Hydrocarbon

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Hydrocarbons are organic compounds made up of two elements (carbon and hydrogen) only, hence their source name. The main source of hydrocarbons is crude oil. There are many hydrocarbons. They can be classified into two main classes: aliphatic and the aromatic hydrocarbons.

Aliphatic hydrocarbons are composed of catenated carbon chain. In acyclic compounds, the carbon chain are straight or branched. Important families that belong to this group are the alkanes, alkenes, and alkynes (homologous series) with members (homologue) examples like {methane ethane}, {ethene and propene} and {ethyne and propyne} respectively. These members are d first two members of each of the families mention above. In cyclic aliphatic compounds, the end carbon atoms of an acyclic carbon chain join together to form a ring. Example of such compounds are {cyclo propane, cyclo butane}, {cyclo propene, cyclo butene}, and {cyclo propyne and cyclo butyne}, respectively.

Aromatic hydrocarbons are all cyclic compounds. The basic cyclic structure is the benzene ring. The are compounds that obey the system of delocalized electron. The have alternating double bonds and are stable. Due to their stability, they will not undergo addition reaction. Michael Faraday was the first scientist to discover benzene, while August Kekulé devised the structure. Benzene as form the major building block of all aromatic compounds. Example of such aromatic hydrocarbons include benzene, cyclobutadiene, cyclooctatetraene etc.

Sources of hydrocarbons

The natural sources of hydrocarbons include coal, petroleum, and natural gas. These are often known as fossil fuels because they are the remains of animals and plants which died millions of years ago; their remains have become deposited and transformed into sediment as a result of the great heat and pressure in the earth's crust. They are used as fuels, burnt to release heat and other forms of energy. Coal is a solid fuel, petroleum is a dark and viscous liquid fuel (otherwise called crude oil), and natural gas is a gaseous fuel.

Alkanes, alkenes, and alkynes

Alkanes

These are aliphatic hydrocarbons which form the homologous series of the saturated hydrocarbons. They have the general molecular formula of CnH2n+2. Each of the alkane member has its carbon in SP3 hybridized state in which all carbon atoms are tetrahedrally bonded with a bond angle ranging from 109.28° and 25% S-orbital character and 75% P-orbital character. They are made up of a single (sigma) bond. Examples are methane, ethane, propane, etc. They mainly undergo combustion under much oxygen, substitution or replacement reactions (dehydrohalogenation), and elimination (dehydrogenation) reactions.

Laboratory preparation

Members may be prepared in the laboratory by the heating anhydrous sodium ethanoate with an alkali, e.g. soda lime (preparation of methane).[1][2]

Alkenes

Alkenes are aliphatic homologous series of hydrocarbon which form unsaturated hydrocarbon. They have the general molecular formula CnH2n, where n are positive integers. These compounds are unsaturated because they contain carbon-carbon double bonds where the two carbons are bonded together by one pi bond and one sigma bond. Each of the member of alkene has and SP2 hybridized orbital with a trigonal shape with a bond angle of 120°, given a total of 33.333% S-orbital character and 66.66% P-orbital character respectively. Alkenes undergo reactions such as Addition( hydrogenation, halogenation, hydrohalogenation) reactions, oxidation reaction,polymerization condensation reaction and elimination (conversion of alkene to alkyne) reactions respectively.

Members include ethene 2 -carbons, butene 4 -carbons, pentene 5- carbons, etc.

Laboratory preparation

Members may be prepared by dehydrating alcohols. E.g., dehydrating ethanol with concentrated sulphuric acid gives ethene. Other members can be prepared in similar way but with corresponding higher molecular mass alcohol.[1][2]

Alkynes

Alkynes are the homologous series of unsaturated hydrocarbons with general molecular formula CnH2n-2. They are unsaturated hydrocarbon, each alkyne molecule is composed of carbon-carbon triple bond in which the carbon-carbon bonds is made up of two pi bonds and one sigma bond. They exhibit the SP hybridized state with 50% S-orbital and 50% P-orbital character, the shape is linear and the bond angle is 180°. They are very reactive due to the presence of pi bonds, which are exposed during hybridization. Alkynes have higher degree of unsaturation than alkenes, hence they are chemically more reactive than the corresponding alkanes and alkenes. Alkynes are prone to addition reactions due to their electron rich triple bond. Members of this series include ethyne, propyne, butyne, etc.

Laboratory preparation

Ethyne can be prepared in the laboratory by the action of cold water on calcium carbide. This process is accompanied by the release of large amount of heat.[1][2]

Aromatic Compounds

The most common aromatic compound (benzene) was first discorvered by Michael Faraday who extracted the compound from a liquid residue obtained after heating whale oil to produce gas used to illuminate buildings. Chemists called the name pheno from the greek name phainein which means "to shine". The early scientists called it aromatic because of the pleasant smell, in this way we distinguish aromatic compoinds from aliphatuc, which has higher number of hydrogen to carbon ratio.

Benzene is a cyclic compound with a cyclic cloud of delocalized electrons above and below the plane of the rings, due to this, all the c-c bond have the same length. This compound is also stable as a result of its resonance energy (36kcal/mol or 151kJ/mol)

A compound is said to be aromatic if the following conditions are satisfied

  1. The cloud must contain an odd number of pairs of pi electron
  2. It must have an interrupted cyclic cloud of pi electron (pi-cloud) above and below the plane of the molecule.
  3. It must satisfy Hückel's rule of (4n + 2)pi electrons.[3]

In conclusion, aromaticity is characterized by stability while anti-aromaticity is characterized by instability. Aromatic compounds are characterized by substitution reaction such as nitration, alkylation etc. They possess double bond but will never undergo addition reaction due to their stability. The scope of aromaticity is beyond this topic given, for clear understanding, one must consult higher materials.

Definition of terms

Homologous series
Families of organic compounds with some common properties: general method of preparation, similar chemical properties, general molecular formula, their physical properties changes as the relative molecular mass increases, each member differ from one another by a CH2 group and each member is named homologue.
Homologue
Member of an homologous series e.g methane of alkane series, ethene of alkene series and propyne of alkyne series.
Sigma bond
These are covalent bonds formed as a result of linear overlap or head to head overlap of atomic orbitals. Compounds with sigma bonds are less reactive. E.g., S+S, Px + S, Px + Px, etc.
Pi-bond
These are covalent bonds formed as a result of parallel or side by side overlap of atomic orbitals. Compounds with pi-bonds are reactive. E.g., Pz + Pz, Py + Py, etc.

References

  1. 1.0 1.1 1.2 Chemguide
  2. 2.0 2.1 2.2 Ababio, O. Y., New School Chemistry
  3. Bruice, P. Y., Organic Chemistry, p. 595

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