6.2 alkenes, stereoisomerism & polymers

Question 1

give the general formula for an alkene.

Answer 1

CₙH₂ₙ

Question 2

explain why alkenes are ‘unsaturated’ hydrocarbons.

Answer 2

• alkene molecules all contain at least one C=C bond.
• alkenes are unsaturated hydrocarbons because this C=C bond allows them to form bonds with other atoms in addition reactions.

Question 3

explain when and why covalent bonds form in organic molecules.

Answer 3

• covalent bonds form when atomic orbitals from different atoms, each containing a single electron, overlap, causing the sharing of a pair of electrons.
• a covalent bond forms because the nuclei of the two atoms are electrostatically attracted to the bonding pair of electrons.

Question 4

give the two types of covalent bond present in an organic molecule.

Answer 4

sigma (𝝈) bonds and pi (𝝅) bonds.

Question 5

define the term ‘bond enthalpy’.

Answer 5

bond enthalpy refers to the amount of energy required to break a covalent bond in one mole of a molecule in a gaseous state.

Question 6

explain why 𝝈 bonds have a high bond enthalpy.

Answer 6

• 𝝈 bonds form when two atomic orbitals overlap in a way that gives the highest electron density between the two positive nuclei.
• the high electron density between the nuclei causes a strong electrostatic attraction between the nuclei and the shared pair of electrons.
• as a result, 𝝈 bonds have a high bond enthalpy.

Question 7

explain why 𝝅 bonds have a relatively low bond enthalpy as opposed to 𝝈 bonds.

Answer 7

• a 𝝅 bond is formed when the lobes of two atomic orbitals overlap sideways.
• in a 𝝅 bond, the electron density is spread out above and below the nuclei, which causes the electrostatic attraction between the nuclei and the shared pair of electrons to be weaker in a 𝝅 bond.
• as a result, 𝝅 bonds have a relatively low bond enthalpy compared to 𝝈 bonds.

Question 8

explain the difference between the C-C bond present in alkanes and the C=C bond present in alkenes.

Answer 8

• in an alkane, the C-C bond contains only a 𝝈 bond.
• in an alkene, the C=C bond contains both a 𝝈 bond and a 𝝅 bond.

Question 9

explain why carbon atoms in a C=C bond and the atoms bonded to these carbons form a trigonal planar shape.

Answer 9

• carbon atoms in a C=C double bond and the atoms bonded to these carbons lie in the same plane.
• the attachment of atoms to each double-bonded carbon forms a trigonal planar shape.

Question 10

give the bond angle of the H-C-H bonds found in the planar unit.

Answer 10

120°

Question 11

define the term ‘stereoisomer’, and describe when stereoisomerism occurs.

Answer 11

• stereoisomers are molecules which have the same structural formula but different spatial arrangement.
• stereoisomerism occurs when the two double-bonded carbon atoms each have two different atoms or functional groups attached to them.

Question 12

give an example of stereoisomerism.

Answer 12

E/Z isomerism.

Question 13

give the difference between the ‘Z-isomer’ and the ‘E-isomer’ formed as a result of E/Z isomerism.

Answer 13

• the Z-isomer has the same groups present either above or below the C=C double bond.
• the E-isomer has the same groups positioned across the C=C double bond.

Question 14

give the rule used to work out the E/Z isomerism for any alkene.

Answer 14

the Cahn-Ingold-Prelog (CIP) rule.

Question 15

if the carbon atoms have at least one group in common, the isomers formed will be either ‘cis’ or ‘trans’. give the difference between a cis isomer and a trans isomer.

Answer 15

• cis isomers have the same atom or functional group present on the same side of the C=C double bond.
• trans isomers have the same atom or functional group present on opposite sides of the C=C double bond.

Question 16

give the name of the reaction which alkenes participate in.

Answer 16

electrophilic addition reactions.

Question 17

define the term ‘electrophile’, and give two examples of electrophiles.

Answer 17

• electrophiles are electron-pair acceptors.
• electrophiles include positively charged ions, such as H⁺, and polar molecules.

Question 18

explain why alkenes participate in electrophilic addition reactions.

