Chemistry · Grade 12 · Organic Chemistry · Term 3

Reactions of Hydrocarbons

Examine the characteristic chemical reactions of hydrocarbons, including substitution reactions of alkanes, addition reactions of alkenes and alkynes, and elimination reactions.

TL;DR:Move beyond simply naming and drawing molecules to explore how they actually behave and transform. This topic uncovers the fundamental rules that govern how hydrocarbons react, forming the basis of organic synthesis.

• Ontario Curriculum Expectations
• Ontario Curriculum: SCH4U, Strand B - Organic Chemistry, B2: Investigate the molecular structures and the properties of organic compounds, and carry out organic reactions.

About This Topic

This topic delves into the characteristic chemical reactivity of hydrocarbons, a cornerstone of organic chemistry within the Grade 12 Canadian curriculum. Building on students' prior knowledge of organic nomenclature and structure, this unit transitions from static molecules to dynamic transformations. The exploration is typically framed around three core reaction types: substitution, addition, and elimination. For alkanes, their relative inertness and saturated nature are highlighted, leading to an examination of free-radical substitution, often initiated by UV light. This provides a foundational understanding of reaction mechanisms involving initiation, propagation, and termination steps.

The focus then shifts to the more reactive unsaturated hydrocarbons. The presence of the electron-rich pi bond in alkenes and alkynes makes them susceptible to electrophilic addition reactions. Students will investigate how molecules like hydrogen halides, halogens, and water add across the double or triple bond. A key concept introduced here is Markovnikov's rule, which governs the regioselectivity of additions to unsymmetrical alkenes. Understanding the stability of carbocation intermediates is crucial for explaining this rule, moving students beyond rote memorization to a deeper mechanistic understanding. Finally, elimination reactions are presented as a method for creating unsaturated compounds from saturated ones, effectively acting as the reverse of addition reactions. This topic is vital for understanding organic synthesis and has direct links to major Canadian industries like petrochemicals and polymer manufacturing.

Key Questions

1. Compare the mechanism of a free-radical substitution reaction with an electrophilic addition reaction.
2. Explain Markovnikov's rule for the addition of hydrogen halides to asymmetrical alkenes.
3. Identify the products formed from the complete and incomplete combustion of a hydrocarbon.

Learning Objectives

• Differentiate between substitution, addition, and elimination reactions by identifying reactants and products.
• Predict the major product(s) of the addition of hydrogen halides, halogens, and water to alkenes and alkynes, applying Markovnikov's rule where applicable.
• Outline the free-radical mechanism for the substitution reaction of an alkane with a halogen.
• Compare the relative reactivity of alkanes, alkenes, and alkynes based on their bonding.
• Draw the structural formulas for reactants and products in common hydrocarbon reactions.

Key Vocabulary

Substitution Reaction	A chemical reaction during which one functional group in a chemical compound is replaced by another functional group.
Addition Reaction	An organic reaction where two or more molecules combine to form a larger one, breaking a multiple bond in the process.
Elimination Reaction	A type of organic reaction in which two substituents are removed from a molecule, leading to the formation of a multiple bond.
Markovnikov's Rule	A principle stating that with the addition of a protic acid to an unsymmetrical alkene, the acid hydrogen attaches to the carbon that already holds the greater number of hydrogen atoms.
Carbocation	An ion with a positively-charged carbon atom. The stability of carbocations is key to predicting reaction outcomes.

Watch Out for These Misconceptions

Common Misconception

Alkanes undergo addition reactions just like alkenes.

What to Teach Instead

Alkanes are saturated, meaning they only have single bonds and cannot 'add' more atoms without first breaking a strong C-C or C-H bond. They undergo substitution, where one atom is swapped for another.

Common Misconception

Markovnikov's rule is just a random rule to memorize.

What to Teach Instead

Markovnikov's rule is based on the stability of the carbocation intermediate formed during the reaction. The reaction proceeds through the most stable intermediate (tertiary > secondary > primary), which leads to the formation of the major product.

Common Misconception

In an addition reaction, the double bond simply disappears.

What to Teach Instead

The double bond consists of one sigma bond and one pi bond. The weaker, more accessible pi bond breaks to form two new, stronger sigma bonds with the atoms being added. The original sigma bond between the carbons remains intact.

Active Learning Ideas

Collaborative Problem-Solving

Modelling Reaction Mechanisms

Students use molecular model kits to physically break and form bonds for a substitution and an addition reaction. This hands-on approach helps them visualize the movement of atoms and the structural changes that occur.

30 min·Pairs

Collaborative Problem-Solving

Markovnikov's Rule Challenge

Provide students with worksheets containing various unsymmetrical alkenes and reagents (e.g., HBr, H2O). In small groups, they predict the major and minor products, drawing the carbocation intermediates to justify their answers.

25 min·Small Groups

Collaborative Problem-Solving

Reaction Type Card Sort

Students are given a set of cards, each with a different hydrocarbon reaction. They must sort these cards into three categories: Substitution, Addition, and Elimination, and then justify their reasoning to their group.

20 min·Small Groups

Real-World Connections

• The production of polymers like polyethylene (plastic bags, bottles) and PVC (pipes, flooring) from alkene monomers through addition polymerization.
• The manufacturing of margarine by the hydrogenation of unsaturated vegetable oils, which is an addition reaction that adds hydrogen across double bonds.
• The process of 'cracking' in petroleum refineries, which uses elimination reactions to break down large, less useful hydrocarbon chains into smaller, more valuable ones like ethene and propene.
• The synthesis of halogenated alkanes, such as chloroform (a solvent) and halothane (an anaesthetic), via free-radical substitution reactions.
• The production of ethanol for fuel and beverages through the hydration (addition of water) of ethene, a key industrial process.

Assessment Ideas

Quick Check

Use 'Predict the Product' exit tickets. Provide students with the starting hydrocarbon and a reagent, and ask them to draw and name the major organic product.

Quick Check

A unit test section containing problems where students must identify the reaction type, predict products for a variety of reactions (including applying Markovnikov's rule), and draw a simple reaction mechanism.

Quick Check

Provide a checklist of the learning objectives. Students rate their confidence level (e.g., from 1 to 5) for each objective, such as 'I can explain why a tertiary carbocation is the most stable'.

Frequently Asked Questions

Why are alkenes and alkynes more reactive than alkanes?

The presence of pi bonds in alkenes and alkynes makes them more reactive. The electrons in pi bonds are more exposed and less tightly held than electrons in sigma bonds, making them an easy target for electron-seeking species called electrophiles.

What's the difference between a major and minor product?

In many organic reactions, more than one product is possible. The major product is the one that is formed in the highest quantity. This is usually because it is formed via a more stable intermediate or a lower energy pathway, as seen with Markovnikov's rule.

Can an elimination reaction be considered the opposite of an addition reaction?

Yes, in many ways. An addition reaction breaks a pi bond to form two new sigma bonds, adding atoms to a molecule. An elimination reaction does the reverse: it removes atoms from adjacent carbons and forms a new pi bond between them.