Hydrogen peroxide is an electron hog – let me explain in terms of a butt-sniffing dog! I hope you don’t think my excitement is disproportionate! But for this oxidizing (AND reducing) agent, I am fortunate – from cleaning wounds, to bleaching hair, to showing us what protein’s where!

Hydrogen Peroxide (H2O2) is made up of 2 oxygen atoms and 2 hydrogen atoms. You can think of atoms as “dog walkers” (protons) walking dogs (electrons). The protons are positively charged and concentrated at the center of the atom (along with non-charged subatomic particles called neutrons). And whizzing around them are negatively-charged electrons. We can predict where electrons are most likely to hang out and represent such “homes” as orbitals, but just like you’re not always at your home, the electrons aren’t either – they’re constantly moving around and the orbitals are just where they spend the most time. 

Because opposite charges attract, the proton can hold in the electrons. Electronegativity is a measure of how tightly the dog walker can pull on the leashes. The more electronegative the atom, the tighter the electrons are held and the harder it is for them to escape. In terms of “pulling strength” for some of the common ones we encounter in biochemistry: H < C < S < N < O. So hydrogen (H) has a loose leash whereas oxygen (O) keeps its electrons on a tight leash. 

Different atoms have different numbers of protons and this number defines the element (e.g. oxygen always has 8 protons & hydrogen always has 2 protons). But the number of electrons can vary – the same dog walker can walk different numbers of dogs.

Protons & electrons have equal but opposite charges. When the number of electrons equals the number of protons, these charges cancel out & the atom’s neutral (overall) but more electrons gives you a negatively-charged molecule (ANION) & fewer electrons and you get a + charged CATION.

The more energetic the dog, the more it can resist the leash and the further from the walker they can get. Similarly, the more energy the electrons have, the further from the nucleus they live. We call the outermost electrons VALENCE ELECTRONS and they’re the ones that are most likely to be involved in reactions – they’re the most accessible & have the most energy.

And speaking of energy – adding energy – such as with light or heat – is like giving the dogs caffeine. Give them enough and they can pull so hard on the leash the leash “breaks” and they can escape. When a dog escapes (an atom loses electrons) we say the molecule got OXIDIZED.

The dog can get “adopted” by another walker – and the molecule that adopts it, gaining electrons, is REDUCED. 

Together, we call this sort of reaction a REDOX REACTION. And you can remember it by OIL RIG: Oxidation Is Loss (of electrons); Reduction is Gain (of electrons)

If you don’t give them enough energy to actually break the leash they can tug a bit harder and get a little further away from the walker (electron gets promoted to a higher energy level) – but then they’ll crash – and fall back down like the leash was bungee – and when they do, they give back that energy – sometimes they give it back as light, and that’s the principle behind fluorescence. 

Each dog walker is restricted to being able to walk a certain number of dogs & they have an “ideal number” of dogs they like to walk. For a lot of the molecules we talk about in biochemicals (like carbon (C), nitrogen (N), oxygen (O) & sulfur (S)) they try to get 8 valence electrons – the so-called “octet rule.” Hydrogen (H) is too small for that many electrons – it only has a single proton, so there’s no way it could keep a hold on 8! Instead, it shoots for 2.  But how to get there?

Valence electrons can get attracted to positive or partly positive things, like a dog that detects a nearby dog butt to sniff & goes to check it out. If the dog takes the rest of the pack with it, 2 molecules can combine in a covalent bond. But the dog can also go it alone. 

If a single dog that was previously in a pair escapes, it leaves behind a pairless pup – we call molecules with lone electrons RADICALS. And they’re highly reactive. It’s like they’re on butt-sniffing high alert! Paired electrons are less reactive (they have a butt right next to them to sniff already!) but lone electrons are on the lookout!

Something interesting can happen when atoms join together to make molecules. There it’s like dog walkers come together to walk together and pairs of dogs – 1 from 1 walker and 1 from the other walker – hang out to form covalent bonds

Now there’s more coordination required because the molecules have to walk “in step” – like a 3-legged race – this constraint is a decrease in entropy (randomness/disorder). Such a decrease in entropy is unfavorable (2nd law of thermodynamics) BUT joining together helps “lighten the leash load” for the walkers. When the walkers are joined, they kinda help each other hold in the dogs, but when they split, each walker has to reign their dogs in on their own. 

So, going back to hydrogen peroxide (H2O2). Oxygen dog walkers keep a tight leash because O is highly electronegative. And in H2O2 you have 2 of them right next to each other connected by a single bond (2 shared electrons) (this is the definition of a peroxide).

This bond is really vulnerable because it’s kinda like 2 dog owners trying to pull their butt-sniffing dogs apart. But because the oxygens are pulling in opposite directions, the bond between them is weakened and more vulnerable to splitting up if you provide enough energy to let the walkers win (energy that can come from heat (thermal decomposition) or light (photodecomposition))

And when they split up this way, they it splits evenly – each oxygen getting 1 of the pair of electrons they shared (homolytic cleavage) -> produces hydroxy radicals 2 OH*

And those radicals are now hyper-sniffers so they go on the hunt for things to sniff. And, like really energetic dogs on tight leashes, they drag the rest of the pack with them to find a new butt to sniff – the radicals join to another molecule

When a radical joins to a non-radical, you still have a radical – you just pass the lone electron to another atom making that one more reactive. This can set off a chain reaction like that which causes acrylamide monomers to polymerize to make PAGE gels we use to separate proteins.

But when a radical joins to another radical, they un-radicalize because lone + lone = no longer alone!

If you have a solution of H2O2, where the only other thing around is more H2O2 & water, if that H2O2 breaks (decomposes) into radicals those can react w/other radicals or it can react with water or O2. You can have all sorts of radical reactions, but if it’s totally efficient you end up with the most stable form of how they can divy up the electrons – water (H2O) and oxygen gas (O2).

