CHEM 2 - Haloalkanes, mechanisms

AQA CHEM 2 - Haloalkanes, 

Mechanisms:

- Nucleophillic subsitution

- Elimination (OH- acts as base)

- Electrophillic addition of alkenes

- Free radicals (CFCs)

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  • Created by: Franklin
  • Created on: 14-04-14 22:56

Haloalkanes

Prefixes - -fluoro, -chloro, -bromo and -iodo indicate a halogen is present in the molecule. (Written in alphabetical order)

  •  C-X bond (where X is a halogen) is polar, halogen is more electronegative than carbon, partial charges on C and X
  • C-X bond not polar enough to be soluble in water
  • Boiling point increases with chain length and halogen. Down the group halogen have greater vdW
  • Haloalkanes have higher boiling points than their alkane counterparts due to higher RAM and polar bonds
  • C in C-X bond is electron deficient and so can be attacked by electron-rich reagents
  • These electron-rich reagents are called nucleophiles. They are electron donors
  • Reactivity of C-X bond increases down the group
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Nucleophillic Substitution

Nucleophiles are electron-rich reagents that attack partially positive carbon atoms

(http://www.chemguide.co.uk/mechanisms/nucsub/nucleophiles.GIF)

  • Nucleophile can either be a negatively charged (partial or full) atom or charged atom within a complex ion
  • C-X bond is always the bond which is broken. Stronger bond = slower reaction, C-F bond the strongest 
  • In terms of reactivity, bond strength is more important than polarity. So even though C-F bond is the most polar, it is the least reactive

(http://www.chemguide.co.uk/mechanisms/nucsub/sn2gen.GIF)

  • Nucleophile approaches the delta+ carbon's backside and ends up forming a bond with it.
  • Simultaneously the electron pair in the C-X bond gets pushed towards the X and the X comes of as a -ve ion, X-
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NucSub 2

(http://www.chemtube3d.com/model/ALevel/sn2rxn.GIF)

SN2  - Subsitution, Nucleophillic, 2 species involved 

Conditions:

  • Aqueous nucleophile-containing reagent
  • Dissolved in ethanol (since haloalkane is insoluble in water, hydrolysis reaction)
  • Water encourages substitution
  • Room Temp
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(Base) Elimination

OH- acts as a base removing a H

 Conditions:

  • Elimination favours:
  • Heat
  • Pure ethanol, no water
  • Concentrated KOH/NaOH

(http://www.chemguide.co.uk/mechanisms/elim/elimpropm.GIF)

(http://www.chemguide.co.uk/mechanisms/elim/elimeq1.GIF)

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