What is an Addition Reaction of Hydrocarbons?

S6-SA4-0299

Grade Level:

Class 10

AI/ML, Physics, Biotechnology, Space Technology, Chemistry, Engineering, Medicine

Definition

What is it?

An addition reaction of hydrocarbons is a chemical reaction where an unsaturated hydrocarbon (like alkenes or alkynes) reacts with another molecule (like hydrogen, halogens, or hydrogen halides) to form a single, saturated product. In simpler terms, multiple molecules combine to form one bigger molecule, usually breaking a double or triple bond.

Simple Example

Quick Example

Imagine you have a two-wheeler (an unsaturated hydrocarbon with a double bond). If you add a sidecar to it (another molecule), you get one combined three-wheeler (a saturated product). The two-wheeler is now 'full' and cannot add anything more, just like how a double bond becomes a single bond and is 'saturated'.

Worked Example

Step-by-Step

Let's see how ethene (an alkene) reacts with hydrogen gas.

STEP 1: Identify the reactants. We have ethene (C2H4), which has a carbon-carbon double bond, and hydrogen gas (H2).
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STEP 2: Understand the condition. This reaction usually needs a catalyst like Nickel (Ni) and heat.
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STEP 3: The double bond in ethene breaks, making two 'empty' spots (valencies) on the carbon atoms.
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STEP 4: The hydrogen molecule (H2) also breaks apart into two individual hydrogen atoms (H).
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STEP 5: Each of the two hydrogen atoms attaches to one of the 'empty' spots on the carbon atoms.
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STEP 6: A new molecule, ethane (C2H6), is formed, which has only carbon-carbon single bonds. This is a saturated hydrocarbon.

ANSWER: C2H4 (ethene) + H2 (hydrogen) --(Ni/Heat)--> C2H6 (ethane)

Why It Matters

Addition reactions are super important in making many things we use daily. In Biotechnology, they're used to create new compounds. Chemical engineers use these reactions to produce plastics like polythene for packaging, and medicines. They help us understand how to convert raw materials into useful products.

Common Mistakes

MISTAKE: Thinking addition reactions only happen with hydrogen | CORRECTION: Addition reactions can happen with other molecules like halogens (e.g., Cl2, Br2) and hydrogen halides (e.g., HCl, HBr) too, not just hydrogen.

MISTAKE: Confusing addition reactions with substitution reactions | CORRECTION: In addition reactions, atoms are ADDED across a double/triple bond to form a single product. In substitution reactions, one atom/group is REPLACED by another, often in saturated compounds.

MISTAKE: Not recognizing that the product is always saturated | CORRECTION: The defining feature of an addition reaction involving unsaturated hydrocarbons is that the double or triple bond is broken, resulting in a single, fully saturated product.

Practice Questions

Try It Yourself

QUESTION: What type of bond is broken in an addition reaction of an alkene? | ANSWER: A carbon-carbon double bond.

QUESTION: When propene (CH3-CH=CH2) reacts with bromine (Br2), what type of reaction is it, and what is the general name of the product formed? | ANSWER: It is an addition reaction. The product formed is a dihaloalkane (specifically, 1,2-dibromopropane).

QUESTION: Ethene reacts with hydrogen gas in the presence of nickel catalyst. Write the balanced chemical equation for this reaction and name the product. | ANSWER: C2H4 + H2 --(Ni)--> C2H6. The product is ethane.

MCQ

Quick Quiz

Which of the following compounds will undergo an addition reaction?

Methane (CH4)

Ethane (C2H6)

Ethene (C2H4)

Propane (C3H8)

The Correct Answer Is:

Ethene (C2H4) is an unsaturated hydrocarbon with a carbon-carbon double bond, which can break to allow addition. Methane, ethane, and propane are saturated hydrocarbons with only single bonds, so they typically undergo substitution reactions.

Real World Connection

In the Real World

You know how some cooking oils are 'vegetable ghee' or 'vanaspati ghee'? This is made using an addition reaction called hydrogenation! Liquid vegetable oils (which have unsaturated fatty acids) are reacted with hydrogen gas in the presence of a catalyst. This converts some of their double bonds into single bonds, making them solid or semi-solid at room temperature, which is why vanaspati ghee is solid.

Key Vocabulary

Key Terms

UNSATURATED HYDROCARBON: A hydrocarbon containing carbon-carbon double or triple bonds. | SATURATED HYDROCARBON: A hydrocarbon containing only carbon-carbon single bonds. | CATALYST: A substance that speeds up a chemical reaction without being consumed itself. | HYDROGENATION: An addition reaction where hydrogen is added across a double or triple bond. | HALOGENATION: An addition reaction where a halogen (like chlorine or bromine) is added across a double or triple bond.

What's Next

What to Learn Next

Great job understanding addition reactions! Next, you should explore 'Substitution Reactions of Hydrocarbons'. This will help you compare and contrast the two major types of reactions hydrocarbons undergo, giving you a complete picture of their chemical behavior.