WBCS Preliminary (Chemistry): Organic Chemistry

Organic chemistry is that branch of chemistry which deals with the study of compounds of carbon with hydrogen (hydrocarbons), and their derivatives. Presently about five million organic compounds are known. Organic compounds were found to contain mainly hydrogen and carbon. Therefore, organic chemistry is defined as the study of hydrocarbons and their derivatives. Most atoms are only capable of forming small molecules. However one or two can form larger molecules. By far and away the best atom for making large molecules with is Carbon. Carbon can make molecules that have tens, hundreds, thousands even millions of atoms! The huge number of possible combinations means that there are more Carbon compounds that those of all the other elements put together! A single Carbon atom is capable of combining with up to four other atoms. We say it has a valency of 4. Sometimes a Carbon atom will combine with fewer atoms. The Carbon atom is one of the few that will combine with itself. In other words Carbon combines with other Carbon atoms. This means that Carbon atoms can form chains and rings onto which other atoms can be attached. This leads to a huge number of different compounds. Organic Chemistry is essentially the chemistry of Carbon. Carbon compounds are classified according to how the Carbon atoms are arranged and what other groups of atoms are attached.

• Hydrocarbons: The simplest Organic compounds are made up of only Carbon and Hydrogen atoms only. Even these run into thousands! Compounds of Carbon and Hydrogen only are called Hydrocarbons.
• 1. Alkanes: In the alkanes, all four of the Carbon valency bonds are taken up with links to different atoms. These types of bonds are called single bonds and are generally stable and resistant to attack by other chemicals. Alkanes contain the maximum number of Hydrogen atoms possible. They are said to be saturated. The simplest Hydrocarbon is:
     • Methane: CH₄ This is the simplest member of a series of hydrocarbons. Each successive member of the series has one more Carbon atom than the preceeding member.
     • Ethane: C₂H₆.
     • Propane –( heating fuel): C₃H₈.
     • Butane – (lighter / camping fuel): C₄H₁₀.
     • Pentane: C₅H₁₂.
     • Hexane: C₆H₁₄.
     Polythene is a very large alkane with millions of atoms in a single molecule. Apart from being flammable, alkanes are stable compounds found underground.
  2. Alkenes: Another series of compounds is called the alkenes. These have a general formula: C_nH_2n. These compounds are named in a similar manner to the alkanes except that the suffix is -ene. Alkenes have fewer hydrogen atoms than the alkanes. The extra valencies left over occur as double bonds between a pair of Carbon atoms. The double bonds are more reactive than single bonds making the alkenes chemically more reactive. The simplest alkenes are listed in the table below:
     • Ethene ( used as an industrial starter chemical): C₂H₄.
     • Propene: C₃H₆.
     • Butene: C₄H₈.
     • Pentene: C₅H₁₀.
     • Hexene: C₆H₁₂.
  3. Alkynes: A third series are the alkynes. These have the following formula: C_nH_2n-2. These highly reactive substances have many industrial uses. Again the naming of these compounds is similar to the alkanes except that the suffix is -yne. Alkynes have two carbon atoms joined by a tripple bond. This is highly reactive making these compounds unstable. Examples of alkynes are:
     • Ethyne - better known as acetylene which is used for welding underwater: C₂H₂
     • Propyne: C₃H₄
     • Butyne: C₄H₆
     • Pentyne: C₅H₈
     • Hexyne: C₆H₁₀
  4. Carbon Rings: Alkanes, alkenes and alkynes all contain Carbon atoms in linear chains. When rings are combined with chains, the number of hydrocarbons is virtually infinite. There are also hydrocarbons arranged in rings. Some examples follow:
     • Cyclohexane - a saturated hydrocarbon with the atoms arranged in a hexagonal ring: C₆H₁₂
     • Benzene - an industrial solvent. The Benzene Ring is one of the most important structures in organic chemistry. In reality, its alternate double and single bonds are "spread around" the ring so that the molecule is symmetrical: C₆H₆
     • Toluene - an important solvent and starter chemical: C₇H₈
     • Naphthalene - used in moth balls. This can be depicted as two fused Benzene Rings: C₁₀H₈
• Carbon, Hydrogen and Oxygen: When Oxygen atoms are added, the variety of compounds grows enormously. Here are some examples where each molecule has a single functional group.
