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chemistry a - level > Topic 6D - Halogenoalkanes (Organic Chemistry) > Flashcards

Topic 6D - Halogenoalkanes (Organic Chemistry) Flashcards

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1
Q

Are Halogenoalkanes soluble?

A

No (they are slightly soluble)

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2
Q

Why are Halogenoalkanes only slightly soluble in water?

A

This is due to the fact the polar interaction between the water molecule and Halogenoalkane are weaker than the hydrogen bonding present in water. Additionally, Halogenoalkanes cannot form hydrogen bonds as they do not contain an H - F,O or N.

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3
Q

What are hydrogen bonds?

A

They are the strongest type of intermolecular force/bonding

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4
Q

How are hydrogen bonds formed?

A

When you have an H - F,O or N in a molecule it is able to form hydrogen bonds. The hydrogen is attached directly to an extremely electronegative
element causing the hydrogen to gain a delta + charge. Each of the elements that the Hydrogen is attached to also has at least one ‘active’ lone pair. As the delta + hydrogen is formed it becomes extremely attracted to the delta - element of another molecule, for example; water.

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5
Q

What is a primary Halogenoalkane?

A

The halogen atom is attached to the first carbon in the chain (the Hl - C bond has one carbon attached to the bonding carbon)

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6
Q

What is a secondary Halogenoalkane?

A

The halogen atom is attached to a carbon that is not on the end of the chain but not at a branch (the Hl - C bond has two carbons attached to the bonding carbon)

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7
Q

What is a tertiary Halogenoalkane?

A

The halogen atom is bonded to a carbon at a branch in the chain (the Hl - C bond has three carbons attached to the bonding carbon)

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8
Q

Do the isomers 1-bromobutane, 2-bromobutane, and 2-bromo-2-methylpropane decrease in boiling point as you go down them?

A

Yes

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9
Q

Why do the isomers 1-bromobutane, 2-bromobutane, and 2-bromo-2-methylpropane decrease in boiling point as you go down them?

A

This is because as the Halogenoalkane becomes more branched as the surface area of it decreases (for the same number of electrons in the molecule) which in turn decreases the strength of London dispersion forces as there is a smaller area over which they can form. Therefore the intermolecular forces are easier to overcome in branched molecules.

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10
Q

What are London Forces?

A

The weakest type of intermolecular force.

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11
Q

How do London Forces form?

A

In molecules their are constant electron fluctuations that cause instantaneous dipoles to form within them leaving the molecule with a delta positive and negative side. This dipole can then induce another dipole ( an induced dipole ) into another molecule.

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12
Q

Do Halogenoalkanes have higher or lower boiling points that alkanes? Why?

A

They have higher boiling points as the polarity of the carbon - halogen bond present leads to stronger intermolecular forces

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13
Q

What is the definition of electronegativity?

A

The tendency of an atom to attract a bonding pair of electrons in a covalent bond

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14
Q

What dipoles are present in Halogenoalkanes?

A

There will be a dipole in the C- X bond ( halogen ) the X will have a delta - pole and the C will have a delta + pole.

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15
Q

What is a nucleophile?

A

A nucleophile is an ion or compound with a negative charge and a lone pair of electrons (electron donating) that has a tendency to attack atoms which are electron deficient, with a partially positive charge for example; Hydroxide ion, Water Molecule, Cyanide ion, Ammonia molecule

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16
Q

The carbon with a delta + charge in halogenoalkanes is susceptible to be attacked by…

A

Nucleophiles such as; OH-, NH3, H2O

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17
Q

Halogenoalkane + potassium Hydroxide -> Alcohol + potassium Halide is what type of reaction?

A

Nucleophillic substitution

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18
Q

What reagant is need for the Nucleophillic substitution reaction between a Halogenoalkane and KOH/ NaOH -> Alcohol + Metal Halide?

