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Halogen Derivatives Flashcards

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1
Q

What are halogen derivatives?

A

Organic compounds that contain at least one C-X bond where X is a halogen

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2
Q

What are the three types of halogen derivatives?

A

alkyl halides, aryl halides, acyl halide

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3
Q

What is the general formula for alkyl halides?

A

R-X

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4
Q

What is the general formula for aryl halides?

A

Ar-X

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5
Q

What is the general formula for acyl halide?

A

RCOX or ArCOX

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6
Q

What affects the boiling point of alkyl halides?

A

As the size of halogen atom increases, the boiling point increases

As the size of halogen atom increases, the number of electrons in the molecule increases and the electron cloud of the molecule become increasingly more polarisable, the id-id between the molecules become increasingly stronger and require an increasing amount of energy to overcome

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7
Q

What happens to the boiling point of alkyl halide as the number of carbon atom increases?

A

As the number of carbon atoms in the molecules increases, the number of electrons in the molecule increases and the electron cloud of the molecule becomes increasingly more polarisable, hence the id-id between molecules become increasingly stronger and require an increasing amount of energy to overcome

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8
Q

Are halogenoalkane soluble in water and why?

A

Although halogenoalkanes are polar, they are insoluble in water because the energy released from the formation of new interactions between halogenoalkane and water molecule is insufficient to compensate for the energy required to overcome the original interactions between halogenoalkane molecules and between water molecules

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9
Q

What are halogenoalkanes soluble in?

A

Organic solvents such as ethanol

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10
Q

Are halogenoalkanes denser than water?

A

Halogenoalkane with one Cl less denser
The rest denser

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11
Q

How to prepare halogen derivatives?

A
  1. Free radical substitution of alkanes
  2. Electrophilic addition to alkenes
  3. Nucleophilic substitution of -OH group in alcohols by X
  4. Electrophilic substitution of arenes to produce aryl halides
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12
Q

What are the types of electrophilic addition of alkenes to prepare halogen derivatives?

A
  1. Addition of HX
  2. Addition of X2
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13
Q

What are the reagents and conditions for free radical substitution of alkanes?

A

X2 (g), UV light
mono-substituted product: limited X2 or excess alkane
poly-substituted product: excess X2

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14
Q

What are the reagents and conditions for addition of HX to alkenes?

A

HX (g), room temperature

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15
Q

What are the reagents and conditions for addition of X2?

A

Cl2 (g) or Br(l), room temperature

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16
Q

What are the types of nucleophilic substitution of -OH group in alcohols by -X?

A
  1. Using HX
  2. Using PX3
  3. Using PCl5
  4. Using SOCl2
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17
Q

What are the reagents and conditions for nucleophilic substitution of -OH by -X using HX?

A

HX (g), heat or
conc. HCl, ZnCl2 catalyst, heat under reflux

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18
Q

What is the equation for the preparation of halogen derivatives using HX through nucleophilic substitution?

A

R-OH + HX → R-X + H2O

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19
Q

What is the order of reactivity of HX with R-OH and why?

A

HI > HBr > HCl
The bond energy increases from HI to HCl because the size of halogen atom decreases from I to Cl, and the valence orbitals become less diffused. This results in more effective overlap of orbitals between the small H atom and the larger halogen atom and thus more energy is required to break the stronger H-X bond

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20
Q

What is the order of reactivity of alcohol and why?

A

tertiary>secondary>primary
tertiary alcohols form tertiary carbocations which is more stable as it has more electron-donating alkyl groups reduces the positive charge and stabilises the carbocation, thus it is most reactive

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21
Q

How to prepare HCl and HBr?

A

HX can be prepared in situ using solid KCl or KBr and concentrated sulfuric acid

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22
Q

How to prepare HI?

A

HI can be prepared in situ using solid KI and concentrated H3PO4

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23
Q

What are the reagents and condition for the nucleophilic substitution of -OH by -X using PX3?

A

PX3, room temperature

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24
Q

What is the equation for the preparation of halogen derivatives using PX3?

A

3R-OH + PX3 → 3R-X + H3PO3

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25
Q

How are PBr3 and PI3 prepared?

A

In situ by heating mixtures of red phosphorous (P4) and Br2 or I2

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26
Q

What are the reagents and conditions for the nucleophilic substitution -OH by -X using PCl5?

A

solid anhydrous PCl5, room temperature

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27
Q

What is the equation for the preparation of halogen derivatives using PCl5?

A

R-OH + PCl5 → R-Cl + HCl +POCl3

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28
Q

What is the observation for the preparation of halogen derivatives using PCl5?

A

white fumes (HCl)

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29
Q

What are the reagents and conditions for the nucleophilic substitution of -OH by -X using SOCl2?

A

SOCl2, room temperature

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30
Q

What is the equation for the preparation of halogen derivatives using SOCl2?

