15 - Haloalkanes

Question 1

Define nucleophile

Answer 1

electron pair donor

Question 2

Examples of nucleophiles

Answer 2

Hydroxide ions / water molecules / ammonia molecules

Question 3

How are haloalkanes classifies

Answer 3

Same as alcohols = primary/secondary/tertiary

Question 4

How do haloalkanes react

Answer 4

Nucleophilic Substitution

Question 5

Why are haloalkanes reactive

Answer 5

• Halogens more reactive than carbons = polar bond
• Carbon = slightly positive so can attract species with lone pairs
• When a haloalkane reacts with a nucleophile, the nucleophile replaces the halogen in a substitution reaction = NUCLEOPHILLIC SUBSTITUTION

Question 6

Define hydrolysis

Answer 6

involves water or aqueous solution of a hydroxide.

Question 7

Product of hydrolysis of haloalkanes

Answer 7

Alcohols + sodium halide

Question 8

Conditions for hydrolysis of haloalkanes

Answer 8

aqueous sodium hydroxide, heated under reflux (

Question 9

Why do we heat under reflux

Answer 9

Reaction slow at room temp

Question 10

Equation for 1-bromobutane reacting with sodium hydroxide

Answer 10

.1-bromobutane + NaOH -> butan-1-ol + NaBr

Question 11

What type of haloalkane does nucleophilic substitution work on

Answer 11

Primary

Question 12

How to draw nucleophilic substitution mechanism

Answer 12

• Only need to show the OH ion
• Draw dipole between C-X (halogens are more electronegative than C)
• Curly arrow from lone pair on OH- to C𝛅+
• Curly arrow from C-X bond to X
• Draw final product and the X- ion

Question 13

Draw the mechanism for 1-bromoethane reacting with sodium hydroxide

Answer 13

Question 14

What does the rate of hydrolysis depend on

Answer 14

C-X bond strength

Question 15

Strengths of all relative bonds

Answer 15

• C-F = very strong bond = unreactive
• C-Cl = strong bond = relatively unreactive
• C-Br = weaker bond = reacts faster
• C-I = weakest bond = reacts even faster

Question 16

Does the rate of hydrolysis increase or decrease as c-x bind strength increases

Answer 16

Decreases

Question 17

Why does the rate of hydrolysis increases as the C-X bond strength decrease

Answer 17

as less energy is required to break the C-X bond.

Question 18

How to measure the rate of hydrolysis in primary haloalkanes

Answer 18

• Undergo the reaction with aqueous silver nitrate
- add ethanol

Question 19

What acts as the nucleophile in the reaction with aqueous silver nitrate

Answer 19

o water (from the nitrate) acts as a nucleophile

Question 20

Why do we add ethanol

Answer 20

• Haloalkanes are insoluble in water = Need ethanol (solvent) to allow the water and haloalkane to mix (prevents the mixture from separating into two layers)

Question 21

What would we see in the reaction with silver nitrate

Answer 21

silver halide precipitate forms
• Ag+(aq) + X-(aq) = > AgX(s)

Question 22

Colours if precipitates

Answer 22

• Chlorine = white
• Bromine = cream
• Iodine = yellow

Question 23

Which precipitate forms more quickly

Answer 23

The yellow precipitate will form the fastest - Idiome

Question 24

Why would the yellow precipitate form the fastest

Answer 24

because the C-I bond has the lowest bond enthalpy, so it is the easiest to break and will cause the I- ions to form the fastest

Question 25

Strength of c-x bond not only factor effecting rate of hydrolysis?

Answer 25

Structural isomers

Question 26

Do Tertiary haloalkanes more readily accept OH - ion

Answer 26

Yes = because more stable than primary

Question 27

How does the c-x bond break

Answer 27

heterolytic fission

Question 28

Draw the mechanism for 2-chloro-2-methyl propane

Answer 28

Question 29

Define organohalogen

Answer 29

at least one halogen atom joined to carbon chain

Question 30

Uses of organohalogen

Answer 30

General solvents, dry cleaning solvents, making polymers, flame retardants, refrigerants, pesticides

Question 31

Why have they become a focus of concern

Answer 31

• Unnatural and not broken down by environment

Question 32

Where is the ozone layer found

Answer 32

in the outer edge of the stratosphere

Question 33

• Only a tiny fraction of gases making up the ozone layer is

Answer 33

ozone (O3)

Question 34

Function of ozone gas

Answer 34

absorbs most of the UV-B from sun

Question 35

What is UV-B

Answer 35

biologically damaging, causes sunburn

Question 36

Therefore, why is the depletion of the ozone layer dangerous

Answer 36

More UV-B = could lead to increased genetic damage and/or an increased risk of skin cancer.

Question 37

R/s between ozone + UV

Answer 37

• Ozone is continually formed and broken down by UV

Question 38

Equation for ozone continually being formed and broken down by UV

Answer 38

• O = RADICAL THIS IS NORMAL

Question 39

Initially, what breaks down to get the O radical

Answer 39

high energy UV O2 -> 2O THIS IS NORMAL

Question 40

What are CFC’s

Answer 40

are very stable as C-X bond is strong

Question 41

CFC's remain stable until..

Answer 41

they reach the stratosphere

Question 42

What happens when the CFC come into con tact with UV

Answer 42

• UV radiation provides enough energy to break C-X bond by homolytic fission

Question 43

What forms after the C-X bond is broken by homolytic fission

Answer 43

Chlorine radicals

Question 44

Effect of chlorine radicals = catalyse break down of ozone layer

Question 45

Initial step

Answer 45

• Photodissociation – triggered by UV radiation • CF2Cl2 -> CF2Cl∙ + Cl∙ • C-Cl bond has lowest enthalpy thus is the bond that breaks

Question 46

Propagation step

Answer 46

• Very reactive Cl∙ reacts with ozone in 2 step process: • Cl∙ + O3 -> ClO∙ + O2 • ClO∙ + O -> Cl∙ + O2 • OVERALL: O3 + O ->2O2 • This repeats in a cycle = chain reaction.

Question 47

Effect of nitrogen oxide

Answer 47

oxide radicals (NO∙) are formed naturally during lightning strikes and from aircraft travel in stratosphere • Breakdown of ozone is similar reaction to chlorine radicals

Question 48

Probation step with nitrogen oxide

Answer 48

- NO∙ + O3 -> NO2∙ + O2 - NO2∙ + O -> NO∙ + O2 - OVERALL: O3 + O -> 2O2

Question 49

why are oxygen radicals called diradicals

Answer 49

each atom has 2 unpaired electrons