A

Aqueous NaOH or KOH

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19
Q

What conditions are needed for the Nucleophillic substitution reaction between a Halogenoalkane and KOH/ NaOH -> Alcohol + metal halide?

A

Heat under reflux

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20
Q

What is the nucleophile in the reaction of Halogenoalkane and KOH/ NaOH -> Alcohol + metal halide?

A

OH-

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21
Q

How can you test for the presence of Halide ions on the reaction of Halogenoalkane and KOH/ NaOH -> Alcohol + metal halide?

A

By adding silver nitrate (AgNO3) to the solution which will form a precipitate

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22
Q

Which group is the ‘leaving group’ in the reaction of Halogenoalkane and KOH/ NaOH -> Alcohol + metal halide?

A

The Halogen (Chlorine, Bromine, Iodine)

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23
Q

What is the definition of a substitution reaction?

A

It is a chemical reaction where and atom or functional group of a molecule is replaced by another atom of functional group

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24
Q

What is the definition of a ‘leaving group’?

A

The atom or group of atoms substituted out of the molecule that takes a pair of electrons with it.

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25
Q

What are the colours of the precipitates that from when Silver Nitrate is added to a solution that contains a Halide?

A

Chlorine - White precipitate of Silver Chloride
Bromine - Cream precipitate of Silver Bromide
Iodine - Yellow precipitate of Silver Iodide

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26
Q

What does the mechanism of the Nucleophillic substitution of Halogenoalkanes depend on?

A

Whether the Halogenoalkane is primary, secondary or tertiary

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27
Q

What Nucleophillic substitution reaction mechanism do primary/ secondary Halogenoalkanes use?

A

Sn2
S - substitution
N - Nucleophillic
2 - number of species acting in the initial stage of the reaction

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28
Q

What Nucleophillic substitution reaction mechanism do tertiary/ secondary Halogenoalkanes use?

A

Sn1
S - substitution
N - Nucleophillic
1 - number of species acting in the initial stage of the reaction

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29
Q

How many stages does Sn2 have?

A

1

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30
Q

How many stages does Sn1 have?

A

2

31
Q

What determines the rate of any reaction?

A

The rate of the slowest step

32
Q

What determines the rate of the Nucleophillic substitution reaction via an Sn2 mechanism?

A

The reaction only has one step, therefore the rate of reaction is dependant on;
- the strength of the nucleophile (OH- is a stronger nucleophile in comparison to Water molecules)
- How readily the leaving group leaves

33
Q

What determines the rate of the Nucleophillic substitution reaction via an Sn1 mechanism?

A

The rate determining step is the first step. Therefore how fast the reaction happens is governed by how fast the Halogenoalkane ionises.

34
Q

What happens in the first stage of the Sn1 mechanism in the reaction of a tertiary Halogenoalkane and KOH/ NaOH -> Alcohol + metal halide?

A

In the first stage a small proportion of the Halogenoalkane ionises to give a carbocation (intermediate) and Halide ion. This stage is relatively slow.

35
Q

What happens in the second stage of the Sn1 mechnism in the reaction of a tertiary Halogenoalkane and KOH/ NaOH -> Alcohol + metal halide?

A

In the second step, one the carbocation has formed is reacts immediately with a Nucleophile. The lone pair of electrons on the nucleophile are strongly attracted towards the positive carbon and forms a new covalent bond with it.

36
Q

Why is the Sn1 mechanism only likely to be used for tertiary Halogenoalkanes?

A
  • A 3° carbocation is more stable that a 2° carbocation which is more stable than a 1° carbocation
  • This is due to the carbocation having more alkyl groups that, through the positive inductive effect, donate electrons to the positive carbon and stabilise the +ve charge
    This means that the 1st step of the Sn1 mechanism is more likely to happen fo r the 3° carbocation and so the carbocation is able to persist for nucleophilic attack
37
Q

What is the trend in bond polarity for halogenoalkanes with the same structure but a different halogen from lowest to highest?