2 H2O2 -> 2 H2O + O2

this overall reaction is a DISPROPORTIONATION REACTION – redox reactions always involve both oxidation & reduction, but in disproportionation reactions, the same starting material gets both oxidized & reduced. (you can see from the 2’s involved that you have multiple H2O2 contributing to this overall reaction and in an H2O2 solution you’ll have lots some of which will act as an oxidant and others as a reductant).

To understand this remember what oxidation and reduction mean. Oxidation Is Loss – we say loss of electrons but what we really mean is loss of electron density – basically the electrons don’t hang out with you as much. And Reduction Is Gain – a molecule is reduced when it convinces electrons to hang out with it more.

In the reactant, H2O2, each oxygen has to share electrons with 1 hydrogen & 1 oxygen. In one of the products (H2O) that oxygen now has to share with 2 hydrogens but only 1 O – H is less electronegative, so it has less pull on the O’s electrons than another O would. And, since the H has a “weak leash” the H’s electron hangs out more towards the O than the H. So the O has more electrons “to itself” so we say it’s been REDUCED (Reduction is Gain). 

But the other product (O2) now has to share with another oxygen which keeps a tight leash on its electrons. So the O has less “to itself” so we call it OXIDIZED (Oxidation is Loss)

If you want to get all formal about it, you can assign oxidation numbers which is the hypothetical charge a molecule’d have if all its bonds in were ionic (in an ionic bond, 1 atom gives up an e⁻ BUT in a covalent bond they share both). More on how to do that at the end of the post so people who don’t want to nerd out quite so much don’t have to read through it…

If H2O2 can oxidize AND reduce why do we call it an oxidant? Turns out it’s a lot better at the oxidizing part. And when it’s not just H2O2 & water in the mix there can be opportunities to oxidize other things so you don’t have to go this decomposition into H2O & water route.

But if you start un-evening the extents of oxidation & reduction you don’t have enough to get the nice H2O & water byproducts. And radicals can go after other things like proteins & DNA. So your body stocks up on extra reducing agents like glutathione 

Free-radical forming isn’t the only route from H2O2 to H2O & O2 and your body takes great precautions so that if it does neutralize it through free radicals, those radicals can’t get far – thanks metals! And, thankfully for us, that radical way, if there isn’t a catalyst (speeder-upper) involved is really slow. Because although the final products (H2O & O2) are much happier than the initial reactant (H2O2), that initial H2O2 doesn’t know that those product options exist. And before it can get there it has to get the activation energy it needs to break. So the spontaneous decomposition is really slow.  

So H2O2 will only slowly oxidize over time if you leave it alone – and to help really leave it alone you keep it in a brown bottle that keeps out the light (remember light’s a form of energy).

So why does it decompose (hence the bubbles) when you put it on your cuts? There’s a protein enzyme (reaction mediator) in your blood, catalase, that makes the decomposition easier – but safer because it happens in a controlled way. And then you get the subsequent stuff & oxygen gas forms and this is why you see bubbles. And that bubbling can help push out dirt. Since the catalase keeps the radicals mostly contained, there shouldn’t be many free radicals but if there are, it’s not just H2O2 & water they radicals meet – the radicals can meet & mess with bacterial cell walls, etc. so it can be used as a disinfectant. But apparently it might also harm new cells, preventing healing, so use with caution

Catalase isn’t the only enzyme that’s “got the power!” Another enzyme that makes H2O2 more reactive is horseradish peroxidase (HRP) which is a nice cooperative little protein that gives us the ability to control where oxidation will occur. If we attach it to invisible things we’re interested in, then we add something that changes when oxidized – and importantly changes in a way that we can detect – we can then tell where the thing we were interested in is. This is the rationale behind using horseradish peroxidase (HRP)-conjugated secondary antibodies in Western Blots. 

And speaking of being able to see things… The reason things look certain colors is that the molecules in that thing absorb light of a particular wavelength (like removing a slice of a rainbow). Double bonds (often in rings) often are responsible for absorbing light making things look colored. This is the case for the melanin giving your hair color. Oxidation can “un-double” those bonds, “un-coloring” them so the hair gets “bleached”

now for the formal stuff. The oxidation number is the HYPOTHETICAL charge a molecule would have if all it’s bonds in were ionic (in an ionic bond, 1 atom gives up an e⁻ BUT in a covalent bond they share both)

for each bond, assign shared e⁻ to more electronegative (e⁻-hogging) atom H < C < S < N < O. The more electronegative one gets to “own” the pair of e⁻ they share (2 pairs 4 a double bond). An exception is bonds between 2 atoms of same element – in those each can “own” 1 e⁻

Next subtract # “owned” from # of valence e- the “natural” element has to get the oxidation #

So, in H2O2, the O is bound to 1 O and 1 H. It gets to “own” both electrons from the O-H bond (so 2) But it doesn’t get any “extra” from the O-O bond, just the 1 it contributes. And then it has 2 lone pairs. So it “owns” 2 +1 + 2(2) = 7 electrons.  “Elemental” oxygen has 6 valence electrons. 6-7 = -1

How about in water (H2O)? The O is bound to 2 H’s and it gets to “own” both electrons from both of those bonds – and it still has the 2 lone pairs, so 2(2) + 2(2) = 8 and 6-8 = -2. This is less than -1, so the O has been REDUCED!

And in O2? Any element in its free element state has an oxidation number of 0. So, by definition, the oxidation state of O in O2 is 0. And this makes sense because it’s sharing with itself so they pull equally & share fairly. And 0 is more than the -1 it started at, so it’s been OXIDIZED. 

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