electrophilic-addition-reaction-diagram

Think about how vegetable oil turns into margarine or how simple plastic products are made.

These everyday transformations happen through addition reactions. In these reactions, molecules combine to form larger ones. From the hydrogenation of oils to the creation of polymers like plastic, addition reactions are all around us.

Let’s explore the addition types, like electrophilic addition, nucleophilic addition, and hydrogenation.

Addition: Quick Summary

Do you just need the basics? Here’s a simple explanation of addition:

🟠 Addition reactions occur when two molecules combine, usually breaking a double or triple bond to form a larger molecule.

🟠 Electrophilic addition happens when an electron-poor molecule attacks an electron-rich area like a double bond.

🟠 Nucleophilic addition involves an electron-rich molecule attacking a carbonyl group, often seen in aldehydes and ketones.

🟠 Hydrogenation adds hydrogen to unsaturated compounds, turning them into saturated ones, like in the production of margarine.

🟠 Addition polymerization involves small monomers joining to form long polymer chains, such as in the creation of polyethylene.

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What Are Addition Reactions?

Addition reactions occur when two or more molecules combine to form a larger molecule. These reactions are common in compounds that have double or triple bonds, such as alkenes and alkynes. In an addition reaction, the multiple bond breaks, and new atoms bond to the carbons that were part of the original bond.

Types of Addition Reactions

1. Electrophilic Addition

  • An electron-poor molecule (electrophile) reacts with an electron-rich double or triple bond.
  • Example: Hydrogen bromide ($HBr$) adds to an alkene, breaking the double bond to form a new compound.

2. Nucleophilic Addition

  • A nucleophile (electron-rich molecule) attacks an electron-deficient site, often a carbonyl group in aldehydes or ketones.
  • Example: Hydride ions ($H^-$) add to aldehydes, reducing them to alcohols.

Electrophilic Addition: How It Works

In an electrophilic addition reaction, an electron-poor molecule, called an electrophile, targets an electron-rich area, such as a double bond in an alkene. Alkenes are good targets for these reactions because their double bonds contain pi electrons, which are more exposed than the sigma electrons in single bonds. During the reaction, the double bond breaks, and new atoms bond to the carbons that were part of the original bond. This process converts an unsaturated molecule, like an alkene, into a saturated product.

A common example is the addition of hydrogen halides (HBr, HCl, HI) to alkenes. For instance, when ethene reacts with HBr, it forms bromoethane.

The mechanism occurs in two main steps: electrophilic attack and nucleophilic attack.

Step 1: Electrophilic Attack

In the first step of the reaction, the pi electrons from the double bond in the alkene react with the hydrogen atom. The double bond breaks as one of the carbon atoms bonds to the hydrogen. This leaves the second carbon with a positive charge, forming a carbocation, a highly reactive intermediate because it lacks electrons.

For example, when ethene ($C_2H_4$) reacts with HBr, the pi electrons from the double bond are attracted to the hydrogen ($H^+$) from HBr. The bond between the two carbons breaks, forming a new bond between the hydrogen and one carbon atom.

The second carbon, now short of electrons, becomes a carbocation intermediate ($C_2H_5^+$). Carbocations are important in many organic reactions because they act as powerful electrophiles in the next step.

Step 2: Nucleophilic Attack

Once the carbocation is formed, it becomes the target for a nucleophile, an electron-rich species. In this case, the nucleophile is the bromide ion ($Br^-$) left over from the hydrogen halide. The bromide ion attacks the positively charged carbon in the carbocation, creating a new sigma bond between the carbon and bromine.

For instance, after the carbocation forms in the reaction between ethene and HBr, the bromide ion ($Br^-$) is attracted to the positively charged carbon. It donates its pair of electrons to form a sigma bond with the carbon, resulting in the final product, bromoethane ($C_2H_5Br$). The addition of HBr to ethene is now complete, with the alkene fully saturated and the halogen added.

Nucleophilic Addition: A Closer Look

In a nucleophilic addition reaction, a nucleophile (an electron-rich species) targets an electron-deficient site in a molecule. This reaction typically occurs with carbonyl groups ($C=O$) found in compounds like aldehydes and ketones.

The carbon in the carbonyl is electron-poor, making it a natural target for a nucleophile, which donates electrons to form a new bond.

Unlike electrophilic addition, which involves breaking a double bond in alkenes, nucleophilic addition focuses on adding electrons to the carbonyl carbon, altering the compound’s structure.

Electrophilic addition targets pi electrons in double bonds, while nucleophilic addition reacts with the electron-poor carbonyl carbon. The result is the transformation of the carbonyl into another functional group, often an alcohol or a cyanohydrin.

Let’s explore two key examples: hydride addition and the formation of cyanohydrins.

Example: Addition of Hydrides

In nucleophilic addition, hydrides ($H^-$), negatively charged hydrogen ions, add to carbonyl groups in aldehydes or ketones, reducing them to alcohols. This reaction is commonly achieved using reducing agents like sodium borohydride (NaBH₄) or lithium aluminum hydride (LiAlH₄).

For example, when NaBH₄ is added to an aldehyde, the hydride ion ($H^-$) attacks the carbonyl carbon, breaking the $C=O$ bond. This results in a primary alcohol if the starting material is an aldehyde. In the case of ketones, the product is a secondary alcohol.

This reaction is widely used to reduce carbonyl compounds into alcohols, an essential process in organic synthesis.

Example: Cyanohydrin Formation

Another example of nucleophilic addition is the formation of cyanohydrins. In this reaction, the cyanide ion ($CN^-$) acts as a nucleophile, attacking the carbonyl group in aldehydes or ketones. The cyanide adds to the carbonyl carbon, breaking the double bond between carbon and oxygen and forming a new bond with the carbon.

