D. halogenoalkanes

Question 1

What are halogenoalkanes

Answer 1

alkanes where one or more hydrogen atoms are substituted for a halogen, usually represented by an X
- can be classified as primary, secondary or tertiary

Question 2

What is the trend in bond polarity down the group of hydrogen halides

Answer 2

bond polarity decreases down the group, with fluoroalkanes being the most polar, and iodoalkanes being the least polar
- iodoalkanes aren’t polar molecules, however the large number of electrons on iodine results in strong london forces, outweighing the absence of a permanent dipole

Question 3

Explain the trend in boiling points of haloalkanes

Answer 3

as chain length increases, the boiling point increases due to stronger london forces
polar haloalkanes also experience dipole-dipole interactions, increasing the boiling point
as the chain length increases, the contribution from permanent dipole interactions are less significant

Question 4

Explain the solubility of haloalkanes

Answer 4

they are insoluble in water as not enough energy is released from the interactions with water to break the hydrogen bonds between water molecules
- haloalkanes are soluble in organic solvents

Question 5

How are haloalkanes synthesised

Answer 5

1) free radical substitution from an alkane
2) nucleophilic substitution from an alcohol
3) electrophilic addition from an alkene

Question 6

What is the reagent in chlorination

Answer 6

phosphorus pentachloride (PCl5)

Question 7

What are the conditions of chlorination

Answer 7

room temperature

Question 8

What is the reaction that takes place in chlorination

Answer 8

alcohol + PCL5 -> haloalkane (l) + POCl3 (l) + HCl (g)

Question 9

What observation can be used to identify a chlorination reaction

Answer 9

HCl gas is observed as ‘misty fumes’

HCl reacts with NH3 to form ammonium chloride, which is observed as ‘white smoke’

Question 10

Give two issues with the chlorination reaction

Answer 10

1) reaction produces HCl which is harmful
2) reaction has a low atom economy

Question 11

Describe the reaction in which a chloroalkane can be formed from a tertiary alcohol

Answer 11

alcohol + HCl -> chloroalkane + H2O

Question 12

What are the reagents in bromination

Answer 12

50% concentrated sulfuric acid and Na/KBr

Question 13

Give the two reactions that take place to form a bromoalkane

Answer 13

H2SO4 + 2NaBr -> 2HBr + Na2SO4
HBr + alcohol -> bromoalkane + H2O

Question 14

Why is hydrogen bromine made in situ in bromination

Answer 14

it is a toxic and corrosive gas

Question 15

Why is 50% concentrated sulfuric acid used in bromination

Answer 15

more concentrated acid would oxidise bromide ions to bromine

Question 16

What are the reagents in iodination

Answer 16

red phosphorus (P) and I2

Question 17

What are the conditions for iodination

Answer 17

heating under reflux

Question 18

Give the two reactions that form an iodoalkane

Answer 18

2P + 3i2 -> 2PI3
PI3 + 3(alcohol) -> 3(iodoalkane) + H3PO3

Question 19

Why is phosphorus (III) iodide make in situ in iodination

Answer 19

it is unstable

Question 20

Why is hydrogen iodide not used as a reagent in iodination

Answer 20

HI is unable to be used as it is unstable and is easily oxidised to iodine

Question 21

What is a nucleophile

Answer 21

a species that is attracted to positive charge

Question 22

What is the reagent in nucleophilic substitution with water to form alcohols

Answer 22

water

Question 23

What are the conditions in nucleophilic substitution with water to form alcohols

Answer 23

heat under reflux
ethanol as a mutual solvent

Question 24

Give the reaction between a halogenoalkane and water

Answer 24

R-X + H2O -> R-OH + HX (aq)

Question 25

Give a limitation of the reaction between haloalkanes and water

Answer 25

the reaction is slow as water is not a good nucleophile

Question 26

Describe how to test for the reaction between halogenoalkanes and water

Answer 26

add silver nitrate AgNO3(aq) halide ions will react with the silver ions to form: AgCl - white precipitate AgBr - cream precipitate AgI - yellow precipitate

Question 27

What is the reagent in nucleophilic substitution with hydroxide ions to form alcohols

Answer 27

either NaOH or KOH (aqueous)

Question 28

What are the conditions in nucleophilic substitution with hydroxide ions to form alcohols

Answer 28

room temperature 50/50 ethanol and water

Question 29

Give the reaction for nucleophilic substitution with hydroxide ions to form alcohols

Answer 29

haloalkane + NaOH -> alcohol + NaX

Question 30

Describe how to test for the reaction between halogenoalkanes and hydroxide ions

Answer 30

silver nitrate can be added to identify the halogen and compare rates of reaction

Question 31

Give the reagent for a substitution with cyanide ions to form a nitrile

Answer 31

KCN in ethanol

Question 32

Give the conditions for a substitution with cyanide ions to form a nitrile

Answer 32

heat under reflux - condenser is placed vertically to prevent loss of volatile vapours or reactants/products

Question 33

What is a benefit of a reaction between cyanide ions and a haloalkane

Answer 33

increases the length of the carbon chain

Question 34

Give the reagents in a substitution with ammonia to form an amine

Answer 34

excess of ethanolic ammonia

Question 35

Why is an excess of ammonia used to form an amine

Answer 35

to prevent multiple substitutions from occuring

Question 36

Why is ethanolic ammonia used to form an amine

Answer 36

absence of water to prevent water from reacting with the haloalkane

Question 37

What are the conditions in a substitution with ammonia to form an amine

Answer 37

heat in a sealed tube - to maintain pressure - so no reactants are lost

Question 38

Give the reaction between a haloalkane and ammonia

Answer 38

R-X + NH3 -> R-NH2 + HX

Question 39

What is formed in the elimination reaction of a haloalkane

Answer 39

an alkene

Question 40

What are the reagents in the elimination of a haloalkane

Answer 40

ethanolic NaOH or KOH

Question 41

What are the conditions in an elimination reaction of a haloalkane

Answer 41

heat under reflux / heat

Question 42

What class of haloalkanes react with the SN2 mechanism

Answer 42

primary (and secondary)

Question 43

Why is the mechanism called SN2

Answer 43

there are two species in the rate determining step (the nucleophile and the haloalkane)

Question 44

Which mechanism occurs in a one step reaction

Answer 44

SN2 - transition state is formed which we are unable to isolate

Question 45

What class of haloalkanes react with the SN1 mechanism

Answer 45

tertiary (and secondary)

Question 46

Why do tertiary haloalkanes react with the SN1 mechanism

Answer 46

tertiary have bulky R groups which causes steric hindrance and prevents attack by the nucleophile - therefore they must react in a two step reaction

Question 47

Why is the mechanism called SN1

Answer 47

there is one species involved in the rate determining step (the haloalkane)

Question 48

Describe the SN1 mechanism

Answer 48

step 1 - the haloalkane loses its halogen and becomes a carbocation (slow step) step 2 - the nucleophile is attracted to the carbocation

Question 49

Why are tertiary carbocations the most stable

Answer 49

due to the increased inductive effect of the alkyl groups - electron density from neighbouring groups is drawn towards the carbocation and stabilises it

Question 50

Describe the energy profile of the SN1 mechanism

Answer 50

two bumps in the profile two activation energies as the reaction occurs in two steps the plateau between the two bumps is the energy of the intermediate (carbocation)

Question 51

Describe the energy profile of the SN2 mechanism

Answer 51

one bump as reaction occurs in one step the peak of the bump is the energy of the transition state