Organic Chemistry I: Hydrocarbons - SS2 Chemistry Past Questions and Answers - page 4

Nomenclature is a system of naming organic compounds that allows for clear and standardised communication in the field of organic chemistry. The International Union of Pure and Applied Chemistry (IUPAC) provides guidelines and rules for naming organic compounds. The key nomenclature rules include:

1.    Identify the parent chain: The parent chain is the longest continuous carbon chain in the molecule. It serves as the main framework for naming the compound.

2.    Number the carbon atoms: Assign numbers to the carbon atoms in the parent chain to locate the position of substituents.

3.    Name substituents: Substituents are atoms or groups of atoms attached to the parent chain. They are named as prefixes, indicating the type and number of atoms present.

4.    Indicate multiple bonds: Double bonds are indicated with the suffix "-ene," while triple bonds are indicated with the suffix "-yne."

5.    Consider functional groups: Functional groups are specific arrangements of atoms in a molecule that determine its chemical properties. They are indicated as suffixes or prefixes in the compound's name.

6.    Use proper punctuation and word order: Commas, hyphens, and proper word order are used to ensure clarity in the compound's name.

Systematic naming is important as it allows chemists to convey the structure and composition of a compound accurately. It provides a common language that facilitates communication and ensures clarity in chemical discussions and literature. Examples of nomenclature application include:

●     Ethanol (C2H5OH): The parent chain is a two-carbon chain (eth-), and the -OH functional group is indicated by the suffix "-ol." Therefore, the compound is named ethanol.

●     2,4,6-trichlorobenzene (C6H3Cl3): The parent chain is a benzene ring, and the chlorine atoms are substituted at positions 2, 4, and 6. The compound is named as 2,4,6-trichlorobenzene.

Isomerism refers to the phenomenon where compounds have the same molecular formula but differ in their arrangement or connectivity of atoms. Isomers exhibit distinct chemical and physical properties due to their structural differences. In organic chemistry, three main types of isomerism are observed:

1.    Structural Isomerism: Structural isomers have the same molecular formula but differ in the arrangement of atoms within the molecule. This can occur due to different carbon skeleton arrangements, functional group positions, or bond connectivity. Examples include:

●     Butane (C4H10) and 2-methylpropane (C4H10): Both compounds have the molecular formula C4H10, but butane has a straight-chain structure, while 2-methylpropane has a branched structure.

●     Ethanol (C2H5OH) and dimethyl ether (C2H6O): Both compounds have the same molecular formula, but ethanol has an -OH functional group, whereas dimethyl ether has an -O- functional group.

2.    Stereoisomerism: Stereoisomerism arises from the different spatial arrangements of atoms in a molecule, resulting in compounds with identical connectivity but differing in their three-dimensional orientation. There are two types of stereoisomerism:

●     Geometric Isomerism (Cis-trans isomerism): It occurs in compounds with restricted rotation around a double bond or a ring structure. In cis-trans isomerism, the groups on either side of the double bond or ring can be arranged differently. Examples include cis- and trans- isomers of 2-butene.

●     Optical Isomerism (Enantiomerism): Optical isomers, also known as enantiomers, are non-superimposable mirror images of each other. They have a chiral centre, which is a carbon atom bonded to four different groups. Enantiomers have identical connectivity but differ in their spatial arrangement. An example is D-glucose and L-glucose.

3.    Functional Group Isomerism: Functional group isomers have the same molecular formula, but different functional groups are present in the molecule. Examples include:

●     Propanal (C3H6O) and acetone (C3H6O): Both compounds have the same molecular formula, but propanal contains an aldehyde functional group (-CHO), while acetone contains a ketone functional group (-C=O-).