Chapter 15 Hydrocarbons

Question 1

how are alkanes produced

Answer 1

• Hydrogenation & Cracking
• Alkanes are hydrocarbons that can be produced by the addition reaction of hydrogen to an alkene or by cracking of longer alkane chains

Question 2

Production of alkanes from addition reactions

Answer 2

=Alkenes are unsaturated organic molecules and contain C-C double bonds

• When hydrogen gas and an alkene are heated and passed over a finely divided Pt/Ni catalyst, the addition reaction produces an alkane:
• The Pt/Ni catalyst is finely divided to increase its surface area and therefore increase the rate of reaction

Question 3

hydrogenation

Answer 3

=The addition reaction of alkenes with hydrogen

• Hydrogenation is often used in the manufacture of margarine from vegetable oil
• Vegetable oil is an unsaturated organic molecule with many C-C double bonds
• When these are partially hydrogenated, their hydrocarbon chains become straighter
• This raises the melting point of the oils which is why margarine is a soft solid and vegetable oil a liquid at room temperature

Question 4

Production of alkanes from cracking

Answer 4

• In cracking large, less useful hydrocarbon molecules found in crude oil are broken down into smaller, more useful molecules
• The large hydrocarbon molecules are fed into a steel chamber and heated to a high temperature and then passed over an aluminium oxide (Al2O3) catalyst
• The chamber does not contain any oxygen to prevent combustion of the hydrocarbon to water and carbon dioxide
• When a large hydrocarbon is cracked, a smaller alkane and alkene molecules are formed
• Eg. octane and ethene from decane

Question 6

Complete combustion

Answer 6

• When alkanes are burnt in excess (plenty of) oxygen, complete combustion will take place and all carbon and hydrogen will be oxidised to carbon dioxide and water respectively
• For example, the complete combustion of octane to carbon dioxide and water

Question 7

Incomplete combustion

Answer 7

• When alkanes are burnt in only a limited supply of oxygen, incomplete combustion will take place and not all the carbon is fully oxidised
• Some carbon is only partially oxidised to form carbon monoxide
• `For example, the incomplete combustion of octane to form carbon monoxide

Question 8

Carbon monoxide cause and effects

Answer 8

• is a toxic gas as it will bind to haemoglobin in blood which can then no longer bind oxygen
• As no oxygen can be transported around the body, victims will feel dizzy, lose consciousness and if not removed from the carbon monoxide, they can die
• Carbon monoxide is extra dangerous as it is odourless (it doesn’t smell) and will not be noticed
• Incomplete combustion often takes place inside a car engine due to a limited amount of oxygen present
• alkane + oxygen –> carbon monoxide + water

Question 9

Free-radical substitution of alkanes

Answer 9

• Alkanes can undergo free-radical substitution in which a hydrogen atom gets substituted by a halogen (chlorine/bromine)
• Since alkanes are very unreactive, ultraviolet light (sunlight) is needed for this substitution reaction to occur
• The free-radical substitution reaction consists of three steps

Question 10

The free-radical substitution reaction consists of three steps

Answer 10

• In the initiation step, the halogen bond (Cl-Cl or Br-Br) is broken by UV energy to form two radicals
• These radicals create further radicals in a chain type reaction called the propagation step
• The reaction is terminated when two radicals collide with each other in a termination step

Question 11

Free Radical Substitution Mechanism

Answer 11

• Alkanes can undergo free-radical substitution in which a hydrogen atom gets substituted by a halogen (chlorine/bromine)
• Ultraviolet light (sunlight) is needed for this substitution reaction to occur
• The free-radical substitution reaction consists of three steps

Question 12

Initiation step

Answer 12

• In the initiation step the Cl-Cl or Br-Br is broken by energy from the UV light
• This produces two radicals in a homolytic fission reaction

Question 13

Propagation step

Answer 13

• The propagation step refers to the progression (growing) of the substitution reaction in a chain type reaction
• Free radicals are very reactive and will attack the unreactive alkanes
• A C-H bond breaks homolytically (each atom gets an electron from the covalent bond)
• An alkyl free radical is produced
• This can attack another chlorine/bromine molecule to form the halogenoalkane and regenerate the chlorine/bromine free radical
• This free radical can then repeat the cycle

