Alkenes

Section 1 of 23Sigma and pi bonds

1Carbon bonding

• Carbon bonding carbon (1s² 2s² 2p²) has four outer electrons shared in covalent bonds to reach a full outer shell

2Sigma bond (σ)

• Sigma bond (σ) formed by overlap of orbitals between atoms

3Pi bond (π)

• Pi bond (π) formed when two p orbitals overlap sideways
• Each carbon uses three electrons in σ bonds, leaving one spare electron in a p orbital.
• The π bond contains two electrons.
• The p orbitals lie above and below the plane of the carbon atoms to maximise overlap.

4Ethene

• Ethene each carbon forms two σ bonds to hydrogen and one σ bond to the other carbon, and the fourth electron forms the π bond

5Multiple bonds

• Multiple bonds a double bond is one σ and one π bond, a triple bond is one σ and two π bonds
• Bond strength increases single < double < triple because of higher electron density between the carbons.

Section 2 of 23Shape of alkenes

Bonding pairs

• Bonding pairs each carbon of the C=C has three σ bonding pairs in the plane of the molecule
• They repel each other, giving a planar arrangement.
• The bond angles are 120°.

Pi bond

• Pi bond the fourth bonding pair forms the π bond above and below the plane, which together with the C–C σ bond makes the double bond

Section 3 of 23Cis/trans isomerism

Stereoisomerism

• Stereoisomerism alkenes show E/Z or cis/trans isomerism

Restricted rotation

• Restricted rotation free rotation occurs about C–C σ bonds in saturated compounds
• In unsaturated compounds the π bond prevents rotation about the C=C, so the attached groups are fixed in position.

Cis

• Cis two groups on the same side of the double bond
  • (both above or both below)

Trans

• Trans two groups on opposite sides of the double bond

Condition

• Condition two different atoms or groups must be on each carbon of the C=C
• 2-methylpropene has no cis/trans isomers because both methyl groups are on the same carbon.
• But-2-ene and 1-chloroprop-1-ene do have cis/trans isomers.

Limits of cis/trans

• Limits of cis/trans it works for three groups only if two are the same and sit on opposite sides
• 1,2-dichloropropene can be named cis/trans.
• 1-bromo-2-chloropropene, with all groups different, needs E/Z.

Section 4 of 23E/Z isomerism and CIP priority

E/Z isomerism and CIP priority

E/Z system

• E/Z system it covers all cases with 2, 3 or 4 different groups and has superseded cis/trans

CIP priority rules

• CIP priority rules higher atomic number of the first atom attached means higher priority
• If the first atoms are the same, compare the second atoms, ignoring identical ones.

E isomer

(entgegen, opposite) — the highest-priority groups are on opposite sides of the C=C

Z isomer

(zusammen, together) — the highest-priority groups are on the same side

E/Z isomerism and CIP priority

On each carbon of the C=C, compare the two attached groups using atomic numbers.

Mark the higher priority group on each carbon.

If they are on the same side the isomer is Z; if opposite it is E.

Examples

• In 1-bromo-1-propen-2-ol, Br beats H and O beats C;
  • highest priority groups opposite gives E and on the same side gives Z.
• If both attached atoms are carbon, go to the next atoms: CH₂Br beats CH₂CH₃ because Br outranks C, and CH₂OH beats CH₃ because O outranks H.

Old term

• Old term geometric isomerism is no longer used

Section 5 of 23Reactivity of alkenes

Cause

• Cause alkenes are more reactive than alkanes because of the π bond
• The π electrons are concentrated above and below the plane of the σ bond and so are exposed.

Bond enthalpies

• C–C is 347 kJ mol⁻¹ and C=C is 612 kJ mol⁻¹
  • (equal to the σ part of a C=C)
• The π bond is about 612 − 347 = 265 kJ mol⁻¹.
• This is weaker than a C–C single bond, so the π bond breaks more easily and reacts more readily.

Section 6 of 23Hydrogenation

Reaction

• Reaction an alkene reacts with hydrogen to form an alkane
  • (hydrogenation or reduction)
• Conditions are a nickel catalyst, 200 °C and 1000 kPa.

Margarine

• Margarine unsaturated vegetable oils are hydrogenated to make margarine
• Margarine has a higher melting point because of stronger London forces.
• Controlling conditions limits the number of C=C bonds broken, giving partially hydrogenated oils with the desired texture.

Section 7 of 23Halogenation

• Reaction
  • an alkene reacts with a halogen to give a dihaloalkane
  • It is an electrophilic addition, with the halogen as the electrophile.
  • The C=C is broken and a new single bond forms from each carbon.
  • It occurs readily at room temperature and is the basis of the test for unsaturation.

Section 8 of 23Test for unsaturation

Reagent

• Reagent bromine water, an orange or yellow solution of Br₂

Test

• Test shake the unknown compound with bromine water
• An unsaturated compound undergoes addition and the solution decolourises.
• This is the standard test for unsaturation in alkenes.

