14 - hydrocarbons

alkanes

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

their general formula is : C_nH_2n+2

naming alkanes - rules (applicable to other organic compounds too)

 Select the longest carbon chain that contains the highest number of substituents.

 Number the carbon chain in order to give the substituents the lowest possible numbers

 Place the substituents according to the alphabetical order as prefixes indicating their respective

positions.

 If more than one substituent of a given type is present, indicate their number by the numerical

terms , di ( two ) , tri ( three ) ,tetra ( four ) and so on.

 Numbers and names are separated by a hyphen ( – ) and numbers are separated by a

comma.

 End the name of the alkane with the suffix , ane.

cycloalkanes

Cycloalkanes are a group of saturated hydrocarbons that share the same general formula CnH2n with alkenes.

Production of alkanes from addition reactions

• Alkenes are unsaturated organic molecules containing at least one C=C double bond

• Alkenes can be heated with hydrogen gas and a Pt/Ni catalyst

  • The Pt/Ni catalyst is finely divided to increase its surface area

  • This increases the overall rate of reaction

•  This is an addition reaction which forms an alkane from an alkene, for example:

butene + hydrogen →(Pt H2)  butane

• Alkanes are formed from hydrogenation. - The addition reaction of alkenes with hydrogen is called hydrogenation

where is hydrogenation used in the industry

Margarine Production from Vegetable Oil

• Vegetable oils are typically derived from plant sources and consist mainly of unsaturated fatty acids—hydrocarbon chains that contain one or more C=C double bonds.

• These double bonds introduce kinks in the chains, which prevent the molecules from packing closely together. As a result, vegetable oils are liquids at room temperature.

• In the production of margarine, these oils undergo partial hydrogenation:

  • Hydrogen gas (H₂) is bubbled through the oil.

  • A nickel catalyst is used.

  • The reaction is carried out at around 150–200°C under high pressure.

• During this process:

  • Some of the C=C double bonds are converted into C–C single bonds as hydrogen atoms are added across the double bonds.

  • This makes the hydrocarbon chains more saturated and straighter.

• As the chains become straighter:

  • They can pack more tightly together.

  • Intermolecular forces (Van der Waals) between the molecules become stronger.

  • This results in a higher melting point.

• Therefore, the product (margarine) becomes a semi-solid at room temperature, making it spreadable, unlike the original liquid vegetable oil.

⚠ Note: Complete hydrogenation would make the product too hard, so only partial hydrogenation is used.

• In older methods, partial hydrogenation sometimes produced trans fats (trans isomers), which have negative health effects. Modern manufacturing aims to minimize or eliminate trans fat formation.c

cracking definition and why we crack hydrocarbons

Cracking is a process where large hydrocarbon molecules (usually alkanes) are broken down into smaller, more useful hydrocarbons — like shorter-chain alkanes and alkenes.

• Long-chain alkanes from crude oil are less useful (too viscous, hard to ignite).

• We crack them to make short-chain alkanes (fuels) and alkenes (feedstock for polymers and chemicals).

• Cracking increases the supply of shorter-chain hydrocarbons, which are in higher demand.

• Cracking also increases the supply of alkenes, which do not occur in crude oil naturally but are very important for industrial chemistry

production of alkanes from cracking

• In cracking large, less useful hydrocarbon molecules found in crude oil are broken down into smaller, more useful molecules - primarily smaller alkanes and alkenes.

• The large hydrocarbon molecules are fed into a steel chamber, 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

  • E.g. octane and ethene from decane

cracking in detail

• The large hydrocarbon is vaporised (turned into gas) by heating to about 450–750°C.

• It is then passed over a hot catalyst, typically aluminium oxide (Al₂O₃) or silicon dioxide (SiO₂) — often supported on a ceramic base.

• The reaction chamber is kept in anaerobic conditions (no oxygen) to prevent combustion (oxidation to CO₂ and H₂O).

• Bonds in the large alkane molecules break heterolytically or homolytically, depending on conditions, producing smaller alkanes and alkenes.

