Bond fission and reaction intermediates

Why watch how a bond breaks

A reaction breaks old bonds and makes new ones. Between the start and the end, there is often a short-lived piece that exists for a tiny fraction of a second. Chemists call it a reaction intermediate.

You cannot keep an intermediate in a bottle. Yet it decides what the reaction makes. So it pays to know three of them well: the free radical, the carbocation and the carbanion.

Two ways to break a bond

A covalent bond is a shared pair of electrons. When it breaks, the pair can go two ways.

Homolytic fission splits the pair evenly. Each atom keeps one electron.

Heterolytic fission gives both electrons to one atom. That atom becomes a negative ion. The other atom is left with an empty orbital and a positive charge.

The curved arrows

Chemists draw the movement of electrons with curved arrows. There are two kinds.

• A full-headed arrow shows a movement of a pair of electrons.
• A half-headed arrow, shaped like a fish hook, shows a movement of one electron.

An arrow starts where the electrons are now, such as a bond or a lone pair. It points to where they go. In homolysis there are two half-headed arrows, one for each electron of the bond. In heterolysis there is one full-headed arrow, pointing at the atom that keeps the pair.

What decides the way

• A bond between two similar atoms, such as Cl-Cl, is nonpolar. It tends to break evenly, in the gas phase or in a nonpolar solvent, with light, heat or a peroxide to start it.
• A polar bond, such as C-Br, tends to break unevenly, in a polar solvent. The more electronegative atom takes the pair.

Free radicals

A free radical is a neutral piece with an unpaired electron. It is written with a dot, as in $\text{Cl}^{\bullet}$ and $\text{CH}_3^{\bullet}$.

An unpaired electron wants a partner. So radicals are very reactive and live for a very short time. A radical usually reacts by pulling an atom, with its electron, from another molecule. The new piece is itself a radical. So a reaction of radicals runs as a chain:

1. Initiation. Light splits $\text{Cl}_2$ into two $\text{Cl}^{\bullet}$.
2. Propagation. $\text{Cl}^{\bullet}$ pulls a hydrogen from $\text{CH}_4$ to make $\text{CH}_3^{\bullet}$. Then $\text{CH}_3^{\bullet}$ pulls a chlorine from $\text{Cl}_2$ to make $\text{CH}_3\text{Cl}$ and a fresh $\text{Cl}^{\bullet}$. The cycle repeats.
3. Termination. Two radicals meet and join, and the chain ends.

Alkyl radicals follow the same order as carbocations: tertiary is more stable than secondary, which is more stable than primary.

Carbocations

A carbocation has a carbon with a positive charge and only six electrons. It forms when a leaving group takes the bonding pair away.

The positive carbon is $\text{sp}^2$ hybridised. Its three bonds lie in one plane at $120^\circ$. An empty $\text{p}$ orbital stands at right angles to that plane. This empty orbital is what makes the ion so keen to react.

We name a carbocation by the number of carbon atoms joined to the positive carbon: methyl (none), primary (one), secondary (two) and tertiary (three).

Stability order:

Two things explain it. Both work by sending electron density towards the positive carbon.

• Inductive effect. An alkyl group pushes electrons a little towards the carbon it is joined to. The more alkyl groups there are, the more the positive charge is spread out.
• Hyperconjugation. A C-H bond on a neighbouring carbon can overlap with the empty $\text{p}$ orbital and share its electrons. Each such bond helps. A tertiary ion has the most.

Practise with new numbers

Two other things make a carbocation more stable: a double bond next to the positive carbon, as in the allyl ion, and a benzene ring next to it, as in the benzyl ion. Both let the charge spread by resonance, a topic of the next lesson.

Carbanions

A carbanion is the opposite. It is a carbon with a negative charge and a lone pair, so it has eight electrons. It forms when the carbon keeps the bonding pair. The carbon is $\text{sp}^3$ hybridised, and the ion is pyramidal like ammonia, with the lone pair in the fourth position.

An alkyl group pushes electrons towards the carbon. For a carbanion, that is bad. The carbon already has too many electrons, and the push crowds it further. So the order reverses:

A group that pulls electrons away from the carbon helps a carbanion. A carbon with more $s$ character also holds the lone pair more tightly, which is why the ion of ethyne is more stable than the ion of ethane.

Try it in Python

The program counts the helpful C-H bonds of four carbocations and ranks them.

Common mistakes

• Reversing the order for a carbanion. Alkyl groups help a carbocation and hurt a carbanion.
• Mixing up the arrows. A full head moves a pair. A half head moves one electron.
• Drawing a flat carbanion. It is pyramidal, like ammonia. The carbocation is the flat one.
• Counting the hydrogen on the positive carbon. Only C-H bonds on the neighbouring carbons can hyperconjugate.
• Calling an intermediate a product. It is made and used up on the way, and it is not kept.

Remember these

• Homolysis gives two free radicals, with half-headed arrows. Heterolysis gives a cation and an anion, with a full-headed arrow.
• A free radical has an unpaired electron. It is neutral and very reactive.
• A carbocation has six electrons and is flat. Stability: tertiary, secondary, primary, methyl.
• A carbanion has a lone pair and is pyramidal. Stability: methyl, primary, secondary, tertiary.
• Hyperconjugation and the inductive effect stabilise the cation.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. In heterolysis of the bond $\text{R-X}$, X takes both electrons. What are the two pieces?

2. What does a half-headed curved arrow show?

3. Why is a tertiary carbocation more stable than a primary one?

4. What is the shape of a carbocation, and why?

5. Which is more stable, the methyl carbanion or the tertiary butyl carbanion?