Alkenes

Question 1

what are alkenes

Answer 1

unsaturated hydrocarbons that contain a C=C double bond

Question 2

describe the bonding in alkenes

Answer 2

• the C=C consists of a σ bond - the direct overlap of s orbitals between bonding atoms
• also contains a π bond - the sideways overlap of of adjacent p-orbitals above and below the plane of the molecules

Question 3

why are pi bonds much weaker than sigma bonds

Answer 3

• e- density is delocalised above and below plane of molecule
• this means that the electrostatic attraction between the nuclei and a shared pair of e- is weaker, so pi bonds have relatively low bond enthalpy

Question 4

why do sigma bonds (strongest type of covalent bond) have high bond enthalpy

Answer 4

• high e- density between the nuclei means there is a strong electrostatic attraction between the nuclei of the bonded atoms and shared pair of e-

Question 5

what shape does the C of the double bond have, and what’s the angle

Answer 5

• trigonal planar
• 120 degrees

Question 6

why does C in the double bond have its shape

Answer 6

• 3 areas of e- density (bonding pairs) around C=C atoms repel equally
• 0 lone pairs
• lone pairs repel more than bonding pairs

Question 7

why are alkenes relatively reactive

Answer 7

• carbon carbon double bond has high electron density and so pi bond can be attacked by an electrophile
• π bond has low bond enthalpy

Question 8

why can alkenes exhibit stereoisomerism

Answer 8

C=C restricts rotation

Question 9

define stereoisomerism

Answer 9

compounds with the same molecular and structural formula, but a different arrangement of atoms in space

Question 10

what’s E-Z isomerism

Answer 10

stereoisomerism that arises due to the restricted rotation about the C=C bond when there are 2 different groups attached to each carbon of the C=C

Question 11

how to use E-Z system

Answer 11

1. assign priority to each group attached to the first C (atom with the highest atomic number)
2. assign priority to each group attached to second C
3. two priority groups on same side of double bond = Z isomer (zame zide)
   opposite sides = E isomer

Question 12

what’s cis-trans isomerism

Answer 12

special case of E-Z isomerism where 2 groups attached to each C of the C=C bond are the same

Question 13

how can alkenes readily undergo addition reactions

Answer 13

• π bond e- can be used to form new bonds with an attacking molecule

Question 14

general equation for hydrogenation and condition

Answer 14

alkene + H2 –(Ni)–> alkane

• reactants heated

Question 15

what can hydrogenation be used for

Answer 15

• in manufacture of margarine from vegetable oils
• vegetable oils are unsaturated and contain C=C
• hydrogenation converts these into C-C and changes the physical properties, so oil converted into solid margarine

Question 16

define electrophile

Answer 16

e- pair acceptors attracted to areas of high e- density

Question 17

what’s the reaction of alkenes with electrophiles

Answer 17

• electrophilic addition
• electrophiles are attracted to e- in C=C

Question 18

general equation for the bromination of alkenes

Answer 18

alkene + bromine —-> dibromoalkane

Question 19

how does bromination of alkenes occur

Answer 19

alkene polarises bromine molecule, which then acts as an electrophile and adds to the alkene

Question 20

what’s bromination of alkenes used to test for

Answer 20

• unsaturation
• bromine water will go from orange to colourless if Br is added to alkene

Question 21

how does the Br-Br bond break

Answer 21

• via heterolytic fission
• both bonding e- go to same Br atom, forming Br- ion

Question 22

general equation for addition reaction of alkenes with steam

Answer 22

• alkene + H2O (g) <–(H3PO4)–> alcohol
  conditions: 300C, 60 atm
• reversible

Question 23

addition reaction alkenes with steam is also an example

Answer 23

hydration reactions

Question 24

with any unsymmetrical molecule there is possibility of forming …. products in addition reactions

Answer 24

2

Question 25

what’s type of reaction occurs between hydrogen halides and alkenes and what does it form

Answer 25

• addition
• forms haloalkanes

Question 26

define carbocation

Answer 26

molecule that contains a C atom with a +ve charge

Question 27

what makes a carbocation more stable and why

Answer 27

• more alkyl groups (C) attached to +vely charged C
• alkyl groups are elctron-releasing so they help spread the charge

Question 28

what’s Markownikoff’s rule

Answer 28

• major product is formed via the most substituted carbocation intermediate
• 1* carbocations are the least stable, 3* are most stable

Question 29

what are addition polymers (of alkenes)

Answer 29

• saturated chains formed from alkene monomers
• alkene double bonds open and form the long chain polymers

Question 30

what are the 4 methods that scientists have developed to make polymer use more sustainable

Answer 30

• recycling
• incineration
• organic feedstock
• bioplastics: biodegradable and photodegradable polymers

Question 31

description, benefits and issues of recycling polymers

Answer 31

• polymer waste tediously and inefficiently sorted before being processed and used to make new materials
• conserves natural resources
• some polymers like PVC, difficult to recycle as toxic and corrosive additives first need to be removed

Question 32

description, benefits and issues of incineration

Answer 32

• polymer waste combusted to convert it into heat energy for use in electricity generation
• produces toxic waste gases (HCl) which can be removed from incinerator chimneys

Question 33

description, benefits and issues of organic feedstock

Answer 33

• polymer waste broken down into gases, mainly H2 and CO or oil
• these can be used in other chemical reaction like cracking, or as raw materials for new plastics and other chemicals

Question 34

description of biodegradable polymers

Answer 34

• plant based, broken down by microbes

Question 35

description of photodegradable polymers

Answer 35

• oil based, but broken down by absorbing light