---

⚗ Types of Cracking (mention briefly):

• Catalytic cracking:
  Uses a catalyst (like Al₂O₃), lower temperature (~450°C), and moderate pressure.
  Produces branched alkanes, aromatic hydrocarbons, and alkenes.
  Used in the petroleum industry for making motor fuel.

• Thermal cracking:
  Uses higher temperatures (up to 900°C) and high pressure, no catalyst.
  Produces mainly alkenes, which are used in polymer manufacture.

what type of reactions are hydrogenation and cracking (exo or end)

hydrogenation is an exothermic reaction and cracking is an endothermic reaction

reactions of alkanes

Alkanes are generally unreactive compounds. The very small difference in electronegativity between carbon and hydrogen makes the C – H bonds non polar and alkane molecules non polar overall. Alkanes are not attacked by electrophiles or nucleophiles.

The reactions of alkanes are limited only to combustion on a large scale for their use as fuels and free radical substitution to form more reactive halogenalkanes.

alkanes in complete combustion

• 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

incomplete combustion of alkanes

• 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

carbon monoxide gas

• Carbon monoxide is a toxic gas as it will bind to haemoglobin in blood which can then no longer bind to 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

free-radical substitution of alkanes

• 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:

  • In the (chain) initiation step, the halogen bond (Cl-Cl or Br-Br) is broken by UV energy to form two radicals through homolytic fission

  • These radicals create further radicals in a chain type reaction called the (chain) propagation step (A chlorine free radical attacks a molecule of methane and continues to propagate the chain reaction. One C – H bond at a time breaks homolytically until CH4 turns in to CCl4. Each step produces a free radical.)

  • The reaction is terminated when two radicals collide with each other in a (chain) termination step

free-radical substitution of alkanes - steps

-

reacting an alkane with bromine

The fact that the bromine colour has disappeared only when mixed with an alkane and placed in sunlight suggests that the ultraviolet light is essential for the free radical substitution reaction to take place

initiation step in free-radical substitution of alkanes

• In the initiation step, the halogen bond (Cl-Cl or Br-Br) is broken by UV energy to form two radicals

• The covalent Cl-Cl bond is broken by energy from the UV light

• Each atom takes one electron from the covalent bond

• This produces two radicals in a homolytic fission reaction

Cl–Cl 2Cl^•

propagation step in free-radical substitution of alkanes

• The halogen free radicals are very reactive and will attack the unreactive alkanes

• One of the methane C-H bond breaks homolytically to produce an alkyl radical

CH₄ + Cl^• → ^•CH₃ + HCl

• The alkyl radical can attack another chlorine molecule to form a halogenoalkane

• This also regenerates the chlorine free radical

^•CH₃ + Cl₂ → CH₃Cl + Cl^• 

• The regenerated chlorine free radical can then repeat the cycle

• For example, the chlorination of ethane is:

ethane + chlorine radical → ethyl radical + hydrogen chloride

CH₃CH₃ + Cl^• → ^•CH₂CH₃ + HCl

ethyl radical + chlorine molecule → chloroethane + regenerated chlorine radical

^•CH₂CH₃ + Cl₂ → CH₃CH₂Cl + Cl^•

• This reaction is not very suitable for preparing specific halogenoalkanes as a mixture of substitution products is formed

• If there is enough halogen present, all the hydrogens in the alkane will eventually get substituted

• For example, the chlorination of ethane could continue:

chloroethane + chlorine radical → radical + hydrogen chloride

CH₃CH₂Cl + Cl^• → ^•CH₂CH₂Cl + HCl

radical + chlorine molecule → 1,2-dichloroethane + regenerated chlorine radical

^•CH₂CH₂Cl + Cl₂ → CH₂Cl₂ + Cl^•

• This process can repeat until hexachloroethane, C₂Cl₆, is formed

last step in free-radical substitution of alkanes

• 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

• For example, the single substitution of ethane by chlorine can form: 

ethyl radical + chlorine radical → chloroethane

^•CH₂CH₃ + Cl^• → CH₃CH₂Cl

ethyl radical + ethyl radical → butane

^•CH₂CH₃ + ^•CH₂CH₃ → CH₃CH₂CH₂CH₃

chlorine radical + chlorine radical → chlorine molecule

Cl^• + Cl^• → Cl₂

examiner tips and tricks - If you are asked to give an equation for the termination step of a free radical reaction / mechanism, you should not give the equation reforming the original halogen as this is often marked as "ignore" on mark schemes.