For instance, when hydrogen cyanide ($HCN$) reacts with an aldehyde, the cyanide ion ($CN^-$) adds to the carbonyl carbon, creating a cyanohydrin. Cyanohydrins contain a hydroxyl group ($-OH$) and a cyanide group ($-CN$), making them important intermediates in many organic reactions.

This step is valuable in organic synthesis because cyanohydrins can be converted into other useful compounds.

Hydrogenation: Adding Hydrogen to Compounds

Hydrogenation is adding hydrogen atoms to unsaturated compounds, like alkenes or alkynes, turning them into saturated compounds. In this reaction, the double or triple bonds between carbon atoms break, allowing the hydrogen to bond with those carbons, creating single bonds and resulting in a more stable compound.

Hydrogenation is important in both industrial applications and organic chemistry because it alters the properties of materials.

One of the major industrial uses of hydrogenation is in food production, where unsaturated fats (like oils) are converted into saturated fats, which are more stable and solid at room temperature.

This process is widely used to produce items like margarine and vegetable shortening. Hydrogenation also plays a big role in chemical manufacturing, producing various materials, such as fuels and plastics.

To speed up hydrogenation, a catalyst (often a metal like palladium or platinum) is typically used. This allows the reaction to proceed more efficiently, often at lower temperatures and pressures.

Hydrogenation of Fats

Hydrogenation converts unsaturated fats, like vegetable oils, into solid saturated fats, commonly used in margarine and shortening. This transformation occurs because the double bonds in unsaturated fats break during hydrogenation, allowing hydrogen atoms to bond with the carbon atoms.

A typical application is the production of margarine and shortening. Hydrogenation increases the shelf life of these products by making them less prone to oxidation and spoilage. However, there is a downside: partial hydrogenation can create trans fats, which have been linked to negative health effects.

For this reason, food producers are reducing or eliminating trans fats in favor of fully hydrogenated oils or alternatives that don’t produce harmful byproducts.

Catalytic Hydrogenation

Catalytic hydrogenation uses metals like palladium or platinum to speed up hydrogenation by lowering the energy needed for hydrogen and the unsaturated compound to react.

In a typical reaction, like the hydrogenation of alkenes, the double bonds between carbon atoms break, and hydrogen atoms attach to those carbons. The catalyst enables the reaction under lower temperatures and pressures than required.

This is especially useful in industrial processes where speed and control are important. For example, hydrogenation is used in the petrochemical industry to convert alkenes into saturated hydrocarbons, which can then be used to make fuels or plastics.

Hydrate Formation: A Special Kind of Addition

Hydrate formation is a type of addition reaction in which water adds to a compound, forming a hydrate. Hydrates contain water molecules chemically bonded to the compound.

This reaction is common with aldehydes and ketones, where water adds to the carbonyl group ($C=O$), turning it into a geminal diol—a molecule with two hydroxyl groups ($-OH$) attached to the same carbon atom.

Unlike other addition reactions that add atoms, hydrate formation incorporates the entire water molecule ($H_2O$). In aldehydes, this process tends to be stable, but in ketones, hydrates are less stable and may revert back. For instance, when formaldehyde ($CH_2O$) reacts with water, it forms methanediol ($CH_2(OH)_2$), a geminal diol often used in further organic reactions.

Addition Polymerization: Building Larger Molecules

In addition polymerization, monomers with double bonds link to form long polymer chains, like polyethylene, without producing by-products.

A common example is polyethylene production. Here, ethylene ($C_2H_4$) molecules add together repeatedly to form polyethylene ($C_2H_4$)_n, used in products like plastic bags and bottles. This process is essential in creating many everyday plastics, making it widely used in industrial manufacturing.

Advance Your Knowledge in Addition Reaction

Addition reactions are a fascinating part of chemistry that helps us transform simple molecules into more complex ones. Whether forming hydrates or producing everyday plastics through polymerization, these reactions show how small changes can lead to big results.

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Addition Reactions: Frequently Asked Questions

1. What is an addition reaction?

An addition reaction happens when two molecules join together, breaking a double or triple bond in one of them.

2. Differences between electrophilic and nucleophilic addition?

In electrophilic addition, a molecule lacking electrons attacks a double bond, while in nucleophilic addition, a molecule with extra electrons attacks a carbonyl group.

3. What happens in hydrogenation?

In hydrogenation, hydrogen atoms are added to unsaturated compounds, such as alkenes, turning them into saturated compounds, such as alkanes.

4. What is catalytic hydrogenation?

Catalytic hydrogenation uses a metal catalyst (like palladium) to help speed up the addition of hydrogen to a double bond.

5. What is hydrate formation?

Hydrate formation occurs when water is added to a carbonyl compound, creating a geminal diol (two hydroxyl groups attached to the same carbon).

6. Can you give an example of nucleophilic addition?

A common example is when a hydride ion ($ H-$) adds to an aldehyde or ketone, turning it into alcohol.

7. What is addition polymerization?

In addition polymerization, small molecules called monomers join together to form long polymer chains, like those produced in the production of plastic.

8. What are geminal diols?

Geminal diols are compounds where two hydroxyl groups ($-OH$) attach to the same carbon atom, usually after adding water to an aldehyde or ketone.

References:

1. LibreTexts Chemistry
2. Britannica
3. Wikipedia

nucleophilic-addition-reaction-example
Nucleophilic addition involves an electron-rich molecule attacking a carbonyl group, often creating an alcohol from a ketone or aldehyde.