Question

Based on the number of bonds and the nature of the bonded atoms, state whether each of the following changes is an oxidation or a reduction: (a) \(=\mathrm{CH}_{2}\) becomes \(-\mathrm{CH}_{2}-\mathrm{OH}\) (b) \(=\mathrm{CH}-\) becomes \(-\mathrm{CH}_{2}-\) (c) \(\equiv \mathrm{c}-\) becomes \(-\mathrm{CH}_{2}-\)

Short answer

(a) Oxidation, (b) Reduction, (c) Reduction

Step-by-step solution

Understanding Oxidation and Reduction

Oxidation involves the loss of electrons, which generally increases the oxidation state of the atom and often corresponds with the gain of oxygen or loss of hydrogen. Reduction involves the gain of electrons, decreasing the oxidation state of the atom, and often corresponds with the gain of hydrogen or loss of oxygen.

Analyze Change (a)

The initial molecule is \(\text{=CH}_2\), and it changes to \(-\text{CH}_2-\text{OH}\). Here, the double bond to carbon (C=C) is broken, and an additional bond to an oxygen (C-OH) is formed. This indicates the addition of oxygen, which is characteristic of oxidation.

Analyze Change (b)

The initial molecule is \(=\text{CH}-\), and it changes to \(-\text{CH}_2-\). This transformation involves breaking a double bond to form single bonds (C=C to C-C), indicating a gain in hydrogen atoms. This is characteristic of reduction.

Analyze Change (c)

The initial molecule is \(\text{≡C}-\), and it changes to \(-\text{CH}_2-\). This transformation involves breaking a triple bond to carbon and forming single bonds (C≡C to C-C), indicating a gain in hydrogen atoms. This is also characteristic of reduction.

Key concepts

Oxidation State

The concept of oxidation state is vital in understanding chemical reactions, especially redox reactions. The oxidation state of an atom is a measure of the degree of oxidation of an atom within a compound. It indicates the hypothetical charge the atom would have if all bonds to atoms of different elements were 100% ionic.
When an atom loses electrons, its oxidation state increases. Conversely, when an atom gains electrons, its oxidation state decreases.

For instance, in the change \( =\text{CH}_2 \) to \( -\text{CH}_2-\text{OH} \), the carbon atom bonds with an additional oxygen. Oxygen is more electronegative and will pull electrons away from carbon, increasing the carbon's oxidation state.
Understanding the shift in oxidation state helps you determine if a redox reaction has occurred and whether it's an oxidation or reduction.

Electron Transfer

Electron transfer is at the heart of redox reactions. It involves the movement of electrons from one atom or molecule to another.
Oxidation means an atom or molecule loses electrons, while reduction means it gains electrons. This flow of electrons is crucial in many chemical processes, including metabolism and energy production in living organisms.

For example, in the change \( =\text{CH}\- \) to \( -\text{CH}_2\- \), electrons are moved as the double bond (which shares more electrons between atoms) changes to a single bond. Here, carbon atoms in a molecule gain hydrogen atoms, which implies gaining electrons, and thus, reduction is evident.

Chemical Bonding

Chemical bonding involves the forces holding atoms together in molecules. Understanding the nature of these bonds is key to predicting and explaining chemical reactions.
Bonds can be single, double, or triple, depending on the number of shared electron pairs. In chemical reactions, breaking and forming these bonds involves energy changes and often results in oxidation or reduction.

For instance, in the reduction example of \( \text{≡C}- \) turning into \( -\text{CH}_2- \), the triple bond converts into single bonds. A triple bond involves sharing three pairs of electrons between two carbon atoms. Moving from a triple bond to single bonds suggests that electrons are redistributed, generally indicative of a reduction (since each hydrogen added is taking up the electrons). The carbon atoms gain electrons as they share fewer with each other, emphasizing the relevance of chemical bonding in redox reactions.