fractional distillation produce

Answer 11

Fractional distillation does not separate each individual alkane
Instead, each fraction contains a number of alkanes with similar boiling points

Question 12

What would need to be done to separate each individual alkane

Answer 12

To separate each individual alkane would require further rounds of fractional distillation

Question 13

Describe the economic reasons from cracking alkanes

Answer 13

The petrol/naphtha fraction is particularly useful
This fraction is used to make petrol for vehicles and is a raw material for the chemical industry

Problem: Crude oil tends to contain a higher proportion of longer chain hydrocarbons than shorter chain. So when we carry out fractional distillation of crude oil we do not produce a large amount of the petrol/naphtha fraction

In contrast, we produce more of the longer chain fractions which are less in demand

So because of this, there is an economic benefit to converting long chain hydrocarbons into shorter chain hydrocarbons. This is done by a process called cracking

Question 14

Benefits of cracking

Answer 14

Cracking converts long chain hydrocarbons into shorter chain hydrocarbons (there is a greater demand for shorter chain hydrocarbons)

As well as producing alkanes, cracking also produces alkenes which are highly reactive molecules. Alkenes are a major feedstock (raw material) for the chemical industry and are used to make a range of products including polymers

Question 15

Describe how alkanes can be cracked by thermal cracking and catalytic cracking

Answer 15

Two methods are thermal cracking and catalytic cracking

Cracking involves breaking C–C bonds in alkanes.

Question 16

Conditions for thermal cracking

Answer 16

Thermal cracking requires both a high temperature and a high pressure

The temperature ranges from around 450 degrees C - 900 degrees C

The pressure is around 70 atmospheres

In thermal cracking, long chain alkanes form both shorter chain alkanes and alkenes

Hydrogen can also be one of the products

The specific products depend on the exact conditions

Question 17

What is the benefit of thermal cracking

Answer 17

The benefit of thermal cracking is that we make a high percentage of alkenes in the products.

Alkenes are very useful molecules due to their high reactivity.

Alkenes are then used to make other chemicals e.g. plastic and polymers

Question 18

Describe what happens during thermal cracking

Answer 18

During thermal cracking, a covalent bond between two carbon atoms splits to form intermediate molecules

Covalent bond - pair of electrons
When the covalent bond splits, both of the intermediate molecules now have one unpaired electron

These molecules are called free radicals

Question 19

Describe conditions for catalytic cracking

Answer 19

Catalytic cracking also requires a high temperature - in this case around 450 degrees C

However, catalytic cracking does not require a high pressure (slight pressure)

Pressure for catalytic cracking is 1-2 atmospheres

Catalytic cracking uses a zeolite catalyst which contains a mixture of aluminium oxide and silicon dioxide

Zeolite has a large SA which helps to make it an effective catalyst

When a long chain alkane undergoes catalytic cracking, the products are often branched chain alkanes

Branched chain alkanes are especially useful for petrol (motor fuels) as they combust very efficiently

Catalytic cracking can also produce cyclic alkanes/cycloalkanes and aromatic hydrocarbons/compounds such as benzene

Question 20

Explain why alkanes are unreactive molecules

Answer 20

Alkanes are unreactive molecules

Since alkanes are non-polar molecules
This is because C atoms and H atoms have a very similar electronegativity

(many molecules react due to their polarity)

So because they are non-polar, alkanes are unreactive

Secondly, the bonds in alkanes are relatively strong and take a lot of energy to break

This makes alkanes unreactive molecules

Question 21

State on way alkanes can react

Answer 21

One way that alkanes can react is via free radicals

Free radicals = radicals

Question 22

What is a free radical

Answer 22

A free radical is any species with an unpaired electron

e.g. bromine free radical and methyl free radical

https://bam.files.bbci.co.uk/bam/live/content/zgxgd2p/small

we show the unpaired electron as a dot

Question 23

Describe the reactivity of free radicals

What can react with free radicals

Answer 23

Free radicals are highly reactive species

So even though alkanes are unreactive molecules, they can react with free radicals

Question 24

Describe free radical substitution of alkanes

Free radical reaction between alkanes and halogens

reaction between Methane + Bromine

Answer 24

methane + bromine -> bromomethane and hydrogen bromide

CH4 + Br2 -> CH3Br + HBr

-> ultraviolet light

A H atom on the methane molecule has been substituted with a Br atom

Because this reaction involves free radicals - this reaction is an example of free radical substitution

Three stages in this reaction.
Initiation
Propagation
Termination

https://chubbyrevision.weebly.com/uploads/1/0/5/8/10584247/1481656.png?572 - curly half arrows

the unpaired electron in a radical is represented by
a dot

Question 25

Initiation stage Free radical reaction between alkanes and halogens reaction between Methane + Bromine

Answer 25

At the start of the reaction - we have a mixture of CH4 and bromine molecules, Br2 In the first stage, we shine ultraviolet light onto the reaction mix The energy of ultraviolet light causes the single covalent bond between the two bromine atoms to break (exactly in half/evenly) Br-Br --> Br` + `Br bromine molecule ultraviolet light bromine free radical - = pair of electrons when bond breaks, one electron now goes to each bromine atom since these now have an unpaired electron, these are now Br free radicals When a covalent bond splits in this way. This is called homolytic fission (even splitting) UV light provides energy to break the bond exactly in half In the initiation stage, we make a pair of Br free radicals We only require a few of the Br molecules to form free radicals in the initiation stage

