Halogenoalkanes

Question 1

General formula for halogenoalkanes

Answer 1

CnH2n+1X (where X is the halogen such as Br or Cl or I)

Question 2

Primary RX

Answer 2

When the carbon atom bonded to the halogen atom is attached to one other carbon atom

Question 3

Secondary RX

Answer 3

When the carbon atom bonded to the halogen atom is attached to two other carbon atoms

Question 4

Tertiary RX

Answer 4

When the carbon atom bonded to the halogen atom is attached to three other carbon atoms

Question 5

What is the reactions of halogenoalkanes called and what happens

Answer 5

Nucleophilic substitution reactions, where the halogen atom is replaced by the nucleophile

Question 6

why is the C-X bond polar

Answer 6

Due to the difference in electronegativity. The halogen atom is more electronegative than the carbon atom

Question 7

What attacks the electron deficient carbon atom

Answer 7

Nucleophiles such as OH-, CN-, NH3 and H20

Question 8

What is a nucleophile

Answer 8

A species that donates a lone pair of electrons. A nucleophile is attracted to an electron deficient centre

Question 9

What happens in the hydrolysis of halogenoalkanes by aqeuous alkali

Answer 9

When a halogenoalkane is heated under reflux with AQ NaOH or KOH, the halogen atom is replaced by the nucleophile, OH-, and an alcohol is produced

Question 10

What happens in the hydrolysis of halogenoalkanes by water

Answer 10

Water is a poor nucleophile but it can slowly displace halide ions from a halogenoalkane

Question 11

What does hydrolysis mean

Answer 11

the C-X bond is broken using water

Question 12

What happens in the reaction between halogenoalkanes with alcoholic solution of potasisium cyanide (CN)

Answer 12

When a halogenoalkane is heated under reflux with KCN dissolved in ethanol, the halogen atom is replaced by the nucleophile CN-, and a nitrile is formed

Question 13

what is the condition needed for nucleophillic substituition

Answer 13

Heat under reflux

Question 14

What organic product is formed by nucleophilic substitution

Answer 14

Alcohol

Question 15

What are halogenoalkanes dissolved in

Answer 15

ethanol since they are insoluble in water

Question 16

What colour precipitate does AgCl form

Answer 16

White

Question 17

What colour precipitate does AgBr form

Answer 17

Cream

Question 18

What colour precipitate does AgI form

Answer 18

Yellow

Question 19

What is the condition needed for nucleophilic substitution of ammonia dissolved in ethanol

Answer 19

heat in a sealed tube

Question 20

What is ammonia used as in the nucleophilic substitution reaction

Answer 20

in the first step - as a nucleophile as it donates a lone pair of electrons

in the second step - as a base as it reacts with a H+ ion to form an NH4 + ion

Question 21

Describe the general test for a halogenoalkane

Answer 21

Add aqeuous sodium hydroxide and heat.

Then add excess dilute nitric acid followed by silver nitrate solution and note the colour of the precipitate formed.

Then add dilute aqeuous ammonia or concentrated aq ammonia to the precipitate and note the solubility of the precipitate

Question 22

When do you use dilute aq ammonia in the test for halogenoalkanes and why

Answer 22

To differentiate between white + cream precipitate or white + yellow precipitate as Cl is solube in dilute aq ammonia and Br is slightly soluble and I is insoluble

Question 23

When do you use concentrated aq ammonia in the test for halogenoalkanes and why

Answer 23

To differentiate between cream and yellow precipitates because Br is soluble in this but I is not soluble

Question 24

Why is sodium hydroxide solution added when testing for halogenoalkanes

Answer 24

To hydrolyse the halogenoalkanes and liberate the halide ion

Question 25

Why is dilute nitric acid added before the silver nitrate solution in the test for halogenoalknes

Answer 25

Nitric acid is added to neutralise the sodium hydroxide. The OH- ions would interfere with the test because they react with Ag+ to form a precipitate

Question 26

What is the trend in reactivity of halogenoalkanes

Answer 26

iodine > bromine > chlorine

Question 27

What bonds do halogenoalkanes contain

Answer 27

Polar bonds

Question 28

Why are iodoalkanes the most reactive

Answer 28

The bond enthalpy of C-I < C-Br < C-CI, so the C-I bond is the weakest. Less energy is needed to break the C-I bond, so iodoalkanes react faster than the others

Question 29

What happens to the C-X bond as you go down group 7

Answer 29

The bond gets larger and therefore weaker as you go down G7

Question 30

What happens in elimination reactions

Answer 30

X leaves as X- OH- removes a H+ ion from the C atom next to the C with the halogen An alkene is formed

Question 31

What is the condition needed for an elimination reaction

Answer 31

Heat

Question 32

How to tell if the reaction is elimination or nucleophilic substitution

Answer 32

Elimination reaction uses a reagent dissolved in ethanol Nucleophilic substitution uses an aq reagent

Question 33

Why are a mixture of alkenes formed when a secondary halogenoalkane reacts with KOH dissolved in ethanol

Answer 33

The H+ ion can be removed from a carbon atom on either side of the carbon atom that is bonded to the halogen atom

Question 34

What does OH- act as when a HX reacts with KOH dissolved in ethanol

Answer 34

a base as it removes a H+ ion

Question 35

What kind of compounds can elimination reactions never form

Answer 35

Branched compounds

Question 36

why were CFC's used as refridgerants

Answer 36

they are unreactive ,not toxic and not corrosive

Question 37

why are CFC'S no longer used

Answer 37

They are the main source of chlorine free radicals which react with ozone and break down the Earth's protective ozone layer

Question 38

Where is ozone formed

Answer 38

naturally in the upper atmosphere

Question 39

Why is ozone beneficial

Answer 39

It absorbs ultraviolet radiation

Question 40

What are the dangers of ultraviolet radiation entering the Earth's atmosphere

Answer 40

causes skin cancer and eye cataracts

Question 41

Why are HFC's used instead of CFC's now

Answer 41

HFC's are not broken down by ultraviolet radiation as the C-F bonds are very strong and no chlorine free radicals can be formed

Question 42

Equations which show how chlorine atoms catalyse decomposition of ozone

Answer 42

Cl* + O3 → ClO* + O2 ClO* + O3 → 2O2+ Cl*

Question 43

What is the overall reaction for the depletion of ozone

Answer 43

2O3 -----> 3O2 and chlorine free radicals are the catalyst

Question 44

Why might excess ammonia be used

Answer 44

To prevent further substitution