Attacking reagents and types of reaction

Who attacks whom

At the heart of most organic reactions, a part that is rich in electrons meets a part that is poor in them. The rich part gives a pair and the poor part takes it. A new bond forms. The reagents come in two kinds.

Nucleophiles

A nucleophile means "nucleus loving". It has a lone pair, or a spare pair, to give. It attacks a centre that is short of electrons.

• Negative ions: $\text{OH}^{-}$, $\text{CN}^{-}$, $\text{Cl}^{-}$, $\text{Br}^{-}$, $\text{RO}^{-}$ and carbanions.
• Neutral molecules with a lone pair: $:\text{NH}_3$, $\text{H}_2\text{O}$ and $\text{ROH}$.

A nucleophile is a Lewis base. Its curved arrow starts at the lone pair.

Electrophiles

An electrophile means "electron loving". It has an empty orbital, or a positive charge, and takes a pair.

• Positive ions: $\text{H}^{+}$, $\text{NO}_2^{+}$ and carbocations such as $\text{CH}_3^{+}$.
• Neutral molecules short of electrons: $\text{BF}_3$, $\text{AlCl}_3$ and $\text{SO}_3$.
• A carbon with a $\delta+$ charge, such as the carbon of $\text{C=O}$.

An electrophile is a Lewis acid. A curved arrow always runs from the nucleophile to the electrophile.

A neutral molecule can be either. Water is a nucleophile through the lone pairs on oxygen. A carbon atom that is joined to a halogen is an electrophile, because the halogen pulls electrons off it.

Four types of organic reaction

Substitution

One atom or group is replaced by another.

Here a nucleophile, $\text{OH}^{-}$, replaces the bromine. Methane and chlorine in light give $\text{CH}_3\text{Cl}$ and $\text{HCl}$. That is a substitution by free radicals. The nitration of benzene is a substitution by an electrophile, $\text{NO}_2^{+}$.

Addition

Two molecules join to make one. A pi bond is used up, and two new sigma bonds form.

Alkenes add electrophiles first. A carbonyl group, $\text{C=O}$, adds a nucleophile at its carbon.

Elimination

A small molecule is removed from the substance, and a pi bond forms in its place.

It is the reverse of addition. The base takes a hydrogen from one carbon, and the halogen leaves the next.

Rearrangement

The atoms of one molecule move to give a new structure, which is an isomer. Nothing is added and nothing is lost.

When ammonium cyanate is heated, it changes to urea. When $\text{AlCl}_3$ is present, n-butane changes to isobutane. A carbocation often rearranges to a more stable one. A hydrogen atom with its bonding pair moves from the next carbon, and a secondary ion becomes a tertiary one.

Telling them apart by counting

• Substitution: two molecules in, two out. The pi bonds stay the same.
• Addition: two in, one out. The pi bonds are fewer.
• Elimination: one in, two out. The pi bonds are more.
• Rearrangement: one in, one out. The pi bonds are usually the same.

The count gives a quick guess. Then check it against what is replaced, joined, removed or moved.

Radical chains

Reactions that run through free radicals, such as the chlorination of methane, are chain reactions. Light starts only a few radicals. Each one then goes round the propagation cycle many times, so a little light makes a lot of product.

Practise with new numbers

Try it in Python

The program names four reactions by counting the molecules that go in and come out, and the pi bonds before and after.

Common mistakes

• Mixing up the two reagents. A nucleophile gives a pair. An electrophile takes one.
• Thinking all nucleophiles are ions. Water and ammonia are neutral nucleophiles.
• Naming a reaction by one product. Count the molecules in and out, and ask what moved.
• Forgetting that the arrow runs from the nucleophile. It points at the electron-poor centre.
• Calling a rearrangement an elimination. Nothing leaves in a rearrangement.

Remember these

• Nucleophile: electron-rich, gives a pair, a Lewis base. Electrophile: electron-poor, takes a pair, a Lewis acid.
• Substitution replaces. Addition joins. Elimination removes. Rearrangement moves atoms inside one molecule.
• Addition uses up a pi bond. Elimination makes one.
• A radical chain has initiation, propagation and termination steps.

Check yourself

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

1. Is $\text{NH}_3$ a nucleophile or an electrophile?

2. Is $\text{AlCl}_3$ a nucleophile or an electrophile?

3. What type of reaction is $\text{CH}_3\text{CH}_2\text{OH} \rightarrow \text{CH}_2\text{=CH}_2 + \text{H}_2\text{O}$?

4. Which type of reaction uses up a pi bond and forms two sigma bonds?

5. Which type of reaction changes the structure but not the formula?