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CHEMISTRY (AS) > Halogenoalkanes > Flashcards

Halogenoalkanes Flashcards

1
Q

What are Halogenoalkanes?

A

⦾ Alkanes where H atoms are replaced with (+)1 halogen atom

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2
Q

What do Halogenoalkanes contain?

A

⦾ Polar bond

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3
Q

What does these polar bonds in Halogenoalkanes result in?

A

⦾ Halogen more electronegative than C
⦾ e- density drawn towards halogen
⦾ 𝛿+ and 𝛿− regions formed

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4
Q

What is a nucleophille?

A

⦾ e- donor
⦾ Lone electron pair attracted to 𝛿+ regions
⦾ Like +ve

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5
Q

Give 3 examples of nucleophilles?

A

⦾ CN:-
⦾ :NH3
⦾ -:OH

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6
Q

What does a carbon-halogen bond look like?

use X as the halogen

A

𝛿+ 𝛿−

C - X

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7
Q

What do nucleophillic substitution reactions do?

A

1 ⦾ Nucleophille attacks polar molecule
2 ⦾ Kicks out functional group
3 ⦾ Replaces it

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8
Q

What is the general equation for a nucleophillic substitution reaction?

A

⦾ CH3CH2X + Nu- → CH3CH2Nu + X-

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9
Q

Explain what generally happens in a nucleophillic substitution reaction?

A

1 ⦾ Lone e- pair on nucleophille attacks 𝛿+ on C
2 ⦾ Creates new bond between nucleophille + C
3 ⦾ C can only bond to 4 other atoms so it breaks bond with halogen (X)
4 ⦾ Pair of e- from C-X bond taken by halogen + become lone pair

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10
Q

What are the conditions for the nucleophillic substitution reaction between Bromoethane and Sodium hydroxide when its changed to ____________?

A

⦾ Warm aqueous NaOH or KOH

⦾ Ethanol

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11
Q

Explain the nucleophillic substitution reaction between Bromoethane and __OH?

A 
1 ⦾ The C-Br bond is polar
2 ⦾ 𝛿+C attracts lone e- pair from OH-
3 ⦾ OH- attacks 𝛿+C 
4 ⦾ New bond forms between C and OH- making alcohol
5 ⦾ C-Br bond breaks 
6 ⦾ Both e- from bond taken by Br
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12
Q

What are the conditions for the nucleophillic substitution reaction between Bromoethane and ethanolic potassium cyanide when its changed to _________ and _______?

A

⦾ Warm halogenoalkane with ethanolic KCN
⦾ Under reflux
⦾ Propanenitrile and Potassium bromide

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13
Q

What happens to ethanolic potassium cyanide?

A

⦾ Dissociates into K+ and CN-

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14
Q

Explain the nucleophillic substitution reaction between Bromoethane and ethanolic potassium cyanide?

A

⦾ Lone e- pair on CN- attacks 𝛿+C
⦾ C-Br bond breaks
⦾ Br leaves

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15
Q

Why do you have to use ethanol and not water as a solvent for the nucleophillic substitution reaction between Bromoethane and ethanolic potassium cyanide?

A

⦾ Water could act as competing nucleophile

⦾ You’d get some alcohol product

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16
Q

What is an amine?

A

⦾ R3N

⦾ N always has lone e- pair

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17
Q

What are the conditions for the nucleophillic substitution reaction between Bromoethane and ethanolic ammonia when its changed to _________ and _______?

A

⦾ Warm halogenoalkane with excess ethanolic ammonia
⦾ Sealed tube
⦾ amine and ammonium ion → Ammonium bromide

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18
Q

Explain the nucleophillic substitution reaction between Bromoethane and ethanolic ammonia?

A

1 ⦾ N from NH3 donates lone e- pair to 𝛿+C
2 ⦾ Br removed + replaced by NH3
2 ⦾ Neutral N lost e- so now left with +ve charge
3 ⦾ 2nd ammonia removes H from NH3 to form NH4+ ion + amine
4 ⦾ NH4+ donates lone e- pair to H to form bond so N in ammonium now has +ve charge + amine has no charge
5 ⦾ Ammonium ion reacts with Br- ion to form Ammonium bromide - held together by ionic bond

19
Q

What is the overall reaction for the nucleophillic substitution reaction between Bromoethane and ethanolic ammonia?

A

⦾ CH3CH2Br + 2NH3 + ethanol → CH3CH2NH2 + NH4Br

20
Q

Which is the strongest C-X bond from the halogens?

A

⦾ C-F

⦾ Highest bond enthalpy

21
Q

What does the C-F bond being the strongest from the halogens mean for when it undergos nucleophillic substitution reaction?

