Problem 29 - Chemistry The Molecular Nature of Matter

Question

Which isomer of butanol cannot be oxidized by dichromate ion? Write its structure and IUPAC name.

Step-by-Step Solution

Answer

The isomer of butanol which cannot be oxidized by dichromate ion is tert-butanol, with the structure (CH3)3COH and IUPAC name 2-methylpropan-2-ol.

Step 1: Understanding the Chemistry Involved

Recognize that primary and secondary alcohols can be oxidized by dichromate ions (Cr2O72-), while tertiary alcohols cannot. Butanol isomers include primary, secondary, and tertiary alcohols.

Step 2: Identifying the Isomers of Butanol

Identify the four isomers of butanol: 1-butanol (primary), 2-butanol (secondary), isobutanol (secondary), and tert-butanol (tertiary).

Step 3: Determining the Isomer that Cannot be Oxidized

Analyze the structure of each isomer to find the one without a hydrogen atom bonded to the carbon bearing the hydroxyl (OH) group. This structure corresponds to a tertiary alcohol, namely tert-butanol.

Step 4: Writing the Structure of the Non-oxidizable Isomer

Draw the structural formula for tert-butanol as (CH3)3COH, indicating the central carbon atom with three methyl groups and one hydroxyl group.

Step 5: Assigning the IUPAC Name

The IUPAC name for tert-butanol is 2-methylpropan-2-ol.

Key Concepts

Dichromate Ion and Its Role in Oxidation

The dichromate ion (Cr₂O₇^2-) is a powerful oxidizing agent commonly used in organic chemistry to induce the oxidation of alcohols to the corresponding carbonyl compounds. Primary alcohols can be oxidized to aldehydes and then to carboxylic acids, while secondary alcohols are typically oxidized to ketones. However, tertiary alcohols, lacking the necessary hydrogen atom attached to the hydroxyl-bearing carbon, resist oxidation by dichromate ions.

In a reaction, the orange color of dichromate ion solution usually turns green due to the reduction of Cr⁶⁺ to Cr³⁺, providing a visual indication of the reaction's progress. This change is not only key to observing the reaction but also serves as an educational tool in understanding redox reactions.

Understanding Tertiary Alcohols

Tertiary alcohols are a class of alcohols where the carbon atom holding the hydroxyl (OH) group is connected to three other carbon atoms. This structure makes them resistant to oxidation because there is no hydrogen atom on the hydroxyl-bearing carbon that can be taken away, which is a necessary step in the oxidation process.

Tertiary alcohols, such as tert-butanol, are often highlighted in educational contexts to teach about structure-reactivity relationships. Their unique resistance to oxidation by dichromate ions makes tertiary alcohols a topic of interest in both laboratory and theoretical chemistry exercises.

IUPAC Naming System

The International Union of Pure and Applied Chemistry (IUPAC) naming system provides a standardized way to name chemical compounds, ensuring clarity and consistency across the scientific community. For alcohols, the IUPAC name is derived from the longest carbon chain containing the hydroxyl group, with the chain numbered to give the hydroxyl group the lowest number.

For tertiary alcohols like tert-butanol, the IUPAC name reflects the molecular structure: 2-methylpropan-2-ol specifies a propanol chain with a methyl group on the second carbon, and the hydroxyl group also on the second carbon. Becoming familiar with IUPAC naming is crucial for students as it is widely used in all branches of chemistry.

Butanol Isomers

Butanol exists in four isomeric forms, which are structurally different molecules with the same molecular formula (C₄H₉OH). The isomers include 1-butanol, 2-butanol, isobutanol (2-methylpropan-1-ol), and tert-butanol (2-methylpropan-2-ol), illustrating different classes of alcohols (primary, secondary, and tertiary).

Understanding the unique structure of each isomer is essential when analyzing their chemical reactivity and physical properties. For instance, while 1-butanol and 2-butanol can be oxidized to form different products, tert-butanol's structure renders it non-reactive to dichromate ion, emphasizing the importance of structural analysis in predicting the behavior of organic molecules in reactions.