Question 26

Propagation Free radical reaction between alkanes and halogens reaction between Methane + Bromine

Answer 26

Next stage called propagation - 2 steps In the first step of propagation, a Br free radical reacts with a methane molecule a free radical has an unpaired electron To make an electron pair, the Br free radical takes a H atom plus one electron from the methane molecule This reaction produces both hydrogen bromide and a methyl free radical Methane + Br free radical -> methyl free radical + HBr https://bam.files.bbci.co.uk/bam/live/content/zgxgd2p/large CH4 + `Br -> `CH3 + HBr In propagation step 2, the methyl free radical now reacts with a bromine molecule This produces our end product bromomethane plus another bromine free radical Methyl free radical + Bromine molecule --> bromomethane + bromine free radical https://bam.files.bbci.co.uk/bam/live/content/zr6rwmn/large 'CH3 + Br2 -> CH3Br + `Br If we look at propagation steps 1 and 2 together, we can see that they form a chain reaction The Br free radical formed in propagation step 2 can now go back and react with methane in propagation step 1 So this reaction will continue until the final stage takes place which is called termination Bromine free radical is a like a catalyst they are regenerated during the chain propagation step, effectively participating multiple times. This regeneration is similar to a catalyst, as it doesn't get used up in the overall reaction but continues to drive the reaction forward

Question 27

Termination Free radical reaction between alkanes and halogens reaction between Methane + Bromine

Answer 27

In termination, two free radicals react together to form a molecule with no unpaired electrons This is now a stable molecule and no longer takes part in the reaction There are three possible reactions in termination Two bromine free radicals can form a bromine molecule `Br + `Br -> Br2 Two methyl free radicals can form a molecule of ethane `CH3 + `CH3 -> C2H6 Or a methyl free radical and a bromine free radical can form a molecule of bromomethane `CH3 + `Br -> CH3Br https://bam.files.bbci.co.uk/bam/live/content/zctcjxs/large

Question 28

Describe the problem with free radical substitution

Answer 28

There is one big problem with free radical substitution of alkanes That is that we get a whole range of side products E.g. if a bromine free radical reacts with a molecule of bromomethane, then we make dibromo methane We can get further reactions to form tribromomethane and tetrabromomethane In the termination step, we can make ethane. This ethane can also react with Br free radicals forming a wholer range of molecule e.g. Bromoethane all the way to Hexabromoethane If we carry out this reaction with longer chain alkanes such as pentane, we could produce a huge range of products, including different isomers So at the end of this reaction, we need to separate out our product molecules.

Question 29

excess bromine + methane bromine + excess methane

Answer 29

can make tetrabromomethane CBr4 a series of substitution reactions occur, resulting in the formation of various brominated methanes, including (CH3Br), (CH2Br2), (CHBr3), and (CBr4) Bromine + excess methane Br + CH4

Question 30

Describe the structure of alkenes including what is meant by a pi bond

Answer 30

Alkenes are unsaturated hydrocarbons with the general formula CnH2n Bonding in alkenes involves a double covalent bond, a centre of high electron density. Ethene is a planar molecule Planar molecules are flat with all of the atoms lying on the same plane Both single and double bonds are bonding regions There are three bonding regions around each carbon atom These bonding regions repel and move as far apart as possible This means that the bond angles around the carbon atoms are around 120 degrees Bonding in ethene is different to ethane. Due to the double bond Just like in ethane, there are sigma bonds between the C atoms and the H atoms C-H bond However the double bond consists of two types of covalent bonds. When the double bond forms, we make one sigma bond between the carbon atoms (C-C) At this point, each carbon atom now has one electron left in a p orbital These p orbitals lie above and below the plane of the molecule The two p orbitals now overlap sideways. They form a covalent bond called a pi bond which is above and below the sigma bond Even though the overlap occurs both above and below the sigma bond, we have only formed one pi bond. Double bond in alkenes consists of both a sigma bond and a pi bond This has a major affect on both the structure and the reactivity of alkenes Unlike sigma bonds, a pi bond cannot rotate. This is because any rotation would reduce the overlap of the p orbitals Because the pi bond cannot rotate This means that the structure of an alkene across the double bond is effectively locked Since the double bond cannot rotate, alkenes can form steroisomers ( alkene molecules can exist as two stereoisomers E and Z ISOMER)

Question 31

Describe why alkenes are reactive molecules

Answer 31

Alkenes are highly reactive molecules This is due to the double bond The bond enthalpy of the pi bond is less than the sigma bond That is because a pi bond is a sideways overlap of orbitals, whereas in a sigma bond, the orbitals directly overlap Because the orbitals overlap sideways, the pi bond is easier to break Because it takes less energy to break the pi bond, it is more likely to take part in reactions' Secondly the double bond contains two pairs of electrons A pair of e- in sigma bond and a pair of e- in pi bond Because of this, the double bond is a region/centre of high electron density This high electron density makes alkenes highly reactive molecules