4.1.3 Alkenes

Question 1

Describe the basic structure of alkenes

Answer 1

• **unsaturated **- at least one double bond
• three sigma bonds around a carbon
• fourth electron is not involved in sigma bonds - in the p orbital and the overlap of two p-orbitals forms a** pi bond **

Question 2

Define Pi bond and describe what it is

Answer 2

a Pi bond is formed by the sideways overlap of two adjacent p-orbitals
-electron density is concentrated above and below the bonding line
-Pi bond locks the carbon atoms in position and stops them rotating

Question 3

Compare the strength of sigma bonds and Pi bonds

Answer 3

• Pi bonds are weaker than sigma bonds
• as electron density is spread out
• reduced electrostatic attraction between nuclei and shared pair of electrons
• reducing the bond enthalpy

Question 4

What shape do alkenes have?

Answer 4

trigonal planar
-three regions of electron density around each carbon atom (double bond is the same as the single bond)
-equal repulsion so each bond angle is 120 degrees

Question 5

Define stereoisomerism
- What are the two types ?

Answer 5

same structural formula but a different arrangement of atoms in space
-E/Z isomerism = only occurs in compounds with a C=C double bond
-Optical isomerism = wider range of compounds

Question 6

What two conditions must be satisfied for E/Z isomerism to occur?

Answer 6

• C=C double bond
• two DIFFERENT groups attached to each carbon atom

Question 7

Describe the shape of the Z-isomer

Answer 7

same groups both ABOVE or BELOW the double bond (so on the same side)

Question 8

Describe the shape of the E-isomer

Answer 8

same groups on opposite sides of the double bond (across it)

Question 9

What are the rules for naming E/Z isomers when there are multiple different groups?

Answer 9

1. look at the atoms directly bonded to the carbon atoms - the atom with the higher atomic number is given priority
2. Decide which two atoms have highest priority
3. If the atoms on one side are the same look for the **first point of difference **and use this
4. Decide if it is a Z or E isomer

Question 10

When is the Cis/trans naming system used?

Answer 10

one of the attached groups on each carbon atom must be the same
-Cis (same side) isomer is the Z isomer
-Trans (opposite sides) isomer is the E isomer

Question 11

Are alkenes more or less reactive than alkanes?
Why?

Answer 11

More reactive
-Pi bond is more exposed so it is more likely to be atttacked by electrophiles and undergo electrophillic addition reactions
-The Pi bond also has a low bond enthalpy

Question 12

Describe the hydrogenation reaction of alkenes
-What conditions are needed?
-What is produced?

Answer 12

• Needs a nickel catalyst and 150 degrees heat
• produces an **alkane **

Question 13

Describe the halogenation reaction of alkenes
-What conditions are needed?
-What is produced?

Answer 13

• Electrophillic addition reaction
• needs gaseous chlorine or bromine at room temp
• produces dihaloalkanes

Question 14

What is the test for an unsaturated hydrocarbon (alkene)?

Answer 14

add bromine water - should go from orange to colourless

Question 15

Describe the reaction of alkenes with hydrogen halides?

Answer 15

• needs gaseous hydrogen halides
• produces haloalkanes
  (same reaction occurs with hydrochloric or hydrobromic acid) - TWO possible products

Question 16

Describe the hydration reaction of alkenes

Answer 16

• needs steam and a phosphoric acid catalyst
• steam adds across the double bond to form an alcohol - TWO possible products

Question 17

Define electrophile

Answer 17

partially or fully positively charged atom or group of atoms which is** attracted to an electron rich centre** and** attracts an electron pair**

Question 18

Where is the electron density located in an alkene?

Answer 18

double bond is a region of high electron density due to the Pi electrons - this area attracts electrophiles

Question 19

Describe the mechanism for the electrophillic addition of but-2-ene with hydrogen bromide (or another POLAR molecule)

Answer 19

• hydrogen bromide is polar and** hydrogen has a partial positive charge** (acts as an electrophile)
• H-Br accepts a pair of electrons from the Pi bond of C=C
• bond forms between carbon and hydrogen (from hydrogen bromide)
• H-Br bond breaks - via heterolytic fission - electron pair goes to the bromine atom
• bromide ion and carbocation are formed - these react to form the addition product

Question 20

Describe the mechanism for the electrophillic addition of propene with bromine (or another NON POLAR molecule)

Answer 20

*** Bromine molecule approaches the alkene and the high electron density of the Pi bond induces a dipole on the C=C bond
* electron pair in Pi bond is attracted to the partially positive bromine (acts as an electrophile) - double bond breaks
* bond forms between carbon atom and bromine atom
* Br-Br bond breaks via heterolytic fission - electron pair moves to Br (slight positive) end
* Br - ion reacts with carbocation to form the addition product

Question 21

What is the rule when a hydrogen halide reacts with an **unsymmetrical alkene **?

Answer 21

hydrogen halide will attach to the carbon with the greatest number of hydrogen atoms and smallest number of carbon atoms/alkyl groups

Question 22

How does the number of alkyl groups impact carbocation stability?
- What impact does the size of the alkyl group have?

Answer 22

• more alkyl groups means **charge is more spread out **(as each alkyl group donates and pushes electrons towards the positive charge of the carbocation) - this is the inductive effect
• this makes the** ion more stable** - THE MOST STABLE CARBOCATION IS THE MOST LIKELY TO FORM AND BE THE MAJOR PRODUCT
  Size - larger alkyl groups = increased stability

So… tertiary carbocations are more stable than secondary carbocations which are more stable then primary carbocations

Question 23

Define addition polymerisation

Answer 23

unsaturated alkene molecules are joined together to produce saturated alkane chains (no double bonds)

Question 24

Give three ways of processing waste polymers

Answer 24

• recycling
• can be used as fuels - due to high energy content (but must be carefully controlled to prevent the release of toxic gases)
• feedstock recycling (eg they can be cracked into monomers) - can be used as raw materials to produce new polymers

Question 25

Give the benefits of biodegradable and photodegradable polymers

Answer 25

Bio - can be broken down by microorganisms so leave no toxic residue
Photo - bonds absorb light which causes them to start to degrade