Hydrocarbons

What is an alkane

A hydrocarbon is a compound containing carbon and hydrogen only. Examples of hydrocarbons are alkanes, alkenes and alkynes.

Alkanes

Alkanes are saturated hydrocarbons, the carbon atoms are joined to each other vua single covalent bond only. They form the homologous series with a formula of CnH2n+2

Physical properties

Alkanes are non-polar, thus they are soluble in non-polar solvents like CCl4, They are, in general, insoluble and less dense than water
The boiling point of straight-chain alkanes increases with the size of molecule. This is because the number of electrons in each molecule increases, Resulting in the increases strength of van der Waals. Thus more energy is required to break these forces
For a branched alkane, it is more volatile and its boiling point is lower than its counterpart with the same number of carbon atoms. This is because they are more spherical and have lower surface area. Thus less temporary dipoles can be set up and the van der Waals forces are weaker, less energy is required to overcome it.
The density of liquid alkanes increases slightly. The mass of each succeeding molecule increases but due to the increased strength of van der Waals forces, The molecules are more closer together, This results in higher ratio of mass/volume
Cycloalkanes are alkanes that joined up as a ring, The smallest possible cycloalkane is cyclopropane. Cycloalkanes have two hydrogen less than the corresponding straight chain alkane.

Reactivity of alkanes

Alkanes are saturated and generally unreactive because they are non-polar, hence they are unattractive towards nucleophiles and electrophiles
Alkanes will only react with non-polar reagents in the presence of heat or ultraviolet light

Combustion

Alkanes undergo complete combustion under excess oxygen gas to give carbon dioxide and water

CH4 + 2O2 = CO2 + 2H2O

Larger alkanes are more difficult to ignite. This is because alkanes only burn in the gaseous state and larger alkanes have stronger van der Waals forces that hold them together, Therefore more energy is required to vaporise it.
When the supply of oxygen is limited, alkanes undergo incomplete combustion, The possible products are carbon monoxide, carbon and water

2CH4 + 3O2 = 2CO + 4H2O

CH4 + O2 = C + 2H2O

Halogenation

Alkanes undergo halogenation via free-radical substitution to give a range of products. In free-radical substitution, hydrogen atoms in the molecules are gradually substituted by halogen atoms.
Reagent: Chlorine or bromine gas (Cl2 or Br2)

Condition: The presence of ultraviolet light or sunlight

Product: Halogenolalkanes

Free-radical substitution proceeds via a chain reaction. During a chain reaction, for every reactive species you start of with, a new one is generated at the end and this keeps the process going.
When methane reacts with chlorine in sunlight, the greenish-yellow color fades and steamy acidic fumes of hydrogen chloride can be observed

CH4 + Cl2 = CH3Cl + HCl

However, this chain reaction does not end there. More hydrogen atoms will be substituted as long as there are sufficient chlorine atoms.

CH3Cl + Cl2 = CH2Cl2 + HCl

CH2Cl + Cl2 = CHCl3 + HCl

CHCl3 + Cl2 = CCl4 + HCl

Free-radical substitution

Step 1:

The reaction is initiated by the homolytic fission of chlorine molecule in the presence of UV light, forming chlorine free radicals

Step 2:

The free radicals then attack the methane molecules, leading to a chain reaction

Step 3: Termination

The reaction ends with the free radicals joining up with each other

Alkenes

Alkenes are unsaturated hydrocarbons which contains at least one carbon-carbon double bond. They form the homologous series with the general formula of CnH2n.
The physical properties of alkenes are similar to that of alkanes

Reactions of alkenes

Electrophilic addition

Electrophilic addition is a reaction between an electrophile and nucleophile, adding to double or triple bonds. An electrophile is defined by a molecule with a tendency to react with other molecules containing a donatable pair of electrons .Alkenes are converted into alkanes in this reaction. The extra bonds from the carbon-carbon double bond will be filled by hydrogen.

