Hydrocarbons

Question 1

Draw the structure of displayed and 3D formula of the alkane ethane

Question 2

Draw the general formula of an alkane

Answer 2

Cn H2n + 2

Question 3

What shape do the covalent bonds around each carbon atom form

Answer 3

A tetrahedral shape

Question 4

What are the bond angles of a covalent bond around a carbon atom on an alkane

Answer 4

109.5 degrees

Question 5

What are the bonds found within alkanes

Answer 5

Sigma bond

Question 6

Explain a sigma bond

Answer 6

Sigma bonds form when electrons 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 sigma bond lie directly between the bonding atoms.

Question 7

What is a key feature of sigma bonds

Answer 7

They are fully rotational so 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.

Question 8

Explain how alkanes are a non-polar molecule

Answer 8

The similar electronegativity between a carbon and hydrogen means that alkanes are non-polar molecules

Question 9

Explain how alkanes are insoluble in water

Answer 9

Water molecule will form hydrogen bonds with each other. However, since alkanes have no permanent dipoles they cannot form hydrogen bonds. Therefore, alkanes cannot dissolve in water

Question 10

Explain how alkanes are generally unreactive

Answer 10

The strong covalent bonds within an alkane molecule works to prevent alkanes from reacting. However, will under certain conditions

Question 11

Explain the pattern in the boiling point of alkanes

Answer 11

Short chain alkanes have low boiling points and are gases at room temperature.However, longer chain alkanes have higher boiling points.

As the carbon chain increases we find liquid and solid alkanes at room temperature

Question 12

How can the increasing boiling point of increasing number of carbons in alkanes be explained

Answer 12

Alkanes are non-polar molecules thus the intermolecular forces acting between alkanes molecules are induced dipole-dipole interactions.

This is also known as London or dispersion forces

London forces are weak and do not take much energy to break thus short chain hydrocarbons have low boiling points.However, once carbon chain increases the strength of London forces also increase thus longer chain hydrocarbons have a greater boiling point.

Question 13

What are the two reason why London forces also increase when chain length increases

Answer 13

Longer chain alkanes have more electrons than shorter chain alkanes. The strength of London forces increase as the number of electron increases.

Longer chain alkanes have a greater surface area than shorter chain alkanes (greater surface area). This means that there are many points along the molecule where they can form London forces

Question 14

Explain why branched chain alkanes have a lower boiling point than straight chain alkanes

Answer 14

Branches prevent alkane molecules from getting close together

London forces are the strongest over short distances. Thus in branched chains the London forces are reduced.

Question 15

What are the uses of alkanes

Answer 15

Alkanes are used for:

Fuels in vehicles

Starting materials for the production of a whole range of organic molecules including pharmaceutical

Question 16

What is crude oil

Answer 16

Crude oil is a fossil fuel that has formed underground. Over millions of years , heat and pressure convert the chemicals in these remains into crude oil.

Question 17

Why is crude oil considered non-renewable

Answer 17

Crude oil is being used at a faster rate then is being formed

Question 18

What is crude oil composed of

Answer 18

Mixture of straight chain and branched chain alkanes with other chemical such as sulfur

Question 19

Explain the stages of fractional distillation of crude oil

Answer 19

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 into a gas

The crude oil vapors and liquid pass into the fractionating column. The column is hotter at the bottom and becomes progressively cooler going upwards.

The crude oil vapors makes it way up the column.

There are collection trays at different levels of the fractionating column. These trays have bubble caps which allow vapor to pass upwards

AS each alkane moved up the column it will reach a temperature which is cooler than its boiling point

It will then condense back into a liquid and pass out of the column

Short chain hydrocarbons (low b.p) collected at the bottom. Long chain hydrocarbons (high b.p) collected at the the top

Alkanes with very long chains form a thick liquid called bitumen collected at the bottom

Very short alkanes are collected at the top of the column as gases

Question 20

Misconceptions of fractional distillation

Answer 20

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

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

Question 21

What is the a problem with the proportion of long chain hydrocarbons and short chain hydrocarbon in crude oil

Answer 21

Crude oils contains a higher proportion of longer chain hydrocarbons than short chain hydrocarbons

Question 22

Why is there a high demand for short chain hydrocarbons

Answer 22

Short chain hydrocarbons are used for fuel for vehicles.

