Developing Fuels: Alkenes

Question 1

How are covalent bonds formed

Answer 1

Overlap of orbitals

Question 2

Describe electrophilic addition mechanism with Br2 and ethene ( 6 marks )

Answer 2

1. Bromine molecule comes close to ethene molecule = dipole induced in bromine molecule
2. A pair of electrons from 1 of the C-C bonds in ethene is attracted to (+ve delta ) bromine atom
3. Bonding pair in of electrons in bromine molecule is repelled to (-ve delta) bromine atom = bond between 2 bromine atoms breaks

( free bromine atom receives both bonding electrons and becomes a bromide ion)

4. Covalent bond forms between 1 of the carbon atoms and ( +ve delta ) bromine atom = ethene becomes +ve charge ion = carbocation
5. the lone pair of electrons in the bromide ion is attracted to +ve charge on the carbocation
6. Bromide ion moves towards the +ve charge carbon atom = covalent bond forms between them

Question 3

What is the name of the intermediate formed in an addition reaction

Answer 3

Carbocation

Question 4

What happens when you add sodium chloride to a solution of ethene and bromine water ???

Answer 4

Bromide (ion) / Br– attacks / bonds with carbocation
OR –
Bromide (ion) / Br attacks / bonds with intermediate A ✓
Chloride (ion) / Cl– attacks / bonds with carbocation OR Chloride (ion) / Cl– attacks / bonds with intermediate A ✓
Attack by Bromide (ion) / Br– gives CH2BrCH2Br
OR – Attack by Chloride (ion) / Cl gives CH2C/CH2Br ✓

Question 5

Describe what a sigma bond is and how it is formed

Answer 5

head on overlap of orbitals forming a covalent bond

Question 6

Describe what a pi bond is and how it is formed

Answer 6

sideways overlap of orbitals to form a covalent bond

Question 7

What are 2 sayings of bonds simplified

Answer 7

• sigma = single bond eg. Cl2

- pi = partner double bond ( sigma &pi ) eg. O2

Question 8

What type of bond is formed from a s and s head on overlap

Answer 8

Sigma

Question 8

What type of bond is formed from a s and p head on overlap

Answer 8

Sigma

Question 9

What type of bond is formed from a p and p head on overlap

Answer 9

Sigma

Question 10

What type of bond is formed from a p and p head on overlap

Answer 10

Pi

Question 11

Explain how you could use a solution of bromine water to show that but-1,3-diene has a greater degree of unsaturation that but-1-ene (3)

Answer 11

1. Same amount of bromine water, but-1,3-diene and but-1-ene
2. But-1,3-diene would go colourless quicker
3. But-1,3-diene has higher no. of saturated bonds

Question 12

Definition of addition reaction

Answer 12

A reaction where 2 or more molecules react to form a single larger molecule

Question 13

Definition of electrophile

Answer 13

+ve ion or a molecule with a partial +ve that is attracted to a
-ve charge region and reacts by accepting a lone pair of electrons to form a covalent bond

Question 14

Definition of carbocation

Answer 14

An ion with a positively charged carbon atom

Question 15

Definition of curly arrow

Answer 15

Represent the movement of a pair of electrons in chemical reactions

Question 16

Describe the ethene and bromine mechanism ( simple terms )

Answer 16

Bromine approaches close to the high electron density of an alkene double bond

Repulsion of the electrons in the Br2 molecule polarises the Br-Br bond so that it can then act as an electrophile

Question 17

Describe the mechanism of ethene and hydrogen bromide = bromoethane ( 5 marker )

Answer 17

1. Pair of electrons is attracted out from the double bond towards the (delta +ve ) H atom.
2. Pair of electrons in the HBr bond is attracted to the Br atom to form a -ve Br ion.
3. A bond forms between C and H.
4. The other C atom is left with a +ve charge = carbocation
5. Lobe pair of electrons of the Br - ion is attracted to +ve charge carbocation to form a covalent bond

Question 18

Production of ethanol from ethene and water in INDUSTRY - write equation + identify the conditions needed

Answer 18

C2 H4 + H2O(steam) ->C2 H5 OH

• high temp
• high pressure
• phosphoric acid on silica

Question 19

Production of ethanol from ethene and water in LAB

Answer 19

Concentrated sulphuric. Acid is added first followed by water

Question 20

Describe production of ethane from ethene and hydrogen ( hydrogenation ) in LAB write equation & conditions

Answer 20

C2 H4 + H2 ——> C2 H6

• catalyst = Nickel (Ni ) powdered
• 150°
• 5 atm

Question 21

What catalyst is used in INDUSTRIAL production of ethane from ethene & hydrogen ( hydrogenation )

What is the reaction used to make

Answer 21

Platinum —> unsaturated fats into saturated fats

Question 22

Explain how experimental evidence supports the electrophilic addition mechanism,using the reaction between hex—1-ene and a mixture of bromine and chloride ions as an example ( 3 marker )

Answer 22

1. 2 products are formed
2. 1,2 dibromohexane and 1-bromine-2-chlorohexane
3. However 1,2- dichlohexane is not formed as the electrophile must first attack to form a carbocation