Conditions:

with H₂: Nickel catalyst, 140 °C
with H₂O: concentrated phosphoric acid (H₃PO₄), 330 °C, 6 MPa (60atm)
with halogens: the halogen is in aqueous state, room temperature
with hydrogen halides (HX): the hydrogen halide is in conc. state, room temperature

Hydrogenation

Reagent: Hydrogen gas

Condition: Heat in the presence of nickel, Ni at (140-150)*C

Product: Alkanes

In hydrogenation of ethene, two hydrogen atoms are added across the C=C double bond and ethane is produced

In this case, although hydrogen molecule has no polarity, it is still able to behave as an electrophile. This is because as the hydrogen molecule approaches the double bond, a dipole is induced due to the repulsion between two double bond pair of electrons

Hydration

Reagent: steam, H2O

Condition: Heat at (300-330)*C and 60 atm in the presence of concentrated H3PO4 as catalyst

Product: Alcohols

In the hydration process of alkene, steam,H-OH is added across the double bond and ethanol is produced

Halogenation

Reagent: Halogen

Condition: Room temp in dark

Product: Halogenoalkanes

When ethene is bubbled into Br2 and CC4 at room temperature in dark, Br-Br is added across the C=C bond. The brown colour of bromine is decolourised at a fast rate.

Br2 in CCl4 is used as a test to differentiate alkanes and alkenes as they are both colourless. Alkanes will decolourise the brown colour of bromine whereas alkane will not

Hydrogen halide

Reagent: Hydrogen halide

Condition: Room temp

Product: Halogenoalkanes

When ethene reacts with hydrogen bromide, 1-bromoethane, a halogenoalkane is produced

Markovnikoff rule

The most general statement of Markonvikov's rule is that when an unsymmetrical reagent adds to an unsymmetrical alkene, the more positive part of the agent goes to the carbon that has more hydrogen atoms. Since ethene is a symmetrical alkene, Markovnikov's rule does not apply.

Oxidation by cold and dilute potassium manganate(VII)solution, KMnO4

Reagent: Cold and dilute potassium manganate(VII) solution

Condition: Room temp

Product: Alcohol(diols)

When ethene reacts with cold, dilute acidified KMnO4 solution, the purple colour is decolourised at a fast rate and ethane-1,2-diol is produced

This a redox reaction, he carbon species is oxidised while the oxidising agent is reduced
This is another test used to differentiate alkanes and alkenes. Only alkenes will decolourise the purple colour of KMnO4 whereas alkane will not

Oxidation by hot and concentrated potassium manganate(VII)solution, KMnO4

Reagent: hot and concentrated potassium manganate(VII) solution

Condition: Room temp

Product: Carbon dioxide/Carboxylic acid/ketones

When ethene reacts with hot and concentrated acidified KMnO4 solution, the C=C bond in alkene ruptures. The purple colour of KMnO4 is also decolourised. The product formed will differ according to the position of the C=C bond

If there are two alkyls groups at one end of the bond, that part of the molecule will give a ketone

If there are one alkyl group and one hydrogen at one end of the bond, that part of the molecule will give aldehyde and will later be further oxidised to carboxylic acid

The final products will be ketone and carboxylic acid

If there are two hydrogens at one end of the bond, that part of the molecule will give carbon dioxide and water

Addition polymerisation

Alkene can open up its C=C bond and join with each other in a long chain to form a polymer. A polymer is a long chain molecule made of repeating units called monomers

Temperature: 60*C

Pressure: a few atmospheres

Catalyst: ziegler-Natta catalysts or metal compounds

Polymer: poly(propene)

Monomer: propene

Use of hydrocarbons

Crude oil is the main source of hydrocarbons, it is found trapped in layers beneath the surface of the earth. Crude oil is a complex mixture of hydrocarbons - alkanes, cycloalkanes and aromatic compounds(attached benzene ring)
These substances can be separated by a technique called fractional distillation. Fractional distillation is the separation of compounds by their boiling points
Crude oil enters the bottom of the fractional column as liquid and vapour. The liquids are drawn off at the bottom while less volatile ones rise up the column. They condense at different levels as the temperature gradually falls and are collected as liquids.
The most volatile short chain hydrocarbons, which are methane and butane leave the top as gases

Catalytic cracking

Cracking breaks less useful hydrocarbons into smaller and more useful ones. Alkenes are normally produced as they are more industrially useful. This is because starting from alkenes, a wide range of organic compounds can be produced
The larger hydrocarbons are fed into a chamber which contains no oxygen, so combustion does not take place, The larger hydrocarbons ar heated at high temperatures about 500*C and passed over zeolite catalyst
Cracking of a hydrocarbon is not unique, different molecules of hydrocarbons can be broken down in different ways, giving rise to different products

Environmental consequences

Alkanes are often used as fuels. This because the combustion of alkanes is an exothermic process and produces a lot of heat energy. The main uses of alkanes as fuels are:

-Generate electricity

-Heat domestic houses and cook food

-Provide energy needed in industrial processes

-Provide power for ships, aeroplanes, trains,... etc

However, the combustion fo alkanes can produce some poisonous gases which can act as pollutants
One such example is carbon monoxide, CO which arises from the incomplete combustion of alkane. Carbon monoxide is a toxic gas that will bind readily to the haemoglobin molecules in the blood and inhibiting oxygen from being transported to actively respiring cells leading to death
The carbon dioxide, CO2 produced from complete combustion of alkane can also act as a greenhouse gas which increases the amount of these greenhouse gases enhancing global warming
Burning fuels in car engines will also oxidise the nitrogen gas in air to produce oxides of nitrogen. These oxides of nitrogen contributes to the formation of acid rain
One solution is installing catalytic converters, it removes carbon monoxide and the oxides of nitrogen.