Question 14

propagation reaction is not very suitable

Answer 14

• for preparing specific halogenoalkanes as a mixture of substitution products are formed
• If there is enough chlorine/bromine present, all the hydrogens in the alkane will eventually get substituted (eg. ethane will become C2Cl6/C2Br6)

Question 15

Termination step

Answer 15

• The termination step is when the chain reaction terminates (stops) due to two free radicals reacting together and forming a single unreactive molecule
• Multiple products are possible

Question 16

Crude oil

Answer 16

• Crude oil is a mixture of hydrocarbons containing alkanes, cycloalkanes and arenes (compounds with a benzene ring)
• The crude oil is extracted from the earth in a drilling process and transported to an oil refinery
• At the oil refinery the crude oil is separated into useful fuels by fractional distillation
• This is a separating technique in which the wide range of different hydrocarbons are separated into fractions based on their boiling points

Question 17

The heavier fractions that are obtained in fractional distillation are further cracked into useful alkane and alkenes with lower Mr values

Answer 17

Question 18

Cracking

Answer 18

• The large hydrocarbon molecules are fed into a steel chamber and heated to a high temperature and then passed over an aluminium oxide (Al2O3) catalyst
• The chamber does not contain any oxygen to prevent combustion of the hydrocarbon to water and carbon dioxide
• When a large hydrocarbon is cracked, a smaller alkane and alkene molecules are formed
• Eg. octane and ethene from decane

Question 19

The low-molecular mass alkanes

Answer 19

• formed make good fuels and are in high demand
• The low-molecular mass alkenes are more reactive than alkanes due to their double bond
• This makes them useful for the chemical industry as the starting compounds (feedstock) for making new products
• Eg. they are used as monomers in polymerisation reactions

Question 20

Unreactivity of Alkanes: Strength of C-H bonds

Answer 20

• Alkanes consist of carbon and hydrogen atoms which are bonded together by single bonds
• Unless a lot of heat is supplied, it is difficult to break these strong C-C and C-H covalent bonds
• This decreases the alkanes’ reactivities in chemical reactions

Question 21

Unreactivity of Alkanes: Lack of polarity

Answer 21

• The electronegativities of the carbon and hydrogen atoms in alkanes are almost the same
• This means that both atoms share the electrons in the covalent bond almost equally
• As a result of this, alkanes are nonpolar molecules and have no partial positive or negative charges (δ+ and δ– respectively)
• Alkanes therefore do not react with polar reagents
• They have no electron-deficient areas to attract nucleophiles
• And also lack electron-rich areas to attract electrophiles
• Due to the unreactivity of alkanes, they only react in combustion reactions and undergo substitution by halogens

Question 22

Oxides of nitrogen

Answer 22

• Normally, nitrogen is too unreactive to react with oxygen in air
• However, in a car’s engine, high temperatures and pressures are reached causing the oxidation of nitrogen to take place:

—N2(g) + O2(g) → 2NO(g)

—N2(g) + 2O2(g) → 2NO2(g)

• The oxides of nitrogen are then released in the car’s exhaust fumes into the atmosphere
• Car exhaust fumes also contain unburnt hydrocarbons from fuels and their oxides (VOCs)

Question 23

effects of oxides of nitrogen: VOCs, PAN and ect

Answer 23

In air, the nitrogen oxides can react with these VOCs to form peroxyacetyl nitrate (PAN) which is the main pollutant found in photochemical smog

PAN is also harmful to the lungs, eyes and plant-life

Nitrogen oxides can also dissolve and react in water with oxygen to form nitric acid which is a cause of acid rain

Acid rain can cause corrosion of buildings, endangers plant and aquatic life (as lakes and rivers become too acidic) as well as directly damaging human health

Question 24

Catalytic removal

Answer 24

• To reduce the amount of pollutants released in cars’ exhaust fumes, many cars are now fitted with catalytic converters
• Precious metals (such as platinum) are coated on a honeycomb to provide a large surface area

Question 25

-The reactions that take place in the catalytic converter include:

Answer 25

• Oxidation of CO to CO2:
• 2CO + O2 → 2CO2

or

• 2CO + 2NO → 2CO2 + N2
• Reduction of NO/NO2 to N2:

