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Organic Chemistry > Halogenoalkanes 1 > Flashcards

Halogenoalkanes 1 Flashcards

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1
Q

What is the general formula of a haloalkane

A

CnH2n+1X Where X is the halogen

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2
Q

What feature determines most of the physical properties of a halogenalkane

A

The polarity of the C-X bond.

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3
Q

Why is the C—X bond in a halogenalkane polar

A

The C—X bond is polar because halogens are more electronegative than carbon.

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4
Q

What happens to the C—X bond as you go down the halogen group (7)

A

It gets less polar

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5
Q

Define electronegativity

A

Electronegativity is a chemical property that describes the tendency of an atom or a functional group to attract electrons towards itself

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6
Q

What are the main intermolecular forces of attraction in halogenalkane

A

Dipole-dipole attractions and Van Der Waals forces

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7
Q

Describe the solubility of halogenalkanes

A
  • The polar C—X bond is not polar enough to make halogenalkanes soluble in water.
  • They are insoluble because they cannot form hydrogen bonds with the water.
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8
Q

Why are halogenalkanes used as dry-cleaning fluids

A

They remove oily stains. Oil is a mixture of hydrocarbons and halogenalkanes mix with hydrocarbons

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9
Q

What two things does the boiling point of a halogenalkane depend on

A
  • the length of the carbon chain
  • the halogen present
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10
Q

How does the chain length affect the boiling point of a halogenalkane

A

The boiling point increases as the chain length increases. This is because the larger the molecule, the greater the number of electrons present and therefore the larger the Van Der Waals forces.

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11
Q

How does the halogen present affect the boiling point of a halogenalkane

A

The boiling point increases going down the halogen group. The size and the molecular mass of the halogen members increase as do the number of electrons present down the homologous series and therefore the Van Der Waals forces are stronger.

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12
Q

How do the boiling points of halogenalkanes compare to alkanes with similar length carbon chains

A

Halogenalkanes have higher boiling points because they have higher relative molecular masses and they are more polar.

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13
Q

Why does increased branching lower the boiling points of halogenalkanes

A

Because branched chains cannot pack together as closely so as unbranched chains and so the Van de Waals forces are not as effective.

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14
Q

What two factors affect how readily the C—X bond reacts

A
  • The C—X bond polarity
  • The C—X bond enthalpy
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15
Q

Define nucleophile

A

A nucleophile is an electron pair donor

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16
Q

Describe how bond polarity affects the reactivity of the C—X bond in a halogenalkane

A
  • The polarity of the C—X bond would suggest that the C—F bond is most reactive as it is most polar.
  • This is because fluorine is the most electronegative halogen.
  • This argument would suggest that the C—I bond is least reactive because it is least polar.
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17
Q

Describe the polarity of the C—X bond in a halogenalkane

A
  • The halogens are more electronegative than carbon.
  • This means that the carbon bonded to the halogen has a partial positive charge- it is electron deficient.
  • This means that it can be attacked by reagents that are electron rich or have electron rich areas (nucleophiles)
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18
Q

Define bond enthalpy

A

Bond enthalpy is the energy required to break a particular covalent bond in one mole of molecule in a gaseous state.

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19
Q

Explain the trend in bond enthalpies for the C—X bond in a halogenalkane as you go down group 7

A

The bond enthalpies get weaker going down the group. This is because in the smallest atoms (fluorine) the shared electrons are strongly attracted to the fluorine nucleus.
- As you go down the group, the shared electrons get further away from the nucleus and therefore the bonds get weaker.

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20
Q

Describe how the bond enthalpies affect the reactivity of the C—X bond

A
  • The bond enthalpies would predict that iodo-compounds, with the weakest bonds are the most reactive and fluoro-compounds, with the strongest bonds are least reactive.
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21
Q

What is the conflict between trends in reactivity due to bond enthalpies and bond polarity and how have experiments helped with this.

A
  • Bond polarity’s suggest that fluoro compounds are most reactive whereas bond enthalpies suggest iodo-compounds are most reactive
  • Experiments have confirmed that reactivity increases going down the group and this means that bond enthalpy is a more important factor than bond polarity.
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22
Q

List the features of a nucleophile

A
  • A nucleophile is either a negatively charged ion or has an atom with a negative enthalpy charge.
  • A nucleophile has a lone (unshared) pair of electrons which it can use to form a covalent bond.
  • The lone pair is situated on an electronegative atom.
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23
Q

Summarise what happens in a nucleophile substitution reaction in a halgoenalkane

A

The nucleophile replaces the halogen atom in a halogenalkane.

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24
Q

What do the curly arrows in reaction mechanisms show

A

The movement of electron pairs in organic reactions.