A

⦾ More slowly than other halogenoalkanes

22
Q

Which is the strongest C-X bond from the halogens?

A

⦾ C-I

23
Q

Nucleophillic substitution reactions can only occur for ___________ and ___________ halogenoalkanes?

A

⦾ Primary

⦾ Secondary

24
Q

What is an elimination reaction?

A

⦾ Halogeoalkanes
⦾ Heated at high temps
⦾ Alcoholic conditions

25
Q

What does a nucleophile act as in an elimination reaction?

A

⦾ Base - accepts proton - removing H from molecule

26
Q

What are the conditions for the elimination reaction of

2-bromopropane with potassium hydroxide?

A

⦾ Potassium hydroxide dissolved in warm ethanol

⦾ Reflux

27
Q

If you ____ halogenoalkane with hydroxide ions dissolved in ethanol instead of water, an elimination reaction occurs and you end up with an ______.

A

⦾ Heat

⦾ Alkene

28
Q

What is the overall equation for the reaction of

2-bromopropane + potassium hydroxide (dissolved in warm ethanol)?

A

⦾ CH3CHBrCH3 + KOH → CH2CHCH3 + H2O + KBr

29
Q

Explain the reaction of 2-bromopropane + potassium hydroxide (dissolved in warm ethanol)?

A

⦾ Elimination reaction
1 ⦾ OH- acts as base + takes proton, H+ from 𝛿+C adjacent to C with Br attached
2 ⦾ 𝛿+C now has spare e- pair so it forms double bond with C with Br attached
3 ⦾ To form double bond, C with Br attached has to let go of Br - Br- kicked out
4 ⦾ Propene, Water and :Br- formed

30
Q

What happens if you used a longer, unsymmetrical secondary alkene? And why?

A

⦾ 2 products formed
⦾ Double bond could go on either side of C with Br attached
e.g. 2-bromobutane forms: but-1-ene + but-2-ene

31
Q

How can you influence whether a nucleophillic substitution or elimination reaction occurs?

A

⦾ Changing the conditions

32
Q

What happens when you react bromoethane with

NaOH or KOH with water under reflux?

A

⦾ Nucleophillic substitution
⦾ Produce ethanol
⦾ Aqueous cond. so OH- acts as nucleophile
⦾ :OH- donates e- pair to 𝛿+C

33
Q

What happens when you react bromoethane with

NaOH or KOH ethanol under reflux?

A

⦾ Elimination reaction
⦾ Produces ethene
⦾ OH- acts as base - removes H from halogenoalkane

34
Q

What would happen if you used mixture of water and ethanol as the solvent?

A

⦾ Both reactions happen

⦾ Mixture of 2 products

35
Q

What does Ozone in the atm do?

A

⦾ Absorbs UV radiation

36
Q

What would UV radiation do if it reached us?

A

⦾ Suburn

⦾ Skin cancer

37
Q

How is Ozone formed?

A

1 ⦾ UV radiation breaks down 1 O2 into 2 free radicals
O2 + hV → O• + O•
2 ⦾ Free radicals attack other O2 forming Ozonw
O• + O2 → O3

38
Q

How is the Ozone layer being destroyed in the upper atm?

A

⦾ Cl-Cl bonds in CFCs split into Cl• by UV radiation
⦾ Cl• react with Ozone (O3) to form intermediate (ClO•)
+ O2

39
Q

What are the reactions for the CFCs and Ozone?

A

CCl3F(g) + UV → •CCl2F(g) + Cl•(g)
Cl•(g) + O3(g) → O2(g) + ClO•(g)
ClO•(g) + O3(g) → 2O2(g) + Cl•(g)

40
Q

What does the Cl• free radical act as?

A

⦾ catalyst

⦾ regenerated

41
Q

What is the overall reaction for Ozone depletion?

A

2O3(g) → 3O2(g)

42
Q

CFCs are pretty __________, ___-_________ and

___-_____?

A

⦾ Unreactive
⦾ Non-flammable
⦾ Non-toxic

43
Q

What were the previous uses of CFCs?

A

⦾ Fire-extinguishers
⦾ Propellants in aerosols
⦾ Coolant gas in fridges
⦾ Added to foam plastics to make insulation + packaging materials

44
Q

What are the safer alternatives developed by chemists for CFCs?

A 
⦾ Contain no Chlorine
⦾ HCFCs
⦾ HCFs
⦾ Pump spray systems for aerolsols
⦾ Nitrogen as propellant
⦾ Ammonia in industrial fridges

CHEMISTRY (AS) (24 decks)

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