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Composition of Hydrocarbons

Paper Type: Free Essay Subject: Chemistry
Wordcount: 4109 words Published: 18th Jan 2018

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  • Samerah Mansha

Chemistry – Hydrocarbons

  1. – Organic compounds are molecules comprising of both carbon and hydrogen atoms. These compounds can be found as gaseous, liquid, or solid.

Organic compounds are vital in life processes as they are found in the products we use daily such as soap, cosmetics, perfumes, plastics, rubber, paper, insecticides. The source of energy we use everyday such as petroleum and coal contain organic compounds. Foods such as margarines and flavourings that we taste are the organic compounds interrelating with our taste buds.Scents in perfumes and food that we detect with our nose are organic compounds interrelating with our receptors.

  1. – The main type of organic compounds are hydrocarbons, which are chemical compounds compromising of carbon and hydrogen atoms. Alkanes and alkenes are two types of hydrocarbons.

Alkanes are saturated hydrocarbons due to containing the full amount of hydrogen atoms possible. Alkanes also contain single bonds. The overall formula of an alkane isCnH2n+2, where n shows the quantity of carbon atoms present.

The chemical and structural formula of alkanes (see figure 1).

Isomers are molecules with an identical chemical formula, but differ in terms of having different structural formula due to varied arrangement of atoms, isomers also have different properties.

The isomer of butane C4H10, is 2methyl-propane. (See figure 2)

These two compounds are isomers of each other due to having the same chemical formula of C4H10, but vary in terms of having different structural formula. The above propane has an added methyl group, thus called methyl propane.

The isomers of hexane C6H14, (see figure 3)

The isomers of pentane C5H12, (see figure 4)

  1. – A homologous series is simply a group of organic chemical compounds which is ordered in increasing size, such as having similar structures but only differ slightly by a CH2 group within their chain. A series of compounds which are related like alkanes are known as homologous series. This is because alkanes have the same general formulas of CnH2n+2, but differ from the next CH2 unit. All the alkanes in the homologous series have similar chemical properties, but have different physical properties such as boiling point and density increase as the number of carbons atoms increase.

Name of Alkane

Number Carbon atoms

Chemical Formula

Simple Structure (Molecular Diagram)




C H4

Methane (Alkane)




Ethane (Alkane)




Propane (Alkane)




n-Butane (Alkane)




n-Pentane (Alkane)




n-Hexane (Alkane)




n-Heptane (Alkane)




n-Octane (Alkane)




n-Nonane (Alkane)




n-Decane (Alkane)


Figure: 5 Example of a homologous series of alkanes with structure of CnH2n+2, Ivy Rose, (n.d)

2.4, 2.5 – As alkanes are saturate hydrocarbons, they are deemed unreactive due to containing single bonds such as C-H and C-C bonds which are quiet strong and difficult to break due to the strong intermolecular force. However the only ways alkanes are able to react is via combustion, chlorination and cracking.

Combustion is exothermic process and is known burning of carbon compounds, in particular hydrocarbons is a vital source of heat energy. For example by combusting alkanes with oxygen it releases energy which can be instantaneously be used as fuels. In combustion of alkanes such as propane, every single covalent bond within the reactants is broken down and forms a new set of covalent bonds within the products. The balanced formula is:

CH3-CH2-CH3 + 5 O2——>3 CO2 + 4 H2O + heat

Combustion also producescarbon dioxideand watervapour which is shown above. Another example of combustion of methane:

CH4(g) + 2O2(g)CO2(g) + 2H2O(g)

(Methane + Oxygen→Carbon dioxide + Water)

If there is not enough oxygen supply within the air, then this would result then would form carbon monoxide gas which is poisons. For example, the below equation shows the partial combustion of methane due to lack of oxygen present:

2CH4(g) + 3O2(g) → 2CO(g)+ 4H2O(g)

(Methane + Oxygen → Carbon monoxide + Water)

However the burning of alkanes has the potential to cause many enviromental problems, such as the toxic waste gas carbon monoxide and unburnt hydrocarbons which are present in cat emissions. Additional excessive combustion of hydrocarbons increases carbon dioxide emissions which contribute to the greenhouse effect.

http://www.chemguide.co.uk/organicprops/alkanes/ch4andcl2.gifAnother reaction of alkanes is called chlorination which is the when alkanes react with chlorine. For example, when the alkane methane and chlorine react together, the hydrogen atoms of the methane are simply replaced one by one by chlorine atoms. Thus forming a combination of compounds of chloromethane, dichloromethane, trichloromethane and tetrachloromethane.

Figure:6 Reaction and products that occur between methane and chlorine, Chem Guide, (2000)

Some fuels which are made from oil mixtures contain large hydrocarbon molecules which are not useful, as they do not flow easily and are quiet difficult to ignite. This is when the process of the cracking, a thermal decompositionreaction comes in.

The process of cracking allows big hydrocarbon molecules to be broken down into much smaller and more useful hydrocarbon molecules. This is achieved by the large hydrocarbon molecules being vaporised and put in a hot catalyst which breakdowns the chemical bonds within molecules thus forming smaller hydrocarbon molecules. For example in the cracking process of the alkane hexane, a smaller alkane is formed plus an alkene. The alkene is formed because the alkane which is this case is the hexane does not contain enough hydrogen atoms in order to produce another alkane.