2CO + 2NO → 2CO2 + N2

-Oxidation of unburnt hydrocarbons:

CnH2n+2 + (3n+1)[O] → nCO2 + (n+1)H2O

Question 27

Pollutants, their effect & removal table

Answer 27

Question 28

Production of Alkenes:

Answer 28

Alkenes can be made by a series of reactions including elimination, dehydration reactions and cracking

Question 29

Production of Alkenes: Elimination

Answer 29

• Alkenes can be produced from the elimination reaction of a halogenoalkane
• An elimination reaction is one in which a small molecule is lost
• In the case of halogenoalkanes, the small molecule that is eliminated is a hydrogen halide, HX, where X is the halogen
• The halogenoalkane is heated with ethanolic sodium hydroxide

–The eliminated H+ in HBr reacts with the ethanolic OH– to form water

–The eliminated Br– in HBr reacts with Na+ to form NaBr

-Note that the reaction conditions should be stated correctly as different reaction conditions will result in different types of organic reactions

–NaOH (ethanol): an elimination reaction occurs to form an alkene

–NaOH (aq): a nucleophilic substitution reaction occurs, and an alcohol is one of the products

Question 30

The eliminated HBr reacts with ethanolic OH– and Na+ to form H2O and NaBr

Answer 30

Question 31

Production of Alkenes: Dehydration Reactions

Answer 31

• Alkenes can also be produced from the elimination reaction of alcohols in which a water molecule is lost
• This is also called a dehydration reaction
• Alcohol vapour is passed over a hot catalyst of aluminium oxide powder (Al2O3)
• Concentrated acid, pieces of porous pot or pumice can also be used as catalysts
• The smaller alkenes (such as ethene, propene and butene) are all gases at room temperature and can be collected over water

Question 32

The smaller alkenes are gases at room temperature and collected over water

Answer 32

Question 33

Production of Alkenes: Cracking

Answer 33

• Alkenes can also be produced from the cracking of long hydrocarbon molecules in crude oil
• An aluminium oxide (Al2O3) catalyst and high temperatures are used to speed up this reaction.
• It is important to ensure that the crude oil doesn’t come into contact with oxygen as this can cause combustion of the hydrocarbons to produce water and carbon dioxide
• The cracking of crude oil produces a smaller alkane and alkene molecules
• The low-molecular mass alkenes are more reactive than alkanes as they have an electron-rich double bond
• They can therefore be used as feedstock for making new products

Question 34

The formation of ethene from ethanol is an example of a dehydration reaction of alcohols

Answer 34

Question 35

Reactions of Alkenes: Electrophilic addition

Answer 35

• Electrophilic addition is the addition of an electrophile to a double bond
• The C-C double bond is broken, and a new single bond is formed from each of the two carbon atoms
• Electrophilic addition reactions include the addition of:

–Hydrogen (also known as hydrogenation reaction)

–Steam (H2O (g))

–Hydrogen halide (HX)

–Halogen

Question 36

Reactions of Alkenes: Oxidation

Answer 36

• Alkenes can also be oxidised by acidified potassium manganate(VII) (KMnO4) which is a very powerful oxidising agent
• Alkenes can be oxidised by both hot and cold KMnO4 which will result in different products being formed
• When shaken with cold dilute KMnO4 the pale purple solution turns colourless and the product is a diol
• When alkenes are reacted with hot concentrated KMnO4 the conditions are harsher causing the C-C double bond to completely break
• The O-H groups in the diol formed are further oxidised to ketones, aldehydes, carboxylic acids or carbon dioxide gas
• The actual products formed depend on what is bonded to the carbon atoms in the alkene

Question 37

The reactions can be used to predict where the double bond in a larger molecule is

Answer 37

The reactions of alkenes with hot concentrated KMnO4 can be used to determine the position of the double bond in larger alkenes

Question 38

Reactions of Alkenes: Addition polymerisation

Answer 38

• Addition polymerisation is the reaction of many monomers containing at least one double C-C bond to form the long-chain polymers as the only product
• Monomers are small, reactive molecules that react together to make the polymer
• A polymer is a long-chain molecule made up of many repeating units (monomers)
• In addition polymerisation reaction, the C-C double bond is broken to link together the monomers and form a polymer
• This is a common method to make plastics
• Other alkenes and substituted alkenes can also polymerise to make polymers with different properties