Section 9 of 23Hydrohalogenation

Reaction

• Reaction alkenes react readily with hydrogen halides such as HCl and HBr to give halogenoalkanes
• It is an electrophilic addition that occurs quickly at room temperature.

Rate

• Rate the order is HI > HBr > HCl
• The weaker the hydrogen–halogen bond, the more easily it reacts.

Section 10 of 23Hydration

Conditions

• Conditions steam at 300 °C, 60 atm and a sulfuric acid or phosphoric acid catalyst

Reaction

• Reaction water adds across the double bond to convert an alkene into an alcohol
• An intermediate forms with H⁺ and HSO₄⁻ added across the C=C, and it is quickly hydrolysed, reforming the acid.

Use

• Use industrial production of large quantities of ethanol, a solvent and fuel
• It is much faster and higher yielding than fermentation.

Section 11 of 23Electrophilic addition

1

Electrophilic addition

Electrophilic addition the addition of an electrophile to a C=C bond

• The C=C is an area of high electron density, so it is attacked by electrophiles.
• The C=C breaks, forming a C–C single bond and two new bonds, one from each carbon.

2

Hydrogen bromide with ethene

• HBr is polar, with H δ+ and Br δ−.
• The H acts as the electrophile and accepts a pair of electrons from the C=C.
• The H–Br bond breaks heterolytically, forming Br⁻ and a reactive carbocation intermediate.
• The Br⁻ bonds to the carbocation, forming bromoethane.

3

Curly arrows

• They must be double headed.
• They must start from a lone pair or an area of high electron density such as the C=C.
• They must move towards a δ+ electrophile or the positive charge of a carbocation.

4

Halogens

• Halogens the mechanism is the same except that a halogen molecule has a temporary induced dipole
• The high electron density of the C=C repels the halogen's electrons, inducing the dipole.
• Hydrogen halides have a permanent dipole.

Section 12 of 23Carbocations and the inductive effect

Carbocation

Carbocation a carbon atom with a positive charge and only three covalent bonds

• It may be primary, secondary or tertiary.
• It is an electrophile because of the positive charge.

Inductive effect

• Inductive effect alkyl groups are electron donating and push electrons towards the positive carbon
• This reduces the positive charge and spreads it, making the carbocation more stable.
• Tertiary carbocations are the most stable because they have three alkyl groups.

Stability order

• Stability order tertiary > secondary > primary

Section 13 of 23Markownikoff addition

Markownikoff addition

Rule for HX

• Rule for HX the halogen ends up bonded to the most substituted carbon

Rule for an interhalogen

• Rule for an interhalogen the more electronegative halogen ends up on the most substituted carbon

Scope

• Scope it applies to unsymmetrical alkenes such as propene and but-1-ene
• Markownikoff addition gives the major product and anti-Markownikoff the minor product.

Mechanism with propene and HBr

• Mechanism with propene and HBr the H can add in two ways
• H on the least substituted carbon gives the more stable secondary carbocation and the major product, 2-bromopropane.
• H on the most substituted carbon gives the less stable primary carbocation and the minor product, 1-bromopropane.
• Br⁻ bonds to the positive carbon of the carbocation.

Deciding the major product

• Deciding the major product it comes from attack on the most stable carbocation

Section 14 of 23Addition polymerisation

Addition polymerisation

Addition polymerisation many monomers containing at least one C=C bond form long chains with a polymer as the only product

• The π bond in each C=C breaks and the monomers link by new C–C single bonds.
• It is the basis of the plastic industry.

Polymer

Polymer a long-chain molecule made of many repeating units

Monomers

Monomers the small, reactive molecules that react to form the polymer

• Poly(ethene) comes from ethene and poly(chloroethene), PVC, from chloroethene.

Repeat unit

Repeat unit the smallest group of atoms that joined in sequence makes up the chain, shown in square brackets

• For a polymer from one alkene monomer the repeat unit is the monomer with the C=C changed to C–C.

Addition polymerisation

Identify the repeat unit, which has only two carbons in the main chain.

Change the C–C bond back to C=C.

Name the resulting monomer.

• Examples
  • Ethenol polymerises to poly(ethenol) with repeat unit CH(OH)–CH₂.
  • Ethene-1,2-diol gives repeat unit CH(OH)–CH(OH).
  • A repeat unit containing a carboxylic acid group comes from prop-2-enoic acid.

Section 15 of 23Waste polymers and recycling

Uses and problems

• Uses and problems polymers are cheap alternatives to metal, glass, paper and cardboard
• Their low reactivity suits uses such as food packaging.
• Many are non-biodegradable, which makes disposal difficult, and waste plastic harms marine animals.

Landfill

• Landfill one disposal method, which is not ideal, so initiatives aim to reduce it

Recycling

• Recycling reduces the polymer waste going to landfill and the use of finite resources
• Many polymers are made from cracking crude oil fractions.
• Recycling is time-consuming because polymers must be sorted by category, shown by recycling symbols and numbers on the product.
• Some landfill sites have recycling points so recyclable materials are removed first.

