A2 - Structures, reactions & properties of commercially important organic compunds

Question 1

name the type of reaction that takes place when an alkane burns in oxygen

Answer 1

Combustion

Question 2

write an equation to show how butane will undergo complete combustion

Answer 2

C4H10 + 6.5O2 —-> 4CO2 + 5H2O

Question 3

write an equation to show how propane will undergo complete combustion

Answer 3

C3H8 + 5O2 —> 3CO2 + 4H2O

Question 4

write an equation to show how decane will undergo complete combustion

Answer 4

C10H22 + 15.5O2 —> 10CO2 + 11H2O

Question 5

put these alkanes in order from highest to lowest boiling point; CH4, C4H10, C512, C2H6 & C3H8

Answer 5

highest boiling point ———— lowest boiling point

C5H12, C4H10, C3H8, C2H6 & CH4

Question 6

explain your answer about the order of alkanes from the highest boiling point to the lowest boiling point

Answer 6

as the size of the molecule increases the boiling point increases - this is because there are more electrons which create stronger IMF attractions

Question 7

explain why C4H10 is a gas but C8H18 is a liquid at room temperature

Answer 7

C8H18 has a higher boiling point than C4H10 because C8H18 has more electrons than C4H10, so C8H18 has stronger IMF attractions

Question 8

why does 2,2-dimethylbutane have a lower boiling point in comparison to hexane

Answer 8

2,2-dimethylbutane has the most branches so it will have the fewest points of contact between its molecules - therefore, it has the weakest IMF attractions

Question 9

what are the bond angles for the orbitals in each type of hybridisation?

Answer 9

sp - 180 sp^2 - 120 sp^3 - 109.5

Question 10

describe the process of sp^3 hybridisation for a carbon atom

Answer 10

• 1 s and 3 p orbitals mix together
• there are 0 spare p orbitals
• this forms 4 sp^3 hybrid orbitals
• the energy of the s orbital increases
• the energy of the p orbital decreases
• each orbital holds 1 electron
• the hybrid orbitals have the same shape

Question 11

what is the definition of a free radical

Answer 11

a free radical is a species with an unpaired electron

Question 12

Br-Br —> Br. + Br.
Identify the type of bond fission shown in the initiation step

Answer 12

homolytic fission

Question 13

State what happens to the free radicals in a termination step

Answer 13

free radicals combine and form a molecule

Question 14

Describe how ultraviolet light causes a reaction to begin

Answer 14

the UV light breaks the halogen bond - this forms two radicals

Question 15

Name the compound
H H H H
| | | |
H — C—C—C—C— H
| | | |
H H Br H

Answer 15

2-bromobutane
Start from RIGHT SIDE ~ use lowest no.

Question 16

CH3CH2CH3 + Br2 —> CH3CH2CH2Br + Hbr
Describe how ultraviolet light causes reaction to begin

Answer 16

UV light breaks halogen bone, Br2 which is needed for homolytic fission

Br2 —> 2Br •

Question 17

Why does propene able to form 2 bromoalkanes?

Answer 17

Because it has an asymmetrical structure ~ 2 different haloalkanes can be made when alkene is asymmetrical

Question 18

Name the bromoalkane
H H Br
| | |
H—C—C—C—H
| | |
H H H

Answer 18

1-bromopropane

Question 19

H H H H H H
| | | | | |
H —C—C—C + H—C—C—C—H
| | | | + |
H H H H H
Carbocation A Carbocation B

Compare stability of carbocation A with carbocation B.

Answer 19

Carbocation A is less stable than carbocation B ~ it is only bonded to 1 carbon ~ named primary, 1^o
Whereas carbocation B (C+) is bonded to 2 carbons ~ so it’s MORE stable ~ named secondary, 2^o ~ so there’s a greater POSITIVE INDUCTIVE EFFECT (more e- donated to C+)

Question 20

What is the type of reaction used in free radical polymerisation of alkenes?

Answer 20

Addition

Question 21

discuss the advantages & disadvantages of manufacture of ethanol by hydration if ethene

Answer 21

• fermentation is irreversible - all reactants become products
• hydration is reversible - not all reactants become products
• hydration requires MORE raw materials; ethene and H2O
• fermentation uses renewable sources whilst crude oil is NON-RENEWABLE, plants take in CO2 when growing (carbon neutral - does NOT release CO2)
• fermentation has SLOW rate of reaction
• impure ethanol made needs DISTILLING
• hydration requires higher temps - increases cots but this does INCREASE rate of reaction
• hydration - phosphoric acid is corrosive (more DANGEROUS than yeast)