Hydrocarbons

What is an alkane

A hydrocarbon is a compound containing carbon and hydrogen only. Examples of hydrocarbons are alkanes, alkenes and alkynes.

Alkanes

Alkanes are saturated hydrocarbons, the carbon atoms are joined to each other vua single covalent bond only. They form the homologous series with a formula of CnH2n+2

Physical properties

Reactivity of alkanes

Alkanes are saturated and generally unreactive because they are non-polar, hence they are unattractive towards nucleophiles and electrophiles

Alkanes will only react with non-polar reagents in the presence of heat or ultraviolet light

Combustion

Alkanes undergo complete combustion under excess oxygen gas to give carbon dioxide and water

Larger alkanes are more difficult to ignite. This is because alkanes only burn in the gaseous state and larger alkanes have stronger van der Waals forces that hold them together, Therefore more energy is required to vaporise it.

When the supply of oxygen is limited, alkanes undergo incomplete combustion, The possible products are carbon monoxide, carbon and water

Halogenation

Alkanes undergo halogenation via free-radical substitution to give a range of products. In free-radical substitution, hydrogen atoms in the molecules are gradually substituted by halogen atoms.

Reagent: Chlorine or bromine gas (Cl2 or Br2)

Condition: The presence of ultraviolet light or sunlight

Product: Halogenolalkanes

Free-radical substitution proceeds via a chain reaction. During a chain reaction, for every reactive species you start of with, a new one is generated at the end and this keeps the process going.

When methane reacts with chlorine in sunlight, the greenish-yellow color fades and steamy acidic fumes of hydrogen chloride can be observed

However, this chain reaction does not end there. More hydrogen atoms will be substituted as long as there are sufficient chlorine atoms.

Free-radical substitution

Step 1:

Alkenes

Alkenes are unsaturated hydrocarbons which contains at least one carbon-carbon double bond. They form the homologous series with the general formula of CnH2n.

The physical properties of alkenes are similar to that of alkanes

Reactions of alkenes

Electrophilic addition

Conditions:

with H2: Nickel catalyst, 140 °C

with H2O: concentrated phosphoric acid (H3PO4), 330 °C, 6 MPa (60atm)

with halogens: the halogen is in aqueous state, room temperature

with hydrogen halides (HX): the hydrogen halide is in conc. state, room temperature

Hydrogenation

Reagent: Hydrogen gas

Condition: Heat in the presence of nickel, Ni at (140-150)*C

Product: Alkanes

Hydration

Reagent: steam, H2O

Condition: Heat at (300-330)*C and 60 atm in the presence of concentrated H3PO4 as catalyst

Product: Alcohols

Halogenation

Reagent: Halogen

Condition: Room temp in dark

Product: Halogenoalkanes

Hydrogen halide

Reagent: Hydrogen halide

Condition: Room temp

Product: Halogenoalkanes

Markovnikoff rule

Oxidation by cold and dilute potassium manganate(VII)solution, KMnO4

Reagent: Cold and dilute potassium manganate(VII) solution

Condition: Room temp

Product: Alcohol(diols)

Oxidation by hot and concentrated potassium manganate(VII)solution, KMnO4

Reagent: hot and concentrated potassium manganate(VII) solution

Condition: Room temp

Product: Carbon dioxide/Carboxylic acid/ketones

Addition polymerisation

Temperature: 60*C

Pressure: a few atmospheres

Catalyst: ziegler-Natta catalysts or metal compounds

Use of hydrocarbons

Catalytic cracking

Environmental consequences

Videos

Fractional distillation of crude oil

Free radical substitution

Hydrocarbon discussion

Reactions of alkene

Mechanism of electrophilic addition

Oxidation of alkene

Free radical substitution vs electrophilic addition

with H₂: Nickel catalyst, 140 °C

with H₂O: concentrated phosphoric acid (H₃PO₄), 330 °C, 6 MPa (60atm)