Topic 6.3

Question 1

General formula for halogenoalkanes

Answer 1

CnH2+1X

Question 2

Why is the carbon to halogen bond polar

Answer 2

As the pair of electrons in the covalent bond between the carbon atom and the halogen atom is closer to the halogen than the carbon, which results in the halogen atom to have a slight negative charge and for the carbon atom to have a slight positive charge

Question 3

Why do halogenoalkanes have higher melting and boiling points than normal alkanes

Answer 3

As normal alkanes only contain London forces which are relatively weak and require little energy to break whilst halogenoalkanes contain London forces and because of the polarity of the carbon to halogen bond there is also permanent dipole - dipole interactions, which are stronger than London forces and require more energy to break so will have a higher melting and boiling point

Question 4

Why does the boiling point increase as you go down group 7 halogenoalkanes

Answer 4

As the number of electrons increase as we go down the group so the size and strength of the London forces increase as you go down the group so it requires more energy to break the London forces so bp increases

Question 5

Why are haloalkanes insoluble in water

Answer 5

As they can’t form hydrogen bonds with water molecules

Question 6

What are the three types reactions that can produce halogenoalkanes

Answer 6

1) reaction from alkanes
2) reactions from alkenes
3) reaction from alcohols

Question 7

How does the reaction from alkanes form halogenoalkanes

Answer 7

The reaction of alkanes with chlorine or bromine either on heating or with the exposure of UV light leads to free radical substitution in which the halogen atoms replace the hydrogen atoms

Question 8

How does the reaction with alkenes form halogenoalkanes

Answer 8

• Reaction with alkenes with hydrogen halides at room temperature produces halogenoalkanes with one halogen
• alkenes can react with halogen atoms to form halogenoalkanes with two halogens

Question 9

How do halogenoalkanes form from the reaction of alcohols and sulfuric acid

Answer 9

1) The sulfuric acid reacts with the compound containing the halogen to form a hydrogen halide
e.g NaBr + H2SO4 ——> HBr + NaHSO4
2) The hydrogen halide reacts with the alcohol to form a halogenoalkane and water via a substitution reaction (as the bromine substituted the OH group of the ethanol)
e.g C2H5OH (ethanol) + HBr —-> C2H5Br (bromoethane) + H2O
3) then we distil or solution to seperate the halogenoalkane

Question 10

How do halogenoalkanes form from the reaction of alcohols and phosphorus (v) chloride/ phosphorus pentachloride

Answer 10

Reacting an alcohol with phosphorus (v) chlorine will produce, POCl3 (phosphorus trichloride), HCl (hydrogen chloride) and a halogenoalkane

Question 11

How do halogenoalkanes form from the reaction of alcohols and phosphorus (lll) iodide

Answer 11

Phosphorus reacts with iodine to make phosphorus (lll) iodide
Reacting phosphorus (lll) iodide with an alcohol makes and halogenoalkanes containing iodine and phosphonic acid (H3PO3) this reaction is carried out by heating under reflux followed by distillation to separate the iodoalkane

Question 12

What are nucleophiles

Answer 12

They are Electron pair donors

Question 13

How do nucleophiles work

Answer 13

They have a lone pair of electrons which is attracted to the electron deficient carbon atom which is positively charged, the nucleophile donates the pair of electrons to form a covalent bond between the nucleophile and the carbon atom

Question 14

What happens in the nucleophilic substitution reaction between hydroxide ions and halogenoalkanes (Sn1 and Sn2)

Answer 14

LEARN TO DRAW THIS

The lone pair of electrons on the oxygen of the hydroxide ions (the nucleophile) is attracted to the electron deficient positive carbon atom (which is attached to the halogen) this results in the hydroxide ion to donate its lone pair of electrons to form a covalent bond with the electron deficient carbon atom. As the carbon atom can only form 4 covalent bonds at the same time the covalent bond between the carbon atom and the halogen breaks, with the pair of electrons moving onto the halogen atom by heterolytic fission, at the end of the reaction an alcohol and a halide ion is formed

Question 15

Conditions in the nucleophilic substitution reaction between hydroxide ions and halogenoalkanes

Answer 15

• aqueous solution of hydroxide ions
• ethanol - this solubilises the haloalkane so it can react with the aqueous hydroxide ions
• reaction is heated under reflux - to increase the rate of the reaction

Question 16

How to measure the rate of hydrolysis of halogenoalkanes

Answer 16

1) place 1cm3 of ethanol in 3 different test tubes (ethanol acts a solvent to dissolve the halogenoalkane allowing the halogenoalkanes to mix with aqueous solutions)
2) add 0.1cm3 of the halogenoalkanes to each test tube then place the test tube into a water bath at 60 degrees
3) in a separate test tube add aqueous silver nitrate and place it into the same water bath then wait 10 mins so they all reach the same temp
4) then add 1 cm3 of the aqueous silver nitrate to each test tube and time how long it take for the halide ions to form by the nucleophillic substitution of water (as it is in an aqueous solution so contains water) and produce their different precipitates

Question 17

Why can’t we use hydroxide ions to measure the rate of hydrolysis

Answer 17

As the hydroxide ions will react with the silver ions to form insoluble silver hydroxide

Question 18

Why does the chloroalkane have the slowest rate of hydrolysis

Answer 18

As the carbon to chlorine bond enthalpy is the highest so it takes a lot of energy to break the carbon to chlorine bond so the rate of hydrolysis is very slow

Question 19

What does the hydrolysis of halogenoalkanes using water makes

Answer 19

Alcohol, hydrogen ion and halide ion

Question 20

What happens in the nucelophillic substitution reaction of ammonia and halogenoalkanes

