Electron-Pair Sharing Reactions

Introduction: How atoms become more stable 🌟

students, many important chemical changes happen because atoms want a more stable outer electron arrangement. In electron-pair sharing reactions, two atoms share one or more pairs of electrons instead of completely transferring electrons. This idea is central to organic chemistry and helps explain how molecules form, break, and rearrange during reactions.

By the end of this lesson, you should be able to:

• explain what is meant by electron-pair sharing reactions,
• use key terms such as covalent bond, lone pair, electrophile, and nucleophile,
• describe how this reaction type fits into the broader topic of chemical change,
• apply the idea to common examples in IB Chemistry HL, and
• connect mechanism ideas to evidence from structure and reactivity.

A useful way to think about these reactions is to imagine electrons as a shared resource. Instead of one atom completely taking from another, both atoms contribute electrons to a new bond. This is especially important in molecules made of nonmetals, where covalent bonding is common.

What does electron-pair sharing mean? 🔬

In an electron-pair sharing reaction, a bond forms when two electrons are shared between two atoms. The shared pair is attracted by both nuclei, and this attraction holds the atoms together. Such a bond is called a covalent bond.

A simple example is the formation of hydrogen gas molecules. Two hydrogen atoms each contribute one electron to make a shared pair:

$$\mathrm{H\cdot + \cdot H \rightarrow H-H}$$

This is a basic example of electron sharing, but in IB Chemistry HL, the term is often used more broadly to describe reaction mechanisms where a pair of electrons moves from one species to another. In these mechanisms, electron movement is shown using curved arrows, and the arrows always start at an electron pair and point to where that pair goes.

This is important because it helps chemists explain not just what changes occur, but how they occur. A mechanism is a step-by-step description of bond breaking and bond making.

Key vocabulary and ideas 🧠

To understand electron-pair sharing reactions, students, you need to know a few key terms:

• Covalent bond: a bond formed by the sharing of one pair of electrons.
• Lone pair: a pair of electrons on an atom that is not being used in bonding.
• Nucleophile: a species that donates an electron pair. It is electron-rich.
• Electrophile: a species that accepts an electron pair. It is electron-poor.
• Curved arrow: a diagram symbol showing the movement of an electron pair in a mechanism.

A nucleophile is often negatively charged or has a lone pair, such as $\mathrm{OH^-}$, $\mathrm{NH_3}$, or a halide ion like $\mathrm{Br^-}$. An electrophile often has a positive charge, a partial positive charge, or an electron-deficient atom. For example, the carbon in a carbonyl group is electrophilic because the oxygen atom pulls electron density toward itself.

Understanding these terms helps you explain why reactions happen at specific places in a molecule rather than everywhere at once.

How electron-pair sharing appears in reaction mechanisms ⚗️

Many important organic reactions are explained using electron-pair sharing ideas. In these mechanisms, one species donates an electron pair to another species, forming a new bond while an old bond may break.

A common example is the reaction between a nucleophile and an electrophile. Suppose hydroxide ion attacks a carbon atom in an alkyl halide. The carbon attached to the halogen is slightly positive because the halogen is more electronegative. The lone pair on $\mathrm{OH^-}$ can be used to form a new bond to carbon.

A simplified mechanism can be described like this:

1. The nucleophile approaches the electrophilic carbon.
2. A lone pair from the nucleophile is donated to form a new bond.
3. The leaving group departs if a bond is broken at the same time or in a later step.

This kind of explanation is especially useful for substitution and addition reactions.

For example, in the reaction of bromine with ethene, the electrons in the $\pi$ bond of ethene are attracted to the bromine molecule. The $\pi$ bond acts as a nucleophile, and the bromine molecule becomes polarized. This leads to addition across the double bond.

$$\mathrm{C_2H_4 + Br_2 \rightarrow C_2H_4Br_2}$$

The key point is that the reaction occurs because electrons are redistributed in a controlled way.

Why bond polarity matters 🌍

Electron-pair sharing reactions depend strongly on bond polarity. When atoms in a bond have different electronegativities, the shared electrons are pulled more toward one atom. This creates partial charges, written as $\delta^+$ and $\delta^-$. These partial charges help predict where nucleophiles and electrophiles will react.

For example, in a carbonyl group such as $\mathrm{C=O}$, oxygen is more electronegative than carbon. So oxygen carries a partial negative charge, and carbon carries a partial positive charge. That makes the carbonyl carbon a target for nucleophiles.