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25
Q

What is the leaving group in the nucleophile substitution of halogenalkanes

A

The halide ion

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26
Q

Describe what the rate of substitution depends on and the pattern in the nucleophilic substitution of halogenalkanes

A
  • The rate of substitution depends on the halogen.
  • Fluoro compounds are un reactive due to the strength of the C—F bond.
  • Then, as you go down the group the rate of substitution increases because the strength of the C—X bond decreases.
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27
Q

What is the nucleophile in the substitution of a halogenalkane with sodium hydroxide

A

Hydroxide ion= - :OH

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28
Q

What are the conditions needed for the nucleophilic substitution of a halogenalkane with sodium or potassium hydroxide

A
  • The NaOH or KOH must be aqueous
  • The mixture must be warmed as the reaction occurs too slowly at room temperature.
29
Q

What is the product of the nucleophilic substitution of halogenalkanes with sodium or potassium hydroxide

A

An alcohol

30
Q

Summarise what happens during the nucleophilic substitution of a halogenalkane with sodium or potassium hydroxide

A

The halogen atom is replaced by an OH group to form an alcohol.

31
Q

What is the overall equation for the nucleophilic substitution of a halogenalkane with sodium or potassium hydroxide

A

R—X + NaOH —> R—OH + NaX

32
Q

List the features of the nucleophilic substitution of a halogenalkane with sodium or potassium hydroxide

A
  • Nucleophile: Hydroxide ion - :OH
  • Reagent: NaOH or KOH
  • Conditions : the reagent must be aqueous and the mixture must be warm
  • Product: Alcohol
  • What happens: The halogen atom is replaced by an OH group to form an alcohol
  • Overall equation: R—X +NaOH —> R—OH + NaX
  • Mechanism : Nucleophilic substitution.
33
Q

What is the nucleophile in the substitution reaction of halogenalkanes with potassium cyanide

A

Cyanide ion= - :CN

34
Q

What are the conditions needed for the nucleophilic substitution of halogenalkanes with potassium cyanide

A

Warmed in an aqueous alcoholic solution of potassium cyanide.

35
Q

What is the product of the nucleophilic substitution of halogenalkanes with potassium cyanide

A

Nitrile

36
Q

Summarise what happens in the nucleophilic substitution of halogenalkanes with cyanide ions

A

The halogen atom is replaced by the CN group to form a nitrile.

37
Q

What is the overall equation for the nucleophilic substitution of halogenalkanes with potassium cyanide

A

R—X + KCN —> R—CN + KX

38
Q

List the features of the nucleophilic substitution of a halogenalkane with potassium cyanide

A
  • Nucleophile: Cyanide ion :CN-
  • Reagent: Potassium cyanide
  • Conditions: warmed in an aqueous alcoholic solution of potassium cyanide
  • Product: Nitrile
  • What happens: the halogen atom is replaced by the CN group to form a nitrile.
  • Overall equation: R—X + KCN —> R—CN + KX
  • Mechanism: Nucleophilic substitution
39
Q

What is the nucleophile in the substitution reaction of a halogenalkane with ammonia

A

Ammonia :NH3

40
Q

What are the conditions needed for the nucleophilic substitution of halogenalkanes with ammonia

A

Reaction of the halogenalkane with an excess concentrated solution of ammonia in ethanol. Reaction should be carried out at pressure in a sealed container.

41
Q

What is the product of the nucleophilic substitution of a halogenalkane with ammonia

A

An amine

42
Q

Summarise what happens in the nucleophilic substitution reaction of a halogenalkane with ammonia

A
  • The first molecule of NH3 causes the halogen atom to be replaced by an NH2 group.
  • The second molecule of NH3 leads to the formation of NH4X
43
Q

What is the overall equation for the nucleophilic substitution of a halogenalkane with ammonia

A

R—X + 2NH3 —> R—NH2 + NH4X

44
Q

List the features of the nucleophilic substitution of a halogenalkane with ammonia

A
  • Nucleophile= :NH3
  • Reagent= Ammonia, NH3
  • Conditions= reaction of the halogenalkane with an excess concentrated solution of ammonia in ethanol. Reaction should be carried out at pressure in a sealed container.
  • Product: Amine
  • What happens: the first molecule of NH3 causes the halogen atom to be replaced by an NH2 group. The second molecule of NH3 leads to the formation of NH4X
    -Overall equation: R—X +2NH3 —> R—NH2 + NH4X
  • Mechanism: Nucleophilic substitution
45
Q

Why are nucleophilic substitution reactions useful

A

They are a way of introducing new functional groups into organic compounds

46
Q

Describe simply what happens when halogenalkanes undergo an elimination reaction

A

A hydrogen halide is eliminated from the molecule, leaving a double bond in its place so that an alkene is formed.

47
Q

Define elimination reaction

A

A reaction where a molecule is lost from a saturated molecule to form an unsaturated molecule.