Answer 20

1) as the nitrogen atoms of ammonia have a lone pair of electrons it can act as a nucleophile
2) the lone pairs of the nitrogen atom is attracted to the positively charged carbon atom in the haloalkane, this causes the nitrogen atom to donate its lone pair of electrons to form a covalent bond to the carbon atom
3) as the carbon atom can only have 4 covalent bonds the bond between the carbon atoms and the halogen breaks and through heterolytic fission both of the electrons move onto the halogen
4) this forms a molecule with a positive charged nitrogen atom and a halide ion
5) a second molecule of ammonia removes a hydrogen ion from the positive nitrogen ion this results in the covalent bind between the nitrogen and hydrogen to break with both electrons moving onto the nitrogen
6) this forms an amino-alkane ( a primary amine) and ammonium halide

Question 21

Conditions for the substitution reaction of ammonia and halogenoalkanes

Answer 21

• carried out in a sealed tube To increase the pressure of the reaction and prevent ammonia escaping as a gas
• use excess ammonia - as the nitrogen atom in the primary amine still has a lone pair of electrons so can react with any unreacted halogenoalkane so by using excess ammonia we make more likely that our halogenoalkane reacts with ammonia rather than the primary amine product

Question 22

What happens you react a halogenoalkane with a cyanide ion then heat under reflux (nucleophilic substitution of halogenoalkane with a cyanide ion)

Answer 22

The halogen is replaced with a CN group to form a nitrile and a halide ion

Question 23

Why do we use a reflux condenser in the substitution of halogenoalkane with a cyanide ion

Answer 23

To ensure that volatile substances condense, the halogenoalkane and ethanol, drip back into the mixture

Question 24

What happens in the elimination reaction between a hydroxide ion and a halogenoalkane

Answer 24

The hydroxide ion acts as a base and eliminate the hydrogen and halogen on adjacent carbonatoms from the halogenoalkane to form an alkene water and a metal halide

Question 25

What reaction occurs with hot and cold aqueous hydroxide ions

Answer 25

Warm - substitution
Cold - elimination

Question 26

how halogenoalkanes are used

Answer 26

• solvents
• refrigerants
• fire extinguishers

Question 27

what are chlorofluorcarbons

Answer 27

• compounds containg only chlorine fluorine and carbon
• they are unreactive, do not burn completely and are not toxic
• they contribute to global warming by effecting the ozone layer

Question 28

physical properties of alcohols

Answer 28

• less volatile than hydrocarbons as the hydrogen bonding between OH groups is much stronger than london forces between alkanes

Question 29

what forms in the combustion of alcohols

Answer 29

carbon dioxide and water

Question 30

how do you form bromoalkanes (substitution reaction)

Answer 30

by heating an alcohol with hydrogen bromide in the presence of sulfuric acid this forms a bromoalkane and water

or by reacting an alcohol using phosphorous(v) bromide this forms a bromoalkane and H3PO3

Question 31

how do you form iodoalkanes (substitution reaction)

Answer 31

by reacting alcohols with phosphorous (III) iodide to make iodoalkanes and H3PO3
- cant use hydrogen iodide as it is unstable so easily oxidises to iodine

Question 32

how do you form chloroalkanes

Answer 32

by reacting an alcohol with phosphorous (v) chloride and forming POCl3 (phosphonic acid) , chloroalkane and hydrogen chloride

Question 33

functional group of aldehydes

Answer 33

CHO e.g propanal

(c is double bonded to o)

Question 34

functional group of ketone

Answer 34

C=O e.g propanone

Question 35

what is formed in the oxidation of primary alcohols

Answer 35

aldehydes and water

Question 36

what is formed in the oxidation of secondary alcohols

Answer 36

ketones and water

Question 37

what is happens in the oxidation of K2Cr2O7/H+ acidified potassium dichromate with ethanol

Answer 37

Stage 1:
the oxidising agent (K2Cr2O7/H+) is reduced from dichromate (VI) which is orange to chromium (III) which is green and the aldehyde ethanal formed is immediately removed from the reaction to prevent further oxidation of the aldehyde by distillation to form ethanal and water (the whole reaction occurs in the distillation apparatus)
Stage 2
the aldehyde collected is then further oxidised using sulfuric acid and excess potassium dichromate (VI) (excess to ensure complete conversion) to make a carboxylic acid , ethanoic acid, this whole reaction is heated under reflux

Question 38

formula for acidified potassium dichromate

Answer 38

K2Cr2O7/H+

Question 39

formula to represent an oxidising agent

Answer 39

[O]

Question 40

why cant ketones oxides any further

Answer 40

as after the 1st oxidation there is no more hydrogen atoms bonded to the carbon that is bonded to the oxygen atom so we cannot oxidise ketones any further as no more hydrogen can be removed via oxidation

Question 41

why do aldehydes and ketones have low boiling points and are volatile

Answer 41

as they cannot form hydrogen bonds with water

Question 42

tests to distinguish between primary secondary and tertiary alcohols

Answer 42

acidified potassium dichromate - turns from orange to green in primary and secondary alcohols but stays orange in tertiary

Question 43

test to distinguish between secondary and primary alcohols

Answer 43

• by testing the aldehydes and ketones produced by primary and secondary alcohols
• fehlings solution turns from blue to an orange brown precipitate of copper (II) ions in aldehydes stays blue in ketones

Question 44

what forms in the dehydration of alchohols by concentrates phosphoric acid

Answer 44

forms an alkene and water

Question 45

what are dehydration reaction

Answer 45

when a substance loses water by the removal of water molecules from crystals or the h atom and the adjacent oh to form a double bond

Question 46

Draw sn1 and sn2 reaction and which halogenoalkanes undergo sn1 or sn2 reaction pathways

Answer 46

Primary halogenoalkane = Sn2
Tertiary halogenoalkane = Sn1
Secondary = both