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Chemistry U2 > 2.6 halogenoalkanes > Flashcards

2.6 halogenoalkanes Flashcards

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1
Q

What are halogenoalkanes?

A

compounds in which a halogen atom has replaced at least one of the hydrogen atoms in an alkane chain

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2
Q

What is a nucleophile?

A

An electron pair donor

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3
Q

what is the difference between primary, secondary and tertiary halogenoalkane?

A

primary - only one alkyl group attached to the carbon to which the halogen is bonded
secondary - two alkyl groups
tertiary - three alkyl groups

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4
Q

what is produced when a halogenoalkane reacts with an aqueous alkali like KOH?

A

an alcohol is produced.
the OH- ion acts as a nucleophile and a nucleophilic substitution takes place

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5
Q

what is formed when a halogenoalkane reacts with ethanolic potassium hydroxide? what type of reaction is this?

A

an alkene is produced
the reaction is an elimination reaction

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6
Q

what is formed when a halogenoalkane reacts with warm ethanolic ammonia?

A

an amine is produced
ammonia acts as a nucleophile in a nucleophilic substitution reaction

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7
Q

what is formed when a halogenoalkane reacts with alcoholic potassium cyanide?

A

a nitrile is produced
the cyanide ion acts as a nucleophile = nucleophilic addition

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8
Q

write the chemical equation for the reaction between bromoethane and ethanolic potassium hydroxide

A

CH3CH2Br + KOH –> CH2CH2 + H2O +KBr

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9
Q

what reaction can be carried out to increase the number of carbons in the halogenoalkane chain?

A

nucleophilic substitution of the halogenoalkane with a cyanide ion in alcoholic conditions.
the cyanide ion CN- contains a carbon atom so the carbon chain length increases by one

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10
Q

how can halogenoalkanes be qualitatively identified?

A

add NaOH, followed by aqueous silver nitrate solution to the halogenoalkane.
if halogenoalkane is present then a silver halide precipitate will form in the solution

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11
Q

what colour precipitate will form when chloroethane is added to silver nitrate? + will it dissolve in dilute ammonia?

A

AgCl - white precipitate
dissolves in dilute NH3

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12
Q

what colour precipitate will form when bromoethane is added to silver nitrate?+ will it dissolve in NH3?

A

AgBr - cream precipitate
dissolves in concentrated NH3

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13
Q

what colour precipitate will form when iodoethane is added to silver nitrate?+ will it dissolve in NH3?

A

AgI- yellow precipitate
does NOT dissolve in NH3

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14
Q

what is the name of the reaction between a halogenoalkane and ammonia?

A

nucleophilic substitution

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15
Q

give the steps for the prac (hydrolysis) of identifying the 3 halogenoalkanes

A
  1. add NaOH and heat
  2. add HNO3
  3. add AgNO3
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16
Q

what is the trend in reactivity of primary, secondary and tertiary halogenoalkanes?

A

tertiary > secondary > primary
the tertiary is most reactive and the primary is least reactive

17
Q

what is the trend in reactivity of chloro-, bromo- and iodo- halogenoalkanes? explain in terms of bond enthalpy

A

iodo- > bromo- > coloro- >
iodo halogenoalkanes are the most reactive because the C - I bond is the weakest (low bond enthalpy) and so it is broken easily.

Chloro- halogenoalkanes are least reactive because the C- Cl has a high bond enthalpy and therefore requires a lot of energy to break

18
Q

what are some uses of halogenoalkanes?

A
  • solvents
  • refrigerants
  • anaesthetics
19
Q

why are there tight regulations surrounding the use of halogenoalkanes?

A

halogenoalkanes are very toxic and have a negative impact on the environment, so their use is regulated

20
Q

what are CFCs?

A

CFC stands for Chlorofluorocarbon. This is a type of
hydrocarbon that is used in most types of refrigerants for
building applications such as centrifugal chillers,
refrigerators, and humidifiers. Most applications were
installed prior to ozone concerns and they are currently
being phased out by the Montreal Protocol.

21
Q

what is the environmental problem associated with the use of CFCs?

A

CFCs damage the ozone layer. the ozone layer is the upper atmosphere and it absorbs a lot of UV radiation

CFCs are broken down by UV light, releasing a chlorine radical reacts with ozone and breaks it down to oxygen. this meanss there is less protection

22
Q

give the equation for the reaction between a chlorine free radical and ozone during propagation

A

Cl* + O3 ➣ O2 + ClO*
ClO* + O3 ➣ 2O2 + Cl*

23
Q

give the equation for the reaction between a chlorine free radical and ozone during the termination stage

A

2O3 ➣ 3O2

24
Q

give the equation of the CFCs during the initiation stage

A

initiated by the uv radiation in the upper atmosphere
CFCl3 –> Cl* + CFCl2*

25
Q

explain the impact of the bonds C-H, C-F and C-Cl on the atmosphere in reference to the strength of the bond

A

the C-Cl bond is the weakest of the three bonds so it breaks first. This means the C-Cl bond has the greatest impact on the environment as the free radical is produced most easily. The C-H and C-F bonds are much stronger with C-F having the highest bond strength

26
Q

what is a reflux setup?

A

reflux is the boiling setup with a vertical condenser that allows the vapours to return to the same mixture once they condense .

27
Q

why are halogenoalkanes good refrigerants?

A
  • boiling point close to room temp so easily vaporised
  • non toxic
  • non flammable
28
Q

why are halogenoalkanes good solvents?

A

they are polar compounds but insoluble in water , able to mix with polar and non-polar organic molecules

29
Q

what type of sodium hydroxide is needed for elimination reactions?

A

ethanolic NaOH

30
Q

what type of sodium hydroxide is needed for substitution reactions?

A

aqueous NaOH

31
Q

why can nucleophilic substitution reactions occur?

A

due to the polar carbon-halogen bond as the nucleophile is attracted to the partially positive carbon in the bond

Chemistry U2 (5 decks)

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