Answer 18

• in an electrophilic addition reaction, the alkene double bond separates, and atoms are bonded to the carbon atoms.
• alkenes participate in electrophilic addition reactions because the C=C bond contains a lot of electrons, so is easily attacked by electrophiles.

Question 19

ethene reacts with hydrogen gas in an electrophilic addition reaction to produce ethane. give the reagent and conditions required for this reaction of occur.

Answer 19

• a nickel catalyst.
• a temperature of 150°C.

Question 20

what is a carbocation?

Answer 20

a carbocation is an organic ion containing a positively charged carbon atom.

Question 21

halogens react with alkenes to form dihalogenoalkanes. give the reagent, and describe the test used to test for the presence of a C=C double bond.

Answer 21

• bromine water is used to test for the presence of a C=C double bond.
• when an alkene is shaken with bromine water, the solution quickly decolourises.
• this occurs because bromine is added across the C=C double bond to form a colourless dihalogenoalkane, dibromoalkane.

Question 22

alcohols can be manufactured from alkenes via steam hydration. give the reagent and conditions required for this reaction to occur.

Answer 22

• a solid phosphoric (V) acid catalyst.
• H₂O (l)
• a temperature of 300°C.
• a pressure of 60 - 70 atm.

Question 23

alkenes can be oxidised to produce diols (alcohols with two -OH groups present) give the reagent required for this reaction to occur, and the result of the reaction.

Answer 23

• reagent: acidified potassium manganate (VII)
• result: shaking an alkene with the acidified potassium manganate will cause the purple solution to decolourise.

Question 24

adding hydrogen halides to unsymmetrical alkenes forms two products. the amount of each product formed depends on how stable the carbocation formed in the middle of the reaction is. explain why a secondary or tertiary carbocation is more likely to form than a primary carbocation.

Answer 24

• secondary and tertiary carbocations contain two and three alkyl groups respectively, as opposed to primary carbocations, which only contain one alkyl group.
• carbocations with more alkyl groups are more stable because the alkyl groups feed electrons towards the positive charge.
• therefore, a secondary or tertiary carbocation is more likely to form than a primary carbocation, due to having more stability.

Question 25

state Markownikoff’s rule of the addition of hydrogen halides to unsymmetrical alkenes.

Answer 25

“the major product from addition of a hydrogen halide (HX) to a unsymmetrical alkene is the one where hydrogen adds to the carbon with the most hydrogens already attached.”

Question 26

landfill is used to dispose of waste plastics. give three instances when waste plastics would be buried in landfill rather than being repurposed.

Answer 26

• when the waste plastic is difficult to separate from other waste.
• when the waste plastic is not in sufficient quantities to make separation financially worthwhile.
• when the waste plastic is too difficult to recycle.

Question 27

give two ways in which waste plastics can be repurposed to reduce landfill.

Answer 27

• some plastics can be recycled, by melting and remoulding them into new materials.
• other plastics can be cracked into monomers, which can then be used as an organic feedstock for the production of more plastics or other chemicals.

Question 28

waste plastics can also be burned, and the heat used to generate electricity. polymers that contain chlorine, such as PVC, produce HCl when burned. give one method which can be used to remove toxic byproducts produced from burning polymers.

Answer 28

• one way in which toxic byproducts produced by burning polymers can be removed is by passing waste gases from the combustion through a ‘scrubber’, such as dilute NaOH.
• these scrubbers neutralise toxic gases, such as HCl, by allowing them to react with a base.

Question 29

give three ways in which chemists can manufacture polymers sustainably, to reduce the impacts on human health and the environment.

Answer 29

• by using renewable raw materials to manufacture polymers where possible.
• by limiting the amount of energy needed to manufacture the polymer, such as through using a catalyst to lower energy usage.
• by limiting the waste products produced, particularly those which are hazardous to human health or the environment.

Question 30

give two advantages and one disadvantage of using raw materials to manufacture biodegradable polymers.

Answer 30

advantages

• raw materials are renewable and have less of an impact on the environment.
• the CO₂ produced through the decomposition of plant-based polymers will be reabsorbed by other plants, rather than ending up trapped in the atmosphere.

disadvantage

• biodegradable polymers are currently more expensive to manufacture than non-biodegradable polymers.