AQA Chemistry Unit 2: 15 Alkenes

Alkenes are unsaturated hydrocarbons

• Made only of Carbon and Hydrogen
• They have one or more Carbon Double Bonds
• The double bond makes the molecule more reactive then alkanes
• The general formula: CnH2n

Shape of Alkenes

• Ethene is a planar molecule so its angles between each bond is 120 degree
• There is no rotation around the double bond
• There is no rotation due to the pi bond formed from the cloud of electron density above and below the single bond

Isomers

• Alkenes with more than 3 carbons can form different isomers
• Two types of isomers can be formed involving the double bond:
  • Position Isomers
  • Geometrical Isomers
• Position isomers are where the double bond is positioned differently
• Geometrical isomers are a form of stereoisomerism, they stereoisomers have the same structural formula but different bonds are arranged differently, it occurs around the double bond
• When two types of the same group e.g. -CH3 are on the same side of a double bond this is called Z and when the groups are on opposite sides this called E
  • When atoms of higher atomic number are on the same side this is Z
  • When atoms of higher atomic number are opposite sides this is E

Physical Properties

• Van der Waals are the only intermolecular forces acting between alkene molecules
• Physical properties are similar to alkanes
• The melting and boiling point increase with the number of carbon atom
• Alkenes are not soluble in water as they are non-polar

How Alkenes React

• The bond enthaply for a C=C bond is almost double of a C-C bond
• Alkenes are more reactive than alkanes
• C=C forms an electron rich area in a molecule which can be easily attacked by a positively charged reagent called an electrophile
• Electrophiles are electron pair acceptors such as H+
• The reaction is known as electrophilic addition

• Alkenes can burn in air to form CO2 and H2O

Electrophilic Addition Reaction

• The four electrons in the C=C bond make an alkene a centre of high electron density
• Electrophiles are attracted to it and form a bond by using 2 of its 4 electrons in the C=C bond
• The Mechanism:
  • Electrophile is attracted to the double bond
  • Electrophiles are positively charged and accept a pair of electrons from the double bond
  • A positive ion (carbocation) is formed
  • A negatively charged ion forms a bond with the carbocation

Reaction with Hydrogen Halides

• Form a haloalkane
• In a hydrogen halide the halogen is more electronegative
• The electrophile is the H delta plus
• The H is attracted to the C=C due to its high electron density
• One of the Carbons bonds with the hydrogen to form a carbocation
• The electrons in the H-Halogen bond are drawn towards the halogen
• The bond in the H-Halogen breaks heterolytically so both electrons go to the halogen since it is more elctronegative
• The Halogen ion then attaches to the positively charged carbon of the carbocation forming a bond with one of its electron pairs

Asymmetrical Alkenes

• When a double bond isn't exactly in the middle of the chain there are more than one possible product
• Some alkyl groups have a tendency to release electrons this is positive inductive effect
• The electron releasing effect tends to stabilise the positive charge of the carbocation
• The more alkyl groups the more stable the carbocation
• This means that a tertiary carbocation is more stable to a primary carbocation

Reaction of Alkenes with Halogens

• Alkenes react rapidly with chlorine gas, or solutions of bromine and iodine
• A dihaloalkane is formed
• The halogen acts as an electrophile as it has an instantaneous dipole
• The delta positive end attracts to the carbon double bond of the alkene
• The electrons in the double bond repel the electrons of the Br-Br bond which strengthens the dipole of the bromine molecule
• One of the bromine ions joins with a carbon from the double bond leaving a carbocation
• The Br- ion that is now formed then joins to the carbocation forming the dihaloalkane

The Addition takes place in two steps:

• The formation of the carbocation by electrophilic substitution
• Rapid reaction with a negative ion

Test for a Double Bond

• Add a few drops of bromine solution and with a double bond it will decolourise from red

Reaction with Concentrated Sulfuric Acid

• Reaction occurs at room temperature and is exothermic
• The electrophile is the partially positively charged hydrogen atom from the sulfuric acid
• The sulfuric acid molecule: H-OSO3H
• The carbocation formed then reacts rapidly with the negatively charged hydrogensulfate ion

Addition of Water

• When water is added an alcohol is formed alongside sulfuric acid

• When an alkene reacts with steam it forms an alcohol
• A suitable temperature and pressure is used
• An acid catalyst such as phosphoric acid is used

Additional Polymerisation

• Alkenes can polymerise, joining together to from a long chain with high relative molecular masses
• Ethene is a monomer that can form Poly(ethene)
• This reaction is additional polymerisation
• The polymer is an alkane so is unreactive

The Repeating Unit

• The smallest group of atoms that produce the polymer when repeated over and over
• In polyethene the repeating unit is CH2CH2
• Different forms of poly(ethene) can be made depending on the conditions of temperature, pressure and catalyst
• The forms differ in chain length and branching