Question

Give the structural formula of: (a) 2, 3 -dimethylbutane (b) 2,2 -?imethylpropane (neopentane) (c) 2 -ethy1-3-me thylhexane (d) 2,4 -dimethy1-4-ethylheptane.

Short answer

(a) 2,3-dimethylbutane: CH3-CH(CH3)-CH(CH3)-CH3 (b) 2,2-dimethylpropane (neopentane): CH3-C(CH3)2-CH3 (c) 2-ethyl-3-methylhexane: CH3-CH(CH2-CH3)-CH(CH3)-CH2-CH2-CH3 (d) 2,4-dimethyl-4-ethylheptane: CH3-CH(CH3)-CH2-CH(CH3)(CH2-CH3)-CH2-CH2-CH3

Step-by-step solution

Identify the main chain

The name of the alkane is 2,3-dimethylbutane. "butane" implies a main chain of 4 carbon atoms.

Identify the substituents

We have two "methyl" substituents, which means, two -CH3 groups at positions 2 and 3 in the carbon main chain.

Draw the structural formula

Combine the main chain and substituents, placing the -CH3 groups at positions 2 and 3. CH3-CH(CH3)-CH(CH3)-CH3 (b) 2,2-dimethylpropane (neopentane)

Identify the main chain

The name of the alkane is 2,2-dimethylpropane. "propane" implies a main chain of 3 carbon atoms.

Identify the substituents

We have two "methyl" substituents, which means, two -CH3 groups at position 2 in the carbon main chain.

Draw the structural formula

Combine the main chain and substituents, placing the -CH3 groups at position 2. CH3-C(CH3)2-CH3 (c) 2-ethyl-3-methylhexane

Identify the main chain

The name of the alkane is 2-ethyl-3-methylhexane. "hexane" implies a main chain of 6 carbon atoms.

Identify the substituents

We have one "ethyl" substituent, which means a -CH2-CH3 group at position 2; and one "methyl" substituent, which means a -CH3 group at position 3 in the carbon main chain.

Draw the structural formula

Combine the main chain and substituents, placing -CH2-CH3 group at position 2 and -CH3 group at position 3 of the carbon main chain. CH3-CH(CH2-CH3)-CH(CH3)-CH2-CH2-CH3 (d) 2,4-dimethyl-4-ethylheptane

Identify the main chain

The name of the alkane is 2,4-dimethyl-4-ethylheptane. "heptane" implies a main chain of 7 carbon atoms.

Identify the substituents

We have two "methyl" substituents, which means, two -CH3 groups at positions 2 and 4; and one "ethyl" substituent, which means a -CH2-CH3 group at position 4 in the carbon main chain.

Draw the structural formula

Combine the main chain and substituents, placing -CH3 groups at positions 2 and 4, and -CH2-CH3 group at position 4 of the carbon main chain. CH3-CH(CH3)-CH2-CH(CH3)(CH2-CH3)-CH2-CH2-CH3

Key concepts

IUPAC Nomenclature for Organic Compounds

Understanding the International Union of Pure and Applied Chemistry (IUPAC) nomenclature is crucial for students to correctly name organic compounds. This system provides a standard way to name organic chemical compounds to avoid confusion. When naming an alkane using IUPAC rules, first identify the longest continuous chain of carbon atoms; this becomes the base name of the compound.

For example, in '2,3-dimethylbutane', 'butane' refers to a chain of four carbon atoms. Next, identify any substituents (side chains) attached to the main chain. In this case, there are two 'methyl' (-CH₃) groups. The numerical prefixes (2,3-) indicate their position on the main chain. If there are substituents at equivalent positions, use di-, tri-, etc., to indicate how many are present. By understanding these naming conventions, students can systematically approach the naming of even more complex organic molecules.

Alkane Substituents

Substituents are atoms or groups of atoms that are attached to the main carbon chain of an alkane. They are considered when the carbon chain has branches or functional groups that modify the main structure. In organic chemistry, common alkane substituents include methyl (-CH₃), ethyl (-CH₂CH₃), propyl (-CH₂CH₂CH₃), and others.

When analyzing compounds like '2-ethyl-3-methylhexane', you'll find the ethyl group at the second carbon and the methyl group at the third carbon of a hexane chain. Naming alkane substituents accurately is integral to IUPAC nomenclature. Their identification not only supports proper naming but also aids in visualizing the molecular structure of the compound.

Drawing Organic Molecules

Drawing organic molecules involves representing the carbon skeleton and its substituents on paper or digitally, providing a visual of the molecular structure. While drawing, the longest chain of carbon atoms, determined during nomenclature, forms the backbone. Substituents are then drawn at their respective positions on the main chain. Begin with simple straight lines to denote bonds between carbon atoms, adding lines or the letters 'CH₃', 'CH₂', etc., to indicate methyl groups or other alkyl groups.

For instance, '2,4-dimethyl-4-ethylheptane' starts with a heptane chain, and then methyl groups are added at the 2nd and 4th carbons, along with an ethyl group at the 4th carbon. Learning to draw these structures assists students in visualizing and understanding molecular shapes and the spatial arrangement of atoms in organic chemistry.