Alkenes

Question 1

general formula of an alkene

Answer 1

CnH2n

Question 2

cycloalkene formula

Answer 2

CnH2n-2

Question 3

are alkenes saturated or unsaturated and why

Answer 3

unsaturated, contains a c=c double bond

Question 4

in terms of bonding what does it mean to have a double bond

Answer 4

more bonds can be made with additional electrons in addition reactions

Question 5

describe the double bonds in terms of sigma and pi bonds

Answer 5

the double bond is made up of sigma and pi bonds
- a pi bond is formed when two lobes of two orbitals overlap sideways
- pi bond electron density is spread out above and below the nuclei
(the electrostatic attraction between the nuclei and shared pair of electrons is weaker resulting in a relatively low bond enthalpy)

Question 6

which is stronger a single or a double bond and why

Answer 6

the sigma single bond is stronger
- two orbitals overlap in a straight line in the same space
- gives the highest possible electron density between the two positive nuclei, resulting in a strong electrostatic attraction between the nuclei and the shared pair of electrons
resulting in a strong bond enthalpy. whereas, due to its spread out electron density, the pi bond in the double bond has a much lower bond enthalpy so is a weaker covalent bond

Question 7

what is electrophilic addition

Answer 7

when an alkene double bond opens up and atoms are added to the carbon atoms

Question 8

describe electrophilic addition

Answer 8

• alkene double bond has lots of electrons, easily attracted by electrophiles who lack electrons
• e.g. positively charged ions, polar molecules

Question 9

what is an electrophile

Answer 9

electron-pair acceptors, short of electrons so are attracted to where there’s plenty (such as alkene double bond)

Question 10

what is produced when alkene reactions with hydrogen

Answer 10

an alkane

Question 11

what state must alkene be in to react with hydrogen

Answer 11

gas

Question 12

conditions for addition reaction with hydrogen

Answer 12

• 150 degrees celcius
• nickel catalyst

Question 13

describe how margarine is created

Answer 13

• its double bond is removed to raise its melting point so it becomes solid at RT
• made by hydrogenation of unsaturated vegetable oils

Question 14

what type of reaction are reactions with halogens

Answer 14

electrophilic addition reaction

Question 15

alkene + halogen ->

Answer 15

dihalogenoalkane

Question 16

describe the test for alkenes

Answer 16

bromine water is used. bromine is added across the double bond to form colourless dibromoalkane

Question 17

how are alcohols formed

Answer 17

steam hydration

Question 18

what state do alkenes need to be in to be hydrated by steam

Answer 18

gas

Question 19

conditions for steam hydration

Answer 19

• 300 degrees celcius
• Phosphoric (V) acid catalyst
• 60-70 atm

Question 20

how are diols formed

Answer 20

oxidation

Question 21

what is a diol

Answer 21

an alcohol with two -OH groups

Question 22

conditions required for oxidation

Answer 22

• alkene react with acidified potassium manganate (VIII)

Question 23

colour change when alkene reacts with acidified potassium manganate (VIII)

Answer 23

purple- colourless