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A level- Chemistry > Haloalkanes > Flashcards

Haloalkanes Flashcards

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1
Q

How do you classify haloalkanes as primary or secondary or tertiary

A

You have to sonsider the carbocation that would be formed if you removed the halogen atom.
If the carbon that was connected to the halogen has:
1 carbon attached to it- primary
2 carbons attached to it- secondary
3 carbons attached to it- tertiary

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2
Q

Compare the boiling points of the haloalkanes to their corresponding alkanes and alcohols

A

Haloalkanes have higher boiling points than alkanes but lower than alcohols

  1. Alkanes are non-polar so only have weak london fores so their boiling point is the lowest
  2. Chloroalkanes are polar molecules so have permanent dipole-dipole forces which are stronger than London forces in this case
  3. Alcohols can form hydrogen bonds which is the strongest force so more energy is needed to break the bonds.
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3
Q

Describe the preparation of chloroethane from an alkane

A
  1. Use ethane and Cl2
  2. conditions- UV light
  3. Cl2 + C2H6 –> C2H5Cl +HCl
  4. name- radical substitution
  5. Initiation, Propagation and Termination
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4
Q

Describe the preparation of chloroethan from an alkene

A
  1. Use ethene and HCl
  2. Conditions- Room temp + pressure
  3. C2H4 + HCl –> C2H5Cl
  4. Name- Electrophilic addition
  5. Double bond breaks and Cl and H atoms join on
  6. 100% atom economy
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5
Q

Why are haloalkanes generally more reactive than alkanes

A

Because the c-x bond strength is generally lower than the C-H in a normal alkane.
But C-F is stronger so fluoroalkanes are less reactive.

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6
Q

Define electronegativity

A

The ability of an atom to attract a shared pair of electrons in a covalent bond.

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7
Q

Define a hydrolysis reaction

A

Hydrolysis is a chemical reaction involving water or an aqueous solutions of a hydroxide that causes the breaking of a bond in a molecule.

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8
Q

Write the equations in skeletal form of these reactions:
1-bromopropane + water
2- Iodobutane +NaOH (aq)
2-Chloro-2-methylpropane + KOH (aq)

A
  1. forms propan-1-ol + HBr
  2. Forms Butan-2-ol + NaI
  3. Forms 2-methylpropan-2-ol
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9
Q

What type of reaction is hydrolysis of haloalkanes

A

It is substitution - halogen molecule is replaced by an -OH group

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10
Q

What reagent and conditions do you need to prepare an alcohol by hydrolysis of a haloalkane

A

Reagent- NaOH (aq)

Conditions- Heat under reflux

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11
Q

Define the term reflux

A

The continual boiling and condensing of a reaction mixture back to the original container to ensure that the reaction takes place without the contents of the flask boiling dry.

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12
Q

Why do organic reactions often require heating under reflux

A

Organic compounds have simple molecular structures so have lower boiling points compared to the salts which have ionic compounds.

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13
Q

Define a nucleophile

A

A nucleophile is an electron pair donor

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14
Q

Draw the mechanism for the reaction of 1-chloropropane with aqueous sodium hydroxide

A

p17 of study pack

  1. carbon has delta + sign
  2. Chlorine has delta - sign
  3. OH has a - sign and a pair of electrons with a curly arrow from them pointing to the carbon
  4. There is also a curly arrow pointing from the carbon to chlorine bond to the chlorine atom
  5. It forms propan-1-ol + Cl-
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15
Q

What type of bond breaking is involved in the hydrolysis reaction of a haloalkane

A

Heterolytic fission

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16
Q

Describe the two reaction used to compare the rate of hydrolysis of primary haloalkanes

A
  1. First reaction: nucleophilic substitution using water as the nucleophile - haloalkane + water –> alcohol + H+ + X-
  2. Second reaction: X- (aq) + Ag+ (aq) –> AgX (s)
17
Q

Which haloalkane reacts fastest

A

Iodo then bromo then chloro because the C-Cl bond is much stronger than C-I and C-Br.

18
Q

Why is a hot water bath used in the reaction to find the rate of hydrolysis

A

To keep the temp constant of different experiments and to speed up the rate of reaction.

19
Q

What co-solvent is used and why in the rate of hydorlysis experiment

A

Ethanol- as AgNO3 and haloalkane are not both soluble in water, so you use ethanol as well as water.

20
Q

What do you use as a source of OH- ions in the rate of hydrolysis experiment

A

H2O is used- the O has a spare pair of electrons

It is used because OH- ions react with Ag+ ions to form a brown solid mess.

21
Q

Explain the roles played by the aqueous silver nitrate solution in the rate of hydrolysis experiment

A
  1. The aqueous solution provides the water as the reactant in a hydrolysis experiment.
  2. The silver ions from the AgNO3 traps the halide ion released on reaction by forming a precipitate, allowing us to see that the hydrolysis reaction has taken place.
22
Q

Why did AgNO3 not form a precipitate immediately when added to haloalkanes

A

Because first nucleophilic substitution needs to happen so the C-X bond needs to break before the ions can react with it.

23
Q

Write an experiment to find the rate of hydrolysis using different haloalkanes

A
  1. Reagents- AgNO3 (aq)
  2. Conditions- ethanol as a co-solvent and warming
  3. Observations- Measure time taken for a silver halide precipitate to form
  4. Equation for hydrolysis- CH3CH2X + H2O –> CH3CH2OH + H+ + X-
  5. Equation from precipitate reaction- Ag+ + X- –> AgX
  6. Trend in reactivity- Iodo is fastest chloro is slowest
  7. Why- The strength of the C-X bond affects the reactivity. C-I is a weaker bond strength than C-Cl (strength depends on size of atom not polarity)
24
Q

Where is ozone layer found

A

In the stratosphere approx 10-40 km above the Earth.

25
Q

Write equations to show how ozone is formed

A
  1. O2 –> 2O. (UV light over arrow) Oxygen molecules are broken down using UV radiation to form radicals.
  2. O2 + O. <=> O3 (UV light over arrow) Oxygen molecules combine with the oxygen radicals to form ozone.
26
Q

What would the dangers be in the abcense of the ozone layer

A

Increased genetic damage and greater risk of skin cancer in humans.

27
Q

What do CFC and HCFC stand for

A

CFC- Chlorofluorocarbons

HCFC- Hydrochlorofluorocarbon

28
Q

Why do CFCs and HCFCs persist in the lower atomsphere

A

Because the C-F and C-Cl bonds are strong so the molecules don’t break down easily.

29
Q

Write equation to show formation of chlorine radicals and name it

A

Photodissociation
CF2Cl2 → CF2Cl• + Cl•
using UV light and homolytic fission

30
Q

Write equations to show breakdown of ozone using Cl radicals

A

Cl• + O3 → ClO• + O2

ClO• + O → Cl• + O2

O3 + O → 2O2 overall

31
Q

What role does the Cl radical take in this reaction

A

A Catalyst - unchanged at the end.

32
Q

Where does NO in the stratosphere come from

A

From lightning and aircraft travel

33
Q

Write equation to show the breakdown of ozone using NO radicals

A

NO• + O3 → NO2• + O2

NO2• + O → NO• + O2

O3 + O → 2O2 overall

A level- Chemistry (31 decks)

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