  1. Alcohols: Alcohols have the OH (hydroxyl) group in the molecule. A group of atoms that gives an organic series its distinctive character is called a functional group. These have a general formula: C_nH_2n+1OH. Examples: Methanol (wood alcohol) CH₃OH, Ethanol(drinking alcohol) C₂H₅OH, Phenol(carbolic acid - used as disinfectant) C₆H₅OH.
  2. Ethers(Ethers have an O atom attached to two hydrocarbon chains) (C_nH_2n+1)₂O. Examples: Dimethyl Ether(a gas) (CH₃)₂O, Diethyl Ether (a liquid used as an anaesthetic) (C₂H₅)₂O
  3. Ketones(Ketones have a CO group attached to two hydrocarbon chains) . These have a general formula: (C_nH_2n+1)₂CO.Example: Dimethyl Ketone (Also known as acetone: nail-varnish remover), CH₃COC H₃
  4. Aldehydes(Aldehydes have a CHO group attached to a hydrocarbon chain). These have a general formula: C_nH_2n+1CHO. Example: Formaldehyde (preservative in labs) HCHO, Acetaldehyde- CH₃CHO.
  5. Fatty Acids(Fatty Acids contain the CO₂H (or COOH) group attached to a hydrocarbon chain or ring). These have a general formula: C_nH_2n+1CO₂H. Example: Formic Acid(in ant bites and stinging nettles)- HCO₂H, Acetic Acid( vinegar)- CH₃CO₂H, Butyric Acid( the rancid butter smell)- C₂H₅CO₂H.
  6. Esters (Esters are similar to Fatty Acids except that the H in the COOH group is another hydrocarbon chain. They are usually very sweet smelling liquids used in perfumes). These have a general formula: RCO₂R’( R and R’ are Hydrocarbon chain or rings). Examples: Methyl Methoate (essence of pear drops) - CH₃CO₂CH₃.
• It is possible to have two or more functional groups on a molecule. These can be the same group (as in Oxalic Acid - a poison found in rhubarb leaves - which has two fatty acid groups) or different (as in Hydroxymethanoic Acid - which has a hydroxyl group and a fatty acid group): Oxalic Acid- (COOH)², Hydroxymethanoic Acid- CH₂OHCOOH.
• The most famous compounds containing Carbon, Hydrogen and Oxygen are the Carbohydrates. An example is the common sugar, Sucrose (C₁₂H₂₂O₁₁).
• Isomerism: An interesting phenomenon with organic molecules is called isomerism. Let us look at two compounds introduced earlier. Dimethyl Ether: (CH₃)₂O and Ethanol: C₂H₅OH. The first is a gas which will knock you out if inhaled. The second is common alcohol drunk in spirits. Both compounds contain 2 Carbon atoms, 6 Hydrogen atoms and 1 Oxygen atom. Even though the atoms are the same, they are arranged differently. This yields two different compounds with the same number of atoms. These compounds are isomers and the phenomenon is called Isomerism. Isomerism increases the number of Organic compounds. The more Carbon atoms in a compound, the more ways of arranging the atoms and the larger number of isomers.
• Adding Nitrogen: Many very important organic compounds contain Nitrogen. This produces more series of compounds.
  1. Amines (Amines have one or more of the Hydrogen atoms in Ammonia (NH3) replaced by a Hydrocarbon chain or ring). These have a general formula: C_nH_2n+1NH₂. Examples: Methylamine (a pungent, water soluble gas)- CH₃NH₂.
  2. Cyanides (Cyanides have the CN group). These have a general formula: C_nH_2n+1CN. Examples: Methyl Cyanide- CH₃CN.
  3. Amino Acids (Amino Acids have two functional groups: the amine (HN₂) group and the fatty acid (COOH) group. These have a general formula: C_nH_2nNH₂COOH. Examples: Glycine (the simplest amino acid)- CH₂NH₂COOH.
  4. A famous compound containing Nitrogen is Trinitro Toluene (C ₆H₂CH₃ (NO)₃) - usually abbreviated to TNT). This is an artificially made explosive.
• The vast majority of organic compounds contain Carbon, Hydrogen, Oxygen and Nitrogen. Other types of atoms can be included to form even more compounds. These can contain atoms like Phosphorus, Sulphur (e.g. Thiamine,), Chlorine (e.g. Chlorophyll-CHCl₃, Dichloro Diphenyl Trichloro Methane – DDT-C ₁₄H₉Cl₁₅) and Iron (e.g. Haemoglobin).