One of my student from Raffles Institution Junior College (RJC) has recently Whatsapp messaged me that she just realised that she does not know how to determine the oxidation states (also known as oxidation numbers) of different carbon atoms in organic compounds. She just joined our weekly JC2 H2 Chemistry Tuition Class not too long ago and missed out my teachings on determining the oxidation state of atoms in complex molecules and ions.
First of all, oxidation state / number (O.S.) is formally known as the charge of an atom would have if it existed as an ion.
In order to master the skills of determining the oxidation state of carbon atoms in organic compounds (in fact, you can determine the oxidation state of ALL atoms in ALL substances), you need to learn 3 sets of rules.
Rule 1: Basic Oxidation States
All atoms in elements, ions or compounds (covalent or ionic) can be given an oxidation state by applying the following basic rules below:
- All free elements → O.S. of zero
- All simple ions → O.S. similar to the charge of the ion
- All neutral molecules → sum of O.S. of all the atoms equals zero
- All polyatomic ions → sum of O.S. of all the atoms equals the charge of ion
Rule 2: Fixed Oxidation State of Certain Elements in a Compound
You need to also appreciate the following fixed oxidation state of certain elements when they are present in a compound:
- Group 1 element in a compound → O.S. = +1
- Group 2 element in a compound → O.S. = +2
- Group 17 element in a compound → O.S. = +1 (only for fluorine), O.S. = variable (rest of the halogens)
- Oxygen atom in a compound → O.S. = -2 (most compounds), O.S. = -1 (in peroxides such as H2O2)
- Hydrogen atom in a compound → O.S. = +1 (most compounds), O.S. = -1 (in metal hydrides such as NaH)
You can check out a Youtube Video on Oxidation States which i have created for my Sec 4 O-Level Pure Chemistry and IP Chemistry students:
Rule 3: Oxidation State of Atoms in Complex Molecules or Polyatomic Ions
This applies to questions which asked about the oxidation state of sulfur in the following complex molecules/ions: SO42-, S2O32-, S4O62-, etc. This is usually covered in JC1 syllabus in your school / junior college when your Chemistry lecturers and tutors are teaching Redox Equations & Titrations.
Same rule applies to organic compounds also.
Questions on determining the oxidation state of carbon in organic compounds came out in 2009 Paper 3 Q2 (Hydroxy Compounds), 2011 Paper 3 Q1 (Carboxylic Acids and Derivatives), 2015 Paper 1 B36 (Introduction and Isomerism) and 2018 Paper 3 Q2 (Introduction and Isomerism).
Will you be tested on oxidation state of carbon atoms in organic compounds in GCE A-Level H2 Chemistry (syllabus code: 9729) this year? Why not!
So, how do we assign the oxidation states of atoms from the structure of compound?
We apply the following steps:
4 Steps to Assign the Oxidation States of Atoms in Complex Molecules and Ions
- Bonding electrons are assigned to the more electronegative atom in a covalent bond
- The more electronegative atom “gains” an electron and oxidation state decreases by -1
- The less electronegative atom “loses” an electron and the oxidation state increases by +1
- If a bond is formed between two atoms of the same element, then there is no “gain” or “loss” of electron
The above steps are really useful for any compound where an element occurs several times in the compound and would have different oxidation states.
Let’s look at some examples to see how we apply them to solve Chemistry exam questions.
Example 1: What is the oxidation state of carbon in methane, CH4?
Carbon is more electronegative than hydrogen. Each hydrogen atom has an O.S. of +1. Therefore, O.S. of carbon = -4.
Example 2: What is the oxidation state of carbon in tetrachoromethane, CCl4?
Chlorine is more electronegative than carbon. Each chlorine atom has an O.S. of -1. Therefore, O.S. of carbon = +4.
Example 3: What is the oxidation state of carbon in dichoromethane, CH2Cl2?
All the hydrogen and chlorine atoms are covalently bonded to the central carbon atom. Hydrogen is more electronegative than carbon. Each hydrogen atom has an O.S. of +1. Chlorine is more electronegative than carbon. Each chlorine atom has an O.S. of -1. Therefore, O.S. of carbon = 0.
Example 4: What is the oxidation state of carbon in ethanol, C2H5OH ?
First of all, if you are not sure how the atoms are covalently bonded to each other, it is always good to draw out the structure of the organic compound.
As we can see, there are two carbon atoms in the structure of ethanol. Each of them has a different environment around them (i.e. different types of atoms covalently bonded). As such, usually, we are going to expect two different oxidation states for the two carbon atoms. Let’s call them C1 and C2 which represents the carbon atom bonded to the three hydrogen atoms and the carbon atom bonded to the hydroxyl (OH) group respectively.
Check out my workings below.
For C-H covalent bonds, the hydrogen atoms will have an O.S. of +1. For O-H covalent bond, hydrogen will have an O.S. of +1 while oxygen will have an O.S. of -2. For C-C covalent bond, there is no “gain” and “loss” of electrons, so you can ignore how how one carbon atom will affect the O.S. of the other carbon atom.
With that, we can work out the O.S. of C1 to be -3 and and the O.S of C2 to be -1 respectively. When you add up the O.S. of every atoms present in ethanol, it will be 0.
Let me give you a question so that you can practice it on your own.
Question: What is the oxidation state of bromine in CH3CONHBr? [modified from 2011 GCE A-Level H2 Chemistry Exam Paper 3 Q1]
Do work on it and share your suggested answer in the Comment Section right below this blog post. Look forward to hearing from you.
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