Section 16 of 23Mechanical recycling

1

Process

• Process after sorting, polymers are chopped, washed, dried, melted and cast into pellets for reuse
• Each polymer type has its own recycling symbol, e.g. for polyethylene terephthalate.

2

Problems

• Mixed polymers make the process wasted because they give an unusable mix.
• Some polymers cause problems because of their composition, e.g. PVC contains a large amount of toxic chlorine that can be released.
• Modern techniques solve the PVC problem by dissolving the polymer and precipitating out the recycled material.

Section 17 of 23Combustion of waste polymers

Use

• Use some petroleum or natural gas derived polymers are difficult to recycle but store a lot of energy, so they can be incinerated
• The heat boils water and the steam turns turbines in a power station, like a coal-fired station.

Drawbacks

• Drawbacks it still causes pollution
• Carbon in the polymer is released as carbon dioxide, contributing to global warming.
• Toxic products form, such as hydrogen chloride from burning PVC.

Section 18 of 23Feedstock recycling

1Process

• Process waste polymers are broken down by chemical and thermal processes into monomers, gases and oils
• These are raw materials for new polymers and other organic chemicals.

2Major benefit

• Major benefit it works with unsorted and unwashed polymers, unlike other disposal methods

Section 19 of 23Bioplastics

Bioplastics

Bioplastics polymers made from plant starch or cellulose, plant oils and plant proteins

Advantage

• Advantage a renewable, sustainable alternative to current polymers, which are mostly based on finite resources such as crude oil

Section 20 of 23Biodegradable polymers

1Biodegradable polymer

Biodegradable polymer a polymer that microorganisms break down over time

• Common products are carbon dioxide, water and other organic compounds.

2Polyesters and polyamides

• Polyesters and polyamides condensation polymers considered biodegradable because hydrolysis breaks them down
• This is a major advantage over polyalkenes made from alkene monomers.
• In landfill they break down easily and their products can be used elsewhere.

Section 21 of 23Compostable and photodegradable polymers

Compostable polymers

• Compostable polymers commonly plant based and degrade naturally leaving no harmful residues
• Plant starch is used for biodegradable bin liners.
• Sugar cane fibres replace polystyrene in disposable plates and cups.

Photodegradable polymers

• Photodegradable polymers contain bonds weakened by absorbing light, which starts the breakdown
• Many are oil based.
• An additive that absorbs light may be mixed in to promote degradation.

Section 22 of 23Key terms

Term	Meaning
Pi bond	A bond formed by sideways overlap of p orbitals, above and below the plane of the nuclei
Stereoisomers	Compounds with the same structural formula but different arrangement of atoms in space
CIP priority rules	Rules ranking groups by atomic number of the attached atoms
E isomer	The highest priority groups are on opposite sides of the C=C
Z isomer	The highest priority groups are on the same side of the C=C
Hydrogenation	Addition of hydrogen across a C=C bond

Term	Meaning
Electrophile	A species that accepts a pair of electrons
Carbocation	A positively charged carbon atom with three bonds
Inductive effect	Electron donation by alkyl groups that stabilises a carbocation
Markownikoff rule	The halogen adds to the most substituted carbon of the alkene
Monomer	A small molecule that joins with others to form a polymer
Repeat unit	The smallest group of atoms repeated along a polymer chain

Term	Meaning
Feedstock recycling	Breaking waste polymers into monomers, gases and oils for new products
Bioplastic	A polymer made from plant starch, cellulose, oils or proteins
Biodegradable polymer	A polymer broken down by microorganisms
Photodegradable polymer	A polymer with bonds weakened by absorbing light
Compostable polymer	A plant-based polymer that degrades naturally leaving no harmful residues

Section 23 of 23Exam technique

1

Saying the π bond is stronger than a C–C σ bond — it is weaker, which makes alkenes reactive

2

Using cis/trans for compounds with three or four different groups — use E/Z

3

Assigning E/Z by position of the alkyl chain rather than CIP priority

4

Saying free rotation occurs about C=C — the π bond restricts it

1

Starting a curly arrow at the electrophile instead of the C=C bond

2

Saying the major product comes from the primary carbocation — the more stable secondary or tertiary carbocation gives the major product

3

Giving the monomer as the repeat unit — the repeat unit has C–C and the monomer has C=C

4

Saying polymerisation produces a second product — addition polymerisation has only one product

1

Saying all polymers can be recycled together — mixed polymers give an unusable mix

2

Saying incineration has no environmental cost — it releases CO₂ and toxic products such as HCl from PVC

3

Describing polyalkenes as biodegradable — polyesters and polyamides are, because they can be hydrolysed

4

Thinking feedstock recycling needs sorted polymers — it works with unsorted, unwashed polymers