Therefore, there is an economic benefit for cracking long chain hydrocarbons which are less in demand in order to create short chain hydrocarbon which are high in demand. This process is called cracking.

Question 23

what are the benefits of cracking

Answer 23

Firstly, cracking converts long chain hydrocarbons into shorter chain hydrocarbons.

Cracking produces alkanes and alkenes which are highly reactive and are used as major feedstock (raw materials;) for the chemical industry to make a range of products e.g polymers.

Question 24

Explain the process of thermal cracking

Answer 24

Thermal cracking requires both high temperature and high pressure (450-900c) and the pressure is 70atm.

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

Hydrogen can also be one of the products

Question 25

Explain the benefits of thermal cracking

Answer 25

A high percentage of alkenes is formed in the products. (Useful molecule due to their high reactivity)

Question 26

How is a radical created during thermal cracking

Answer 26

During thermal cracking a covalent bond spilts to form intermediate molecules.

When the covalent bond splits, both of the intermediate molecules now have one unpaired electrons.

This is called a free radical

Question 27

Explain the process of catalytic cracking

Answer 27

Catalytic cracking also require a high temperature (450c). However, does not require high pressure.

In fact the pressure for catalytic cracking is 1-2 atmospheres.

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

Zeolite has large surface area thus supports to make it an effective catalyst

When a product undergoes catalytic cracking the product are often branched chained alkenes which are especially useful for petrol as they combust very easily

Catalytic cracking can produce cyclic alkanes and aromatic hydrocarbons such as benzene

Question 28

What is a free radical

Answer 28

A free radical is any species with an unpaired electron. They are highly reactive

Question 29

Draw the word equation for the reaction between methane and bromine

Answer 29

Methane + Bromine -UV light—> Bromoethane + Hydrogen bromide

Question 30

Explain what happens in the reaction of methane and bromine

Answer 30

The is the initiation stage: The mixture of methane and bromine have ultraviolet light shined into the reaction mix. The energy of ultraviolet light causes the single covalent bond between the two bromine atoms to break.

A single covalent bond consist of a pair of electrons. When a bond breaks like this, one electron now goes to each bromine atom because these now have an unpaired electron, these are now bromine free radicals. This is called homolytic fission. This is called the initiation stage.

The propagation stage starts where a bromine free radical reacts with a methane molecule. The bromine free radical take a hydrogen atom plus one electron from the methane molecule.

The hydrogen atom on the methane molecule has been substituted with a bromine atom because this reaction involves free radicals. This reaction is an example of free radicals substitution.

This produces both hydrogen bromide and a methyl free radical

The methyl free radical now reacts with a bromine molecule. This produces the end product bromomethane plus another bromine free radical

Propagation step 1 and 2 can form a chain reaction. (Bromine free radical can react with the methane in propagation step 1). This reaction will continue until termination

In termination , the two free radical 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 different reaction which can take place:

Two bromine free radicals can form a bromine molecule

Two methyl free radicals can form a molecule of ethane

A methyl free radicals and bromine free radicals can form a molecule of bromoethane

Question 31

Explain why there is one big problem with free radical substitution of alkanes

Answer 31

We get a whole range of side products.