This is why many reactions of aldehydes and ketones begin with nucleophilic attack at the carbonyl carbon. The electron-pair sharing mechanism explains the reactivity pattern much better than simply memorizing products.

In acid-base chemistry, electron-pair sharing also appears when a base uses a lone pair to bond to a proton. A proton, $\mathrm{H^+}$, has no electrons and is therefore an electrophile. A base such as $\mathrm{NH_3}$ donates a lone pair to form a new bond:

$$\mathrm{NH_3 + H^+ \rightarrow NH_4^+}$$

This is a clear example of electron-pair donation leading to bond formation.

Interpreting curved arrows correctly ✏️

Curved arrows are a major tool in mechanism diagrams, so students, it is important to read them correctly. Each curved arrow represents the movement of a pair of electrons, not the movement of atoms.

Remember these rules:

• the arrow starts where the electrons are,
• the arrow ends where the electrons will be used,
• one arrow represents one electron pair,
• arrows help show bond making and bond breaking step by step.

If a mechanism shows a lone pair on $\mathrm{OH^-}$ attacking a carbon atom, the arrow begins at the lone pair and ends at the carbon. If a bond breaks, an arrow begins at that bond and ends on the atom that receives the electrons.

This notation helps make mechanisms precise and testable. It is not just a drawing style; it is a record of electron movement.

Real-world examples of electron-pair sharing reactions 🧪

Electron-pair sharing reactions are used in many real chemical processes.

1. Making esters

In ester formation, a carboxylic acid reacts with an alcohol. The alcohol oxygen uses a lone pair to form a bond with the carbonyl carbon. After several steps, water is eliminated and an ester is formed. This is a classic example of electron-pair movement in organic synthesis.

2. Addition to alkenes

Alkenes contain a $\pi$ bond, which is electron-rich and can react with electrophiles. This is why alkenes undergo addition reactions easily. The $\pi$ bond is weaker than a $\sigma$ bond and can be broken as new bonds form.

3. Hydrolysis of halogenoalkanes

A hydroxide ion can act as a nucleophile and replace a halogen atom in an alkyl halide. This reaction is useful in the preparation of alcohols. It depends on the electron pair from $\mathrm{OH^-}$ being donated to carbon.

These examples show that electron-pair sharing is not just a theory. It explains a wide range of lab reactions and industrial processes.

How this fits into Reactivity 3: mechanisms of chemical change 🔗

This lesson connects directly to the broader theme of Reactivity 3 because it explains the mechanism behind many types of chemical change. In acid-base reactions, electron pairs are transferred to protons. In organic reactions, electron pairs move between nucleophiles and electrophiles. In both cases, bond making and bond breaking can be explained using electron-pair movement.

It also supports later topics in IB Chemistry HL, such as reaction pathways, reaction conditions, and the interpretation of structural formulas. When you understand electron-pair sharing, you can predict why certain compounds react, why some reactions need catalysts, and why products form in specific ways.

Mechanistic explanations are powerful because they connect structure to reactivity. If you know where electron density is high, you can predict where attack is likely. If you know where electron density is low, you can predict where a nucleophile may react.

Conclusion: A small idea with big power ✅

Electron-pair sharing reactions describe how atoms and molecules form new bonds by donating and accepting electron pairs. This concept explains covalent bonding, nucleophilic attack, electrophilic behavior, and many common organic reactions. It is one of the most important ideas in understanding how chemical change happens at the molecular level.

For IB Chemistry HL, students, the most useful habit is to look for electron-rich and electron-poor sites in a structure. That habit helps you predict mechanisms, write correct curved-arrow diagrams, and explain why reactions occur the way they do.

Study Notes

• Electron-pair sharing forms covalent bonds.
• A nucleophile donates an electron pair.
• An electrophile accepts an electron pair.
• Curved arrows show the movement of electron pairs.
• Bond polarity creates partial charges that guide reactivity.
• The $\pi$ bond in alkenes is often a reaction site because it is electron-rich.
• Carbonyl carbon atoms are often electrophilic because oxygen pulls electron density away.
• Acid-base reactions can also be explained using electron-pair donation to $\mathrm{H^+}$.
• Mechanisms explain how reactions happen step by step, not just the final products.
• Understanding electron-pair sharing helps connect organic chemistry, acid-base chemistry, and reaction pathways within Reactivity 3.