Free radical substitution using bromine instead of chlorine is possible and follows similar initiation, propagation and termination steps

-

further substitution in alkanes

• Often, free radical reactions are not very suitable 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

• For example, methane could be substituted to become chloromethane and then further substituted
  
  (Free radical substitution is a very reactive and uncontrolled process.

  Once it starts, it doesn't just stop at making one product.

  Instead, it keeps going and can replace more than one hydrogen atom in the alkane.

  So, you don’t just get the product you want — you get a mixture of different halogenoalkanes.)

• Single substitution:

CH₄ + Cl^• → ^•CH₃ + HCl

^•CH₃ + Cl₂ → CH₃Cl + Cl^• 

• Second substitution:

CH₃Cl + Cl^• → ^•CH₂Cl + HCl

^•CH₂Cl + Cl₂ → CH₂Cl₂ + Cl^•

• Third substitution:

CH₂Cl₂ + Cl^• → ^•CHCl₂ + HCl

^•CHCl₂ + Cl₂ → CHCl₃ + Cl^•

• Complete substitution:

CHCl₃ + Cl^• → ^•CCl₃ + HCl

^•CCl₃ + Cl₂ → CCl₄ + Cl^•

examiner tips and tricks

You could be asked to draw the mechanism for initiation and termination steps for free radical substitution

This mechanism will use half-headed arrows to show the movement of one electron (double-headed arrows show the movement of a pair of electrons)

A half-headed arrow is known as a ‘fish hook’ arrow. 

Initiation:

Termination:

The key is the use of the ‘fish hook’ arrow to show the homolytic fission of the bond in initiation and the formation of the bond in termination.  

crude oil

• Crude oil is a natural, unrefined fossil fuel formed over millions of years from the remains of ancient marine organisms (like plankton) buried under sedimentary rock. Under high pressure and temperature, these remains were chemically transformed into 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

how is crude oil separated

• At the oil refinery, the crude oil is separated into useful fuels by fractional distillation

• This is a separating technique in which a wide range of different hydrocarbons are separated into fractions based on their boiling points

• Crude oil is heated and partially vaporised.

• The vapour enters a fractionating column — hot at the bottom, cooler at the top.

• As vapours rise, they cool and condense at different levels depending on their boiling points.

• The hydrocarbons are collected as fractions, each containing molecules of similar chain length and boiling range.

main fractions in fractional distillation with their uses

Fraction	Carbon atoms	Use
Refinery gas	C1–C4	Bottled gas, LPG
Petrol (Gasoline)	C5–C12	Car fuel
Naphtha	C5–C10	Petrochemicals
Kerosene	C10–C16	Jet fuel
Diesel	C14–C20	Lorry and bus fuel
Fuel oil	C20–C70	Ship/industrial fuel
Bitumen	C70+	Road surfacing

why is cracking sometimes used instead of fractional distillation

• However, the smaller hydrocarbon fractions (such as gasoline fractions) are in high demand compared to the larger ones

• Therefore, some of the excess heavier fractions are broken down into smaller, more useful compounds

• These more useful compounds include alkanes and alkenes of lower relative formula mass (M_r)

• This process is called cracking

cracking process

• The large hydrocarbon molecules are fed into a steel chamber, 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

  • E.g. octane and ethene from decane

 Cracking of long-chain hydrocarbons

low-molecular mass alkanes and alkenes formed from cracking

• The low-molecular mass alkanes 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

  • E.g. they are used as monomers in polymerisation reactions

Using alkenes to form other useful compounds

strength of c-h bonds in alkanes

• Alkanes consist of carbon and hydrogen atoms which are bonded together by single bonds, known as sigma (σ) bonds, formed from the direct overlap of atomic orbitals.

• Unless a lot of heat is supplied, it is difficult to break these strong C–C and C–H covalent bonds, which have high bond enthalpies.

• This decreases the alkanes’ reactivities in chemical reactions, making them relatively inert under normal conditions.

