Question

Which of the following is incorrect statement? (A) \(\mathrm{C}_{3} \mathrm{H}_{7}-\) represents two alkyl groups (B) \(\mathrm{C}_{4} \mathrm{H}_{9}-\) represents four structurally different alkyl groups (C) \(\mathrm{C}_{5} \mathrm{H}_{11}-\) represents eight structurally different alkyl groups (D) 2 -Iodo, 2 -methyl propane is the correct IUPAC of \(n\)-propyl iodide

Short answer

The incorrect statement is (D) 2 -Iodo, 2 -methyl propane is the correct IUPAC of \(n\)-propyl iodide.

Step-by-step solution

Statement A: \(\mathrm{C}_{3} \mathrm{H}_{7}-\) represents two alkyl groups

The molecular formula \(\mathrm{C}_3 \mathrm{H}_7-\) represents an alkyl group with 3 carbon atoms. The two possible structures are 1) propyl and 2) isopropyl. Hence, this statement is correct.

Statement B: \(\mathrm{C}_{4} \mathrm{H}_{9}-\) represents four structurally different alkyl groups

The molecular formula \(\mathrm{C}_{4} \mathrm{H}_{9}-\) represents an alkyl group with 4 carbon atoms. There are four different structures: 1) butyl, 2) isobutyl, 3) sec-butyl, and 4) tert-butyl. Thus, this statement is also correct.

Statement C: \(\mathrm{C}_{5} \mathrm{H}_{11}-\) represents eight structurally different alkyl groups

The molecular formula \(\mathrm{C}_{5} \mathrm{H}_{11}-\) represents an alkyl group with 5 carbon atoms. There are eight possible structures: 1) Pentyl, 2) Isopentyl, 3) Neopentyl, 4) 2,2-dimethylpropyl (commonly referred to as neopentyl), 5) 1,1-dimethylpropyl, 6) 1,2-dimethylpropyl, 7) 1,1,2-trimethyl ethyl (also known as tert-pentyl), 8) 2,2-dimethyl-2-methylpropyl Hence, this statement is correct.

Statement D: 2 -Iodo, 2 -methyl propane is the correct IUPAC of \(n\)-propyl iodide

The compound "2 -iodo, 2 -methyl propane" represents the molecular formula \(\mathrm{C}_4\mathrm{H}_9\mathrm{I}\). Its structure is such that a methyl group and an iodine atom are both bonded to the second carbon atom in a propane chain. However, \(n\)-propyl iodide is an iodine-substituted alkyl group with 3 carbon atoms. Its IUPAC name is "1-iodopropane" with the molecular formula \(\mathrm{C}_3\mathrm{H}_7\mathrm{I}\). Since these two compounds are different, the statement is incorrect. In conclusion, the incorrect statement is (D) 2 -Iodo, 2 -methyl propane is the correct IUPAC of \(n\)-propyl iodide.

Key concepts

Understanding Alkyl Groups

Alkyl groups are simplified forms of hydrocarbons that are missing one hydrogen atom. This missing hydrogen atom creates an opportunity for the group to bond with other atoms or groups. They form the basis for naming various organic compounds, using the IUPAC system. Here are some essential points about alkyl groups:

• They are generated by removing a hydrogen atom from an alkane.
• This removal leads to a general formula of \( ext{C}_n ext{H}_{2n+1}\) instead of the alkane's \( ext{C}_n ext{H}_{2n+2}\).
• Common alkyl groups include methyl \( ext{-CH}_3\), ethyl \( ext{-C}_2 ext{H}_5\), and propyl \( ext{-C}_3 ext{H}_7\).

There are different types of alkyl groups based on their structures, including normal (n-), iso-, and tert-alkyl groups. These variations occur due to branching in the carbon chain, resulting in different physical and chemical properties. Understanding these groups is essential for mastering organic chemistry nomenclature, as it allows you to identify and classify organic molecules.

Decoding Molecular Formulas

Molecular formulas represent the actual number of each kind of atom present in a molecule. Unlike structural formulas, they do not provide information on how the atoms are connected. However, they are crucial in chemical nomenclature.

• Molecular formulas give direct insight into the composition of a molecule by revealing the types and quantities of atoms present.
• This formula is distinct from the empirical formula, which shows the simplest ratio of atoms.
• An example: the molecular formula \( ext{C}_4 ext{H}_9\) indicates a group with four carbon atoms and nine hydrogen atoms.

The ability to interpret these formulas helps in determining possible structural isomers and underlying alkyl groups. For instance, the formula \( ext{C}_3 ext{H}_7\) can lead to different bonding structures like propyl and isopropyl, each with unique characteristics. Mastering molecular formulas is a stepping stone to understanding more complex chemical structures.

Exploring Structural Isomers

Structural isomers are compounds that share the same molecular formula but differ in how their atoms are arranged. This differences lead to variations in their physical and chemical properties.

• They are also known as constitutional isomers.
• The differing arrangements can affect boiling points, melting points, and reactivity.
• An example: \( ext{C}_5 ext{H}_{11}\) has eight structural isomers, ranging from pentyl to various branched forms.

Understanding structural isomers is crucial when discussing alkyl groups and molecular formulas. They help explain why two compounds with identical molecular formulas can behave differently. This knowledge is instrumental when identifying errors in chemical naming, such as the discrepancy between 2-iodo, 2-methyl propane and \(n\)-propyl iodide. By grasping the concept of isomerism, students can better appreciate the diversity and complexity of organic molecules.