Alkenes Starter Questions

Alkenes Starter Questions

• Name the compound CH₃CH=CH₃CHBrCH₃

• Draw the structural isomer 2-chlorobut-2-ene

• Draw a functional group isomer of but-1-ene

• Draw the product of ethene reacting with bromine (Br₂)

Topic Objectives

• Alkenes are hydrocarbons containing a carbon-carbon double bond (C=C)
• The double bond is an area of high electron density, causing high reactivity
• The double bond forms from the overlap of p orbitals, creating a pi (π) bond

Structure and Bonding

• Alkenes have the general formula CₙH₂ₙ
• The C=C bond is planar with bond angles of 120°
• There is restricted rotation around the C=C bond

Recap: Naming and Drawing Alkenes

• Example: But-2-ene (CH₃CH=CHCH₃)
• Example: 2-methylpent-2-ene (CH₃C(CH₃)=CHCH₂CH₃)

Recap: Isomerism Questions

• Structural isomerism involves compounds with the same molecular formula but different structural formulas
• Chain, position, and functional group isomers can be drawn

Isomerism

• There are two types of isomers: structural and stereoisomers
• Structural isomers have the same molecular formula but different structural arrangements
  • Chain isomerism: differing carbon chain arrangement
  • Position isomerism: differing position of a functional group
  • Functional group isomerism: differing functional groups
• Stereoisomers have the same structural formula but different spatial arrangements
  • Geometric isomerism (E/Z): differing arrangement around a double bond
  • Optical isomerism: differing arrangement around a chiral carbon

Geometrical (E/Z) Isomerism

• E/Z isomers arise from restricted rotation around a C=C double bond
• The Cahn-Ingold-Prelog priority rules determine E/Z configurations
  • Higher atomic numbers have higher priority
  • If highest priority groups are on the same side, Z; on opposite sides, E

Quick Check - E/Z or Neither?

• Practice problems are presented to discern E/Z configurations from structures

Naming Isomers

• Priority rules for naming E/Z isomers are given using atomic numbers

Knowledge Check

• Two conditions required for geometric isomerism:
  • Restricted rotation around C=C
  • Two different atoms/groups attached to C of the double bond

Cahn-Ingold-Prelog (CIP) Priority Rules

• Used for determining the configuration of E/Z isomers when there are more than two different groups

Worked Example - Is the Molecule E or Z?

• Examples shown to practice applying the CIP rules to classify E/Z isomers

Quick Check - on Whiteboards

• Additional structure analysis problems for applying E/Z classifications

Alkene Table

• A table for drawing both displayed and skeletal formulas for identifying E/Z isomers of but-2-ene and 1-chloroprop-1-ene

Exercises – Geometric Isomerism

• Example problems for drawing and classifying additional E/Z isomers

Name the Molecule

• Problems given to practice using the CIP rules to name alkenes

High Challenge, Low Stress

• More complex problems using CIP rules to name alkenes

Link – 3.3.3 Halogenoalkanes

• Mechanism: Elimination—reactions drawing the skeletal formulas and naming the three alkene isomers produced from the reaction of 2-bromohexane with NaOH

High Challenge, Low Stress

• Problem for drawing a possible halogen alkane to form Z-1-chlorobut-1-ene, drawing other possible products

Reactions of Alkenes—Electrophilic Addition

• Electrophilic addition mechanism for reaction of alkenes
• Alkenes are very reactive compared to alkanes due to the high electron density in the C=C π bond
• Electrophiles are electron pair acceptors that are attracted to the electron-rich C=C region
• Electrons move, causing the breaking of the C=C bond

With Hydrogen Halide

• Reactions between alkenes and hydrogen halides (HBr, HCl, or HI) produce bromoalkanes, chloroalkanes, or iodoalkanes, in either the gas phase or concentrated aqueous solutions

Electrophilic Addition Mechanism

• Detailed mechanism steps explaining the reaction of alkenes with an electrophile

With Bromine Water

• Alkenes react with bromine water; the red-brown color is decolorized, showing the presence of a C=C double bond (an unsaturated compound)
• Bromine acts as an electrophile

With Sulfuric Acid

• Alkenes react with sulfuric acid (H₂SO₄) to form alkyl hydrogen sulfates via electrophilic addition

Hydrolysis of Alkyl Hydrogen Sulfates

• Addition of ethyl hydrogensulfate with water causes hydrolysis, forming ethanol and H₂SO₄

With Steam

• Alkenes react with steam (H₂O) in the presence of a catalyst like concentrated sulfuric acid (H₂SO₄) to produce alcohols (direct hydration)

Electrophilic Addition Summary

• Summary table of reagents, conditions, starting compounds, and products for specific alkene reactions

Questions

• Various questions outlining alkene reactions with different reagents (Br₂, HCl, H₂SO₄, H₂O)

Outline a Mechanism of But-2-ene with

• Mechanism outlines for but-2-ene reactions with Br₂, HCl, H₂SO₄, and H₂O

High Challenge, Low Stress

• Problems outline the mechanism for the reaction of propene and sulfuric acid to yield 2-propyl hydrogensulfate according to proper carbocation placement

A Molecule with Molecular Formula C₄H₈

• Mechanisms outlining the reaction between C₄H₈ and HBr to yield an optical isomer

Identifying Unknown Molecules

• Problems identifying unknown molecules in reactions with different reactants and conditions

Recap Review Pages 1–9

• Review of concepts including electrophile definitions, reaction mechanisms for specific alkene reactants, and the reactions overview

Stretch Q – Outline Mechanisms

• Problems outlining the mechanisms for specific reactions given chemical structures

Lesson Outcomes

• Summary of learning objectives, covering electrophilic addition to alkenes, major and minor products, and properties of addition polymers

Unsymmetrical Alkenes

• Stability analysis of carbocations: primary (1°), secondary (2°), and tertiary (3°)
• The major product forms from the most stable carbocation intermediate, while the minor product originates from the less stable one.

Quick Check

• Quick analysis problems, classifying carbocations and identifying their resulting stability

On Whiteboards - Name the Major Product

• Lab problems identifying and naming the major products in various reactions

Example 2

• Mechanism of addition reaction of an alkene with concentrated sulfuric acid, outlining the formation of the major product and illustrating the stability of a secondary carbocation intermediate

Question

• Mechanisms for the formation of major/minor products from alkene reactions with specialized reagents

Other Examples of Addition Polymerisation

• Illustrative examples depicting polymer formation from different monomers showing different polymer repeating units

Polymer Table

• Table summarising examples of monomers and repeating units for polymers

Question

• Repeat units of a given polymer structure are given

Uses of Polymers

• Table of monomer, repeating unit and typical uses for several common polymers are presented.

PVC (Poly(chloroethene))

• Details on properties, uses (plasticized and unplasticised), and common plasticisers for PVC

Problems With Plastics

• General issues regarding the use of single-use plastics, their sustainability, and impacts are discussed

Additional Topics

• Brief history of plastic
• Plastic use in fabrics