14.1 Organic Compounds

What is organic chemistry?

1) Organic chemistry is the study of carbon compounds (excluding simple compounds like CO, CO2, CO3²⁻, HCO3⁻).

Formulae of organic compounds

• There are five ways in which organic compounds/molecules can be presented:
  1. Empirical formula
  2. Molecular formula
  3. Structural formula
  4. Displayed formula
  5. Skeletal formula
  6. Empirical formula gives the simplest ratio of the number of atoms of each element present in the compound.
  7. Molecular formula gives the actual number of atoms of  each element present in the compound.
  8. These two formulae are least useful and rarely used in organic chemistry because they do not reveal much information about the compounds

• Structural formula shows the order of the atoms joined together in an organic compound. These formulae are usually written in one line. Examples:
  Ethane                   – CH3CH3
• Ethene           – CH2=CH2 (only double and triple bonds are shown) Ethanoic acid – CH3COOH2-methylpentane  – CH3CH(CH3)CH2CH3

   

• Displayed formula shows the order of the atoms joined together, as well as the orientation of the atoms and bond angles

• Examples:

• Skeletal formula shows only the functional groups. The carbon and hydrogen atoms are simplified and hidden. In a skeletal formula:
  1. there is a carbon atom at each junction and at the end of the bond, there is no carbon atom at a place occupied by another functional group.
  2. there is enough hydrogen atoms bonded to each carbon so that each carbon has four bonds only
• It is essential to learn to convert between different formulae of the same compound.

Names of organic compounds

• Organic compounds are named systematically according to the IUPAC (International Union of Pure and Applied Chemistry) nomenclature
• For details, refer to the attached document at the end of the notes
• Some examples to fill the page:

Functional groups

• A functional group is a group of atoms in an organic compound that determines the chemistry of that particular compound

• A homologous series is a series of compounds with the same functional group, and the adjacent members differ in the number of carbon atoms

[ Note: You do not need to memorise the table above at this moment. ]

Bonding in organic molecules

• Carbon atoms which are sp³ hybridised(typically forming 4 bonds) have the atoms bonded to it arranged in tetrahedral shape with a bond angle of 109.5°.
• One such example is ethane. In an ethane molecule, the two carbon atoms undergo sp³ All other atoms are bonded to the carbon atoms via σ bonds. All bond angles are 109.5°.
• If the carbon chain is longer, all the bond angles are still 109.5°, the molecule has a zig-zag shape

• Carbon atoms which are sp² hybridised(typically forming 3 bonds) have the atoms bonded to it arranged in a trigonal planar shape with bond angle 120°.
• One such example is ethene. In an ethene molecule, the two carbon atoms undergo sp² The two carbon atoms are connected via σ and π bonds, a double bond is formed between the two carbon atoms. The bond angle is 120°.
• If the carbon chain is longer, depending on the number of double bonds, a mixture of trigonal planar and tetrahedral shape is possible. However, each carbon atoms can only have either one

• Observe the but-1,2-ene molecule beside. Carbon 1 and 2 are sp² hybridised, so the atoms around it have a trigonal planar arrangement. Carbon 3 and 4 are sp³ hybridised, so the atoms around it have a tetrahedral  arrangement.[ Note : For greater detail on hybridisation and bonding, refer Chapter 4 ]