E.g-if a bromine free radical reacts with a molecule of bromoethane then we make dibromoethane

A huge range of products can be formed including isomers. Therefore, at the end of this reaction, we need to separate out our product molecules

Question 32

Explain what are alkenes

Answer 32

Alkenes are unsaturated hydrocarbons with the general formula of CnH2n

Question 33

Explain planar molecules

Answer 33

Planar molecules are flat with all of the atoms lying on the same plane

When looking at molecular shapes we treat both single and double bonds as bonding regions

There are three bonding regions around each carbon atom. These repel as far away as possible

Question 34

What are the bond angles of a covalent bond around a carbon atom of an alkene

Answer 34

The bond angles around the carbon atom are 120

Question 35

Explain boding regions in alkenes

Answer 35

When looking at molecular shapes we treat both single and double bonds as bonding regions

There are three bonding regions around each carbon atom. These repel as far away as possible

Question 36

Explain how a pi bond appears in a double bond

Answer 36

When the double bond forms there is one sigma bond between the carbon atom. Each carbon atom will have one electron left in the p-orbital .

These 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 (both above and below the sigma bond)

Question 37

Explain how the double bonds can cause differences in reactivity and structure of alkenes

Answer 37

Pi bonds cannot rotate- rotation will reduce the overlap of p-orbitals. The structure across the double bond is effectively locked. Thus are able to form stereoisomers

Alkenes are very reactive. The bond enthalpy of the pi bond is less than the sigma bond. That’s because pi bond is a sideways overlap of orbitals whereas in a sigma bond, the orbitals directly overlap. Thus the pi bond is easier to break as it takes less energy resulting in it to be more likely to be used in reactions

The double bond contains two pairs of electrons ( a pair in the pi bond and sigma bond). The double bond is a region of high energy electron density. This makes it a highly reactive molecule.

Question 38

Explain why the double bond has a high region of electron density

Answer 38

A double bond contains a sigma bond and pi bond which both contain a pair of electrons

The double bond in alkenes actually consist of four electrons.

This mean that the double one is a region of high electron density.

Question 39

Key idea behind halogen atom during the electrophilic addition of hydrogen halides

Answer 39

Halogen atoms are more electronegative than hydrogen. This means that hydrogen atoms molecules have a permanent dipole.

The hydrogen atom has a small positive charge whereas the halogen atom has a small negative charge.

When a hydrogen halide is added to a symmetrical alkene, we can only make one product.

When an asymmetrical alkenes reacts with a hydrogen halide molecule we can form two different isomers. This depends on what cabin atom has the positive charge in the carbocation intermediate.

We do not create an equal amounts of product e.g (2-bromobutane is a major product whereas 1-bromobutane is a minor product)

Question 40

Explain the reaction mechanism of the electrophilic addition of hydrogen halide to alkenes

Answer 40

The hydrogen halide molecule approaches the alkene.

The positive hydrogen on the hydrogen halide is attracted to the high electron density of the double bond. This is referred to as an electrophile (positive ion/molecule attracted to a region of high electron density)

The positive charge on the hydrogen atom attracts the pair of electrons in the pi bond of the alkene.This pair of electrons move toward the hydrogen atom. (Curly arrow is used to show this)

A covalent bond forms towards the hydrogen atom and hydrogen halide. This creates a problem as a hydrogen atom only has one covalent bond. At the same time the pair of electrons between the hydrogen and halide now move onto the halide atom. (This is shown as a curly arrow). When covalent bonds break like this, with both electrons going to one atom this is called heterocyclic fission.

A positively charged intermediate molecule is formed from the alkene. This is called the carbocation intermediate. This contains a positively charged carbon atom as it has lost its pair of the electron in the pi bond.

A negatively charged halide ion. Both the electrons that were in the covalent bond were now on the bromide ion.

The electron pair on the halide ion are attracted to the positive carbon atom in the carbocation. ( shown through a curly arrow).

Lastly a product is formed

Question 41

Key point about the reaction of a hydrogen halide to an alkene

Answer 41

Key points- covalent bonds is a pair of electrons

when a curly arrow is used make sure it starts where the electrons are moving from and ends where they are moving to.

The hydrogen halide is added to the alkene. This reaction is electrophilic addition.