A

R-OH + SOCl2 → R-Cl + SO2 + HCl

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31
Q

What is the observation of preparing halogen derivatives using SOCl2?

A

white fumes (HCl)

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32
Q

What is the advantage of preparing halogen derivatives using SOCl2?

A

convenient way of producing liquid halogenoalkanes as the by-products are gases

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33
Q

What are the reagents and conditions for electrophilic substitution of arenes to produce aryl halides?

A

X2, Fe or anhydrous FeX3 or anhydrous AlX3

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34
Q

Are alkyl halides reactive?

A

Alkyl halides are fairly reactive because C-H bond is polar

35
Q

What do alkyl halides tend to react with?

A

C atom is electron-deficient, thus alkyl halides tend to react with nucleophiles and bases (lewis bases are electron pair donors)

36
Q

What are the main types of reactions alkyl halides undergo?

A
  1. nucleophilic substitution
  2. elimination reaction
37
Q

What is the order of reactivity of alkyl halides (in decreasing order) and why?

A

R-I > R-Br > R-Cl > R-F
bond energy of C-I is the lowest and increases to C-F, thus C-I is most reactive

38
Q

Are fluoroalkanes reactive and why?

A

Unreactive because C-F bond is very strong

39
Q

Are aryl halides more or less reactive than alkyl halides and why?

A

Aryl halides are very much less reactive than alkyl halides
1. Partial double bond character of C-X bond
A lone pair in the p orbital of halogen atom is delocalised into the pi electron cloud of the benzene ring as the p orbital can overlap sideways with the pi electron cloud, this strengths the C-X bond and causes the partial double bond character, thus halogen atom in aryl halide is more difficult to be substituted compared to halogen atom in alkyl halide
2. decreased polarity of C-X bond
Interaction of the p orbital of the halogen atom with the pi electron cloud of benzene ring causes a drift of electron density from X towards the electron deficient C atom, reducing the polarity of the bond
3. presence of electron rich ring
Hinders nucleophilic attack due to electronic repulsion between electrons in the ring and approaching electron rich nucleophile

40
Q

What are the neutral nucleophiles?

A

H2O, NH3, ROH, RNH2

41
Q

What are the anionic nucleophiles?

A

OH-, CN-, X-, RO-

42
Q

What are nucleophiles?

A

Electron-rich species which have at least one lone pair of electrons capable of forming a dative covalent bond with an electron-deficient C atom

43
Q

What are the nucleophilic substitution reactions of alkyl halides?

A
  1. Reaction with OH-
  2. Reaction with CN-
  3. Reaction with ethanolic ammonia
  4. Reaction with ethanolic amine
44
Q

What are the reagents and conditions for the reaction between alkyl halides and OH-?

A

NaOH(aq) or KOH(aq), heat under reflux

45
Q

Why can’t the reaction of alkyl halides and OH- be in alcoholic medium?

A

alkyl halide will undergo elimination to form alkene

46
Q

What happens when a geminal diol is formed when alkyl halide react with OH-?

A

It is not stable, therefore will be converted to carbonyl with the elimination of water

47
Q

What happens when a geminal triol is formed?

A

It is not stable, therefore will be converted to carboxylic acid with the elimination of water

48
Q

What is the reagents and conditions of the reaction between alkyl halide and CN-?

A

ethanolic NaCN or ethanolic KCN, heat under reflux

49
Q

What are the reagents and conditions of the reaction between alkyl halide and NH3?

A

excess ethanolic NH3, heat in sealed tube

50
Q

Why is there a need for heat in sealed tube for the reaction between alkyl halide and NH3?

A

To prevent the escape of volatile NH3 during heating

51
Q

What is the equation for the reaction between alkyl halide and NH3?

A

R-X + NH3 → R-NH2 + HX

52
Q

What happens after the reaction between alkyl halide and NH3?

A

Amine that is produced is a nucleophile and is more reactive than ammonia, it can react and further substitute the alkyl halide

R-X + NH3 → RNH2 + HX
R-X + RNH2 → R2NH + HX
R-X + R2NH → R3N + HX
R-X + R3N → R4N+X- (quartenary ammonium salt)

53
Q

How to get a higher yield of primary amine from the reaction of alkyl halide and NH3?

A

Use excess NH3

54
Q

Are amines or ammonia a better nucleophile and why?

A

Amines are better nucleophiles than ammonia as the alkyl groups are electron-donating, thus enhancing the availability of the lone pair of electrons on the nitrogen atom

55
Q

What is the order of the strength of nucleophile for ammonia and amines?

A

NH3 < primary amine < secondary amine < tertiary amine

56
Q

What is the reagents and conditions for the reaction between alkyl halides and ethanolic amines?

A

alcoholic amine, heat under reflux

57
Q

What are the reagents and conditions for elimination reaction of alkyl halides?