C6H14 C4H10 + C2H4

Figure: 7 The cracking reaction of the alkane hexane, BBC Bitesize, (n.d)

3.1 – Alkenes are also hydrocarbons compromising of carbon and hydrogen atoms, but also have one or more double bonds present within the carbon chain. Alkenes are also a series of compounds within a homologous series but have a different general formula of CnH2n.

Examples of alkenes and their chemical and structural formula (See figure 8)

Geometrical isomerisms are formed when atoms or groups of molecules are arranged in a different way due to limited amount of rotation of the bond or bonds within a molecule.

For example, there are two geometrical isomers for the alkene 2-butene, as there are two different spatial arrangements of methyl groups and double bond, thus forming geometrical isomers called cis-2-butene and trans-2-butene, these are formed because the CH3 groups are found on opposite of the double bond. (See figure 9)

Another example is the 1,2-dichloroethene, forms the trans-1,2 dichoethne and Cis-1,2 dichroloethee gemotical isomers, (See figure 10)

One of the isomer, the two chlorine atoms are locked upon opposite sides of double bond, thus called theTransisomer whereas in the other isomer the two chlorine atoms are locked within the same side of the double bond thus known as cisisomer. (See figure 10).

3.2, 3.3 – A single covalent bond is simply when one pair of electrons (2 electrons) are shared amongst two atoms. It typically consists of one sigma bond. On the other hand, bonds with more than one mutual pair of electrons are called multiple bonds, in particular sharing two pairs of electrons is called a double bond where 4 electrons are covalently bonded together, the double bonds typically comprises of one sigma and one pi bond.

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Alkanes are saturated compounds with single covalent sigma bonds, thus are more stable than alkenes as high amount of energy would be needed to break these. Alkenes are unsaturated with the presence of carbon-carbon pi-bonds and sigma bonds which connects to carbon atoms which makes them highly more reactive than alkanes due to the existence of double bonds and because they are unstable as they want to achieve full saturation. Another reason alkenes are ore reactive is because in double bond there is a lone pair of free electrons which is easily lost thus highly reactive as its easily lost in order to be covalently single bonded Thus when alkenes react in such reactions such as combustion or additions, it mainly involves the rupture of the pi bond, thus forming newsingle bonds. Within the alkenes, the double bond allows more electrons to be put into the joining within the two carbon atoms, thus held less strongly making them more likely to react with less input of energy needed to overcome the activation energy of reaction

3.4, 3.5 – The main reaction of alkenes is the addition reaction which occurs due to the extra electron found within the C=C double bond which causes alkenes to be attacked by species called electrophiles in which “like” positve charged electrons. Due to the electrophile having an positive charge it is attracted towards the alkene which causes the electrons within the pi bond to break, then electrons from it are used in order to form a bond to the positive end joining the two carbon atoms to other things.


Figure: 11 Example of an addition reaction using X-Y, Chem guide, (2003)

Another example of addition reactions is hydrogen and hydrogen chloride.

ethene +hydrogen→ethane = C2H4+H2→C2H6

Due to alkenes containing pi-bonds (double or triple) which can be broken easily due to their overlay, it makes them the prime target for addition reaction, which stabilises the molecular orbitals by forming sigma-bonds, which are stronger.

Another example ofan addition reactionis the bromine test which is simply used in order to tell the difference between an alkane and an alkene.

Alkenes undergo an addition reaction when combined with bromine water which is usually orangey-brown in colour, but when it is shaken with an alkene, the solution becomes colourless as alkene decolourise bromine water as it reacts with the double bond, which indicates the presence of a pi bond. However if bromine water was shaken with an alkane, the bromine water remains brown as there is no double bond present

For example, the bromine is decolourised because a colourless dibromethane compound forms. For example:

ethene + bromine → dibromoethane

C2H4+ Br2→ C2H4Br2 (See figure 12)


An ethene monomer has four hydrogen atoms and two carbon atoms that are joined together with a double bond. After polymerisation, the monomer forms a repeating unit of polyethene which has single bonds between the carbon atoms. A chloroethene monomer has three hydrogen atoms, one chlorine atom and two carbon atoms. The carbon atoms are joined together with a double bond. After polymerisation, the monomer forms a repeating unit of polychloroethene that has single bonds between the carbon atoms3.6 – Polymerisation is an important reaction of alkenes. During polymerisation all the atoms within the alkenes are used to form a polymer, a polymer is a large long-chain of molecule of repeated units which formed when smaller molecule called monomers join together. For example, several chloroethene monomers join end-to-end to make poly(chloroethene), which is also known as PVC via the polymerisation process. (See figure 13)

Figure: 14 – Addition polymerisation reaction, BBC Bitesize, (n.d)

Alkenes act as monomer due to the presence of the double bond. Different polymers have different properties, so they have different uses which make them important in everyday life. For example, the polymer polyethene is commonly used to make plastic bags and bottles, whereas polyproper is used to make crated and ropes and polycholorethene is uses in water pipes and commonly in insulation of electricity cables.

However regardless of the uses of polymers being useful their disposal creates various problems, due to being non-biodegradable, because being unreactive to majority of chemicals and bacteria. However can only be recycled, but this is a costly process. They can also be burnt which does produce energy but again produces toxic fumes.



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