48
Q

What acts as the base when halogenalkanes undergo elimination

A

:OH-

49
Q

What is the reagent when halogenalkanes undergo elimination

A

Sodium hydroxide (NaOH) or Potassium hydroxide (KOH)

50
Q

What are the conditions needed for halogenalkanes to undergo elimination

A
  • No water present
  • NaOH or KOH dissolved in ethanol and mixed with the halogenalkane
  • The mixture is heated.
51
Q

What is the product of the elimination of halogenalkanes

A

An alkene

52
Q

Describe what happens when halogenalkanes undergo elimination

A
  • The OH- ion uses its lone pair to form a bond with one of the hydrogen atoms on the carbon next to the C—X bond. These hydrogen atoms have a small positive charge.
  • The electron pair from the C—H bond now becomes part of thr C=C double bond.
  • The halogen takes the pair of electrons in the C—X bond and leaves a halide ion (the leaving group)
53
Q

What is the overall equation for the elimination of halogenalkanes

A

R—X(CH2) + KOH —> R=CH2 + KX + H20

54
Q

List the features of the elimination of a halogenalkane

A
  • Base= :OH-
  • Reagent= NaOH or KOH
  • Conditions= No water present. NaOH or KOH dissolved in ethanol and mixed with the halogenalkane. The mixture is heated.
  • Product= Alkene
  • What happens= The OH ion uses its lone pair to form a bond with one of the hydrogen atoms on the carbon next to the C—X bond. These hydrogen atoms have a small positive charge. The electron pair from the C—H bond now becomes part of a C=C bond. The bromine takes the pair of electrons in the C—X bond and leaves a halide ion.
  • Overall equation= R—X(CH2) + NaOH —>R=CH2 + KX + H20
55
Q

In terms of the hydroxide ions, which conditions favour elimination and which favour nucleophilic substitution

A
  • The hydroxide ions being at room temperature and dissolved in water (aqueous) favours nucleophilic substitution.
  • The hydroxide ions being at an high temperature and dissolved in ethanol favours elimination.
56
Q

What are chlorofluorocarbons

A

Halogenalkanes containing both chlorine and fluorine atoms, but no hydrogen.

57
Q

How reactive are CFCs under normal conditions

A

Very unreactive.

58
Q

What were CFCs used for

A
  • The short chain ones are gases and were used for aerosol propellants, refrigerants and blowing agents.
  • the longer chain ones were used as dry cleaning and de-greasing solvents.
59
Q

Why are CFCs bad

A

They end up in the atmosphere and the chlorine atoms decompose ozone O3 in the stratosphere. This has caused a hole in the earths ozone layer.

60
Q

Explain why haloalkanes have a higher boiling point than their equivalent alkanes

A
  • Alkanes are non-polar, so only Van Der Waals forces act between the molecules which are relatively weak and require little energy to break.
  • This means alkanes have relatively low boiling points.
  • Haloalkanes also contain Van Der Waals.
  • Because of the polarity of the C—X bind, haloalkanes also have permanent dipole-dipole interactions acting between molecules.
  • Permanent dipole-dipole interactions are stronger than Van Der Waals and require more energy to break so haloalkanes have higher boiling points than alkanes.
61
Q

Summarise what we can do to measure the rate of hydrolysis (nucleophilic substitution with -:OH of a haloalkane)

A

To measure the rate of hydrolysis, we monitor the formation of the halide ion using aqueous silver nitrate.

62
Q

What does the rate of hydrolysis in a haloalkane depend on

A

The halogen present.

63
Q

Describe the experiment that allows us to measure the rate of hydrolysis of a haloalkane

A

1) In three different test tubes add 1cm^3 ethanol
2) Add 0.1cm^3 of each haloalkane to these test tubes (an iodoalkane, chloroalkane and bromoalkane preferably with the same number of carbon atoms)
3) place each treat tube in a water bath at 60 degrees Celsius.
4) In a separate test tube, add aqueous silver nitrate and place into the same water bath and wait for ten minutes until all of the solution reach the same temperature.
5) Add 1cm^3 of aqueous silver nitrate to each test tube and start timing.
6) The water reacts with the haloalkane by nucleophilic substitution which will then react with the silver iterate to form the coloured precipitate. Whichever precipitate forms most rapidly indicates the haloalkane in which the rate of substitution is highest.

64
Q

Sate what results you would obtain from the experiment measuring the rate of hydrolysis in different haloalkanes

A
  • Iodine forms its yellow precipitate most rapidly, then bromine forms its cream, then chlorine it’s white.
  • This is due to the C—X bond enthalpies.
65
Q

Why must an excess ammonia be used in the nucleophilic substitution of ammonia with a haloalkane

A
  • Because in the product amine, the nitrogen atom still has a lone pair of electrons.
  • This means it can also act as a nucleophile, reacting with any unreacted haloalkane.
  • By using excess ammonia, you make it more likely that the haloalkane reacts with ammonia instead of the product amine.
66
Q

What is the formula of the chemical ozone

A

O3

67
Q

Give three equations that show how chlorine free radicals destroy the ozone layer

A

1) *Cl + O3 —> *ClO +O2
2) *ClO + O —> *Cl + O2
3) *ClO + O3 —> *Cl +2O2

68
Q

What is the overall equation for the decomposition of ozone

A

2O3 —> 3O2

Organic Chemistry (14 decks)

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