Unsaturated hydrocarbons (at least 1 C=C bond)

Special case of E/Z isomerism Same requirements as E/Z isomerism but one of the attached groups on each C atom of the C=Cmust be the same

Chapter 13 Alkenes Flashcards by Ankita Somisetty

Alkenes

Unsaturated hydrocarbons (at least 1 C=C bond)

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General formula of alkenes

CnH2n

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The nature of the C=C bond

C has 4 electrons and 3 are used in sigma bonds (1 to the other C and the other 2 for other atoms)
The remaining electron is a pi bond

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pi bond formation

Formed by the sideways overlap of 2 electrons in the p-orbitals (one from each C of the C=C bond)

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Where is the pi-electron density concentrated

above and below the line joining the nuclei of the bonding atoms

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What does the pi bond do to the C atoms

Locks the C atoms in positions and prevents them from rotating around the C=C bond -> geometry of alkenes is different to that of alkanes

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Alkenes in the natural world

Carotene = makes flamingos pink
Limonene = causes scent of oranges and lemon

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Stereoisomers

Have the same structural formula but a different arrangement of the atoms in space

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Types of stereoisomerism

E/Z isomerism
Optical isomerism

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Why does stereoisomerism occur?

The rotation around the C=C bond is restricted so the groups attached to those Cs are therefore fixed relative to eachother

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When will a molecule have E/Z isomerism

When the molecule has:
* A C=C bond (restricted rotation)
* 2 different groups attached to each C atom of C=C bond

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Cis-trans isomerism

Special case of E/Z isomerism
Same requirements as E/Z isomerism but one of the attached groups on each C atom of the C=Cmust be the same

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Relationship between cis/trans isomerism and E/Z isomerism

When there is a H on each C=C bond
* Cis isomer = Z isomer
* Trans isomer = E isomer

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Cahn-Ingold-Prelog Rules

Z isomer: If the groups of highest priority are on the same side of the C=C bond
E isomer: If the groups of highest priority are diagonally places across the C=C bond

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How to assign priority

As atomic number increases, priority increases
If it is a large atom, find the first point of difference and use this to compare priority

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Why are alkenes much more reactive than alkanes

pi bond -> the electrons in the pi bond are more exposed than the electrons in the sigma bond. The pi bond readily breaks and alkenes undergo addition reactions relatively easily

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Addition reactions of the alkenes

hydrogen in the presence of a nickel catalyst
halogens
hydrogen halides
steam in the presence of an acid catalyst

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Hydrogenation of alkenes

Alkene + hydrogen -> alkane

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What is the product of an addition reaction?

Always saturated

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Halogenation of alkenes

Alkene + bromine -> dihaloalkane

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Test for unsaturation (C=C bond)

Add bromine water
orange -> colourless

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Hydrogen halides

Hydrogen + halogen
e.g. HCl

Alkenes + hydrogen halides

haloalkane

Hydration of alkenes

Alkene + Water -> alcohol

Electrophilic addition

Reaction mechanism for addition reactions of alkenes

Electrophile

* Electron pair acceptor (attracted to an electron-rich centre) * Usually a positive ion, or a molecule containing an atom with a δ+ charge

Reaction between but-2-ene + hydrogen bromide

1. Bromine is more electronegative than hydrogen, so hydrogen bromide is polar (contains dipole Hδ+ - Brδ-) 2. The electron pair in the pi bond is attracted to the Hδ+ -> causes the double bond to break 3. A bond forms between the H of the H-Br and a C that was part of the C=C bond 4. The H-Br bond breaks by heterolytic fission, with the electron pair going to the bromine atom 5. A Br- ion and carbocation are formed. These react together to form the addition product

How does polarisation of bromine occur (bromine is a non-polar molecule)

The pi electrons interact with the electrons in the Br-Br bond -> causes Brδ+ -Brδ-

Markownikoff's rule

When a hydrogen halide reacts with an unsymmetrical alkene, the hydrogen from the hydrogen halide attaches to the carbon with more hydrogen and less carbon

Steps of electrophilic addition

Sometimes, 3 different carbocations can be produced: * Primary carbocation (has 1 alkyl group) * Secondary carbocation (has 2 alkyl groups) * Tertiary carbocation (has 3 alkyl groups)

Alkyl group

Polymers

Extremely large molecules from many repeat units (monomers)

What polymerisation do alkenes undergo?

Addition polymerisation -> produces long chains of saturated chains with no C=C bonds

Characteristics of addition polymers

* High molecular masses * Have prefix poly-

Examples of synthetic addition polymers

* Poly(ethene) * Poly(chloroethene) * Poly(propene) * Poly(styrene) * Poly(tetrafluoroethene)

Environmental concerns of polymers

* Disposing of waste polymers * Recycling * PVC recycling * Using waste polymers as fuel * Feedstock recycling

Biodegradable and photodegradable polymers

* Biodegradable polymers * Photodegradable polymers

Environmental concerns of polymers: Recycling

Reduces environmental impact by conserving finite fossil fuels and decreasing amount of waste going to landfill

Environmental concerns of polymers: PVC recycling

Disposal and recycling of PVC is hazardous due to high chlorine and additive content * New technology uses solvents to dissolve the polymer

Environmental concerns of polymers: Using waste polymers as fuel

As polymers are derived from natural gas, they have a high stored energy value -> so can be incinerated to produce heat -> electricity

Environmental concerns of polymers: Feedstock recycling

Describes the chemical + thermal processes that can reclaim monomers, gases or oil from waste polymers

Advantage of feedstock recycling

Can handle unsorted and unwashed polymers

Bioplastics

* Produced from plant starch, cellulose, plant oils and proteins * Offer a renewable + sustainable alternative to oil-based product, and protects our environment and conserves our valuable oil reserves

Biodegradable polymers

* Broken down by microorganisms into water, carbon dioxide and biological compounds

What are compostable polymers

Based on poly(lactic acid)

Photodegradable polymers

Contain bonds that are weakened by absorbing light to start the degradation

Catalyst for hydration of alkenes

Phosphoric acid

Differences between sigma bonds and pi bonds

* pi bond is weaker than sigma bonds * pi bond is above and below the bonding atoms whereas sigma bond is between bonding atoms

When there is a possibility for different haloalkanes to be produced in a reaction, which one will be produced in greater quantity?

The tertiary haloalkane because it is more stable than the secondary and primary haloalkane

Advantages of combusting polymers

Produces electricity

Disadvantages of combusting polymers

Produces Hcl or chlorine (however this can be neutralised using an alkali)

Bonding angle around addition polymer

Tetrahedral 109.5