2.4 Isomerism

Isomerism in alkanes
■ Isomerism

	►	Phenomenon in which two or more compounds having the same molecular formula but different structural formula.
	►	Isomer are molecules that have the same molecular formulas but different structures.
	►	Isomers have the same chemical properties.
	►	Isomerism is common in the compounds of carbon due to the ability of carbon atoms to form a variety of molecular structures such as chains and rings of carbon atoms of various types through covalent bonds. ○ Butane : C4H10 → 2 isomer ○ Pentane : C5H12 → 3 isomer ○ Hexane : C6H14 → 5 isomer ○ Heptane : C7H16 → 9 isomer

■ This video contains information on the isomerism in alkanes.

■ Alkyl group

	►	A group of atoms that is produced when one hydrogen atom is removed from an alkane.
	►	General formula: CnH2n+1 with n = 1, 2, 3, ....
	►	Naming of alkyl groups Number of carbon, n Prefix The word 'ane' of the alkane is replaced with 'yl'. 1 Meth Methyl 2 Eth Ethyl 3 Prop Propyl 4 But Butyl 5 Pent Pentyl 6 Hex Hexyl 7 Hept Heptyl 8 Oct Octyl 9 Non Nonyl 10 Dec Decyl

■ Method of drawing the structural formula of alkane isomers

	►	Alkanes begin showing isomerism starting from butane.
	►	For example, n-butane and 2-methylpropana C4H10 with the same formula are both alkanes. So, the isomers in the same homologous series will have chemical properties similar but its different physical properties.

Nomenclature of alkanes and their isomer
■ Guidelines when naming an alkane isomer

	►	Step 1: Select the longest continuous chain of carbon atoms in the molecule.
	►	Step 2: Number the carbon atoms in the main chain sequence.
	►	Step 3: Identify the branch chain and their position on the parent chain. ○ methyl for 1 carbon atom ○ ethyl for 2 carbon atom ○ propyl for 3 carbon atom ○ butyl for 4 carbon atom
	►	Step 4: Use prefix if necessary. ○ 'di' for two similar branch chain ○ 'tri' for three similar branch chain ○ Arrange the branch chains in alphabetical order if more than one similar branch exist.
	►	Step 5: Naming ○ Use comma to separate numbers and use hyphens to separate words from numbers.

■ Example:

	►	Step 1: Select the longest continuous chain of carbon atoms in the molecule. The longest chain contains 7 carbon atoms. Therefore the name of the parent chain is heptane Step 2: Number the carbon atoms in the main chain sequence. The nearest chain from the left is at carbon number 3. Therefore you should start numbering from the left of the mother chain. Step 3: Identify the branch chain and their position on the parent chain. The branch chain methyl is at carbon number 3. The names of the branch chain together with their position are 3 methyl. Step 4: Use prefix if necessary. No similar branch chain, therefore the name will remain as 3 methyl. Step 5: Naming The name of the alkane is 3-methyl heptane.
	►	The following animation show the names and structural formulae of the first five alkane members.

✍ Worked-example 2.4(a) Name the structural formulae of the following alkane. 2-methylbutane 3-methylhexane 2,3 dimethylpentane Solution: The following animation shows the method used to name the structural formulae of alkanes.

✍ Worked-example 2.4(b) Based on the molecular formula of the given alkanes, draw their structural formulae. 3-methylhexane 2-methylpentane 2,3 dimethylhexane Solution: The following animation shows the method used to draw the structural formulae of alkanes.

Isomerism in alkenes
■ Isomerism of alkenes

►

Alkenes display isomerism starting from butene. ○ Butene : C4H8 → 3 isomer ○ Pentene : C5H10 → 5 isomer

■ This video contains information on the isomerism in alkenes.

■ Method of drawing the structural formula of alkene isomers

	►	Step 1: Draw all the possible structures of the carbon chain.
	►	Step 2: Draw single bonds around every carbon atom.
	►	Step 3: Add hydrogen atom to the carbon atom at each of the single bonds.
	►	For example, butene (C4H8) has 3 isomers as follows:

Nomenclature of alkenes and their isomer
■ Guidelines when naming an alkene isomer

	►	The steps are the same as those used for naming the isomers of alkanes with one additional step(Use a smallest number to indicate the position of the carbon atoms in which covalent bonding begins).
	►	Example: ○ Position of the double bond : C3 Number of the carbon atoms in mother chain : 6 Name of the mother chain : hexane Position of the branch chain : C2 Types of branch chain : 1 methyl group Name : 2-methylhex-3-ene ○ Position of the double bond : C2 Number of the carbon atoms in mother chain :5 Name of the mother chain : Pentane Position of the branch chain : C2 and C3 Types of branch chain :2 methyl group Name : 2,3-dimethylpent-2-ene
	►	The following animation show the names and structural formulae of the first four alkene members.

✍ Worked-example 2.4(c) Name the structural formulae of the following alkenes. 2,3-dimethylbut-1-ene 2,4-dimethylpent-2-ene 2,3,4-trimethylhex-2-ene Solution: The following animation shows the method used to draw the structural formulae of alkenes.

✍ Worked-example 2.4(d) Based on the molecular formula of the given alkanes, draw their structural formulae. 2,3-dimethylpent-1-ene 2,4-dimethylpent-2-ene Solution: The following animation shows the method used to name the structural formulae of alkenes.

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