The alkene in this reaction is Ethene which is symmetrical molecule. Thus only one product can be formed. However, if the reaction was carried out with an asymmetric alkene then two products can be formed

Question 42

Key ideas behind asymmetric alkeness

Answer 42

When an asymmetric alkene reacts with a hydrogen halide molecule we can form two different isomers.

This depends on which carbon atom has the positive charge in the carbocation intermediate.

Equal amount of products between isomers are not created (one isomers is the major product and the other is the minor product)

Question 43

Explain how are major and minor products formed from asymmetric alkene isomers

Answer 43

Carbocation intermediate is an unstable molecule which only exists for a short period of time.

The stability of the carbocation depends on how many alkyl groups are bonded to the carbon atoms with the positive charge.

If the positive carbon atom is bonded to only one alkyl group then we call this a primary carbocation. However, if it is bonded to two alkyls it is called a secondary carbocation

Secondary carbocation are more stable than a primary carbocation. Thus secondary carbocations exist for longer and is more likely to form a product.

Question 44

what is Markownikoff’s rule

Answer 44

When a hydrogen halide reacts with an asymmetric alkene, the hydrogen atom of the hydrogen halide is more likely to bond to the carbon atom which is attached to the greater number of hydrogen atoms

Question 45

Explain Markownikoff’s rule

Answer 45

Secondary carbocation are more stable than primary carbocation. This is because the electrons in alkyl groups can shift towards the positive charge. In order to stabilise the positive charge . (Positive inductive effect)

In primary carbocations, the positive charge is stabilized by only one alkyl group whereas in secondary carbocations, the positive charge is stabilised by two alkyl groups. Therefore, secondary carbocations are more stable and the major product is formed.

Question 46

Describe the reaction mechanism between an alkene and a halogen

Answer 46

In the first stage of the reaction, the halogen molecule approaches the alkene molecule. The halogen does not have a permanent dipole. However, the double bond of the alkene is a region of high electron density.This high electron density repels the electron pair of the covalent bond in the halogen molecule. This means that the halogen molecule has an induced dipole.

The pair of electrons from in the pi bond of the alkene are attracted to the positive halogen. The positive halogen acts as an electrophile. The electron pair then forms a covalent bond to the halogen atom.At the same time, the covalent bond in the halogen molecule now breaks and the pair of electrons move onto the other halogen atom. (When a covalent bond breaks like this with both electron going to the same atom- this is an example of heterolytic fission). We then have a carbocation intermediate and a halogen ion.

The electron pair in the halogen is attracted to the positive carbon atom in the carbocation intermediate. This electron pair now forms a covalent bond and we have our final product. (The halogen atom would add across the double bond. This means that the two halogen atoms end up on two adjacent carbon atoms-both cannot be on the same carbon)

Question 47

Key point of electrophilic addition of alkene and halogens

Answer 47

Halogen molecules add across the double bond. This is because halogen molecules do not have a permanent dipole.

Question 48

Key differences between adding a halogen to an alkene and a hydrogen halide to one

Answer 48

hydrogen haldie molecules have a permanent dipole whereas halogens have induced dipoles

When a hydrogen halide is added to an asymmetric alkene. This makes major and minor product. However, when a halogen is added to an asymmetric alkene, we only make one product.

Question 49

What can the reaction with a halogen be used for

Answer 49

To test for the presence of an unsaturated molecule such as an alkene.

Question 50

Describe how you can test for unsaturated molecules

Answer 50

We use bromine water which has an orange/brown colour.

To test if a substance is unsaturated. We add drops of bromine water and gently shake the test tube.

If our substance is unsaturated, then the bromine will add across the double bond and the product of the reaction will be colourless.

The orange bromine water will decolourlise. If the test substance is saturated, then the bromine will not react and the bromine water will remain orange.

Question 51

What is a hydration reaction

Answer 51

The electrophillic addition of water to alkenes to make alcohols