A

ethanolic NaOH or ethanolic KOH, heat under reflux

58
Q

What does OH- act as in the elimination of alkyl halide?

A

In alcoholic medium, OH- acts as a base instead of a nucleophile

59
Q

What reactions do aryl halides undergo?

A

Electrophilic substitution

60
Q

What does 1 in SN1 represent?

A

unimolecular or first order reaction

61
Q

What does 2 in SN2 represent?

A

bimolecular or second order reaction

62
Q

How many steps does SN2 mechanism have?

A

1 step with the formation of a transition state

63
Q

What does the energy profile diagram of SN2 reaction look like?

A

Reactants to transition state (highest energy) to products (lower energy than reactants)

64
Q

Is there inversion of configuration at the C atom for SN2?

A

Yes

65
Q

What is the order of reaction for SN2 and what is the rate equation?

A

second order
Rate = k[alkyl halide][nucleophile]

66
Q

What is SN2 mechanism favoured by and why?

A

Primary alkyl halide because:
1. lack of bulky substituents make it have less steric hindrance and more susceptible to backside attack
2. primary carbocations are relatively unstable because they have only one electron-donating alkyl group to reduce the positive charge

67
Q

What happens when the nucleophile in SN1 and SN2 is a molecule instead of an anion?

A

An addition step of deprotonation will occur

68
Q

How many steps does SN1 have?

A

2 steps with a formation of carbocation intermediate

69
Q

What is the shape of carbocation in SN1?

A

Trigonal planar

70
Q

Is there inversion of configuration at the C atom for SN1?

A

Can be inverted or retained depending on the direction of attack of the nucleophile

71
Q

What happens if a halogen atom in alkyl halide is not bonded to a chiral carbon for SN1?

A

There will only be one product formed as there are no enantiomers

72
Q

What is the order of reaction and rate equation for SN1?

A

First order
Rate = k[alkyl halide]

73
Q

What does the energy profile diagram of SN1 look like?

A

2 peaks, the first one higher than the other

74
Q

What is SN1 reaction favoured by and why?

A

Tertiary alkyl halide because:
1. electron-donating alkyl group reduces the positive charge on the electron deficient carbon atom in the carbocation and hence stabilises the carbocation
2. The electron-deficient carbon is shielded by bulky substituents, hindering the attack by the nucleophile

75
Q

What are some excepts to SN1 and SN2?

A
  • primary alkyl halide that have a R group that is very bulky does not undergo SN2 due to steric hindrance which prevents backside attack
  • alkyl halide with a benzene ring undergoes SN1 instead of SN2 because carbocation is stabilised by delocalisation of positive charge into the benzene ring
76
Q

What is the chemical test to test for alkyl halide?

A
  1. heat alkyl halide with NaOH (aq) for a few minutes
    R-X + OH⁻ → R-OH + X⁻
  2. Acidify the cooled products with dilute HNO3 to remove excess alkali
  3. Add AgNO3 (aq)
    Ag⁺ + X⁻ → AgX (s)

AgCl: white ppt.
AgBr: cream ppt.
AgI: yellow ppt.

77
Q

What is the chemical test to distinguish alkyl halide and aryl halide?

A

Test: NaOH(aq), heat
Alkyl halide: ppt. formed
Aryl halide: no ppt. formed

78
Q

What are CFCs?

A

halogenoalkanes which contain chlorine and fluorine but no hydrogen

79
Q

What are the uses of halogen derivatives and what are their properties that makes it ideal for the use?

A
  • Refrigerants
  • Aerosol propellant: gas that propel liquid out of spray cans forming fine mist of droplets (CFCs are volatile, inert and non-toxic ideal for this)
  • Fire extinguisher: volatile, dense, inert and non-inflammable nature of CFCs
  • Grease solvents
  • Polymer production: monomer for Teflon which is used as non-stick coating for pans
  • Petrol additives
  • Antiseptics
  • Pesticides
  • Anaesthetics
80
Q

What is the natural process of ozone in the stratosphere like?

A

The formation and destruction of ozone by natural process is a dynamic equilibrium that maintains a constant concentration of ozone in the stratosphere

81
Q

What happens to chlorofluoroalkanes released in the atmosphere?

A

Due to their stability, chlorofluoroalkanes do not break down and eventually find their way into their upper atmosphere, once they reach ozone layer, they cause ozone depletion

82
Q

Why are chlorofluoroalkanes so harmful?

A

Chlorine atoms are not used up in these reagents and are homogeneous catalyst, thus one CFC could destroy up to thousands of ozone molecules

83
Q

Which mechanism can form a racemic mixture?

A

SN1

84
Q

Why can SN1 form a racemic mixture?

A

There is equal probability of attack by the nucleophile from either face of the trigonal planar around the carbonation, which gives rise to equal proportions of both enantiomers, this a racemic mixture is obtained

Chemistry (21 decks)

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