A

CH3I, CH3Br, CH3Cl, CH3F

38
Q

Why does bond polarity increase from CH3I, CH3Br, CH3Cl to CH3F?

A

As you move across the Halogenoalkanes the bond polarity increases due to the difference in electronegativity of the atoms increases

39
Q

What does the polarity of the C - X (halogen) bond explain?

A

The polarity explains why the Halogenoalkanes undergo nucleophilic substitution

40
Q

How do you determine the relative rates of reaction of the Halogenoalkanes?

A

You have to determine how easy it is to break the C - X bond

41
Q

What is the trend in bond enthalpy as you go from a C-F bond to C-I bond?

A

The bond enthalpy decreases

42
Q

Why does the bond enthalpy decrease as you go from a C-F bond to a C-I bond?

A
  • As you go down the group C-X bond length increases
  • Due to the halogen atom having more shells, larger atomic radius and more shielding as the atom has more electrons
  • This means that the bonding pair of electrons are further away from the halogen nucleus and therefore the electrostatic forces of attraction between the C- X nuclei and the bonding pair of electrons is weaker
  • Therefore the covalent bond is weaker so the enthalpy decreases
43
Q

What is the trend in reactivity for halogenoalkanes with the same structure but a different halogen from lowest to highest?

A

CH3F, CH3Cl, CH3Br, CH3I

44
Q

What does the rate of the nucleophilic substitution reaction depend on when the halogen is the same but it has a different structure?

A

Whether the starting Halogenoalkane has a primary, secondary or tertiary structure

45
Q

What is the trend in reactivity of the 1°,2° and 3° Halogenoalkanes from slowest/ least reactive to fastest/ most reactive?

A

1°,2°,3°

46
Q

Why does the trend in reactivity of the 1°,2° and 3° Halogenoalkanes increase from slowest/ least reactive to fastest/ most reactive in that order?

A

This is because the 3° carbocation goes via a more stable carbocation so it reacts the fastest.

47
Q

How to you measure the rate of hydrolysis for different Halogenoalkanes?

A

You add AgNO3 (s) and observe the rate at which the silver halide forms

48
Q

What nucleophile do we use in hydrolysis?

A

H2O: (a lone pair)

49
Q

Why do we use H2O as the nucleophile in hydrolysis?

A

As it is not as good of a nucleophile so you can see the difference in reactivities of the Halogenoalkanes more easily

50
Q

Why can silver nitrate be used in a hydrolysis reaction?

A
  • it plays no part in the actual hydrolysis reaction
  • it forms a precipitate (silver halide) when the halide ion becomes present and has left
  • the halide ion is only present once/ if a substitution reaction has occurred
  • Therefore the rate of precipitate formation is directly related to the rate of substitution
51
Q

What is the type of reaction of; Halogenoalkane + KOH/NaOH (in ethanol) -> Alkene + H2O + halide ion?

A

Nucleophilic elimination

52
Q

What is the reagant in the reaction; Halogenoalkane + KOH/NaOH (in ethanol) -> Alkene + H2O + halide ion?

A

KOH or NaOH

53
Q

What are the conditions for the reaction of; Halogenoalkane + KOH/NaOH (in ethanol) -> Alkene + H2O + halide ion?

A
  • Alcoholic/Ethanolic solutions
  • Heat under reflux
54
Q

What is the reaction in order to produce a primary amine?

A

Halogenoalkane + NH3 -> Amine + ammonium halide

55
Q

What is the reagant needed in the reaction for making a primary amine?

A
  • NH3 dissolved in ethanol
56
Q

What are the conditions needed in the reaction fro making a primary amine?

A
  • Heating under pressure/ in a sealed tube
57
Q

Why is Ammonia heated in a sealed vessel in the reaction; Halogenoalkane + NH3 -> Amine + ammonium halide?