• The electronegativities of the carbon and hydrogen atoms in alkanes are almost the same, with carbon being only slightly more electronegative than hydrogen.

• This means that both atoms share the electrons in the covalent bond almost equally, leading to little to no bond polarity.

• As a result of this, alkanes are nonpolar molecules and have no partial positive or negative charges (δ+ and δ– respectively), so they do not interact strongly with polar solvents or reagents.

• Alkanes therefore do not react with polar reagents, such as acids, bases, or oxidizing agents under standard conditions.

• They have no electron-deficient areas to attract nucleophiles, which are species that donate electron pairs.

• They also lack electron-rich areas to attract electrophiles, which are species that accept electron pairs.

• Due to the unreactivity of alkanes, they only react in combustion reactions and undergo substitution by halogens

examiner tips and tricks

Remember: nucleophiles are negatively charged and are attracted to electron-deficient regions

Electrophiles are positively charged and attracted to electron-rich regions

Combustion of Alkanes & the Environment

• Cars’ exhaust fumes include toxic gases such as carbon monoxide (CO), oxides of nitrogen (NO/NO₂) and volatile organic compounds (VOCs)

• When released into the atmosphere, these pollutants have drastic environmental consequences damaging nature and health

carbon monoxide and its effects

• When oxygen supply is limited, carbon monoxide (CO) forms instead of carbon dioxide:

fuel + oxygen → carbon monoxide + water

• For example, Incomplete combustion of propane:

propane + oxygen → carbon monoxide + water

C₃H₈ (l) + 3½O₂ (g) → 3CO (g) + 4H₂O (l)

• Carbon monoxide is extremely dangerous because it is:

  • Colourless and odourless (can’t be seen or smelled)

  • Hard to detect without a sensor

• It is a toxic and poisonous gas that binds irreversibly to haemoglobin in the blood.

  • This prevents haemoglobin from carrying oxygen.

• Lack of oxygen transport leads to:

  • Dizziness

  • Loss of consciousness

  • Potentially death if not treated

 The effect of carbon monoxide on haemoglobin

The high affinity of CO to haemoglobin prevents it from binding to O2 and CO

oxides of nitrogen

• Normally, nitrogen is too unreactive to react with oxygen in air

• 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)

• In the air, the nitrogen oxides can react with these VOCs to form peroxyacetylnitrate (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, endanger plant and aquatic life (as lakes and rivers become too acidic) and directly damage human health

catalytic converters

• 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

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

  • 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:

C_nH_2n+2 + (3n+1)[O] → nCO₂ + (n+1)H₂O

Pollutants, their effect & removal summary

• Carbon Monoxide (CO)

  • Formation:

    • Incomplete combustion of alkanes in car engines

  • Environmental consequence:

    • Toxic gas

  • Catalytic removal:

    • Oxidised to carbon dioxide (CO₂):

2CO + O₂ → 2CO₂

2CO + 2NO → 2CO₂ + N₂

• Oxides of Nitrogen (NO, NO₂ etc.)

  • Formation:

    • Oxidation of nitrogen in car engines (due to high temperatures)

  • Environmental consequence:

    • Dissolve in water and react with oxygen to form acid rain

  • Catalytic removal:

    • Reduced to nitrogen gas:

2CO + 2NO → 2CO₂ + N₂

• Volatile Organic Compounds (VOCs)

  • Formation:

    • Unburnt hydrocarbons from fuels

    • Oxides of these hydrocarbons formed in car engines

  • Environmental consequence:

    • React with oxides of nitrogen in the atmosphere to form PAN

  • Catalytic removal:

    • Oxidised to CO₂ and H₂O

    • General formula reaction:

C_nH_2n+2 + (3n + 1)[O] → nCO₂ + (n + 1)H₂O

• PAN (peroxyacyl nitrate)

  • Formation:

    • From the photochemical reaction of VOCs and nitrogen oxides in the atmosphere

  • Environmental consequence:

    • Contributes to photochemical smog

  • Catalytic removal:

    • Oxidise unburnt hydrocarbons

    • Reduce nitrogen oxides to prevent PAN formation

examiner tips and tricks : Although CO₂ is not a toxic gas, it is still a pollutant causing global warming and climate change

-