Electron displacement effects

Electrons shift inside a molecule

A molecule is not a set of still pictures. The electrons shift, and the shift decides where a reagent attacks and how stable an ion is. Four effects are worth knowing.

• The inductive effect and resonance are permanent.
• The electromeric effect is temporary. It appears only while a reagent attacks.
• Hyperconjugation is permanent too. It involves C-H bonds.

The inductive effect

In the bond $\text{C-Cl}$, chlorine is more electronegative than carbon. It pulls the shared pair towards itself. Chlorine gets a small negative charge, $\delta-$. Carbon gets a small positive charge, $\delta+$.

That is not all. The carbon with $\delta+$ now pulls a little on the next bond, so the next carbon gets a smaller $\delta+$, and so on. This passing of a pull through single bonds is the inductive effect.

The effect fades quickly with distance. It is strong at the first bond, weaker at the second, and almost gone after the third carbon.

We compare each group with hydrogen, which counts as zero.

• A group that pulls electrons harder than hydrogen has a $-I$ effect. Examples are $-\text{NO}_2$, $-\text{CN}$, $-\text{COOH}$, $-\text{F}$, $-\text{Cl}$, $-\text{Br}$, $-\text{I}$, $-\text{OH}$ and $-\text{OR}$. Among the halogens, the order is $\text{F} > \text{Cl} > \text{Br} > \text{I}$.
• A group that pushes electrons towards the carbon has a $+I$ effect. Alkyl groups are the common case. Their strength runs $(\text{CH}_3)_3\text{C-} > (\text{CH}_3)_2\text{CH-} > \text{CH}_3\text{CH}_2\text{-} > \text{CH}_3\text{-}$.

What it does to acids

Chloroacetic acid is a much stronger acid than acetic acid. The chlorine pulls electrons through the chain, and the O-H bond loses its hydrogen more easily. The same pull steadies the negative ion that is left behind. A group that pushes electrons does the opposite. That is why formic acid is stronger than acetic acid.

The more chlorine atoms, the stronger the acid. The nearer the chlorine is to the acid group, the stronger the acid. The simulation below shows how fast the pull fades.

Practise with new numbers

Resonance

Some molecules cannot be drawn with a single correct structure. The acetate ion is one. We can draw it with the double bond on one oxygen, or on the other. Neither picture is right alone. The real ion is something in between.

This is resonance, also called the mesomeric effect. The real molecule is a resonance hybrid of two or more structures. The structures are not real. They are our way to draw one molecule that our pictures of bonds cannot show in one go. We join them with a double-headed arrow.

Three rules guide the drawing of resonance structures.

• Only the electrons move. The atoms stay where they are.
• Every structure has the same number of paired and unpaired electrons.
• Structures that have more bonds, and charges on the atoms that suit them, count for more.

The hybrid is more stable than any one structure. The difference is called the resonance energy. The charge is spread over several atoms, and a spread-out charge is more stable than a concentrated one.

The effect also changes bond lengths. In the acetate ion the two C-O bonds are equal. They lie between a single and a double bond. In benzene all the C-C bonds are equal too.

Practise with new numbers

Groups that give and groups that take

Resonance also moves electrons between a group and a ring or a double bond.

• A group with a $+R$ effect gives electrons into the system. It has a lone pair or a pi bond to give. Examples: $-\text{OH}$, $-\text{OR}$, $-\text{NH}_2$ and the halogens. In aniline the lone pair on nitrogen flows into the ring, so the ring carbons get more electron density.
• A group with a $-R$ effect takes electrons out of the system. Examples: $-\text{NO}_2$, $-\text{CN}$, $-\text{CHO}$, $-\text{COOH}$ and the group $\text{C=O}$. In nitrobenzene the nitro group takes electrons from the ring.

The electromeric effect

A double or triple bond has a pi pair that can move. When a reagent attacks, the pair shifts completely to one of the two atoms. When the reagent goes away, the pair moves back. So the effect is temporary.

• $+E$ effect. The pair moves to the atom that the attacking reagent joins. An acid, $\text{H}^{+}$, attacking $\text{C=C}$ is the example.
• $-E$ effect. The pair moves away from the atom that the reagent attacks. A cyanide ion attacking the carbon of $\text{C=O}$ is the example. The pair goes to oxygen.

Hyperconjugation

A C-H bond next to an empty $\text{p}$ orbital, or next to a pi bond, can share its electrons with that neighbour. This spreading of sigma electrons is hyperconjugation. It is also called no-bond resonance, because one structure has no bond between carbon and hydrogen.

For propene, the structures are:

The C-H bond is the one on the carbon next to the double bond. We call such a carbon the alpha carbon and its hydrogen atoms the alpha hydrogens. More alpha hydrogens mean more structures, and more stability.

It explains three things.

• Stable carbocations and radicals. A tertiary ion has nine alpha hydrogens, so it is more stable than a primary ion with three.
• Stable alkenes. The more alkyl groups on the double bond, the more alpha hydrogens, and the more stable the alkene. So but-2-ene is more stable than but-1-ene.
• A slightly shorter C-C bond next to a double bond, because it gains a little double bond character.

Try it in Python

The program turns the pKa values of four acids into strengths. It shows what one chlorine atom does as it moves away from the acid group.

Common mistakes

• Thinking resonance structures are real. The molecule is one hybrid. It does not flip between structures.
• Moving atoms. Only electrons move in resonance structures.
• Thinking the inductive effect reaches far. It is strong at the first bond and nearly gone by the fourth.
• Mixing up inductive and resonance. The inductive effect passes along sigma bonds. Resonance passes through pi bonds and lone pairs.
• Calling the electromeric effect permanent. It needs a reagent, and it stops when the reagent leaves.

Remember these

• Inductive effect: a permanent pull or push along single bonds, fading with distance.
• $-I$ groups pull: $-\text{NO}_2$, $-\text{CN}$, halogens. $+I$ groups push: alkyl groups.
• Resonance: electrons spread over several atoms. The hybrid is more stable than any one structure.
• $+R$ groups give electrons. $-R$ groups take them.
• Electromeric effect: a temporary shift of a pi pair under attack.
• Hyperconjugation: sigma electrons of an alpha C-H bond spread out. More alpha hydrogens, more stability.

Check yourself

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

1. Is the chloromethyl group, $-\text{CH}_2\text{Cl}$, a $+I$ group or a $-I$ group?

2. Why is a resonance hybrid more stable than any of its structures?

3. How many alpha hydrogens does propene have?

4. What is the difference between the inductive effect and the electromeric effect?

5. Does the amino group, $-\text{NH}_2$, show a $+R$ or a $-R$ effect?