Organic Chemistry I – Introduction and Alkanes

Summary: Organic chemistry is a subdiscipline of chemistry that studies the structure, properties and reactions of organic compounds, which contain carbon in covalent bonding.

Products from Oil

Coal, Oil and Natural Gas Formation – Fossil Fuels
Just as coal has formed by the action of heat and pressure on the remains of trees and plants on land over millions of years, so oil and natural gas have formed by the action of heat and pressure on the remains of sea plants and animals over millions of years.
They are called fossil fuels because they are buried underground (from Latin fossils – dug up)

The oil is called crude oil (or petroleum, from Latin – rock oil), and has natural gas in it or in a pocket above it trapped by non-porous rock.
Natural gas is mostly methane (CH4)
Crude oil is a mixture of substances (mostly hydrocarbons).

Hydrocarbons

Crude oil is a mixture of substances which are mostly hydrocarbons.
A hydrocarbon is a compound containing hydrogen and carbon only.
Since crude oil is a mixture of different hydrocarbon compounds, the different hydrocarbons will have different boiling points.

Fractional Distillation of Crude Oil

Crude oil is heated until it boils and then the hydrocarbon gases are entered into the bottom of the fractionating column.
As the gases go up the column the temperature decreases.
The hydrocarbon gases condense back into liquids and the fractions are removed from the sides of the column.
The smaller the hydrocarbon molecule, the further it rises up the column before condensing.

The fractionating column operates continuously.
The temperatures shown are approximate.
A sample of crude oil may be separated in the laboratory by fractional distillation.
The collection vessel is changed as the temperature rises to collect the different fractions.

Naming Hydrocarbons

Hydrocarbons are named according to the number of carbon atoms in the molecule.
Depending on the type of bond that exist between the individual carbon atoms, hydrocarbon are classified as:

• Alkanes
• Alkenes
• Alkynes

Alkanes

They are saturated hydrocarbons and they form an homologous series called alkanes with a general formula C_nH_2n+2 Saturated hydrocarbons have no C=C double bonds, only carbon-carbon single bonds, and so has combined with the maximum number of hydrogen atoms.

Naming Alkanes

These are hydrocarbons with a general formula C_nH_2n+2 where n is the number of Carbon atoms in a molecule. The carbon atoms are linked by single bond to each other and to hydrogen atoms.

NOTES ON ALKANES

• The general formula/molecular formula of a compound is the number of each atoms of elements making the compound e.g. Decane has a general/molecular formula C₁₀H₂₂ ;this means there are 10 carbon atoms and 22 hydrogen atoms in a molecule of decane.
• The structural formula is the arrangement / bonding of atoms of each element making the compound.
• Since carbon is tetravalent, each atom of carbon in the alkane MUST always be bonded using four covalent bond /four shared pairs of electrons.
• Since Hydrogen is monovalent ,each atom of hydrogen in the alkane MUST always be bonded using one covalent bond/one shared pair of electrons.
• One member of the alkane differ from the next/previous by a -CH₂– group.
• A group of compounds that differ by a -CH₂– group from the next /previous consecutively is called a homologous series
  NB: A homologous series; differ by a CH2 group from the next /previous consecutively, have similar chemical properties, have similar chemical formula that can be represented by a general formula, and the physical properties (e.g.melting/boiling points) show steady gradual change

Isomers of Alkanes

Isomers are compounds with the same molecular general formula but different molecular structural formula.
Isomerism is the existence of a compounds having the same general/molecular formula but different structural formula.
The first three alkanes do not form isomers.

Isomers are named by using the IUPAC(International Union of Pure and Applied Chemistry) system of nomenclature /naming.
The IUPAC system of nomenclature uses the following basic rules/guidelines;

• Identify the longest continuous carbon chain to get/determine the parent alkane.
• Number the longest chain form the end of the chain that is near the branches so as the branch get the lowest number possible.
• Determine the position, number and type of branches.
• Name them as methyl, ethyl, propyl e.tc. according to the number of carbon chains attached to the parent alkane.
• Name them fluoro-,chloro-,bromo-,iodo- if they are halogens.
• Use prefix di-,tri-,tetra-,penta-,hexa- to show the number of branches attached to the parent alkane.

Naming Procedure for Isomers of Alkanes

• Identify the longest continuous carbon chain to get/determine the parent alkane.
• Number the longest chain form the end of the chain that is near the branches so as the branch get the lowest number possible.
• Determine the position, number and type of branches. Name them as methyl, ethyl, propyl e.tc. according to the number of carbon chains attached to the parent alkane.

Example

Occurrence and Extraction of Alkanes

– Crude oil ,natural gas and biogas are the main sources of alkanes.
– Natural gas is found on top of crude oil deposits and consists mainly of methane.
– Biogas is formed from the decay of waste organic products like animal dung and cellulose.
– When the decay takes place in absence of oxygen , 60-75% by volume of the gaseous mixture of methane gas is produced.
– Crude oil is a mixture of many flammable hydrocarbons/substances.

Physical Properties of Alkanes

• Physical state Lower molecular weight alkanes are gases.
• Methane, ethane, propane and butane are gases at ordinary room temperature.
• Higher alkanes up to those having 17 carbon atoms are liquids; higher alkanes are solids at room temperature.
• Melting and boiling points Homologous alkanes show increase in melting and boiling points.
• Similar to the behavior of elements in the same group in a periodic table.
• Like all other organic chemicals are insoluble in water. They are however soluble in organic liquids.
• Alkanes are non-polar and are hence soluble in other non-polar liquids and not in water, as water is a polar molecule.

Chemical Properties of Alkanes

1. Substitutional reactions of alkanes
   Alkanes are most inert of all homologous series. They are not very reactive unless burned.
   They will react with strong oxidising chemicals like chlorine when heated or subjected to u.v light.
   A substitution reaction occurs and a chloro-alkane is formed e.g a hydrogen atom is swapped for a chlorine atom and the hydrogen combines with a chlorine atom forming hydrogen chloride.
   This process is called halogenation.
   A halogen atom attacks the alkane, substituting itself for a hydrogen atom.
   This substitution may occur many times in an alkane before the reaction is finished.
2. Combustion
   Alkanes, along with all other types of hydrocarbon, will burn in an excess of oxygen to give carbon dioxide and water only as the productsCH₄ (g) + 2O₂(g) → CO₂(g) + 2H₂O(g)The general reaction is as followsC_nH_2n+n (g) + (1.5n+0.5)O₂(g) → nCO₂(g) + (n+1)H₂O(g)If there is not enough oxygen present then instead of carbon dioxide, carbon monoxide, CO, is produced.
   Carbon monoxide is particularly toxic and absorbed into blood, through respiration, very easily.
3. Reactivity
   Alkanes are saturated hydrocarbons
   Molecules of saturated hydrocarbons contain only single bonds between all carbon atoms in the series
   Hence their reactivity with other chemicals is relatively low