A

This is because ammonia is a gas and so otherwise the ammonia would escape without taking part in the reaction

58
Q

What acts as the nucleophile in the first step of the reaction; Halogenoalkane + NH3 -> Amine + ammonium halide?

A

Ammonia, NH3: (lone pair)

59
Q

What acts as a base in the second step of the reaction; Halogenoalkane + NH3 -> Amine + ammonium halide?

A

Ammonia, NH3:

60
Q

What type of reaction is; Halogenoalkane + NH3 -> Amine + ammonium halide?

A

Nucleophilic substitution

61
Q

What happens in the first step of the reaction to make an amine; Halogenoalkane + NH3 -> Amine + ammonium halide?

A

Regular nucleophilic substitution. The NH3 acts as a nucleophile and bonds to the delta + Carbocation removing the halide ion from the Halogenoalkane. This is because as the Nucleophile gets closer to the carbon, the electrons on the nucleophile repel the bonded pair of electrons within the C-X bond. Halogenoalkane + NH3 -> CxHxNH3+ + Halide ion

62
Q

What happens in the second step of the reaction to make an amine; Halogenoalkane + NH3 -> Amine + ammonium halide?

A

In the second step the NH3 acts as a base and removes a hydrogen from the NH3+. This is because the NH3+ is electron deficient and it gets the electron when the hydrogen is removed by the other NH3. CxHxNH3+ + NH3 +Cl- -> CxHxNH2 + NH4(halide)

63
Q

In what conditions will only a primary amine be formed?

A

Excess ammonia

64
Q

In what conditions will a secondary, tertiary etc amine be formed?

A

Excess Halogenoalkane, as this means the N gains more lone pairs so becomes more available to act to act as a nucleophile. Therefore multiple substitution reactions occur.

65
Q

Why is the reaction for forming a nitrile form a Halogenoalkane so beneficial?

A

As it extends the carbon chain by 1 therefore you are ascending the homologous series

66
Q

What are the reagents to form a nitrile form a Halogenoalkane?

A

KCN in ethanol

67
Q

What are the conditions to form a nitrile from a Halogenoalkane?

A

Heat under reflux

68
Q

What type of reaction is it when you go from a Halogenoalkane + KCN -> Nitrile + Halide ion?

A

Nucleophilic substitution

69
Q

What is the mechanism for reaction is it when you go from a Halogenoalkane + KCN -> Nitrile + Halide ion?

A

The nitrile :C (triple bond) N acts as the nucleophile and attacks the carbocation and removes the halide ion

70
Q

Write the reaction of 1-chlorobutane with water

A

CH3CH2CH2CH2Cl + H2O → CH3CH2CH2CH2OH + Cl- +H+

71
Q

Describe the mechanism for the nucleophilic elimination reaction of CH3CH2Br + KOH → CH2=CH2 + Br- + H2O

A
  • The OH- nucleophile attacks the hydrogen bonded to the carbon that is next to the carbon bonded to the halogen. It donates its 2 electrons to the hydrogen atom meaning the carbon has 2 electrons available to form a bond.
  • The halogen is good at leaving and so it takes the pair of electrons from its C-X bond with it leaving the carbon electron deficient by 2
  • Therefore the 2 Carbon atoms form a bond with each other leaving them with a double carbon carbon bond.
72
Q

What does the hydroxide ion act as in elimination reactions?

A

A base

73
Q

Why does the Nitrogen gain a + charge when it bonds to the Carbon atom in the Nucleophilic substitution reaction of ammonia with a Halogenoalkane?

A
  • On the left hand side of the equation you start with two overall neutral molecules. Assuming you forgot about the positive charge, you would end up with a neutral species and a negative ion on the right. Charges must balance in equations, so something is wrong.
  • The nitrogen looks wrong! The nitrogen atom is joined to 4 things rather than its usual 3. Nitrogen can only join to 4 things if it carries a positive charge.

chemistry a - level (26 decks)

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