Question 39

The diagram shows a polymerisation reaction of propene to poly(propene)

Answer 39

Question 40

Poly(chloroethene) is used as plastic

Answer 40

Question 41

Test for Unsaturation

Answer 41

• Halogens can be used to test if a molecule is unsaturated (i.e. contains a double bond)
• Br2(aq) is an orange or yellow solution, called bromine water and this is the halogen most commonly used
• The unknown compound is shaken with the bromine water
• If the compound is unsaturated, an addition reaction will take place and the coloured solution will decolourise

Question 42

Alkenes: Electrophilic Addition

Answer 42

• The double bond in alkenes is an area of high electron density (there are four electrons found in this double bond)
• This makes the double bond susceptible to attack by electrophiles (electron-loving species)
• An electrophilic addition is the addition of an electrophile to a double bond

Question 43

Alkenes: Electrophilic addition of hydrogen bromide

Answer 43

• A molecule of hydrogen bromide (HBr) is polar as the hydrogen and bromine atoms have different electronegativities
• The bromine atom has a stronger pull on the electrons in the H-Br bond
• As a result of this, the Br atom has a partial negative and the H atom a partial positive charge
• In an addition reaction, the H atom acts as an electrophile and accepts a pair of electrons from the C-C bond in the alkene
• The H-Br bond breaks heterolytically, forming a Br– ion
• This result in the formation of a highly reactive carbocation intermediate which reacts with the Br– (nucleophile)

Question 44

Alkenes: Electrophilic Addition: Electrophilic addition of bromine

Answer 44

• Bromine (Br2) is a non-polar molecule as both atoms have similar electronegativities and therefore equally share the electrons in the covalent bond
• However, when a bromine molecule gets closer to the double bond of an alkene, the high electron density in the double bond repels the electron pair in Br-Br away from the closest Br atom
• As a result of this, the closest Br atom to the double bond is slightly positive and the further Br atom is slightly negatively charged
• In an addition reaction, the closest Br atom acts as an electrophile and accepts a pair of electrons from the C-C bond in the alkene
• The Br-Br bond breaks heterolytically, forming a Br– ion
• This results in the formation of a highly reactive carbocation intermediate which reacts with the Br– (nucleophile)

Question 46

Br2 is a non-polar molecule however when placed close to an area of high electron density it can get polarised

Answer 46

Question 47

The stability of the carbocation intermediate is as follows:

Answer 47

• tertiary > secondary > primary
• When more than one carbocations can be formed, the major product of the reaction will be the one that results from the nucleophilic attack of the most stable carbocation.

Question 48

Alkenes: Stability of Cations & Markovnikov’s Rule

Answer 48

• Carbocations are positively charged carbon atoms with only three covalent bonds instead of four
• There are three types of carbocations: primary, secondary and tertiary

Question 49

Inductive effect

Answer 49

The alkyl groups attached to the positively charged carbon atoms are ‘electron donating groups’

This is also known as the inductive effect of alkyl groups

• The inductive effect is illustrated by the use of arrowheads on the bonds
• The alkyl groups push electrons away from themselves towards the positively charged carbon
• This causes the carbocation to become less positively charged
• As a result of this, the charge is spread around the carbocation which makes it energetically more stable
• This means that tertiary carbocations are the most stable as they have three electron-donating alkyl groups which energetically stabilise the carbocation
• Due to the positive charge on the carbon atom, carbocations are electron-loving species (electrophiles)

Question 50

Markovnikov’s rule

Answer 50

• In addition reactions, an electrophile reacts with the double bond of alkenes
• The electrophile will add to the carbon to give the most stable carbocation
• Therefore, the nucleophile will bond to the C-C carbon atom with the highest number of alkyl groups bonded to it
• This is also known as the Markovnikov’s rule which predicts the outcome of addition reactions and states that:
• In an addition reaction of a halogen halide (HX) to an alkene, the halogen ends up bonded to the most substituted carbon atom.

Question 51

The nucleophile ends up to the most substituted C-C carbon atom

Answer 51